Geochronology of the Mines Gaspé porphyry deposit, Québec, Canada

North-central Gaspésie, Québec, has long been a focus of mineral exploration for magmatic-hydrothermal base metal deposits (e.g., Ford 1959; Brummer 1966; Allcock 1982). The region is part of the Gaspé Belt of the Appalachian Province, a package of east-trending moderately dipping Silurian to Devonian sedimentary rocks that were intruded by multiphase syn- to post-orogenic intrusions associated with the later stages of the Acadian orogeny (de Römer 1977; Duquette et al. 1984). The most significantly mineralized intrusions of this suite occur at Mines Gaspé, host to porphyry Cu–Mo and skarn mineralization within the historically mined Copper Mountain porphyry orebody with a historical resource of 209 Mt at 0.40% Cu and 0.02% Mo and the undeveloped Porphyry Mountain orebody with a resource of 437 Mt at 0.89% Cu-equivalent (Martin 2011a, 2011b).

A range of ages have been generated for intrusive rocks in the region using different methods (e.g., Wanless et al. 1966, 1967; de Römer 1969, 1974; Tauchid 1970; Hollister et al. 1974, in Allcock 1982; La Rocque 1986; and Pilote 2005) suggesting a broad range from approximately 410 to 330 Ma.This paper aims to establish more precise dates for intrusive rocks at Mines Gaspé contributing to a better understanding of magmatic processes, as well as the timing of potential mineralizing intrusions in the region. This study presents new geochemical and CA-TIMS U–Pb zircon geochronology data that constrain the ages of the two main causative intrusions at Mines Gaspé: Copper Mountain and Porphyry Mountain. Also presented are new epidote laser ablation inductively coupled mass spectrometry (LA-ICP-MS) U–Pb ages from both Mines Gaspé and the nearby Sullipek deposit to constrain the timing of epidote formation and test whether it can be linked to any of the mineralizing porphyry systems.

Mines Gaspé is hosted in the Canadian Appalachian Province, a Paleozoic orogen bordered by the Grenville Province to the northwest and submarine Paleozoic sedimentary rocks to the southeast (Fig. 1; Williams 1995). The orogen has been divided into several terranes, with the Humber terrane of the Gaspé Peninsula representing the Paleozoic passive margin of eastern North America and the outer terranes a result of accretion to the North American craton (Williams 1979; Williams and Hatcher 1983; Keppie 1985).

In the Gaspésie region of Québec, three major lithotectonic assemblages of the Canadian Appalachians are recognized: the early Paleozoic Humber and Dunnage terranes and the overlying middle Paleozoic Gaspé Belt (Williams 1979; Bourque et al. 2000). The Silurian to Devonian rocks of the Gaspé Belt have been divided into three tectonostratigraphic zones (Fig. 1): the Connecticut Valley–Gaspé zone, the Aroostook–Percé zone, and the Chaleurs Bay zone (Malo and Bourque 1993). The Canadian section of the Connecticut Valley–Gaspé zone stretches from the Eastern Townships of Québec to the tip of Gaspésie, with the American section extending further west into the United States Appalachians and totals more than 1000 km in length (Fig. 1; Williams 1995). The Connecticut Valley–Gaspé zone is bordered by the Shickshock-South Fault to the north and the Restigouche Fault to the south, separating it from the Humber terrane rocks to the north and the Aroostook–Percé zone to the south (Malo and Bourque 1993; Bourque et al. 1995). The Mines Gaspé and Sullipek systems are situated on the northern limb of the Connecticut Valley–Gaspé zone of the Gaspé belt on the north-central part of Gaspésie (Fig. 2).

The Mines Gaspé and Sullipek deposits are hosted in thick sequences of gently folded Silurian to Devonian sandstones, limestones, and calcareous siltstones that dominate the region of north-central Gaspésie (Fig. 2). The stratigraphy around Mines Gaspé consist of the Shiphead, Forillon, and Indian Cove formations of the Upper Gaspé Limestones Group of Gaspésie, which host skarn mineralization. This package is overlain by the York River and York Lake formations of the Gaspé Sandstone Group. Sullipek is hosted in the underlying West Point Formation of the Chaleurs Group (Wares 1988). The Upper Gaspé Limestones and Gaspé Sandstones range in age from Pragian to Eifelian (approximately 410–388 Ma), based on biostratigraphy (Bourque et al. 1995). In northern Gaspésie, the Upper Gaspé Limestone units can be traced approximately 100 km from the Forillon Peninsula to the Murdochville region (Lésperance 1980; Rouillard 1984).

The West Point Formation of the Chaleurs Group consists of fossiliferous limestone conglomerate, with interbedded limestone and siltstone (Bourque and Lachambre 1980). The Forillon Formation of the overlying Upper Gaspé Limestones is a repetitive sequence of shaly, dolomitic, and siliceous calcilutite with black, predominantly calcareous mudstone (Lésperance 1980). The overlying Shiphead Formation has a higher siliciclastic content and consists of siliceous and dolomitic mudstone and limestone with minor calcarenite and sandstone beds (Lésperance 1980). The Indian Cove Formation consists of a homogenous sequence of siliceous or silty calcilutite with minor sandstones and siltstones in the upper part of the stratigraphy (Lésperance 1980). To the southwest of Mines Gaspé, all three formations of the Gaspé Upper Limestones group contain abundant mafic lava flows and pyroclastic units (Lésperance 1980; Bourque et al. 1995; D’Hulst et al. 2008). The York Lake Formation of the Gaspé Sandstone Group consists of alternating siliceous calcilutites with minor quartz arenites and includes the transition zone of limestone-dominated units in the underlying formations to the sandstones of the York Lake and York River formations (Bourque et al. 1995). The York River Formation consists of a mudstone–siltstone–sandstone assemblage with minor calcarenites, and a sandstone dominated assemblage with minor mudstones (Bourque et al. 1995) and includes thick basaltic and rhyolitic volcanic sequences (Doyon 1988; Doyon and Valiquette 1991).

The highly variable metamorphic zoning observed throughout this region, most notably in the Mines Gaspé and Sullipek areas, is the result of the combined effect of several heat sources, with some authors arguing for an abundance of intrusive rocks at depth to explain the metamorphic aureoles (Duquette 1983; Williams-Jones 1986; Wares 1988). The contact metamorphism throughout the region is sporadic and complex, with most impure limestones and siltstones altered to varying calc-silicate assemblages (Allcock 1982; Wares 1988) with little evidence for a regional metamorphic effect.

The Mines Gaspé property contains two different, but genetically related styles of mineralization: porphyry-style copper–molybdenum and skarn (Allcock 1982). These are hosted within the Copper Brook aureole, a region of bleached strata within the property that has a complex history of metamorphism, hydrothermal alteration, and sulphide mineralization (Allcock 1982; Fig. 3). The geology of the mine area has been discussed in detail by Brummer (1966), Allcock (1982), and Shelton (1983) and is only summarized here. Skarn-type mineralization consists of a stacked series of replacement high-temperature garnet–pyroxene skarn orebodies at Needle Mountain and Needle East hosted in the Indian Cove Formation sedimentary rocks as disseminated sulphides and quartz–chalcopyrite–pyrrhotite stockworks (Fig. 3). The east-zone orebodies are located at depth under the town of Murdochville. These were discovered in the 1980s and differ from the other skarn orebodies as they occur within bleached carbonaceous porcellanite host rocks with calcite, tremolite, and diopside (Procyshyn 1987). The combined Needle Mountain and Needle East orebodies have historical resources of 69.5 Mt at 1.50% Cu, whereas the E32 orebody has a historical resource of 4.5 Mt at 2.95% Cu and 15 g/t Ag (Procyshyn 1987).

Porphyry-type deposits within the Mines Gaspé property include Copper Mountain and Porphyry Mountain (Fig. 3). In the Copper Mountain orebody, pyrite, chalcopyrite, and molybdenite occur as disseminations or in veins within and surrounding the Copper Mountain plug, a small porphyritic granite intrusion. The plug and surrounding sedimentary host rocks display hydrothermal alteration characteristic of porphyry copper deposits; however, as they are emplaced into calcareous host rocks a large portion of the alteration consists of varying calc-silicate assemblages along with potassic and phyllic alteration of the intrusive rocks (Allcock 1982). Copper Mountain has a historical resource of 209 Mt at 0.40% Cu and 0.02% Mo, with roughly 40% of the resource contained within intrusive rocks and remaining 60% of the resource in directly adjacent porcellanite and skarn units (Martin 2011a).

In 1994, four exploration drill holes intersected blind porphyry and skarn style Cu–Mo–Ag mineralization to the northeast of Copper Mountain, beneath Porphyry Mountain (Martin 2011b). A resource estimate on the Porphyry Mountain orebody in 2005 reported 437 Mt at 0.89% Cu equivalent, representing the most significant undeveloped resource on the property (Martin 2011b). The bulk of mineralization at Porphyry Mountain comprises sulphide bearing quartz–carbonate rich stringer veins and related disseminated sulphides linked to a porphyry-style stockwork system generated around a discordant quartz–feldspar porphyry intrusion. As at Copper Mountain, stratigraphically controlled Cu mineralized skarn horizons are present within 200 m of the intrusion (Martin 2011b). The Porphyry Mountain deposit contains a vertical thickness of mineralization up to 700 m in length at approximately 1100–1700 m depth (Martin 2011b).

The Sullipek deposit has a historical resource of 0.5 Mt grading 1.4% Cu, 0.025% Mo, and 7 g/t Ag and is located approximately 40 km southwest of Mines Gaspé (Fig. 4; Wares 1983, 1988). It consists of an Fe–Au–Cu skarn hosted in pure marbles of the uppermost Silurian West Point Formation that was intruded by dacitic porphyries that have undergone strong potassic and sodic alteration (Wares 1988). The Sullipek fault system has localized skarn occurrences at Sullipek. The region hosts several varieties of igneous rocks, most of which are intrusive, hypabyssal, and syn- to-post orogenic, similar to those found at Mines Gaspé (Wares 1988). Field observations and cross-cutting relationships indicated that the oldest intrusions are porphyritic gabbro followed by a series of rhyolitic porphyries and porphyritic quartz monzonites (Wares 1988). The main intrusions associated with mineralization are a swarm of dacite porphyry dykes, followed by post-mineralization tonalitic porphyries and granodiorite dykes. Other than the intrusive complex of the McGerrigle mountains to the north, intrusions in the area are generally sills and dykes, typically with orientation controlled by the major northeast-trending structures of the region (Wares 1988). Similar to Mines Gaspé, these intrusions have been emplaced into thick sequences of calcareous sedimentary rocks, forming skarns and porcellanites, that host the majority of the mineralization (Wares 1988). Alteration within the region is similar to that found in and surrounding porphyry Cu deposits, including potassic, phyllic, and sodic alteration, with chlorite and epidote being observed in the propylitic alteration, but also as calc-silicate minerals within skarns (Wares 1988). There is an approximately 100 km², complex metamorphic aureole containing several mineralized systems within the Sullipek region, twice the size of the aureole footprint at Mines Gaspé (Duquette 1983). Duquette (1983) proposed that this large aureole represents metamorphism related to a large igneous body at depth, as suggested for Mines Gaspé (Allcock 1982), or the surface expression of the large McGerrigle igneous complex to the north (Whalen et al. 1991).

There have been several geochronology studies on intrusive rocks in the region; however, most have used isotopic systems that result in significant associated error, making it difficult to constrain magmatic evolution throughout Gaspésie (Table 1). The most commonly cited ages for intrusive rocks at Mines Gaspé are from Tauchid (1970, in Allcock 1982) who reported K–Ar biotite ages of 357 ± 17 Ma for Porphyry Mountain and 358 ± 16 Ma for Copper Mountain (Table 1). La Rocque (1986) reported Rb–Sr ages for several north-central Gaspésie intrusions, including an age of 382 ± 7 Ma for Copper Mountain at Mines Gaspé and 365 ± 2 and 364 ± 3 Ma for two rhyolite porphyries at Sullipek. A more recent project by Stephenson et al. (1998) generated U–Pb zircon ages of 384.9 ± 2.5 Ma for Porphyry Mountain and 384.8 ± 2.8 Ma for Copper Mountain, ages that overlap within error (Table 1). Meinert et al. (2001) was the first study to incorporate U–Pb dating of zircon and hydrothermal minerals, garnet and titanite, from skarn associated with Porphyry Mountain. Uranium–Pb single and multigrain zircon analyses from Porphyry Mountain scatter near concordia between 365 and 385 Ma, with evidence for both zircon inheritance and post-crystallization Pb-loss (Meinert et al. 2001). They reported a zircon age of 371.2 ± 4.7 Ma, based on a U–Pb ion probe single zircon analysis, which is in agreement with a reported titanite U–Pb age of 371.8 ± 8.7 Ma. Nine U–Pb garnet analyses fall on or near concordia between 340 and 380 Ma and they also reported a linear regression garnet age of 362.7 ± 3.5 Ma (Meinert et al. 2001).

A total of 37 samples were collected in the Mines Gaspé and Sullipek regions, with 10 samples from 5 drill holes in the vicinity of the Sullipek deposit (Fig. 4), 26 samples from 7 drill holes from Mines Gaspé within the footprint of the Copper Mountain and Porphyry Mountain deposits, and one surface sample from the Copper Mountain open pit (Fig. 3). Seven intrusive unit samples and six epidote-bearing samples are included here as the remainder were samples of metamorphosed sedimentary rock. Two granite porphyry samples were selected for dating. Sample GA18PH032 was collected from an ∼20 m thick sill in the Copper Mountain pit (Fig. 5) and GA18PH033 was collected from drill core at ∼1650 m down-hole within the Porphyry Mountain orebody. Copper Mountain intrusion material was inaccessible in outcrop at time of sample collection. The plug and associated dykes and sills are believed to be emplaced contemporaneously and representative of the magmatic system that formed the Copper Mountain porphyry-type mineralization (e.g., Allcock 1982). Five additional granite porphyry samples came from various dyke swarms at depth beneath the Copper Mountain pit. Six additional epidote-bearing samples were selected from the altered host sedimentary lithologies from Mines Gaspé (three samples) and the nearby Sullipek deposit (three samples) for epidote U–Pb LA-ICP-MS geochronology.

The seven granite porphyry samples from Mines Gaspé were submitted to Bureau Veritas in Vancouver, British Columbia, for whole-rock geochemical analysis to determine major, minor, and trace element concentrations. Representative whole-rock geochemical data are listed in Table 2. Samples were prepared for analysis by being weighed, dried, and crushed until more than 70% of the sample could pass through a 2 mm mesh. A 250 g portion of the sample was then split and pulverized until more than 85% of the sample could pass through a 75 µm mesh.

For major and minor element analysis, a 0.20 g portion of each sample was mixed with a 0.90 g lithium metaborate/lithium tetraborate flux and fused at 1000 °C in a furnace. The resultant melt was then dissolved in 100 mL of 4% nitric and 2% hydrochloric acid. This solution was then analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES) to determine concentrations of major element and major element oxide concentrations. Trace and rare-earth elements (REEs) were analyzed using inductively couple plasma mass spectrometry (ICP-MS). Aqua regia was used for dissolution of base metal elements in the samples. A 30 g sample was partially digested in a solution of 1:1:1 nitric acid, hydrochloric acid, and water and 37 elements analyzed with ICP-AES and ICP-MS. For both major and trace element calibration standards, verification standards and reagent blanks were included in the sample sequence. Reported detection limits for the major elements are <0.04 wt.% and <0.5 ppm for the majority of the trace elements but <0.1 ppm for the REE.

Epidote mineral chemistry analyses for geochronology were performed on circular polished epoxy mounts (approximate diameter of 2.5 cm). The mounts were cut, prepared, and polished at the Lakehead University lapidary laboratory and analyzed at the Centre for Ore Deposit and Earth Sciences (CODES) Analytical Laboratories, University of Tasmania, Australia. Prior to analysis, mounts were kept in a vacuum desiccator overnight to remove moisture and atmospheric air (Thompson et al. 2016, 2018).

LA-ICP-MS spot data were collected using an Agilent 7900 quadrupole ICP-MS coupled to an Australian Scientific Instruments Resolution S-155 ablation system with Coherent COMPex Pro 110 Ar-F excimer laser operating at a 193 nm wavelength and a pulse width of 20 ns. All instrumentation is housed at the CODES Analytical Laboratories at the University of Tasmania. Laser ablation analyses were performed in an atmosphere of pure He flowing at a rate of 0.35 L/min. Immediately past the ablation point within the cell, He carrier gas was mixed with Ar (1.05 L/min) for improved efficiency of aerosol transport. The laser beam size used was 29–43 µm with a frequency of 5 Hz, and a laser energy density of ∼3.5 J/cm². The analysis time for each sample was 60 s, with the initial 30 s to measure background with the laser turned off, and the remaining 30 s of analysis with the laser on. Only a limited number of masses were analyzed to reduce sweep time (∼0.22 s) with longer counting time on the Pb and U isotopes compared to the other elements.

The data reduction used was based on the method outlined in detail in Thompson et al. (2016) and is based on Chew et al. (2014) where a common Pb correction is done on the calibration standard. Intervals of the laser spectrum were chosen based on the lowest common Pb and avoiding other mineral phases. The downhole fractionation, instrument drift, and mass bias correction factors for Pb/U ratios were calculated using analyses of the AA063 epidote using unpublished values obtained by Isotope Dilution Multi Collector ICPMS (ID-MC-ICPMS) method at the University of Melbourne. The instrument drift and mass bias correction factors for the ²⁰⁷Pb/²⁰⁶Pb ratio (ages) were calculated using analyses of the NIST610 or NIST612 glass, using the Pb isotopic values of Baker et al. (2004). The calibration of the epidote U–Pb ages was checked using several epidote in-house reference materials that were treated as unknowns: RE16JP071 epidote (73.9 ± 3.1 Ma) and LB15DC052 epidote (33.15 ± 0.24 Ma). Ages were calculated using concordia intercept. In all samples there was enough spread on the isochron to negate the need for assuming the common Pb composition.

Uranium–Pb zircon ages were generated for samples of intrusive units at Copper Mountain and Porphyry Mountain. The analyses were conducted using CA-TIMS at the Pacific Centre for Isotopic and Geochemical Research at the University of British Columbia. The CA-TIMS procedures described here are modified from Mundil et al. (2004), Mattinson (2005), and Scoates and Friedman (2008). After rock samples had undergone standard mineral separation procedures, zircons were hand-picked in alcohol and selected based on clarity and lack of cracks and inclusions. After zircons were annealed in quartz glass crucibles at 900 °C for 60 h, hydrofluoric acid and nitric acid were added. After heating in a high-pressure dissolution device, the zircons were rinsed with water and sub-boiled acetone. Five single zircon grains were separated and transferred into crucibles with hydrofluoric acid, nitric acid, and a ^233–235U–²⁰⁵Pb tracer solution (EARTHTIME ET535) and again heated in a high-pressure dissolution device. Resulting solutions were dried and hydrochloric acid was added and again were placed in the high-pressure device. HCl solutions were then dried with phosphoric acid and loaded onto degassed, zone refined Re filaments in a silicic acid emitter (Gerstenberger and Haase 1997). Isotopic ratios were measured with a single collector VG 54R thermal ionization mass spectrometer. Data reduction used the excel-based program of Schmitz and Schoene (2007). Standard concordia diagrams were constructed and regression intercepts and weighted averages were calculated with Isoplot (Ludwig 2003). All errors are reported at the 2 sigma or 95% confidence level. The ages are listed with three error values. The first error value with no brackets corresponds to errors based on analytical precisions only used to compare with results from the same data set or for other U–Pb TIMS ages using the same isotopic tracer (ET535). The second error value with round brackets corresponds to errors based on analytical precisions and isotopic tracer (ET535) spike errors and should be used to compare these results with U–Pb dates not using the ET535 tracer, including non-TIMS U–Pb dates. The third error value with square brackets corresponds to errors based on analytical precisions, ET535 spikes and U decay constant errors and should be used to compare these results with dates generated with other decay systems, such as K–Ar or Ar–Ar.

The intrusive rocks sampled at Mines Gaspé are quartz–feldspar porphyries. Samples range in colour from pink/red to beige to light grey (Fig. 6). Phenocrysts are dominantly plagioclase–quartz, with minor biotite and rare hornblende within a very fine-grained matrix of quartz ± plagioclase ± K-feldspar. Potassic altered porphyry units are typically pinkish-orange to red, commonly hosting veins with purple anhydrite, molybdenite, chalcopyrite, and chlorite (Figs. 6a–6e). Phyllic altered porphyry units typically have moderate to strong sericitic alteration of feldspars in a light grey to beige porphyry (Figs. 6b–6f).

Seven whole-rock geochemical analyses were obtained from samples of both the Copper Mountain and Porphyry Mountain intrusions, five of which are from drill hole intersections of dyke swarms beneath the Copper Mountain pit. Whole-rock geochemical data are presented in Table 2. Samples range in composition from granodiorite to granite (65–80 wt.% SiO₂; Table 2) and all Copper Mountain and Porphyry Mountain intrusive samples are calc-alkaline. All seven samples display moderate alteration with phyllic to potassic assemblages and variable loss on ignition values of 2.1–5.1 wt. %. All samples have similar REE profiles with steep light rare earth element (LREE) and flat heavy rare earth element (HREE) patterns on a chondrite-normalized REE diagram (Fig. 7). The samples have ΣREE values of 63–194 ppm and LREE/HREE values range from 4.8 to 9.0, with (La/Yb)_N ranging from 24.8 to 29.0, with the exception of GA18PH033 at 47.5. Samples are relatively depleted in incompatible elements, with strongly negative Ti and Nb anomalies (Fig. 7).

The results of U–Pb zircon geochronology analyses for samples GA18PH032 and GA18PH033 are presented in Table 3 and Fig. 8. The zircons contain 171–438 ppm U and 10.7–28.7 ppm Th. The modelled Th/U ratio (0.330–0.679, calculated from radiogenic ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²³⁵U age) is greater than 0.1, consistent with a magmatic origin (Hoskin and Schaltegger 2003). The zircons for each sample were split into five fractions, composed of single zircon grains or fragments. The ages listed here are based on the ²⁰⁶Pb/²³⁸U date for the youngest zircon fraction. The age for the sample from a sill in the Copper Mountain pit, GA18PH032, is 377.60 ± 0.45 (0.48) [0.62] Ma. The age for the sample from the Porphyry Mountain deposit, GA18PH033, is 378.80 ± 0.37 (0.41) [0.57] Ma (Fig. 8). The Porphyry Mountain sample also has an inherited zircon population c. 392 Ma.

Mines Gaspé epidote samples occur as disseminated/granular grains within the groundmass of many samples and as larger grains within veinlets or veinlet halos of diopside skarn of the Forillon Formation, often associated with porphyry Cu–Mo mineralization (Figs. 9a–9c). At Sullipek, epidote occurs as massive replacement within siltstones comprising up to 50 modal % in some samples. Occurrence of epidote ranges within samples from fine-grained patches and clusters to elongate interlocking blades and laths (Figs. 9d–9f).

The results of epidote U–Pb geochronology analyses are in Table S1 and concordia diagrams are shown in Fig. 10. Three samples of epidote from Mines Gaspé were dated. Vein-/disseminated replacement-style epidote from sample GA18PH005 yielded a concordia intercept age of 372 ± 26 Ma from 15 grains. Vein-syle epidote from sample GA18PH017 yielded an age of 353 ± 11 Ma from 12 grains. Replacement-style epidote from sample GA18PH020 yielded an age of 364.5 ± 9.5 Ma from 23 grains. Three replacement-style samples of epidote from Sullipek were dated. Sample GA18PH028 yielded a concordia intercept age of 380 ± 12 Ma from 25 grains, sample GA18PH031 an age of 373.4 ± 7.5 Ma from 18 grains, and sample GA18PH036 an age of 396.3 ± 3.7 Ma from 17 grains.

The numerous quartz–feldspar porphyry sills, dykes, and plugs throughout the Mines Gaspé property are part of a multiphase calc-alkaline I-type felsic intrusive complex. The intrusive units at Mines Gaspé vary in composition from granites to granodiorites with similar REE patterns characterized by strong negative Nb and Ti anomalies and enriched LREE trends with respect to HREE with flat to positive Zr and Hf anomalies. This signature is characteristic of calc-alkaline subduction related magmas formed in an arc setting (Pearce 1982). The whole-rock data of intrusive rocks from Mines Gaspé are similar to the findings of Whalen (1993) and Whalen et al. (1994) at other sites within the area, including the McGerrigle Complex, Mont Brown, Mont Hog’s Back, and Mont Vallières-de-Ste-Réal.

A summary of historical ages of intrusive units from Mines Gaspé and Sullipek are provided in Table 1. These previous studies (Wanless et al. 1966, 1967; de Römer 1974; Hollister et al. 1974; and Tauchid 1970, in Allcock 1982) included intrusive rocks from several deposits and mineralized systems in north-central Gaspésie. The K–Ar dates using biotite or whole-rock samples typically have larger error margins than more precise U–Pb dating and are more prone to thermal resetting (Faure 1977). Tauchid (1970, in Allcock 1982) reported ages of 358 ± 16 Ma for Copper Mountain and 357 ± 17 Ma for Porphyry Mountain, which are significantly younger than ages generated in this study. Allcock (1982) argued that most of the biotite at Copper Mountain and Porphyry Mountain have been affected by hydrothermal alteration, suggesting that some of the K–Ar dates may document the time of final closure for argon, rather than the age of crystallization. Other K–Ar ages for Copper Mountain (395 ± 15 Ma, W,anless et al. 1966; 350 ± 15 Ma, Wanless et al. 1967; 346 Ma, Hollister et al. 1974; 378 ± 17 Ma, Allcock 1978) record an imprecise age range of 410 to 335 Ma. La Rocque (1986) generated ages for several intrusions in the region, including Mines Gaspé and Sullipek, using the Rb–Sr whole-rock method. This isotopic system works provided that the systems are closed, but also yielded ages with large margins of error. La Rocque (1986) noted that samples from both Mines Gaspé and Sullipek were hydrothermally altered, likely contributing to inaccuracy in ages. Alternatively, Whalen et al. (1991) suggested that the anomalously young Rb–Sr ages were the result of inhomogeneous initial Sr isotopic compositions.

Stephenson et al. (1998) generated zircon U–Pb ages of 384.9 ± 2.5 and 384.8 ± 2.8 Ma for Porphyry Mountain and Copper Mountain, respectively (L.D. Meinert (personal communication, 2022)). Meinert et al. (2001) reported U–Pb single and multigrain zircon analyses from Porphyry Mountain that scatter near concordia between 365 and 385 Ma, including a U–Pb ion probe single zircon age of 371.2 ± 4.7 Ma. The same study generated a Re–Os molybdenite age of 375 ± 2 Ma (L.D. Meinert (personal communication, 2022)).

The youngest zircon U–Pb population age of 377.60 ± 0.45 Ma for the sample from the Copper Mountain pit and the 378.80 ± 0.37 Ma age of the Porphyry Mountain intrusion are interpreted to represent the emplacement age of the two intrusions. The ages of the two samples do not overlap within analytical error with a 0.38 m.y. gap between samples, for the first time allowing the emplacement history of the two intrusions to be resolved with the Porphyry Mountain intrusion being slightly older than the Copper Mountain intrusion. The Porphyry Mountain intrusion has an inherited zircon population at 392.04 ± 0.41 Ma, which is coincident with a zircon age of 391.3 ± 3.4 Ma from Whalen et al. (1991) for the McGerrigle Complex, approximately 30 km west of Mines Gaspé. The zircon CA-TIMS ages generated in this study are broadly consistent with previous ages, specifically the more-recent ages of Meinert et al. (2001) but provide more precise constrains on the time of intrusion. The new, high precision age for Porphyry Mountain is older than the Re–Os age of 375 ± 2 Ma (L.D. Meinert (personal communication, 2022)) even when the larger error of ±0.55 m.y. is considered, whereas the Copper Mountain age is within error of the mineralization age. The ages are younger than those reported by Stephenson et al. (1998), suggesting either a protracted magmatic history for the two intrusions or possibly that the Stephenson ages included some inherited grains.

Mines Gaspé consists of two, nearly simultaneously emplaced apophyses or plugs within a broad hydrothermal alteration/skarn envelope. Detailed investigation into mineralization paragenesis by previous workers (notably Allcock 1982) concluded that porphyry-type mineralization at Mines Gaspé is late, and post-dates skarn formation and mineralization, similar to findings at other porphyry intrusion related deposits of the Gaspé Peninsula, such as Sullipek (Wares and Williams-Jones 1981) and Patapédia (Willams-Jones 1982).Inherited zircon grains c. 392 Ma at Mines Gaspé could be a marker for early, extensive magmatism that produced the extensive skarn alteration aureoles throughout the Gaspé Peninsula at sites like Mines Gaspé and the nearby McGerrigle Complex (Whalen et al. 1991), followed by significantly later (i.e., >10 m.y.) porphyritic intrusions and associated mineralization that added to existing skarn resources.

Epidote at both Mines Gaspé and Sullipek occur as disseminated/granular crystals within the host groundmass and as larger crystals within veinlets in metasomatised sedimentary rocks. Phillips (2019) and Phillips et al. (2021) showed that U–Pb dating of epidote could be used to resolve the ages of different generations of epidote around the Resolution porphyry system in Arizona, showing that there were two generations of epidote with one related to a Proterozoic intrusive event and the second linked to the ∼65 Ma Resolution porphyry. This is significant given the recent application of epidote mineral chemistry to vector to porphyry mineralization (Cooke et al. 2014, 2020a, 2020b). The northeastern portion of the Connecticut Valley–Gaspé trough in the region of Mines Gaspé underwent at most prehnite-pumpellyite facies metamorphism (Bédard 1986) in the Devonian, in contrast to higher metamorphic grades in western Gaspésie and the northeastern United States (e.g., Perrot et al. 2020). At Mines Gaspé, epidote ages range from 404 to 334 Ma and Sullipek epidote ages range from 414 to 348 Ma, with both sites’ epidote broadly consistent with zircon ages in this study and the window of hydrothermal mineral deposits within the region. Given the absence of greenschist or higher grade metamorphism within the peninsula, it is likely that the epidote dated in this study was associated with propylitic hydrothermal alteration associated with either the skarn or porphyry mineralization. The wide spread of ages and the high analytical errors that result from the high common Pb contents of the epidote, preclude linking the epidote to any specific mineralizing intrusion dates in the region. However, it is possible to draw a number of conclusions from the epidote geochronology. The epidote ages from Mines Gaspé overlap within error and two of the populations overlap with the new U–Pb ages reported in this study and the Re–Os age of 375 ± 2 Ma (L.D. Meinert (personal communication, 2022)). In contrast, the epidote from GA18PH017 do not overlap with the new U–Pb ages at Mines Gaspé, suggesting that there is at least one younger propylitic hydrothermal event in the region. The younger age of GA18PH017 at 353 ± 12 Ma corresponds to epidote peripheral to a quartz–pyrite vein, contrary to spotty/disseminated replacement epidote in the other older epidote samples at Mines Gaspé (Fig. 9). The Sullipek epidotes also show a broad overlap between the three dated sample and overlap with the epidote from Mines Gaspé, suggesting that there be a regional propylitic alteration event. Sample GA18PH036 from Sullipek yielded the smallest error of the three samples and only barely overlaps with the ages at Mines Gaspé, hinting that there may also be an older propylitic alteration event present within the region. Combined, the new epidote U–Pb ages suggest a prolonged and complex history of propylitic alteration but additional geochronology will be required in the region to fully constrain the paragenesis of the epidote and it’s links to specific mineralizing porphyries.

Mines Gaspé hosts numerous multiphase quartz–feldspar porphyry sills, dykes, and plugs representing a multiphase sequence of calc-alkaline intrusions, with a geochemical signature consistent with a supra-subduction zone setting, producing skarn and porphyry mineralization in calcareous host rocks. Two new zircon U–Pb CA-TIMS ages of 377.60 ± 0.45 and 378.80 ± 0.37 Ma for Copper Mountain and Porphyry Mountain, respectively, show that Porphyry Mountain intrusion was emplaced at least 0.38 m.y. before Copper Mountain. The ∼392 Ma inherited zircon grains may constrain the extensive magmatism that produced the extensive skarn alteration aureoles throughout the Gaspé Peninsula at sites like Mines Gaspé and the nearby McGerrigle Complex (Whalen et al. 1991), followed by significantly later (>10 m.y.) porphyritic intrusions and associated mineralization that added to existing skarn resources. Epidote at both Mines Gaspé and Sullipek are broadly coincident with zircon ages in this study and the window of hydrothermal mineral deposits within the region. The epidote ages from Mines Gaspé overlap within error and two of the populations overlap with the new U–Pb ages reported in this study and the Re–Os age of 375 ± 2 Ma (L.D. Meinert (personal communication, 2022)). In contrast, vein associated epidote in GA18PH017 do not overlap with the new U–Pb ages at Mines Gaspé, suggesting that there is at least one younger propylitic hydrothermal event in the region. Sample GA18PH036 from Sullipek yielded the smallest error of the three samples and only slightly overlaps with the ages at Mines Gaspé hinting that there may also be an older propylitic alteration event present within the region. Combined, the new epidote U–Pb ages suggest a prolonged and complex history of propylitic alteration but additional geochronology will be required in the region to fully constrain the paragenesis of the epidote and its links to specific mineralizing porphyries.

This research is a result of an HBSc thesis by Mitch Marcelissen at Lakehead University, Canada. The project was part of the Australian Mineral Industry Research Association (AMIRA) International project P1202 “Far-field and near-mine footprints”, with funding support from sponsors gratefully acknowledged. PH acknowledges support from an NSERC Collaborative Research Development grant. Gord Maxwell and Robert Banville from Glencore are thanked for assistance with sampling. Kristi Tavener and Jonas Valiunas are thanked for sample preparation. Two anonymous reviewers are thanked for their constructive comments.

Data generated or analyzed during this study are provided in full within the published article and its supplementary materials.

Conceptualization: MM, PH, DRC

Data curation: MM, PH, MJB, IB

Funding acquisition: PH

Formal analysis: MJB, IB, EO, RF

Investigation: MM, PH

Methodology: PH, MJB, IB, EO

Project administration: DRC

Supervision: PH, DRC

Writing – original draft: MM

Writing – review & editing: PH, DRC, MJB, IB, EO

Supplementary data are available with the article at https://doi.org/10.1139/cjes-2024-0013.