Michael Schirra
email: michael.schirra@erdw.ethz.ch

Oscar Laurent

Petrochronology of Hydrothermal Rutile in Mineralized Porphyry Cu Systems

Keywords: Rutile, Porphyry Cu deposits, U-Pb dating, Cathodoluminescence, LA-ICP-MS, Mineral chemistry

Hydrothermally formed rutile is a common mineral in porphyry Cu (-Au-Mo) deposits and has the potential to provide versatile information about the physical-chemical conditions and timing of mineralization. In this study, we investigated the trace element composition and U-Pb dates of hydrothermal rutile by LA-ICP-MS together with its petrographic context, in four well-studied porphyry systems: Batu Hijau (Indonesia), Bingham Canyon (USA), El Salvador (Chile) and Northparkes (Australia). A similar sequence of rutile generations occupying comparable textural positions was found to be common to all investigated deposits. We particularly focused on the most common rutile generation that is formed by the breakdown of biotite and temporally closely associated with mineralization as demonstrated by intergrowth relationships between individual, blocky rutile grains and Cu-Fe sulfides as well as cathodoluminescence imaging of mineralized quartz veins.

Compositionally, rutile shows large trace element variations within and among the porphyry deposits, indicating that the compositions of both the local host rock and the fluid phase influence that of hydrothermal rutile. In comparison with non-hydrothermal rutile of various petrogenetic origins, rutile from porphyry Cu deposits show characteristic differences regarding their Sn, W, Si and Fe concentrations, indicating that rutile might be used to fingerprint porphyry-style mineralization. Due to its temporal link to Cu mineralization, texturally controlled in-situ U-Pb dating of blocky rutile grains by LA-ICP-MS provides the possibility to directly determine the absolute timing of Cu mineralization.

Despite variable incorporation of initial Pb in the investigated hydrothermal rutile, we obtained robust lower intercept U-Pb dates that overlap within uncertainty with previously obtained mineralization ages for all four deposits, including those based on high-precision (ID-TIMS) zircon dating of bracketing porphyry intrusions. This demonstrates that rutile can be used to accurately predict the age of porphyry Cu deposit formation and with a precision better than 1.5% relative for deposits older than ca. 10 million years.

Introduction

Porphyry Cu deposits are the products of short-lived magmatic-hydrothermal systems operating in the upper crust and represent the major source for Cu and other critical metals such Au, Mo, Ag, and Re [START_REF] Sillitoe | Porphyry copper systems[END_REF]. Their thermal evolution with time is of fundamental interest for ore formation due

Bingham Canyon

Hydrothermal activity at the Bingham Canyon porphyry Cu-Au-Mo deposit, Utah, is related to the multiphase Bingham Stock, which intruded into a sedimentary sequence [START_REF] Butler | Ore deposits of Utah[END_REF][START_REF] Babcock | Summary of the geology of the Bingham Canyon district[END_REF][START_REF] Redmond | The Bingham Canyon porphyry Cu-Au-Mo deposit. I. Sequence of intrusions, vein formation, and sulfide deposition[END_REF]. The Bingham Stock consists of an early equigranular monzonite (EGM) intrusion, which is crosscut by five successive porphyry intrusions: Quartz monzonite porphyry (QMP), latite porphyry (LP), quartz latite porphyry (QLP), biotite porphyry, and quartz latite porphyry breccia [START_REF] Moore | Compositions of biotites from unaltered and altered monzonitic rocks in the Bingham mining district[END_REF][START_REF] Babcock | Summary of the geology of the Bingham Canyon district[END_REF][START_REF] Redmond | The Bingham Canyon porphyry Cu-Au-Mo deposit. I. Sequence of intrusions, vein formation, and sulfide deposition[END_REF]. The QMP displays the most intense hydrothermal overprint and highest ore grades, which extend into the premineralization EGM and sedimentary units. The other porphyry intrusions are volumetrically minor and show decreasing vein densities, alteration intensity and ore grade towards the youngest, only weakly mineralized and partially unaltered latite porphyry breccia [START_REF] Redmond | The Bingham Canyon porphyry Cu-Au-Mo deposit. I. Sequence of intrusions, vein formation, and sulfide deposition[END_REF].

Zircon TIMS U-Pb dating yields an age of the EGM of 38.55 ± 0.19 Ma, supported by a 38.40 ± 0.19 Ma 40 Ar/ 39 Ar plateau age for primary biotite [START_REF] Parry | U-Pb dating of zircon and 40Ar/39Ar dating of biotite at Bingham[END_REF]. The individual porphyry dykes have been dated by CA-ID-TIMS of zircon indicating a short time interval of 320 ka for porphyry emplacement and Cu-Au ore formation between 38.10 to 37.78 Ma (von Quadt et al., 2013). Recent high-precision CA-ID-TIMS U-Pb zircon dating of the QMP, LP and QLP intrusions refined the maximum duration of Cu-Au mineralization to 662 ka [START_REF] Large | The magmatic to hydrothermal evolution of porphyry Cu-Au deposits -A zircon perspective[END_REF]. A Re-Os age of molybdenite within the QMP revealed an age of 37.00 ± 0.27 Ma [START_REF] Chesley | Preliminary Re-Os dating on molybdenite mineralization from the Bingham Canyon porphyry copper deposit[END_REF].

Several authors have reported the presence of rutile throughout the Bingham Canyon deposit [START_REF] Stringham | Granitization and hydrothermal alteration[END_REF][START_REF] Moore | Compositions of biotites from unaltered and altered monzonitic rocks in the Bingham mining district[END_REF]Lanier et al., 1978;[START_REF] Maughan | Contributions from mafic alkaline magmas to the Bingham porphyry Cu-Au-Mo deposit[END_REF]. In general, two types of rutile have been distinguished: sagenetic rutile, which forms oriented, thin, elongated crystals within igneous biotite in altered and unaltered rocks (e.g., [START_REF] Maughan | Contributions from mafic alkaline magmas to the Bingham porphyry Cu-Au-Mo deposit[END_REF] and blocky/stubby rutile associated with partially altered igneous and hydrothermal biotite (e.g., [START_REF] Stringham | Granitization and hydrothermal alteration[END_REF]). An overall increase in rutile abundance and size of individual crystals with depth and ore grades has been reported by [START_REF] Czamanske | Some geologic and potential resource aspects of rutile in porphyry copper deposits[END_REF].

El Salvador

The porphyry Cu-Mo deposit at El Salvador, Chile, is centered on an Eocene composite porphyritic granodiorite complex that intruded into Cretaceous and lower Tertiary volcanic-sedimentary rocks of the Indio Muerto district. Four porphyritic intrusions of decreasing age, the X, K, L and A porphyries, are related to alteration, veining and mineralization, whereas latite or dacite porphyritic dykes postdate ore formation [START_REF] Gustafson | The porphyry copper deposit at El Salvador[END_REF][START_REF] Cornejo | El Salvador, Chile porphyry copper deposit revisited: Geologic and geochronologic framework[END_REF]. The X porphyry intrusion is interpreted to pre-date the main mineralization event, which was temporally related mainly to the emplacement of the K porphyry and in a lesser extent to the younger L and A porphyries that are associated with lower Cu grades and less intense alteration [START_REF] Gustafson | The porphyry copper deposit at El Salvador[END_REF].

Dating of all intrusions by variable methods indicates an age of about 42 Ma for the El Salvador deposit [START_REF] Gustafson | The porphyry copper deposit at El Salvador[END_REF][START_REF] Mcwilliams | Ar/39Ar results from CODELCO El Salvador Samples[END_REF][START_REF] Mcwilliams | Ar/39Ar results from CODELCO El Salvador Samples[END_REF][START_REF] Cornejo | El Salvador, Chile porphyry copper deposit revisited: Geologic and geochronologic framework[END_REF][START_REF] Gustafson | Multiple centers of mineralization in the Indio Muerto district[END_REF]. More recent, SHRIMP-RG U-Pb zircon dating yielded ages of 43.6 ± 0.4 Ma for the X porphyry followed by the K porphyry at 42.9 ± 0.4 Ma and 42.3 ± 0.5 Ma and the L porphyry at 42.4 ± 0.6 Ma and 42.2 ± 0.5 Ma [START_REF] Lee | Magmatic evolution of granodiorite intrusions at the El Salvador porphyry copper deposit, Chile, based on trace element composition and U/Pb age of zircons[END_REF]. Re-Os ages of hydrothermal molybdenite suggest that molybdenite mineralization occurred between 41.8 and 41.2 Ma (Zimmermann et al., 2014).

The hypogene ore body at El Salvador is zoned similarly to the one at Batu Hijau with a bornite + chalcopyrite-rich core hosted by potassic altered rock grading outwards to chalcopyrite-dominated sulfide assemblages overlapping with the chlorite -sericite alteration zone [START_REF] Gustafson | The porphyry copper deposit at El Salvador[END_REF]. Rutile has been reported to be present within the alteration halos of all vein types in El Salvador [START_REF] Gustafson | The porphyry copper deposit at El Salvador[END_REF][START_REF] Gustafson | Patterns of mineralization and alteration below the porphyry copper orebody at El Salvador[END_REF]. The breakdown of ilmenite to rutile plus hematite and the replacement of sphene by rutile plus anhydrite or calcite are assumed by these authors to be the major rutile-producing processes.

Northparkes

At Northparkes in the Goonumbla district of New South Wales, Australia, composite quartz monzonite porphyries (QMP) host multiple economic Cu-Au anomalies within a small area of around 10 km 2 . All porphyries are rooted in a pre-mineralization, granite to quartz-monzonite pluton, emplaced into Ordovician volcanic rocks [START_REF] Heithersay | Endeavour 26 North: A porphyry copper-gold deposit in the Late Ordovician shoshonitic Goonumbla Volcanic Complex[END_REF][START_REF] Lickfold | Endeavour copper-gold porphyry deposits[END_REF]. The QMP complexes include in relative time sequence early biotite QMP, K-feldspar QMP, and augite-biotite-Kfeldspar QMP [START_REF] Lickfold | Endeavour copper-gold porphyry deposits[END_REF]. The K-feldspar and augite -biotite -K-feldspar QMP host the highest ore grades associated with the highest vein densities and most intense alteration overprint [START_REF] Heithersay | Endeavour 26 North: A porphyry copper-gold deposit in the Late Ordovician shoshonitic Goonumbla Volcanic Complex[END_REF][START_REF] Lickfold | Endeavour copper-gold porphyry deposits[END_REF][START_REF] Lickfold | Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits[END_REF]. Volumetrically minor monzonite porphyry dikes, termed zero porphyries, represent the youngest phase of intrusive activity at Northparkes, postdating the Cu-Au mineralization.

The Northparkes deposits are of Late Ordovician to Early Silurian age and one of the oldest wellpreserved porphyry copper deposits. Radiometric dating studies have produced an age range from 455 to 435 Ma for emplacement of the various intrusions and associated alteration using mainly 40 Ar/ 39 Ar dating of hornblende, biotite and sericite as well as U-Pb SHRIMP dating of zircons [START_REF] Perkins | 40Ar/39Ar and U-Pb geochronology of the Goonumbla porphyry Cu-Au deposits[END_REF][START_REF] Butera | Zircon U-Pb dating of Early Palaeozoic monzonitic intrusives from the Goonumbla area[END_REF][START_REF] Lickfold | Endeavour copper-gold porphyry deposits[END_REF][START_REF] Lickfold | Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits[END_REF]. [START_REF] Lickfold | Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits[END_REF] bracketed the mineralization period by U-Pb SHRIMP zircon dating of the biotite quartz monzonite (444.2 ± 4.7 Ma) and a Zero porphyry dyke (436.7 ± 3.3 Ma). Most recently, [START_REF] Pacey | The anatomy of an alkali porphyry Cu-Au system: Geology and alteration at Northparkes Mines[END_REF] used Rb-Sr dating of epidotes and whole rock samples from the propylitic alteration zones, providing a rough estimate of 450 ± 11 Ma for hydrothermal activity at Northparkes. All deposits at Northparkes share common hydrothermal features, including veining sequence, alteration patterns and mineralization style [START_REF] Heithersay | Endeavour 26 North: A porphyry copper-gold deposit in the Late Ordovician shoshonitic Goonumbla Volcanic Complex[END_REF]. The ore bodies consist of a bornite-rich center, surrounded by a chalcopyrite-rich zone that transitions outward to the pyritedominated periphery. Based on textural observations, ore-grade mineralization is interpreted to be related to quartz -K-feldspar -sulfide veins associated with pervasive potassic alteration [START_REF] Pacey | The anatomy of an alkali porphyry Cu-Au system: Geology and alteration at Northparkes Mines[END_REF]. Rutile is abundant and interpreted to be the result of biotite and titanoferous magnetite alteration. Size and geochemical composition of rutile, in particular high V contents, have been shown to be indicative for porphyry-style mineralization at Northparkes [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF].

Samples and Analytical Methods

The porphyry deposits included in this study were selected based on their spread in reported formation ages (Neogene to Early Silurian/Late Ordovician), their different geotectonic settings (intraoceanic vs. continental arc) and metal endowments (Cu-Au vs. Cu-Mo vs. Cu-Au-Mo). In addition, the timing and duration of hydrothermal activity at Batu Hijau and Bingham Canyon have been precisely bracketed by recent zircon CA-ID-TIMS dating [START_REF] Large | The magmatic to hydrothermal evolution of porphyry Cu-Au deposits -A zircon perspective[END_REF][START_REF] Large | Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U-Pb petrochronology: Geochronology[END_REF], allowing robust assessment of the U-Pb rutile dating accuracy.

Most samples used in this study are inherited from earlier sample collections used for fluid inclusion studies at ETH Zürich. One or several samples were selected for each deposit, preferably from the highgrade ore shell. Polished thin and thick sections were prepared for further petrographic and analytical work. A brief sample description is given in Table 1.

Conventional transmitted and reflected light petrography was carried out at ETH Zürich to find rutile grains and to identify their textural relationship with other alteration minerals and economic sulfides.

Subsequently, Secondary Electron (SE), Back-Scattered Electron (BSE) and CL imaging were conducted using a JEOL JSM 6390 LA secondary electron microscope equipped with a Deben Centaurus CL detector at an acceleration voltage of 15 kV. Some of the identified grains were checked for TiO2 polymorphs by acquiring Raman spectra using a 532 nm DPSS laser and a Dilor LabRam Instrument attached to an Olympus BX40 microscope. Obtained spectra were compared with reference spectra of rutile, anatase and brookite from the RRUFF database [START_REF] Lafuente | The power of databases: the RRUFF project[END_REF]. Only a small fraction (< 5%) of the identified grains were assigned as anatase and these showed no significant differences in trace element composition compared to rutile. Furthermore, anatase could also be microscopically identified with great confidence due to its very pale, slightly greenish color compared to the heavily colored rutile grains. Therefore, we did not check systematically every grain with the Raman. Brookite in contrast was never present in the investigated porphyry samples.

Analyses of rutile U-Pb isotopic compositions and trace element concentrations were carried out at ETH Zürich, Switzerland by laser ablation -inductively coupled plasma -sector field -mass spectrometry (LA-ICP-SF-MS) using a RESOlution S-155 (ASI/Applied Spectra) 193 nm ArF excimer laser system attached to an Element XR (Thermo) sector-field ICP-MS. We used a laser repetition rate of 4 Hz, a spot diameter of 51 μm (for U-Pb dating) or 19 μm (for trace elements) and a laser energy density on sample of ca. 4 J/cm 2 . The sample surface was cleaned immediately before each analysis by three pre-ablation pulses. Ablation was performed in a dual-volume, fast-washout S-155 ablation cell (Laurin Technic) fluxed with carrier gas consisting of ca. 0.5 L/min He and make-up gas consisting of ca. 1 L/min Ar and 2 mL/min N2. The ablated aerosol was homogenized via flushing through an in-house squid device before being introduced in the plasma. The ICP-MS instrument was optimized for maximum sensitivity on Pb, Th and U while keeping low the production of oxides ( 248 ThO + / 232 Th + ≤0.25%) and the U/Th ratio at ca. 1 (on NIST SRM612 glass). A typical measurement consisted in 25 s of background measurement followed by 40 s of sample ablation for U-Pb dating and 30 s for trace elements. For trace elements, we have recorded intensities for the following isotopes: 23 Na, 27 Al, 29 Si, 49 Ti, 51 V, 53 Cr, 55 Mn, 57 Fe, 59 Co, 60 For U-Pb dating, the resulting intensities were processed offline with the Igor Pro Iolite v2.5 software [START_REF] Hellstrom | Iolite: Software for spatially resolved LA-(quad and MC) ICP-MS analysis[END_REF], using the VizualAge data reduction scheme [START_REF] Petrus | VizualAge: A Novel Approach to Laser Ablation ICP-MS U-Pb Geochronology Data Reduction[END_REF]. Background-subtracted intensities were used to calculate isotope ratios, which were corrected for laser-induced Pb/U fractionation (after [START_REF] Paton | Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction[END_REF], instrumental mass discrimination and drift by conventional standard-sample bracketing, against rutile reference material R10 (using isotope ratios from [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF]. No common Pb correction was carried out and dates were interpreted by the lower intercept method in Tera-Wasserburg diagrams. Data from secondary rutile reference materials R13 [START_REF] Schmitt | High-sensitivity U-Pb rutile dating by secondary ion mass spectrometry (SIMS) with an O2+ primary beam[END_REF] and WOD-B (in-house) were processed as unknowns to check the accuracy of the corrections. Age calculations and data plotting were performed using the IsoplotR toolkit [START_REF] Vermeesch | IsoplotR: A free and open toolbox for geochronology[END_REF]. The data from unknowns and rutile reference materials are provided in the Supplementary Table . Systematic uncertainties were propagated on the final dates (as discussed in [START_REF] Zack | situ U-Pb rutile dating by LA-ICP-MS: 208Pb correction and prospects for geological applications[END_REF] using the same protocol as conventionally applied for zircon U-Pb geochronology [START_REF] Horstwood | Community-Derived Standards for LA-ICP-MS U-(Th-)Pb Geochronology -Uncertainty Propagation[END_REF], i.e. by quadratic addition of relative uncertainties on (i) the final lower intercept date obtained using IsoplotR; (ii) the 235 U and 238 U decay constants, known to a high precision (< 0.1% relative; [START_REF] Jaffey | Precision measurement of halflives and specific activities of 235U and 238[END_REF]; (iii) the isotopic ratios of the calibration reference material (set to 0.5% based on ID-TIMS data of [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF] and (iv) the long-term excess variance of secondary rutile reference materials (0.8% based on 8 analytical sessions over a period of 2 years).

For trace elements, the intensities were processed offline using the Matlab-based SILLS software [START_REF] Guillong | SILLS: A Matlab-based program for the reduction of LA-ICP-MS data of homogeneous materials and inclusions[END_REF]. The NIST SRM610 glass [START_REF] Jochum | Determination of reference values for NIST SRM 610-617 glasses following ISO guidelines[END_REF] was used to quantify element concentrations and correct for instrumental drift via conventional standard-sample bracketing. A TiO2 concentration of 99 wt.% was used as internal standard for relative sensitivity correction. A further matrix effect correction factor (of typically < 10% relative) was applied by comparing the average concentrations obtained on rutile reference material R10 (measured as an unknown) and the recommended values of [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF]. The USGS GSD-1G glass [START_REF] Guillong | Preliminary characterisation of new glass reference materials (GSA-1G, GSC-1G, GSD-1G and GSE-1G) by laser ablation-inductively coupled plasmamass spectrometry using 193 nm, 213 nm and 266 nm wavelengths[END_REF] was used as a validation reference material to check accuracy and reproducibility. Data is provided in the Supplementary Table.

Results

Microtextural Context of Rutile

In all investigated porphyry deposits rutile occurs in similar textural positions and mineral parageneses. Morphologically, two types of rutile can be distinguished: 1) needle-like rutile with very high aspects ratios ≥10 (e.g., Fig. 1D), and 2) blocky rutile with aspect ratios ≤ 10 (Fig. 1). Needle-like rutile occurs exclusively within igneous biotite and hydrothermal quartz. In biotite, rutile needles follow cleavage planes, resulting in a triangular mesh network structure (sagenetic rutile, Fig. 1D), whereas rutile needles in quartz are typically randomly oriented and can be slightly bent. Needleshaped rutile was generally too small to be targeted for LA-ICP-MS analyses and is hence not further considered in this study.

Blocky rutile occurs in three different textural positions:

(1) Most commonly, elongated, prismatic or equidimensional, rounded rutile grains are hosted by chlorite and/or sericite partially or completely replacing igneous and hydrothermal biotite (Fig. 1 and2). These rutile grains can be aligned along former cleavage planes or be randomly distributed and their sizes can vary from < 10 to > 500 µm.

Unaltered biotite rarely hosts blocky rutile grains. Most rutile grains that appear to be hosted by biotite are typically associated with biotite-replacing alteration minerals on the micro-scale (e.g., Fig. 2).

(2) Rutile may form in clusters up to a few millimeters in size that are associated with Cabearing minerals, such as anhydrite, calcite and apatite (Fig. 1L). Most individual rutile grains are typically between 20 to 200 µm in size.

(3) Some individual rutile grains are hosted within hydrothermal quartz veins. These are rare compared to the other two textural positions. CL imaging reveals the spatial association of these rutile grains with a specific quartz generation characterized by comparatively dull luminescence. The dull-luminescent quartz generation crosscuts and truncates growth zones of bright luminescent quartz, besides rutile also hosts Cu-Fe sulfides and is present in all investigated porphyry Cu deposits (Fig. 3). Rutile within quartz veins ranges in size from < 10 to 100 µm. In all textural positions, blocky rutile can be intergrown with or included in sulfides (Fig. 1A to C and I to L). Blocky rutile from all deposits contains mineral and fluid inclusions. Mineral inclusions are most commonly ilmenite and monazite-Ce, rarely apatite and anhydrite (e.g., Fig. 1F). Sulfide inclusions have not been observed. Rounded or irregular fluid inclusions (< 10 µm) are exclusively two-phase, highdensity (small bubble, < 30 vol.%), low-to moderate-salinity (no halite cubes, < 26 wt.% NaCleq.) fluid inclusions (Fig. 4). Optically, they resemble aqueous inclusions hosted by dull-luminescent quartz described from Batu Hijau (Schirra et al., in revision).

Optical microscope and SEM observations reveal that rutile grains are mostly unzoned. Only a few grains show irregular, patchy zoning with areas relatively enriched in Si and Fe (based on EDS spectra, Fig. 1F). Oscillatory or sector zoning are rare and, wherever present, also chemically defined by zones enriched in Si and Fe.

LA-ICP-MS U-Pb Dating

More than 500 U-Pb measurements were conducted on hydrothermal rutile grains of clear textural context from all deposits. Results are reported in Table 2 and illustrated in figure 5. The U-Pb data, calculated ages and uncertainties of single-grain analyses can be found in the Supplementary Table .   Only the largest rutile grains (>50 μm) were targeted for U-Pb dating due to the use of a larger spot diameter than for trace elements.

Rutile U-Pb isotopic compositions from all deposits suffer from a strongly variable but generally high proportion of common lead (PbC) as reflected by the spread of the discordant data points in the Tera-Wasserburg ( 207 Pb/ 206 Pb vs. 238 U/ 206 Pb) diagram (Fig. 5). This spread is generally interpreted as reflecting a mixture, in different proportions, of common and radiogenic Pb, in which case the lower intercept of the data regression line with the Concordia marks the inferred crystallization age of the population (e.g., [START_REF] Schoene | U-Th-Pb Geochronology[END_REF]. In theory, a PbC correction based on the 208 Pb intensity is possible in the case of rutile that contains very little, if any Th, so that thorogenic lead can be considered as negligible [START_REF] Zack | situ U-Pb rutile dating by LA-ICP-MS: 208Pb correction and prospects for geological applications[END_REF]. However, this correction still requires assuming a certain PbC isotopic composition [START_REF] Zack | situ U-Pb rutile dating by LA-ICP-MS: 208Pb correction and prospects for geological applications[END_REF], which is not straightforward for such a fluid-mobile element in multicomponent, magmatic-hydrothermal porphyry Cu systems. In addition, uncertainties on the composition and fraction of PbC should be propagated to the final common Pb-corrected date [START_REF] Andersen | Correction of common lead in U-Pb analyses that do not report 204Pb[END_REF]. Given the very large data spread in U/PbC ratios for all samples, the common Pb correction based on 208 Pb would arguably yield less precise results than the lower intercept method, so that the latter was preferred.

For all four deposits, the rutile data linear regression arrays with MSWD´s comprised between 1.0 and 1.7 (Fig. 5). For Batu Hijau, Bingham Canyon and El Salvador, rutile U-Pb data were obtained from three distinct samples, but individual dates were systematically overlapping within uncertainty for each deposit (see Fig. 5). Therefore, we pooled the data from all samples together for these deposits to calculate single lower intercept dates. These are respectively of 3.60 ± 0.15 Ma (2σ; n = 187; MSWD = 1.2) for Batu Hijau; 38.09 ± 0.49 Ma (2σ; n = 114; MSWD = 1.7) for Bingham Canyon; 42.11 ± 0.50 Ma (2σ; n = 144; MSWD = 1.6) for El Salvador, and 442.2 ± 4.8 Ma (2σ; n = 46; MSWD = 1.0) for Northparkes.

Trace Element Systematics

Some trace elements analyzed in rutile grains show suspiciously high concentrations and intercorrelations (e.g., Al and Si; Fig. 6A) indicative for potential contamination of the signal, either by accidental ablation of neighboring mineral phases or due to the presence of micro-inclusions observed by SEM. The resulting element intensity changes are generally easy to detect and exclude from the time-integrated signals (Fig. 7) during data reduction. This is not always the case, especially if the volume of ablated rutile relative to another phase remains roughly constant throughout the ablation, producing an apparently homogeneous, yet contaminated signal. Likewise, numerous tiny (< 1 μm) inclusions would elevate the overall abundance of certain elements without showing clearly distinguishable peaks. Many rutile analyses show unusually high, apparent Al and Si concentrations (> ~1000 μg/g) that are positively correlated to each other, and to some extent to Na contents (Fig. 6B), suggesting that Al-and/or Na-rich silicates (chlorite, sericite, plagioclase) might be common contaminants. Extremely high Si values that do not correlate with Al are most likely caused by partial ablation of quartz. The moderate correlation between Na, Zn and Cu (Fig. 6) points towards a potential ablation of small fluid inclusions. A few unreasonably high Fe contents might be due to partial ablation of ilmenite intergrown with rutile.

These sources of contamination did not affect most of the important rutile-hosted trace elements like Zr, Hf, Mo, W, Sn, Nb, Ta, V, Cr, Y, and REE, so that contaminated rutile grains are not highlighted in other trace element diagrams presented here and not considered for further discussion.

As a first step, principal component analysis (PCA), a statistical method to reduce the dimensionality of multivariate compositional datasets (e.g., [START_REF] Grunsky | The interpretation of geochemical survey data: Geochemistry: Exploration[END_REF], was applied to our hydrothermal rutile trace element database in order to identify similarities and differences between the porphyry Cu deposits as well as trace element intercorrelations (Fig. 8). The PCA results show that trace element concentrations in rutile from the investigated porphyry systems show significant variations, both within and among individual deposits. When rutile grains from the four investigated deposits are considered together, the first two principal components capture approximately 64 % of the data variability (Fig. 8A). PC1 is predominantly defined by the well-correlated HFSE (Ta, Nb, Zr, Hf) as well as refractory metals (Cr, Mo, W), U and to a lesser extent V, whereas PC2 is defined by a combination of mainly Sn and Sb (Fig. 8B). The contributions of each element to the first four principal components are listed in table 3. Differences between individual deposits lie mostly in the PC1 value, which on average decreases from Bingham Canyon to El Salvador and Northparkes, with Batu Hijau clearly showing the lowermost values. In contrast to the other deposits, rutile from Batu Hijau appears to show a greater spread along the PC2 axis in figure 8A, indicating that Sn and Sb, potentially in combination with W and Cr, explain most of the variation.

Figures 8C to F show PCA results for individual deposits, which show significant differences between one another in agreement with the observations above. Indeed, PCA for the rutile data from Batu Hijau (Fig. 8C) indicates that PC1 is defined by positively correlated Sn, V, Sb, Cr, W, and U concentrations, whereas PC2 is defined by well positively correlated concentrations of Zr and Hf with a lesser contribution of Mo. Niobium and Ta are positively correlated to each other but less well with Zr and Hf. For all other deposits (Fig. 8D to F), PC1 is mainly defined by positively correlated HFSE and to lesser extend by U and Mo. At Bingham Canyon, W contributes to PC1 but shows a slight negative correlation with Ta, Nb, Zr, Hf and U (Fig. 8D). PC2 is mainly defined by positively correlated concentrations of V and Sb. At El Salvador in contrast V and Sb are negatively correlated and contribute to a lesser extent to PC2, which is mainly defined by negatively correlated concentrations of Cr and Fe (Fig. 8E). Rutile from Northparkes appears to show positive correlations between Sn, Mo, U, Ta, Nb, Zr and Hf, which collectively define PC1, whereas PC2 is mainly defined by positively correlated Fe and W, which are negatively correlated with V. Some of these general trace element characteristics are also highlighted in figures 9 and 10. In general, rutile from all four porphyry systems are characterized by Nb, Fe, Si, W, V, and Cr concentrations >1000 and up to 10,000 µg/g, and Zr, Al, Ta, and Sn concentrations between 10 and 1000 µg/g. Antimony, Hf and U are typically in the range of 1 to 100 µg/g, and Na and Zn were commonly detected. Metals such as Mn, Cu, and Pb also vary between 1 to 100 µg/g but were detected in less than 50% of the analyses. In the sub-µg/g range, Y and Th were commonly detected, Ag only sporadically. Rare earth elements (REE) were frequently detected with concentrations typically between 0.01 and 10 µg/g. Only La, Ce, and Nd can occasionally reach concentrations up to 100 µg/g, which do not appear to be caused by mineral inclusions in the time-integrated LA-ICP-MS signals. The chondrite-normalized REE+Y spider diagrams show slightly fractionated patterns for rutile from most investigated porphyry Cu deposits (LaN/YbN = 2 to 6), except Bingham Canyon rutile that shows moderate fractionation (LaN/YbN ~20; Fig. 10A). A pronounced negative Y anomaly is visible in all deposits and a slightly positive Eu anomaly in rutile from Batu Hijau and El Salvador (Fig. 10A).

Discussion

Origin of Rutile and Its Relation to Mineralization

Besides the first generation of needle-like rutile in biotite and quartz that formed by exsolution of excess Ti during cooling (e.g., [START_REF] Shau | On oriented titanite and rutile inclusions in sagenitic biotite[END_REF], the other two generations of blocky crystals targeted for analyses most certainly reflect hydrothermal rutile formed by the breakdown of Ti-rich minerals [START_REF] Williams | Rutile and apatite: Useful prospecting guides for porphyry copper deposits[END_REF][START_REF] Czamanske | Some geologic and potential resource aspects of rutile in porphyry copper deposits[END_REF][START_REF] Force | Geology of titanium-mineral deposits[END_REF]. Regarding the clustered rutile grains, the mineral association and textures (rutile cemented by Ca-rich minerals; Fig. 1F) indicate pseudomorphic replacement of igneous titanite as indicated by the di-pyramidal shape of these clusters. Titanite replacement by rutile and Ca-rich minerals occurs during potassic alteration in the presence of oxidizing S-rich high-temperature fluids (e.g., [START_REF] Force | Geology of titanium-mineral deposits[END_REF].

The third rutile generation is spatially associated with chlorite and/or white mica and presumably formed by alteration of Ti-rich biotite (Fig. 1 D to F and Fig. 2). High Ti concentrations in igneous and hydrothermal biotite from Batu Hijau (2 to 3.5 wt.% TiO2; [START_REF] Mitchell | Geological review of the Batu Hijau porphyry copper-gold deposit[END_REF] and Bingham (1.5 to 6 wt.% TiO2; [START_REF] Moore | Compositions of biotites from unaltered and altered monzonitic rocks in the Bingham mining district[END_REF] are typical for biotite from porphyry systems (e.g., [START_REF] Jacobs | Geochemistry of biotite in the Santa Rita porphyry copper deposit[END_REF][START_REF] Selby | Chemical composition of biotite from the Casino porphyry Cu-Au-Mo mineralization, Yukon, Canada: evaluation of magmatic and hydrothermal fluid chemistry[END_REF]. In addition, hornblende, which is replaced by secondary biotite during potassic alteration, might contribute to the formation of rutile (e.g., [START_REF] Force | Exploration for porphyry metal deposits based on rutile distribution -a test in Sumatera[END_REF][START_REF] Rabbia | The El Teniente porphyry Cu-Mo deposit from a hydrothermal rutile perspective[END_REF]. However, relatively high Ti concentrations of several weight percent in secondary biotite (e.g., [START_REF] Czamanske | Some geologic and potential resource aspects of rutile in porphyry copper deposits[END_REF][START_REF] Idrus | Petrography and mineral chemistry of magmatic and hydrothermal biotite in porphyry coppergold deposits: A tool for understanding mineralizing fluid compositional changes during alteration processes[END_REF] indicate that most of the Ti originally hosted in mafic minerals like hornblende is passed on and incorporated in the forming biotite, although minor formation of rutile during potassic alteration cannot be ruled out. Ilmenite laths, rather than rutile, have been found in association with magnetite, indicating that re-equilibration of Ti-rich magnetite probably did not significantly contribute to rutile formation. The formation process of the different rutile generations is schematically illustrated in figure 14.

From the above discussion, rutile forms throughout the lifespan of the hydrothermal system, from the potassic alteration stage at temperatures > 500°C to the sericite alteration stage down to ca. 300°C (Fig. 14). Particularly, blocky rutile that formed predominantly by the breakdown of biotite is considered to form contemporaneously with major Cu-sulfide precipitation at similar temperatures (i.e., 400° to 300°C). Textural relationships clearly confirm the co-existence of rutile and Cu-Fe sulfides at Batu Hijau, Bingham Canyon, El Salvador and Northparkes (Fig. 1 and3). At Batu Hijau at least a significant amount of the economic sulfides formed at moderate temperatures (360° to 310°C), coinciding with biotite-destructive chlorite -sericite alteration and formation of a distinctively dullluminescent quartz generation (Schirra et al., in revision). At Bingham Canyon, a spatial relationship has also been previously reported between dull-luminescent quartz and economic sulfide grains formed at a similar temperature range [START_REF] Landtwing | Copper deposition during quartz dissolution by cooling magmatic-hydrothermal fluids: The Bingham porphyry[END_REF][START_REF] Landtwing | The Bingham Canyon porphyry Cu-Mo-Au deposit. III. Zoned copper-gold ore deposition by magmatic vapor expansion[END_REF]. Blocky rutile formed coevally with such mineralization, as it is systematically associated with both, biotite-destructive alteration mineralogy and dull-luminescent quartz hosting Cu-Fe sulfides in the investigated samples. [START_REF] Czamanske | Some geologic and potential resource aspects of rutile in porphyry copper deposits[END_REF] reported that abundance and grain size of rutile correlate with ore grades in several porphyry Cu deposits, supporting the temporal relationship between rutile formation and sulfide precipitation.

The abundance and distribution of rutile in porphyry systems likely depend on the TiO2 concentration of the host rocks, the nature of the primary Ti-rich minerals, and the alteration type and intensity (e.g., [START_REF] Rabbia | The El Teniente porphyry Cu-Mo deposit from a hydrothermal rutile perspective[END_REF]. Although this cannot be studied in detail here due to the limited amount of samples, the fact that rutile is a product of typical porphyry-related hydrothermal reactions and occupies similar textural positions in all studied deposits supports our first-order conclusion that it formed systematically synchronous with Cu mineralization.

Texturally Controlled U-Pb Rutile Dating of Porphyry-Style Mineralization

In every studied porphyry Cu deposit, rutile U-Pb isotopic data define a single regression array between common and radiogenic Pb compositions with a MSWD close to 1 (Fig. 5), indicating that within uncertainties, the analyzed rutile grains belong to a single, co-genetic population of crystals within each individual porphyry deposit and that they did not lose radiogenic lead. The lower intercept date corresponds to the crystallization age of the rutile population, which, insofar it formed coevally with sulfide precipitation and biotite-destructive alteration, yields robust ages for Cu mineralization.

In the following, we compare our rutile-based mineralization ages with previous age determinations using different dating methods for all investigated porphyry deposits (Fig. 5).

Rutile from Batu Hijau contains the lowest amount of U (Fig. 8A, B and 9F), while fluid and mineral inclusions are abundant (e.g., Fig. 4), which in combination with the young age and related low radiogenic Pb content, result in an elevated age uncertainty of around 4% relative. However, the obtained age of 3.61 ± 0.20 Ma is consistent with high-precision U-Pb zircon dating of zircons from the causative tonalite intrusions, i.e., between the mineralized OT (3.736 ± 0.022 Ma) and the weakly mineralized YT (3.646 ± 0.023 Ma; [START_REF] Large | Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U-Pb petrochronology: Geochronology[END_REF]. Although overlapping within uncertainty, the rutile U-Pb age is slightly shifted to the young side, which might be due to an initial deficit of 230 Th and hence of 206 Pb [START_REF] Schärer | The effect of initial 230Th disequilibrium on young U-Pb ages: the Makalu case[END_REF]. Initial 230 Th/ 238 U disequilibrium may be caused by the distinct solubilities of U and Th in aqueous fluids (e.g., [START_REF] Ludwig | U-Th systematics and 230 Th ages of carbonate chimneys at the Lost City Hydrothermal Field[END_REF] and fractionation during hydrothermal rutile growth [START_REF] Brenan | Zr, U and Th: Implications for high field strength element depletions in island-arc basalts[END_REF]. In the case of Th-poor minerals like rutile, the maximum extent of the age correction for 230 Th disequilibrium is 110 ka [START_REF] Schoene | U-Th-Pb Geochronology[END_REF], which would shift the Batu Hijau rutile age to 3.72 ± 0.20 Ma. The effect of 230 Th/ 238 U disequilibrium on the calculated age becomes negligible for older samples, so that it was not considered in the interpretation of the rutile ages for the other deposits.

At Bingham Canyon, high-precision zircon dating of the porphyry intrusions bracket the mineralization age between the intrusion of the highly mineralized QMP (38.284 ± 0.024 Ma) and the post-mineralization QLP (37.736 ± 0.057 Ma; [START_REF] Large | The magmatic to hydrothermal evolution of porphyry Cu-Au deposits -A zircon perspective[END_REF]. The obtained rutile age of 38.09 ± 0.49 Ma falls within this maximum time span of mineralization but is significantly older than the 40 Ar/ 39 Ar age of hydrothermal biotite from the QMP (37.07 ± 0.21 Ma; [START_REF] Parry | U-Pb dating of zircon and 40Ar/39Ar dating of biotite at Bingham[END_REF] and the Re-Os age of molybdenite (37.0 ± 0.27 Ma; Chesley et al., 1997). Molybdenite mineralization at Bingham Canyon occurs in later quartz-molybdenite veins associated with biotite alteration that crosscut earlier Cu-Fe sulfide-bearing veins as well as the QLP dikes [START_REF] Seo | Separation of molybdenum and copper in porphyry deposits: The roles of sulfur, redox and pH in ore mineral deposition at Bingham Canyon[END_REF]. Consequently, the reported Re-Os and 40 Ar/ 39 Ar ages might not be representative for the apparently older Cu-Au mineralization.

For the El Salvador and Northparkes deposits, high-precision CA-ID-TIMS U-Pb zircon ages are not available. Ages for the porphyry intrusions at El Salvador have been obtained using U-Pb zircon SHRIMP dating and span a range of approximately 3 Ma from the X porphyry at 43.6 ± 0.4 Ma over the K and L porphyries at 42.55 ± 0.75 Ma and 42.35 ± 0.65 Ma, respectively, to a post-mineralization latite dike at 41.6 ± 0.5 Ma [START_REF] Lee | Magmatic evolution of granodiorite intrusions at the El Salvador porphyry copper deposit, Chile, based on trace element composition and U/Pb age of zircons[END_REF]. Hydrothermal rutile indicates a mineralization age of 42.11 ± 0.50 Ma, overlapping with the ages for the K, L porphyries and the latite dike as well as the 40 Ar/ 39 Ar age of sericite (41.16 ± 0.48 Ma; [START_REF] Gustafson | Multiple centers of mineralization in the Indio Muerto district[END_REF] and Re-Os age of molybdenite (41.5 ± 0.3 Ma; Zimmerman et al., 2014). Nevertheless, the better agreement of the average rutile age with field relationships suggests that it might be the best approximation for the mineralization age so far at El Salvador. Like for Bingham Canyon, the slightly younger molybdenite ages might be the result of two temporally separated precipitation events.

At Northparkes, the age of mineralization is not precisely constrained. It occurred within a ca. 16

Ma time span bracketed by U-Pb zircon SHRIMP dating of the pre-mineralization biotite quartz monzonite pluton (444.2 ± 4.7 Ma) and the post-mineralization zero porphyries (436.7 ± 3.3 Ma; [START_REF] Lickfold | Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits[END_REF]. This age can be refined to be in the 449-438 Ma range, defined by 40 Ar/ 39 Ar dating of biotite in the syn-mineralization biotite QMP, both igneous (446.5 ± 2.6 Ma) and hydrothermal associated with potassic alteration (441.3 ± 3.8 Ma; [START_REF] Lickfold | Shoshonitic magmatism and the formation of the Northparkes porphyry Cu-Au deposits[END_REF]; as well as hydrothermal sericite (439.2 ± 1.2 Ma; [START_REF] Perkins | 40Ar/39Ar and U-Pb geochronology of the Goonumbla porphyry Cu-Au deposits[END_REF]. Most recently, Rb-Sr dating of epidote has been applied to propylitic alteration at the Northparkes deposits [START_REF] Pacey | The anatomy of an alkali porphyry Cu-Au system: Geology and alteration at Northparkes Mines[END_REF] but the obtained age of 450 ± 11 Ma does not refine the age of hydrothermal activity. The U-Pb age of 442.22 ± 4.78 Ma obtained here for hydrothermal rutile overlaps within uncertainty with the previously best constrained time span for mineralization (defined by 40 Ar/ 39 Ar dating) and narrows this span down to <10 Ma.

Advantages of U-Pb Rutile Dating in Porphyry Systems

Hydrothermal rutile from porphyry Cu deposits contains enough U, and consequently radiogenic Pb, for a good analytical precision on measured isotope ratios. With the exclusion of Batu Hijau (low U-, young rutile), analytical uncertainties on crystallization dates are <1% relative. When propagated uncertainties on rutile U-Pb ages are taken into account (see Methods for details), the final uncertainties range from 1.1 to 1.3% relative (excluding Batu Hijau). External uncertainties ≥ 1% are also expected for the Re-Os and 40 Ar/ 39 Ar dating methods [START_REF] Chiaradia | How accurately can we date the duration of magmatic-hydrothermal events in porphyry systems? -an invited paper[END_REF], indicating that U-Pb dating of rutile can be as or more precise than the other isotope systems.

For grains with radii > 20 μm and expected lifetimes of hydrothermal systems of < 1 Ma, closure temperatures for U-Pb in rutile are expected to be > 600°C [START_REF] Cherniak | [END_REF][START_REF] Vry | LA-MC-ICPMS Pb-Pb dating of rutile from slowly cooled granulites: confirmation of the high closure temperature for Pb diffusion in rutile[END_REF]. The breakdown of biotite in hydrothermal systems, and hence rutile formation, typically occurs at considerably lower temperatures (< 450°C, e.g., [START_REF] Parry | Geochemistry of hydrothermal chlorite replacing igneous biotite: Clays and Clay Minerals[END_REF][START_REF] Eggleton | The alteration of granitic biotite to chlorite[END_REF].

Therefore, the U-Pb system should not be affected by Pb diffusion, as indicated by the well-defined regressions for all four deposits (Fig. 5). The closing temperature of the 40 Ar/ 39 Ar system within the same conditions is lower than for U-Pb in rutile, in the order of 350° and 300°C for muscovite and biotite, respectively [START_REF] Dodson | Closure temperature in cooling geochronological and petrological systems[END_REF][START_REF] Hames | An empirical evaluation of the argon diffusion geometry in muscovite[END_REF]. Partial or complete resetting by later thermal events, such as post-mineralization intrusions, is therefore more likely.

The Re-Os system in molybdenite has a closure temperature as high as rutile [START_REF] Suzuki | Re-Os dating of molybdenites from ore deposits in Japan: Implication for the closure temperature of the Re-Os system for molybdenite and the cooling history of molybdenum ore deposits[END_REF], and yields precise ages of molybdenite formation (e.g., Zimmerman et al., 2014). In several cases however, Re-Os molybdenite dating yielded diachronous ages covering time spans of up to few million years. For the Escondida and El Teniente porphyry Cu-Mo deposits for example, molybdenite dates cover an interval of 3 m.y. [START_REF] Padilla-Garza | Hypo-gene evolution of the Escondida porphyry copper deposit, Chile. -In: Andean metallogeny: new discoveries, concepts, and updates[END_REF][START_REF] Romero | Molybdenite mineralization and Re-Os geochronology of the Escondida and Escondida Norte porphyry deposits[END_REF] and 2 m.y. [START_REF] Maksaev | New chronology for El Teniente, Chilean Andes, from U-Pb, 40Ar/39Ar, Re-Os, and fission-track dating: implications for the evolution of a super-giant porphyry Cu-Mo deposit[END_REF], respectively. From individual porphyry Cu-Mo systems in Northern Mexico Re-Os molybdenite geochronology also spans variable time intervals from < 2 m.y. up to 7 m.y. [START_REF] Barra | Laramide porphyry Cu-Mo mineralization in Northern Mexico: Age constraints from Re-Os geochronology in molybdenite[END_REF]. The relative timing between Cu(-Au) and Mo mineralization in porphyry systems is not always clear (e.g., [START_REF] Seo | Separation of molybdenum and copper in porphyry deposits: The roles of sulfur, redox and pH in ore mineral deposition at Bingham Canyon[END_REF][START_REF] Spencer | The distribution and timing of molybdenite mineralization at the El Teniente Cu-Mo porphyry deposit[END_REF] and consequently Re-Os ages of molybdenite, despite being accurate and reliable, do not necessarily determine the age of Cu mineralization. At Bingham Canyon for instance, molybdenite Re-Os ages are significantly younger than our rutile U-Pb ages (Fig. 5).

Zircon geochronology by CA-ID-TIMS has been commonly used to determine the maximal duration of hydrothermal activity associated with porphyry intrusions (e.g., von [START_REF] Von Quadt | Zircon crystallization and the lifetimes of ore-forming magmatic-hydrothermal systems[END_REF][START_REF] Buret | From a long-lived uppercrustal magma chamber to rapid porphyry copper emplacement: Reading the geochemistry of zircon crystals at Bajo de la Alumbrera (NW Argentina)[END_REF][START_REF] Large | The magmatic to hydrothermal evolution of porphyry Cu-Au deposits -A zircon perspective[END_REF]. However, zircon crystallized in the parental magma chamber before or during porphyry emplacement and is hence not temporally related to ore formation. Therefore, only bracketing of hydrothermal activity using the porphyry intrusions as time markers is possible.

Moreover, it relies on the assumption that the youngest of the few dated zircon grains within a porphyry sample represents porphyry emplacement; and that individually dated grains are not zoned in age since dissolution of the whole zircon grain is necessary for ID-TIMS analysis. Despite the lower absolute analytical precision of the LA-ICP-MS technique compared to CA-ID-TIMS, rutile dating has the advantage of directly yielding the mineralization age. As a result, the maximum time periods of mineralization obtained by zircon CA-ID-TIMS at Bingham Canyon and Batu Hijau (respectively 662 and 122 ka; [START_REF] Large | The magmatic to hydrothermal evolution of porphyry Cu-Au deposits -A zircon perspective[END_REF][START_REF] Large | Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U-Pb petrochronology: Geochronology[END_REF] are only 1.5 to 3 times smaller than those obtained based on the uncertainties of our rutile LA-ICP-MS U-Pb dating (respectively 980 and 400 ka). Consequently, texturally controlled U-Pb analyses of hydrothermal rutile with LA-ICP-MS provide a new, accurate way to date porphyry-style Cu mineralization with less sample preparation, lower costs and easier accessible analytical setup than more time-and resource-demanding methods. For resolving the entire time scale of magmatic-hydrothermal processes in porphyry Cu deposits from high to low temperatures, a combination of U-Pb, Re-Os and 40 Ar/ 39 Ar methods applied to various minerals is required. Rutile dating thereby complements the current geochronological toolbox for porphyry systems and might be the best option to rapidly and directly date Cu mineralization, provided that petrographic observations confirm co-formation of rutile and Cu-Fe sulfides.

Using direct CA-ID-TIMS dating of rutile, which would require inclusion-free and in the best-case common lead-free crystals, would allow for even more accurate and precise dating of hydrothermal activity in porphyry systems. In theory, the time gap between high-temperature mineralization associated with potassic alteration and hence rutile formation by titanite breakdown and lowtemperature mineralization during biotite-destructive alteration, might be resolvable in future by dating the different rutile generations. Likewise, combining magmatic zircon and hydrothermal rutile U-Pb dating might provide further insights into the duration of porphyry ore formation, especially if the age relationships of different porphyry intrusions are absent or difficult to establish.

Parameters Controlling Trace Element Variability of Rutile in Porphyry Cu Deposits

In the following subsections, the trace element contents of non-and hydrothermally formed rutile in general are firstly compared to each other to investigate the general effects of a fluid phase on the trace elemental composition of rutile. Subsequently, differences and similarities between the studied porphyry Cu deposits regarding rutile composition and its potential implications for mineralization conditions will be discussed. Our analytical work was focused on hydrothermal blocky rutile grains of the third generation, i.e., the ones formed contemporaneously with Cu-Fe sulfides during biotitedestructive alteration. Importantly, in a given deposit, we did not find any significant difference in trace element composition (and U-Pb age) between rutile directly associated with other alteration products of Ti-rich phases (e.g., chlorite or sericite) and rutile enclosed in dull-luminescent quartz. This may indicate that the latter represent crystals that were physically transported in the fluid from their site of formation in the host rock.

Comparison of hydrothermal and non-hydrothermal rutile

Rutile is an accessory mineral in many metamorphic rocks, particularly in high-temperature and high-pressure metapelites, whereas igneous rocks rarely contain rutile [START_REF] Zack | Trace element abundances in rutiles from eclogites and associated garnet mica schists[END_REF]. Siliciclastic rocks typically also contain abundant detrital rutile [START_REF] Force | The provenance of rutile[END_REF][START_REF] Morton | Identifiying provenance-specific features of detrital heavy mineral assemblages in sandstones[END_REF]. Our dataset contains 650 LA-ICP-MS trace element analyses of rutile grains from Batu Hijau, Bingham Canyon, El Salvador, and Northparkes, which are compared with 1550 trace element analyses obtained by EMPA and LA-ICP-MS of non-hydrothermal rutile from pegmatite (Pe-Piper et al., 2019), various metamorphic rocks [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF][START_REF] Meyer | Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism[END_REF][START_REF] Ewing | The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)[END_REF][START_REF] Gao | U-Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogeny[END_REF][START_REF] Sengün | Trace element composition of rutile and Zr-in-rutile thermometry in metaophiolitic rocks from the Kazdağ Massif[END_REF][START_REF] Hart | A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF], highly fractionated igneous rocks from the Bushveld Complex [START_REF] Ver Hoeve | A temperature-composition framework for crystallization of fractionated interstitial melt in the Bushveld Complex from trace element characteristics of zircon and rutile[END_REF], and detrital rutile [START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF][START_REF] Okay | Sinistral transport along the Trans-European Suture Zone: detrital zircon-rutile geochronology and sandstone petrography from the Carboniferous flysch of the Pontides[END_REF][START_REF] Rösel | Interpretation and significance of combined trace element and U-Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia[END_REF]. We also compare our results with rutile data from orogenic Au deposits [START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF] and topaz-bearing quartz veins [START_REF] Santos | LA-ICP-MS U-Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt[END_REF]. The comparison could only be based on the limited number of elements common to all these studies. Combination of our dataset with rutile data from these various geological settings shows that generally hydrothermal and non-hydrothermal rutile can be distinguished based on differences in their trace element characteristics (Fig. 11). Elements like Fe, Sn, W and potentially Si are more enriched in hydrothermally formed rutile, whereas concentrations of other elements like Mo and V are not systematically different (Fig. 9B and11E). Elements considered to be fluid immobile like Nb, Zr, and Cr do not appear to be considerably depleted in hydrothermal rutile (Fig. 9). Rutile from the investigated porphyry Cu deposits is distinct from non-hydrothermal rutile regarding their high Sn and W contents, their low V and Cr concentrations relative to Fe and the generally large compositional variations, even within individual deposits, compared to more uniform concentrations in rutile from metamorphic environments (Fig. 12). This suggests that the chemical composition of hydrothermal rutile is influenced by multiple factors, most importantly the fluid composition and the nature of precursor minerals (e.g., Rabbia and Hernandez, 2012), whereas the composition of non-hydrothermal rutile is well-equilibrated with its host rock by protracted residence at high temperature (e.g., [START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF][START_REF] Watson | Crystallization thermometers for zircon and rutile[END_REF][START_REF] Tomkins | The pressure dependence of the Zirconium-in-rutile thermometer[END_REF]. Elemental compositions of detrital rutile grains generally overlap with those of metamorphic rutile but span wider ranges (Fig. 9 and 12) due to their different origins, potentially including hydrothermal systems.

In addition, the composition of rutile from porphyry Cu deposits and non-hydrothermal rutile define distinctive trends in Fe/V vs. V and Fe/Cr vs. Cr diagrams (Fig. 12B andC). The former show higher V concentrations at a given Fe/V ratio; and lower Cr contents and conversely higher Fe/Cr ratios, except Bingham Canyon that shows similar Fe/Cr ratios as non-hydrothermal rutile yet at higher Cr contents (Fig. 12 B andC). Although elevated V concentrations in rutile from mineralized samples at Northparkes (> 0.4 wt.%) were already reported by [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF] and agree with our analytical results (e.g., Fig. 9B and 12B), extreme V enrichment (up to 6.3 wt.%) as reported from the Pebble porphyry Cu deposit [START_REF] Kelley | Exploration geochemistry at the giant Pebble porphyry Cu-Au-Mo deposit[END_REF] appears not to be characteristic for porphyry systems in general. Contrary, most rutile grains from the investigated porphyry Cu deposits overlap with those from metamorphic or sedimentary environments regarding their V concentrations (Fig. 9B and12B), indicating that hydrothermal processes do not significantly affect total V concentrations in rutile.

Rutile formed in other hydrothermal environments show similarly elevated Si, Fe, Sn, and W concentrations as porphyry Cu rutile (Fig. 11). A major compositional difference is visible in rutile from orogenic Au systems, which is strongly enriched in As and/or Sb [START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF] relative to porphyry Cu rutile (Fig. 13). This emphasizes the influence of the fluid chemistry on rutile composition, as fluids forming orogenic Au deposits are assumed to be rich in As and Sb [START_REF] Phillips | Formation of gold deposits: a metamorphic devolatilization model[END_REF][START_REF] Goldfarb | Orogenic gold: Common or evolving fluid and metal sources through time[END_REF]. Although high Sb concentrations in rutile have been frequently reported from Au-rich hydrothermal systems (e.g., [START_REF] Smith | Sb-rich rutile in the manganese concentrations at St. Marcel-Praborna[END_REF]Clark and Williams Jones, 2004;[START_REF] Porter | Rutile as a pathfinder for metals exploration[END_REF], Sb and As concentrations in rutile from all four porphyry systems significantly overlap with each other (Fig. 13), irrespective of their metal endowment.

Inherited trace element signatures in rutile from porphyry systems

Fluid immobile elements such as Zr, Ta, Nb, V and Cr are not systematically depleted in hydrothermal rutile compared to igneous and metamorphic rutile (Fig. 9 and 12), highlighting the importance of inherited trace element signatures from the host rocks and its minerals, particularly biotite.

Rutile from Batu Hijau generally contains lower concentrations of V, Nb, Ta, Zr, Hf, Mo and U compared to the other deposits (Fig. 8A and9). This might reflect the petrogenesis of the associated tonalitic melts in an intra-oceanic arc system, formed by melting of asthenospheric mantle with only limited subsequent assimilation of crustal material (Fiorentini and Garwin, 2010). This could explain the generally lower contents of incompatible elements in rutile from Batu Hijau compared to the other porphyry deposits, which are situated in continental arc systems where assimilation of continental crust is more likely.

Conversely, rutile from Bingham Canyon is considerably richer in Nb, Ta and Cr compared to rutile from the other deposits, indicating that precursor biotite might have been also enriched in these elements. Partitioning experiments suggest that mica/melt partition coefficients for Nb and Ta are strongly correlated to each other [START_REF] Ewart | Application of proton-microprobe data to trace-element partitioning in volcanic rocks[END_REF][START_REF] Acosta-Vigil | Mechanisms of crustal anataxis: A geochemical study of partially melted metapelitic enclaves and host dacite[END_REF] and to the TiO2 content of biotite [START_REF] Stepanov | Fractionation of Nb and Ta by biotite and phengite: Implications for the "missing Nb paradox[END_REF]. Biotite from Bingham Canyon contains high TiO2 contents of up to 6 wt.% [START_REF] Moore | Compositions of biotites from unaltered and altered monzonitic rocks in the Bingham mining district[END_REF], implying high Nb and Ta concentrations. The high Cr contents (> 1000 µg/g) in rutile from Bingham Canyon are probably also inherited from precursor biotite, possibly reflecting the involvement of more mafic magmas that are described at Bingham Canyon as mafic inclusions and dikes with high Cr contents [START_REF] Keith | The role of magmatic sulfides and mafic alkaline magmas in the Bingham and Tintic mining districts[END_REF][START_REF] Maughan | Contributions from mafic alkaline magmas to the Bingham porphyry Cu-Au-Mo deposit[END_REF][START_REF] Redmond | The Bingham Canyon porphyry Cu-Au-Mo deposit. I. Sequence of intrusions, vein formation, and sulfide deposition[END_REF].

On top of these first-order differences related to host rock petrogenesis and mineral composition, different degrees of biotite replacement would explain the observed ranges of fluid-immobile trace element contents in rutile from all deposits. The high HFSE compatibility in rutile [START_REF] Klemme | Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones[END_REF], which is probably higher than in biotite as demonstrated for Nb [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF], causes the first formed rutile grains to incorporate high amounts of HFSE, while the residual biotite becomes more and more depleted in those elements with progressive replacement (cf., [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF].

Accordingly, rutile grains formed from already HFSE-depleted biotite contain progressively lower concentrations of those elements, explaining the large elemental variations (Fig. 9). The same might apply for the variations of Mo and U in hydrothermal rutile, which both correlate positively with Zr (Fig. 8B, 9D and F), as all of them are tetravalent with comparable ionic radii. Likewise, Mo contents of rutile might be mainly inherited from Ti-rich precursor phases (e.g., [START_REF] Rabbia | The El Teniente porphyry Cu-Mo deposit from a hydrothermal rutile perspective[END_REF] as they do not correlate with Mo grades in the porphyry deposits. For example, rutile Mo concentrations are comparable at Northparkes, a Cu-Au porphyry not particularly enriched in Mo, and at Bingham Canyon and El Salvador, both mined for Mo in addition to Cu. [START_REF] Rabbia | The El Teniente porphyry Cu-Mo deposit from a hydrothermal rutile perspective[END_REF] reported similar Mo concentrations from the El Teniente porphyry Cu-Mo deposit.

Fluid-derived trace element variations in rutile from porphyry systems

Compared to most other elements, Fe contents of hydrothermal rutile are less variable within each of and across the four porphyry Cu deposits (Fig. 9E). Iron has been shown to be predominantly incorporated into rutile as Fe 3+ [START_REF] Murad | The chemical state of Fe in rutile from an albitite in Norway[END_REF][START_REF] Bromiley | Solubility of hydrogen and ferric iron in rutile and TiO2 (II): Implications for phase assemblages during ultrahigh-pressure metamorphism and for the stability of silica polymorphs in the lower mantle[END_REF], via coupled substitutions like Fe 3+ + Nb 5+ ↔ 2Ti 4+ [START_REF] Bromiley | Hydrogen and minor element incorporation in synthetic rutile[END_REF], resulting in positive correlations between Fe 3+ and other trivalent and pentavalent cations. However, no direct correlation between Fe and Nb or Ta concentrations can be observed in the investigated deposits (Fig. 9E). When comparing molar amounts of potentially trivalent and penta-and hexavalent cations, coupled substitutions should be identifiable by linear trends defined by the respective substitution mechanism (e.g., [START_REF] Scott | Rutile compositions in the Kalgoorlie Goldfields and their implication for exploration[END_REF][START_REF] Carocci | Rutile from Panasqueira (Central Portugal): An excellent pathfinder for wolframite deposition: Minerals[END_REF]. Rutile from the investigated porphyry Cu deposits contains significantly higher amounts of trivalent cations compared to penta-and hexavalent cations than expected considering typical coupled substitutions, particularly when V is assumed to be present as trivalent cation (Fig. 15A). Without V, there is a better agreement between analyzed rutile compositions and the linear relationships defined by potential coupled substitutions but many rutile grains, particularly from Batu Hijau, still contain too high amounts of trivalent cations (Fig. 15B). Considering all V being present in the pentavalent state, most rutile compositions overlap with the three shown coupled substitution possibilities, although there is still an excess of trivalent cations in many rutile grains from Batu Hijau (Fig. 15C). This indicates that V in rutile from porphyry Cu systems may be present as a mixture of tetravalent and pentavalent cations, which would also explain the correlations with variable elements such as Sb and Sn (Fig. 8). At Batu Hijau and Bingham Canyon, V is positively correlated with Sb (Fig. 8C andD), suggesting that both elements are introduced into rutile via similar substitutions and hence a considerable portion of V may be present in its highly oxidized form. At El Salvador and Northparkes in contrast, V shows a rather negative correlation with Sb indicating that both elements are part of competing substitution mechanisms and hence V 4+ , which can directly replace Ti 4+ , is the dominant oxidation state of V. In addition, oxygen-vacancies can charge-balance the incorporation of trivalent cations, implying a strong influence of f(O2) on Fe concentrations in rutile [START_REF] Bromiley | Hydrogen and minor element incorporation in synthetic rutile[END_REF][START_REF] Meinhold | Rutile and its applications in earth sciences[END_REF]. High Fe concentrations in rutile from all deposits and the presence of tetra-and pentavalent V point towards high oxygen fugacities during biotite-destructive alteration in the porphyry systems, which is further supported by the presence of texturally associated anhydrite (Fig. 3).

The influence of the fluid phase on rutile geochemistry is particularly well visible in the form of noncharge-and-radius-controlled ("CHARAC"; [START_REF] Bau | Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: Evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect[END_REF] behavior in element ratios and the chondrite normalized REE + Y patterns (Fig. 10B). Concentrations of Zr and Hf as well as Y and Ho are tightly coupled to each other but their ratios diverge from chondritic values (37.1 and 28.75, respectively, McDonough and Sun, 1995). Such elements of nearly identical charge and radii can only be fractionated by hydrothermal processes because they form different complexes in aqueous solutions [START_REF] Bau | Comparative study of yttrium and rare-earth element behaviors in fluorine-rich hydrothermal fluids[END_REF][START_REF] Bau | Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: Evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect[END_REF]. Subchondritic Zr/Hf and Y/Ho ratios (and corresponding negative Y anomaly; Fig. 10B), as well as generally lower Zr contents of hydrothermal vs. non-hydrothermal rutile, suggest that the solubilities of Zr and Y in the hydrothermal solution were lower than those of Hf and Ho.

Although both REE and Y form complexes with F (e.g., [START_REF] Bau | Comparative study of yttrium and rare-earth element behaviors in fluorine-rich hydrothermal fluids[END_REF], REE rather form monofluoride complexes [START_REF] Migdisov | An experimental study of the solubility and speciation of the Rare Earth Elements (III) in fluoride-and chloride-bearing aqueous solutions at temperatures up to 300[END_REF] whereas Y preferentially occurs as di-fluoride complex, so that their relative solubility is strongly correlated to fluorine activity in the fluid [START_REF] Loges | An experimental study of the aqueous solubility and speciation of Y(III) fluoride at temperatures up to 250[END_REF].

The mobility of Zr in hydrothermal solutions is also enhanced by the presence of F [START_REF] Rubin | The mobility of Zr and other "immobile" elements during hydrothermal alteration[END_REF][START_REF] Gysi | Hydrothermal mobilization of pegmatite-hosted REE and Zr at Strange Lake, Canada: A reaction path model[END_REF]. Consequently, limited but sufficient quantities of F in the hydrothermal fluid might have been favorable for the stabilization of aqueous complexes of Y and Zr over those of REEs. Moreover, elevated Hf and REE concentrations in rutile agree with experimentally determined high partitioning coefficients between rutile and fluid [START_REF] Brenan | Zr, U and Th: Implications for high field strength element depletions in island-arc basalts[END_REF].

Due to the non-CHARAC behavior of element partitioning in hydrothermal rutile and the lack of equilibrium with zircon, the Zr-in-rutile thermometer [START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF][START_REF] Watson | Crystallization thermometers for zircon and rutile[END_REF][START_REF] Ferry | New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers[END_REF] is not applicable to hydrothermal rutile (cf., [START_REF] Cabral | Fluidinclusion microthermometry and the Zr-in-rutile thermometer for hydrothermal rutile[END_REF]. Other potential thermometers based on Si and Al in rutile (e.g., [START_REF] Ren | SiO2 solubility in rutile at high temperature and high pressure[END_REF][START_REF] Hoff | Aluminum in rutile as a recorder of temperature and pressure[END_REF] have not been calibrated yet for hydrothermal temperature and pressure conditions but might be applicable to hydrothermal rutile in the future.

Rutile as Exploration Tool for Porphyry Cu Systems

Hydrothermal rutile occurs in various ore deposit types, including Carlin-type Au, sediment-hosted stratiform Cu, VMS, orogenic Au, quartz-vein type W-Sn, IOCG, and porphyry Cu deposits [START_REF] Richards | Fluid inclusion characteristics and U-Pb rutile age of late hydrothermal alteration and veining from the Musoshi stratiform copper deposit, Central Africa copper belt[END_REF][START_REF] Schandl | Are the alteration halos of massive sulfide deposits syngenetic? Evidence from U-Pb dating of hydrothermal rutile in the Kidd volcanic center[END_REF]Clark and Williams-Jones, 2004;[START_REF] Rabbia | The El Teniente porphyry Cu-Mo deposit from a hydrothermal rutile perspective[END_REF][START_REF] Pi | In situ SIMS U-Pb dating of hydrothermal rutile: reliable age for the Zhesang Carlin-type gold deposit in the golden triangle region[END_REF][START_REF] Carocci | Rutile from Panasqueira (Central Portugal): An excellent pathfinder for wolframite deposition: Minerals[END_REF][START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF]. During erosion, rutile becomes concentrated by sedimentary processes as consequence of its high density, resistance to weathering and transport [START_REF] Force | The provenance of rutile[END_REF]. [START_REF] Cox | Descriptive model of porphyry Cu, Mineral deposit models[END_REF] noted that soils above mineral deposits contain anomalous rutile concentrations.

Consequently, numerous studies have focused on the utilization of rutile as pathfinder for mineralization (e.g., Williams and Cesborn, 1977;[START_REF] Force | Exploration for porphyry metal deposits based on rutile distribution -a test in Sumatera[END_REF]Clark and Williams-Jones, 2004;[START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF][START_REF] Scott | Rutile compositions at the Big Bell Au deposit as guide for exploration: Geochemistry: Exploration[END_REF][START_REF] Scott | Rutile compositions in the Kalgoorlie Goldfields and their implication for exploration[END_REF][START_REF] Kelley | Porphyry Cu indicator minerals in till as an exploration tool: Example from the giant Pebble porphyry Cu-Au-Mo deposit[END_REF][START_REF] Carocci | Rutile from Panasqueira (Central Portugal): An excellent pathfinder for wolframite deposition: Minerals[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF][START_REF] Plouffe | Mineral markers of porphyry copper mineralization: Work in progress at the Gibraltar deposit[END_REF][START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF][START_REF] Porter | Rutile as a pathfinder for metals exploration[END_REF].

So far, investigations of rutile from porphyry Cu deposits were mainly qualitative (focused on distribution, crystal color and shape; Williams and Cesborn, 1977;[START_REF] Force | Exploration for porphyry metal deposits based on rutile distribution -a test in Sumatera[END_REF][START_REF] Plouffe | Mineral markers of porphyry copper mineralization: Work in progress at the Gibraltar deposit[END_REF]. As discussed earlier, discrimination of hydrothermal, including rutile formed in porphyry systems, and non-hydrothermal ("background") rutile is possible [START_REF] Carocci | Rutile from Panasqueira (Central Portugal): An excellent pathfinder for wolframite deposition: Minerals[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF][START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF][START_REF] Porter | Rutile as a pathfinder for metals exploration[END_REF]. Rutile from porphyry systems is systematically enriched in Si, Fe, Sn, and W compared to "background" rutile (Fig. 11,12A). Concentrations of Sn and W higher than 100 and 500 μg/g, respectively, occur in approximately 75% of the rutile grains from porphyry systems but are uncommon in "background" rutile (Fig. 12A). Such criteria might be used to identify hydrothermal rutile in detrital datasets. For instance, the dataset of [START_REF] Rösel | Interpretation and significance of combined trace element and U-Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia[END_REF], who concluded that part of the studied detrital material originated from magmatic rocks of the Cadomian arc system, contains rutile displaying high of Sn, W and Fe, indicating that hydrothermal rutile might be present. Significant V enrichment, which has been described from some porphyry systems (e.g., [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF][START_REF] Kelley | Exploration geochemistry at the giant Pebble porphyry Cu-Au-Mo deposit[END_REF] has not been found in our dataset (Fig. 12B). Although some rutile grains from the Nortparkes sample show high V contents of up to 1 wt.%, which is comparable with the data reported by [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF], a systematic enrichment of V in hydrothermal compared to nonhydrothermal rutile seems not to be a general feature of porphyry systems. The spatial variations of V within an individual porphyry system as mentioned by [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF] could not be investigated based on the limited sample material studied from each deposit. Distinctively high Fe concentrations in rutile from porphyry systems result in the relatively steep trends shown for Fe/Cr vs. Cr and Fe/V vs. V when compared to "background" rutile (Fig. 12B andC). Rutile of metamorphic origin is assumed to preserve Cr and V bulk rock characteristics (e.g., [START_REF] Meyer | Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism[END_REF] resulting in less variable Cr and V contents, compared to rutile from porphyry Cu deposits, and in combination with their lower Fe concentrations leads to Fe/Cr vs. Cr and Fe/V vs. V trends that are less steep, making a distinction of metamorphic and hydrothermal rutile possible (Fig. 12). Finally, other elements analyzed here, like Mn, Cu, Zn, Ag, Se, Te, Hg and REE, might be useful for distinguishing rutile associated with porphyry Cu systems, but this could not be assessed because they were not systematically analyzed in other studies.

Besides trace element signatures, the presence, abundance and type of fluid inclusions might be important criteria for distinction of rutile from different settings. Rutile from porphyry systems studied here commonly contains aqueous fluid inclusions consisting of a small bubble (< 30 vol.%) surrounded by liquid (Fig. 4). In contrast, well-preserved fluid inclusions in metamorphic rutile are comparatively rare and typically rich in CO2 (e.g., [START_REF] Ni | Constraining ultrahigh-pressure (UHP) metamorphism and titanium ore formation from an infrared microthermometric study of fluid inclusions in rutile from Donghai UHP eclogites, eastern China[END_REF][START_REF] Cabral | Fluidinclusion microthermometry and the Zr-in-rutile thermometer for hydrothermal rutile[END_REF].

Conclusions and outlook

Hydrothermal rutile forms throughout the hydrothermal evolution of a porphyry system and contains several layers of information, including the absolute timing of Cu-sulfide mineralization, insights into magmatic and hydrothermal processes, fluid composition and mineralizing conditions. Its trace element composition can furthermore be used as pathfinder for hydrothermal activity, which might be related to economic metal accumulation. Specifically, comparison of trace element signatures from hydrothermal and "background" rutile emphasizes its promising potential as pathfinder for blind ore deposits in highly eroded terranes, where the use of rutile as exploration tool was already investigated (e.g., [START_REF] Scott | Rutile geochemistry as a guide to porphyry Cu-Au mineralization[END_REF]Kelly et al., 2010). Different types of ore deposits, like orogenic Au and porphyry Cu, can be clearly distinguished based on rutile geochemistry, which provides additional insights into hydrothermal fluid compositions responsible for different mineralization styles. However, in order to certify rutile as exploration tool, more trace element data from non-hydrothermal rutile including a broader range of elements is needed.

We have obtained LA-ICP-MS U-Pb ages for rutile formation that overlaps in time with Cu mineralization, for the four studied porphyry deposits. Comparison with existing high-precision CA-ID-TIMS data for two of these (Batu Hijau, Bingham Canyon) ascertained the accuracy of our results, and application to two less well dated systems (El Salvador, Northparkes) yielded precise constraints on mineralization ages (ca. 1.1 to 1.3% relative). Using TIMS for dating inclusion-free unzoned rutile grains with low common Pb contents would result in smaller uncertainties, which could open up new possibilities for determining timescales of mineralization in porphyry systems, especially when combined with high-precision zircon dating.

Almost completely unexplored are fluid inclusions hosted in hydrothermal rutile. Fluid inclusions bigger than around 5 µm are not uncommon in hydrothermal rutile and sufficiently large for microthermometric measurements, whereas fluid inclusions with sizes suitable for LA-ICP-MS analysis are rare but not absent in rutile from the investigated porphyry systems. Rutile-hosted fluid inclusions might be less prone to post-entrapment modifications compared to quartz, in which small monovalent cations like Li, Cu, and Ag can diffuse through the quartz lattice distorting the original fluid composition (e.g., [START_REF] Zajacz | Diffusive reequilibration of quartz-hosted silicate melt and fluid inclusions: Are all metal concentrations unmodified?[END_REF][START_REF] Lerchbaumer | High Cu concentrations in vapor-type fluid inclusions: An artifact?[END_REF][START_REF] Seo | Selective copper diffusion into quartz-hosted vapor inclusions: Evidence from other host minerals, driving forces, and consequences for Cu-Au ore formation[END_REF]. Consequently, rutile also bears potential for future fluid inclusion microanalysis to further constrain the fluid evolution of porphyry systems. Fig. 2: Phase map created with iSpectra [START_REF] Liebske | iSpectra: An open source toolbox for the analysis of spectral images recorded on scanning electron microscopes[END_REF] showing textural relationship between rutile and other silicate alteration minerals (Bingham Canyon, sample 162-362). A) K-silicate alteration with sericitic overprint showing remnants of a primary biotite (bi) phenocryst partially overprinted by sericite (ser) in K-Feldspar (ksp) and quartz (qtz matrix). Accessory minerals are rutile (rt), apatite (ap), zircon (zr) and chalcopyrite (cpy). B) Enlarged section of A), emphasizing the spatial relationship between blocky rutile and biotite-replacing sericite.

Using conventional transmitted light microscopy, this textural relationship can be less clear, especially if biotitereplacement is weakly developed, which gives the impression that some blocky rutile is hosted by secondary biotite. (single-column, color) [START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF][START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF][START_REF] Meyer | Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism[END_REF]Okay et al., 2012;[START_REF] Ewing | The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)[END_REF][START_REF] Gao | U-Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogeny[END_REF][START_REF] Sengün | Trace element composition of rutile and Zr-in-rutile thermometry in metaophiolitic rocks from the Kazdağ Massif[END_REF][START_REF] Hart | A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF][START_REF] Rösel | Interpretation and significance of combined trace element and U-Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia[END_REF]. (2-column, color) Fig. 10: REE + Y spider diagram normalized against C1 chondrite [START_REF] Mcdonough | The composition of the Earth[END_REF] and Zr/Hf vs Y/Ho ratios of hydrothermal rutile from Batu Hijau, Bingham Canyon, El Salvador, and Northparkes. CHARAC-field after [START_REF] Bau | Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: Evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect[END_REF], Y/Ho and Zr/Hf ratios of chondrite from [START_REF] Anders | Abundances of the elements: meteoritic and solar[END_REF]. (single-column, color) Fig. 11: Box and Whisker plot for comparison of selected trace elements from hydrothermal and nonhydrothermal rutile. Rutile from porphyry deposits are from this study, whereas other hydrothermal rutile are from numerous orogenic Au deposits [START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF] and imperial topaz-bearing quartz (± calcite) veins from Brasil [START_REF] Santos | LA-ICP-MS U-Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt[END_REF]. Non-hydrothermal rutile includes rutile from pegmatite [START_REF] Pe-Piper | Geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens[END_REF], the Bushveld pluton [START_REF] Ver Hoeve | A temperature-composition framework for crystallization of fractionated interstitial melt in the Bushveld Complex from trace element characteristics of zircon and rutile[END_REF], various metamorphic environments [START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF][START_REF] Meyer | Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism[END_REF][START_REF] Ewing | The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)[END_REF][START_REF] Gao | U-Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogeny[END_REF][START_REF] Sengün | Trace element composition of rutile and Zr-in-rutile thermometry in metaophiolitic rocks from the Kazdağ Massif[END_REF][START_REF] Hart | A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF] and detrital rutile [START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF]Okay et al., 2012;[START_REF] Rösel | Interpretation and significance of combined trace element and U-Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia[END_REF]. Missing boxes indicate that the respective trace element was not included in the element setup of the individual studies, which hampers comparison of rutile data to some extent. Note that chlorite might not necessarily be present in all parts of a given deposit as its formation depends on host rock and fluid composition but is shown here to represent biotitedestructive alteration in general. In cases where chlorite is not present, sericite is the main alteration product replacing primary and secondary biotite. (2-column, color) Fig. 15: Comparison of hydrothermal trace element composition of rutile from porphyry deposit with linear relationships between di-, tri-, penta-and hexavalent elements indicative for coupled substitution mechanisms represented by grey lines with slopes of 2, 1 and 0.5. M represents Nb 5+ , Ta 5+ , Sb 5+ and V 5+ or W 6+ , N can be Al 3+ , Fe 3+ , Cr 3+ and V 3+ and P represents divalent cations such as Fe 2+ . (1 column, color)

Fig. 3 :Fig. 4 :Fig. 9 :

 349 Fig.3: Cathodoluminescence images supporting the association of rutile with Cu-Fe sulfides in the different porphyry systems. A) and B) CL and reflected light image, respectively, of euhedral quartz crystal growing into open space, Batu Hijau. Note that the partially resorbed bright-luminescent core does not contain any sulfide. Chalcopyrite occurs exclusively in the dull-luminescent rim together with blocky rutile and small anhydrite grains (sample 257-694). C) and D) CL and combined transmitted and reflected light image, respectively, of chalcopyrite and rutile hosted by a micro-fracture network filled with dull-luminescent quartz crosscutting earlier brightluminescent quartz, Bingham Canyon (sample 123-210). E) and F) CL and BSE image of a paint vein containing chalcopyrite, rutile, chlorite, sericite, and anhydrite mantled by dull-luminescent quartz, crosscutting earlier bright-luminescent quartz, El Salvador (sample 946-315). G) and H) Bornite, rutile, anhydrite, and sericite hosted by a micro-fracture network filled with dull-luminescent quartz crosscutting earlier bright-luminescent quartz, Northparkes. Abbreviations as in figure1.(1.5-column, color)

  Fig.11: Box and Whisker plot for comparison of selected trace elements from hydrothermal and nonhydrothermal rutile. Rutile from porphyry deposits are from this study, whereas other hydrothermal rutile are from numerous orogenic Au deposits[START_REF] Agangi | Antimony in rutile as a pathfinder for orogenic gold deposits[END_REF] and imperial topaz-bearing quartz (± calcite) veins from Brasil[START_REF] Santos | LA-ICP-MS U-Pb dating of rutiles associated with hydrothermal mineralization along the southern Araçuaí Belt[END_REF]. Non-hydrothermal rutile includes rutile from pegmatite[START_REF] Pe-Piper | Geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens[END_REF], the Bushveld pluton[START_REF] Ver Hoeve | A temperature-composition framework for crystallization of fractionated interstitial melt in the Bushveld Complex from trace element characteristics of zircon and rutile[END_REF], various metamorphic environments[START_REF] Luvizotto | Nb and Zr behavior in rutile during high-grade metamorphism and retrogression: An example from the Ivrea-Verbano Zone[END_REF][START_REF] Meyer | Trace element composition of rutile and the application of Zr-in-rutile thermometry to UHT metamorphism[END_REF][START_REF] Ewing | The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)[END_REF][START_REF] Gao | U-Pb ages and trace elements of metamorphic rutile from ultrahigh-pressure quartzite in the Sulu orogeny[END_REF][START_REF] Sengün | Trace element composition of rutile and Zr-in-rutile thermometry in metaophiolitic rocks from the Kazdağ Massif[END_REF][START_REF] Hart | A window into the lower crust: Trace element systematics and the occurrence of inclusions/intergrowths in granulite-facies rutile[END_REF][START_REF] Plavsa | Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration[END_REF] and detrital rutile[START_REF] Zack | Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer[END_REF] Okay et al., 2012;[START_REF] Rösel | Interpretation and significance of combined trace element and U-Pb isotopic data of detrital rutile: a case study from late Ordovician sedimentary rocks of Saxo-Thuringia[END_REF]. Missing boxes indicate that the respective trace element was not included in the element setup of the individual studies, which hampers comparison of rutile data to some extent. For example,[START_REF] Pe-Piper | Geochronology and trace element mobility in rutile from a Carboniferous syenite pegmatite and the role of halogens[END_REF] did not measure Fe in rutile from pegmatites and Ver Hoeve et al. (2018) did not include Sn in their measurements of igneous rutile from the Bushveld complex.(2-column, color)

  Ni, 63 Cu, 66 Zn,75 As,89 Y, 90 Zr, 93 Nb, 95 Mo, 107 Ag, 118 Sn, 121 Sb, 139 La, 140 Ce, 141 Pr, 146 Nd, 147 Sm, 153 Eu, 156 Gd, 159 Tb, 163 Dy, 165 Ho, 166 Er, 169 Tm, 173 Yb, 175 Lu, 180 Hf, 181 Ta, 182 W, 208 Pb, 232 Th and 238 U. For U-Pb dating, intensities for 238 U, 235 U, 207 Pb and 206 Pb were measured.

  Zimmerman, A., Stein, H.J., Morgan, J.W.,Markey, R.J., and Watanabe, Y., 2014, Re-Os geochronology of the El Salvador porphyry Cu-Mo deposit, Chile: Tracking analytical improvements in accuracy and precision over the past decade: Geochimica et Cosmochimica Acta, v. 131, p. 13-32. Fig. 1: Rutile petrography identifying different textural positions of rutile and mineral associations in Batu Hijau, Bingham Canyon, El Salvador, and Northparkes. A) Transmitted light image (PPL/XPL) of blocky, hydrothermal rutile in chlorite-sericite altered rock from Batu Hijau. Rutile grains are in contact with bornite grain and mantled by chlorite. The small inset show a reflected light image of the two rutile grains and bornite. The small cavity shows a negative crystal shape, possibly representing a breached rutile-hosted fluid inclusion (sample 267-757). B) and C) Transmitted (PPL/XPL) and reflected light images, respectively, showing intergrowth of elongated hydrothermal rutile with chalcopyrite in chlorite-sericite altered rock from Batu Hijau (sample 257-694). D) Transmitted light image of a thick section from Bingham Canyon showing two generations of rutile, needle-like exsolutions (sagenetic texture) and blocky rutile, in biotite phenocryst, partially replaced by sericite (sample 195-991). E) Sericite replacing biotite in the presence of chalcopyrite in Bingham Canyon, producing numerous blocky rutile grains with sizes typically < 25 µm (sample 123-210). F) BSE image of complex zoned anhedral rutile with apatite inclusion associated with partial replacement of biotite by chlorite from El Salvador. Brighter zones are characterized by elevated Si and Fe contents (sample 946-315). G) Combined transmitted and reflected light image showing rutile and chalcopyrite associated with chlorite within a paint vein that crosscuts an earlier quartz vein at El Salvador (sample 1104-4.3). H) Combined transmitted and reflected light image of blocky rutile in chlorite, El Salvador (sample 946-315). I) Combined transmitted and reflected light image of rutile, sericite, albite, and anhydrite intergrown with bornite in a K-feldspar -biotite matrix partially overprinted by later albite and sericite, Northparkes. J) BSE/reflected light image of rutile intergrown with bornite and chalcopyrite, Batu Hijau (sample 257-507). K) Rutile intergrown with and as inclusions in anhedral bornite filling open spaces between euhedral quartz crystals, Northparkes. L) Rutile cluster in chlorite -sericite altered rock from El Salvador. Note that rutile grains are cemented by calcic minerals such as anhydrite, calcite, and apatite in combination with sericite and K-feldspar. Chalcopyrite occurs in between rutile grains, partially surrounding them (sample 946-315).

Abbreviations: Alb -albite, Anh -anhydrite, Ap -apatite, Bi -biotite, Bn -bornite, Ca-calcite, Chl -chlorite, Cpy -chalcopyrite, K-Fsp -K-feldspar, Olg -oligoclase, Qtz -quartz, Rt -rutile, Ser -sericite.

(2-column, color)

Acknowledgments

We would like to thank Christoph Heinrich for access to the studied sample material, which was collected by several of his former Master and PhD students, and his helpful comments on earlier versions of this manuscript, which led to stimulating discussions and overall improvement of this paper. We are thankful for the constructive reviews by Richard Tosdal and Diana Zivak that helped to improve the manuscript. Financial support of this research project by ETH Zürich and the Swiss Science Foundation (Project 166151) to Christoph Heinrich is gratefully acknowledged.