Breccia-hosted chemical and mineralogical zonation patterns of the northeast zone, Mt. Polley Cu-Ag-Au alkalic porphyry deposit, British Columbia, Canada

摘要

The Mt. Polley alkalic Cu-Ag-Au porphyry was emplaced in the late Triassic to early Jurassic Quesnellia island-arc terrane of the Canadian Cordillera. In the Central Quesnel Belt, Middle Triassic fine-grained oceanic sediments ± limestone were overlain by a thick pile of Late Triassic submarine, alkalic basaltic to andesitic volcanics (Nicola Group), with related subvolcanic intrusions and minor limestones of Late Triassic age. The Mt. Polley Complex is a 6.0 km long by 3.5 km wide, north-northwest-trending, composite alkalic intrusive (and breccia) complex; one of several that are scattered along the length of this terrane, making British Columbia the type location for Au-enriched alkalic porphyry deposits. The Mt. Polley Complex contains silica-undersaturated to silica-saturated pyroxenites, diorites and syenites, but is dominated by monzonites, monzodiorites, and associated breccia bodies, all of which were emplaced during the final stages of arc magmatism, between 209.4 Ma and 195.4 Ma. Mineralization occurred between 209 and 204 Ma. A new model Pb age of 207.8 ± 1.8 Ma for a galena vein is consistent with these ages.udThe NEZ is an alkalic Cu-Ag-Au porphyry deposit hosted by the Mt. Polley Complex. Ore is distinctly higher grade than in other ore zones at Mt. Polley, with average Cu grades at 0.8–1.0 percent and Au grades 0.19–0.29 g/t. Mineralization and alteration mostly occurred during magmatic-hydrothermal breccia formation. Chalcopyrite and bornite occur primarily as coarse- to fine-grained breccia cement, with lesser disseminations, veins and replacements. Bornite-rich, pyrite-deficient high-grade zones of mineralization (>1% Cu, up to >5% Cu locally) occur within broader chalcopyrite-rich domains (Cu grades of 0.3 to 1.0 %). Pyrite is the dominant sulfide on the deposit periphery (up to 1–2% locally).udThere were five major mineralization and alteration events in the NEZ: 1) early-stage (pre-breccia), 2) main-stage (syn-breccia), 3) post-brecciation late-stage mineralization, 4) barren intrusions, veins and vein breccia, and 5) epithermal-style veins. Main stage alteration and mineralization assemblages are zoned vertically and laterally through the breccia body. Magmatic-hydrothermal brecciation focused the high temperature mineralizing fluids, forming a core of potassic alteration (K-feldspar-magnetite-albite-calcite ± biotite ± augite ± anhydrite ± epidote and Cu-Fe-sulfides) surrounded by a halo of propylitic alteration (pyrite-chlorite-epidote ± albite ± sericite). Calc-potassic (garnet ± epidote) and sodic (albite) alteration assemblages are variably abundant within the broader domains of potassic and propylitic alteration. Alteration minerals are consistent with temperatures >300°C and near-neutral to alkalic pH fluids during stages 1, 2 and 3. Acidic and lower temperature fluids were associated with stages 4 and 5.udLaser-ablation ICP-MS analyses of sulfide trace element contents of the NEZ have shown bornite is enriched in Ag (average 913 ppm), Bi and Se. Chalcopyrite is enriched in Pb, Zn and Se, with Zn concentrations increasing and Pb concentrations decreasing from the centre of the deposit to its margins. Pyrite is enriched in Cu, Zn, Cd, Co, Ni, Se, Te and Au with these elements substituted into the pyrite structure or evenly disseminated as nano-particles. Pyrite contains abundant micro-inclusions of chalcopyrite, galena,udsphalerite, electrum and tellurides. Gold contents were found to be 1–3 orders of magnitude higher in pyrite (0.011–4.32 ppm, plus one outlier of 483.2 ppm) than in chalcopyrite (0.050–1.29 ppm). Gold was mostly below detection limits in bornite. Gold, Pd- and Pt-bearing inclusions were primarily detected in pyrite on the fringes of the deposit. This contradicts the assay data that shows high gold grades are associated with areas rich in bornite and chalcopyrite. This implies that the Au in the high-grade ore zones does not occur in Cu-sulfides, but in another phase, possibly electrum.udThe (δ^{34}S_{sulfide}) isotopic compositions of main- and late-stage chalcopyrite, pyrite and bornite range from -7.1 to +1.4 per mil, with most between -3 and -4 per mil. Sulfur isotopic compositions of anhydrite and gypsum were between +6.2 and +9.8 per mil, with two outliers of +13.6 and +14.0 per mil. These values, together with the presence of hematite, are consistent with an oxidized (sulfate predominant), high-temperature (>400°C) magmatic-hydrothermal fluid. Limited sulfide geothermometry indicates that ore precipitated at temperatures from ~ 480° to ~ 250°C. The `δ^(34)S` values of main-stage sulfides define zonation patterns across the deposit, from low `δ^(34)S` values in the core to higher `δ^(34)S` values near the deposit periphery. Changes in redox conditions, pH changes, cooling, and water-rock interaction are concluded to have been important processes of ore formation and hydrothermal alteration in the NEZ.udHydrothermal calcite occurs throughout the paragenesis, and several processes may have contributed to its precipitation, including boiling, `CO_2` degassing, pH increase, and water-rock interaction. Calcite `δ^(13)C` values range from -0.2 to -10.5 per mil (average -3.0 ‰), and `δ^(18)O` values from +4.0 to +20.9 per mil (average +15.4 ‰). The C-O isotopic values are not consistent with simple precipitation from a “normal” magmatically-derived source hydrothermal fluid. Enriched `δ^(13)C` values suggest the involvement of a heavy carbon source, such as limestone or seawater. However, `δ^(18)O` isotopic data preclude the involvement of meteoric or seawater in the formation of the NEZ, until stage 4.udLead isotopic data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite, pyrite and late-stage galena have (^{206/204}Pb) values of 18.77–18.92, (^{207/204}Pb) of 15.56–15.59 and (^{208/204}Pb) of 38.22–38.32. Strontium isotopic data (0.70331 to 0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between +5.9 and +6.5, also indicating a depleted mantle source. Stage 5 carbonate (^{206/204}Pb) values of 18.96–19.04, (^{207/204}Pb) of 15.57–15.59 and (^{208/204}Pb) of 38.26–38.36, from epithermal-textured veins suggest that telescoping of an epithermal environment into the NEZ occurred ~100 m.y. after breccia formation.udThe stable and radiogenic isotopic data provide evidence that the silica-undersaturated alkalic Mt. Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain both the `δ^(13)C-δ^(18)O` isotopic data and the silica-undersaturated composition of the magmatic-hydrothermal system. The CO2 released during assimilation of carbonate could have promoted magmatic-hydrothermal brecciation, thereby leading to high-grade ore formation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate or where carbonate platforms are subducted.