Geology, geochronology and alteration of the Lorraine alkalic porphyry Cu-Au deposit, British Columbia, Canada

The present study assesses ore-forming processes and the timing of mineralisation in the Lower Main Zone of the Lorraine porphyry Cu deposit, north-central British Columbia. The deposit is hosted in an intrusion complex that comprises pre-mineral and post-mineral shoshonitic ultrabasic to syenite rock types. New U-Pb zircon ages of pre- and post-mineralisation dykes constrain the timing of mineralisation between 178.8 and 178.4 Ma. Intrusions in the Lower Main Zone have undergone several stages of alteration, which include early-, transitional- and late-stage assemblages. Early stage alteration includes; (1) pervasive biotite and K-feldspar alteration of wall rocks; (2) veins and patches of magnetite-diopside ¬¨¬± albite and (3) early coarse-grained K-feldspar-biotite veins that are locally associated with bornite and chalcopyrite. Transitional-stage mineralisation has produced distinctive sulphide zonation patterns consisting of a bornite-chalcocite core grading outwards to domains of bornite-chalcopyrite, chalcopyrite, chalcopyrite>pyrite, and a peripheral domain of pyrite with minor chalcopyrite. The sulphide zonation pattern is tilted and has overprinted numerous rock-types and the early-formed alteration assemblages. Syenite in the inner bomite-chalcopyrite zone typically contains abundant turbid K-feldspar (i.e. >70 %), whereas syenite marginal to the bomite-chalcopyrite core contains 50-70 % Kfeldspar, indicating an increase in K-metasomatism of syenites towards the core of the deposit. Transitional-stage mineralisation at Lorraine predominantly occurs as fine-grained disseminated sulphides in syenite, biotite pyroxenite and fine-grained K-feldspar biotite rock. Rare net-textured chalcopyrite and bomite mineralisation has also been identified in some biotite pyroxenites. Earlier workers speculated that biotite pyroxenites with net-textured sulphides at Lorraine may represent a 'deep' magmatic component of the porphyry system. However textural analysis of mineralised biotite pyroxenite appears to be at odds with this concept. In particular the current study has shown that: (1) primary diopside, which form contacts with sulphides, have corroded and actinolite-altered margins, suggesting alteration of primary minerals; (2) primary diopside and biotite do not contain primary sulphide inclusions and therefore there appears to be no evidence of a primary magmatic sulphide melt; (3) irregular-shaped relicts of biotite and diopside occur in sulphides, suggesting sulphides have replaced primary minerals and (4) deposit-scale sulphide zonation patterns overprints numerous lithology types, including biotite pyroxenites with net-textured sulphides. These findings support the author's contention that primary magmatic biotite and diopside were replaced or partially replaced by sulphide minerals, and are consistent with an influx of Cu and S during transitional-stage alteration and subsequent replacement of primary magmatic minerals (e.g., biotite) as opposed to the precipitation of primary magmatic sulphides from a Cu-rich ultrabasic magma source. Analysis of oxygen and hydrogen isotopes from F-rich biotite in early altered fine-grained K-feldspar biotite rock and fine-grained biotite in altered syenite has revealed the composition of fluids in equilibrium with early alteration assemblages. Fluid compositions define a range between +6.1 ‚ÄövÑ‚àû and +7.6 ‚ÄövÑ‚àû for ˜í¬•\\(^{18}\\)O, and -53‚ÄövÑ‚àû and -47 ‚ÄövÑ‚àû for ˜í¬•D. These values are consistent with the range for magmatic-derived aqueous fluids or metamorphic fluids. A magmatic fluid source is consistent with the close timing of alteration and mineralisation and the emplacement of syenite intrusions, and biotite geothermometry indicates that early alteration occurred at approximately 550 ¬¨‚àûC. The current study also investigated the chemical composition of unaltered shoshonitic rocks from Lorraine. The study concluded that biotite pyroxenite and syenite from Lorraine have major and trace element compositions that plot along a single trend, which is similar to fractionation trends for arc-related Fijian shoshonite lavas or shoshonitic glasses from the Nicola Group lavas (British Columbia). It is inferred that geochemical trends for Lorraine rocks reflect fractionation trends, and it is also inferred that syenite dykes were sourced from magma chambers at depth, or from fractionating melt in mature crystal mush columns. Ultrabasic rocks at Lorraine (i.e. biotite pyroxenite; ~42 to 48 wt. % SiO\\(^2\\)) include: (1) depletion in high field strength elements in comparison with rare earth elements; (2) enrichment in large ion lithophile elements and P in comparison with rare earth elements; (3) \\(^{87}\\)Sr/\\(^{86}\\) Sr (avg. = 0.7036), ˜í¬µNd (avg. = + 4.7) and stable isotope ˜í¬•\\(^{18}\\)O (diopside = 5.89) compositions that are similar to post-collisional shoshonites from the Mariana arc and Fiji and (4) an abundance of P (avg. ~0.87 wt. % P\\(^2\\)O\\(^5\\)) and Sr (avg. ~1200 ppm), which is higher or similar in abundance to Fijian shoshonites respectively. Some workers have related the high P and Sr in western Pacific shoshonites to the melting of slab-derived carbonate- and phosphorous-rich sediments in the mantle, which could also explain the enrichment of these elements in Lorraine shoshonites.