Mineralization, U-Pb Geochronology, and Stable Isotope Geochemistry of the Lower Main Zone of the Lorraine Deposit, North-Central British Columbia: A Replacement-Style Alkalic Cu-Au Porphyry

Abstract

The Lower Main zone of the Lorraine alkalic porphyry Cu-Au district, BC, Canada, is hosted in an intrusive complex that comprises pre- and late mineral biotite pyroxenites, monzonites, and syenites. Some of the biotite pyroxenites have interstitial sulfides, which appear similar to net-textured sulfides in magmatic ore deposits; however, the pyroxenites have undergone several stages of hydrothermal alteration. Main-stage mineralization produced sulfide zonation patterns consisting of a bornite-chalcocite or bornite-chalcopyrite core grading outward to domains of chalcopyrite, chalcopyrite > pyrite, and a peripheral domain of pyrite with minor chalcopyrite. Syenite in the inner bornite-chalcopyrite zone typically contains abundant turbid K-feldspar (>70%), whereas syenite marginal to the bornite-chalcopyrite core contains less K-feldspar (50–70%), indicating an increase in K metasomatism of syenites toward the core of the deposit. Main-stage mineralization predominantly occurs as fine-grained disseminated sulfides in syenite, biotite pyroxenite, and fine-grained K-feldspar biotite rock. Textural analyses have shown that (1) primary magmatic diopside in contact with sulfides has corroded and actinolite-altered margins, suggesting alteration of primary minerals, (2) scalloped relicts of biotite and diopside occur as inclusions in sulfides, implying that sulfides have replaced primary minerals, and (3) the deposit-scale sulfide zonation patterns overprinted numerous rock types, including the biotite pyroxenites that contain interstitial sulfides. These results provide evidence for replacement-style mineralization at Lorraine, where primary magmatic biotite and diopside were totally or partially replaced by sulfide minerals during main-stage alteration. U-Pb zircon dates from pre- and late mineral syenite dikes show that the timing of main-stage mineralization occurred between 178.8 and 176.0 Ma. Lorraine is hosted within the Quesnel island arc terrane, and mineralization and magmatism at Lorraine postdate accretion of the Quesnel terrane to ancestral North America by approximately 7 to 10 m.y. The Lorraine deposit represents the youngest known alkalic Cu-Au porphyry deposit within the Quesnel terrane, and appears to coincide with the last gasp of alkalic magmatism within the Quesnel terrane during the Mesozoic.