Open Collections

Xu, Huaze

University of British Columbia

The Lorraine alkalic porphyry Cu-Au deposit is located in the Triassic-Jurassic Quesnel arc of north-central British Columbia. Copper-gold mineralization at Lorraine is hosted by syenite and biotite-pyroxenite. This study focused on thin sections and newly obtained whole rock and four-acid lithogeochemical data to investigate the relationship between copper-gold mineralization, lithologies and mineral assemblages. Pyroxenite, syenite, and monzonite are petrographically distinguished based on the proportion of mafic and felsic minerals. Pyroxenite textures are cumulate in nature and can be geochemically subdivided based on variable Fe and P content, reflecting cumulate magnetite-apatite abundance. Syenite are geochemically subdivided into leocratic, mesocratic, and melanocratic syenite based on representative thin sections. Mineral assemblages associated with sulfide mineralization in syenite include K-feldspar±titanite with disseminated bornite-chalcopyrite in syenite, biotite±magnetite±K-feldspar with disseminated to pervasive chalcopyrite-pyrite, and epidote-white-mica-magnetite±garnet±biotite with disseminated to patchy pyrite-chalcopyrite, and bornite-chalcopyrite when garnet exists. These sulfide-associated mineral assemblages are consistent with previous studies but do not show petrographic textural evidence for secondary hydrothermal alteration overprinting primary minerals. Orthomagmatic sulfides are commonly observed in pyroxenite characterized by net-textured chalcopyrite-pyrite in the Lower Main zone and bornite in the Bishop zone. Orthomagmatic net-textured chalcopyrite-pyrite sometimes also occurs in mesocratic and melanocratic syenite and monzonite. Sulfur isotope results are consistent with crustal assimilation that contributed to the formation of orthomagmatic sulfides. Major and trace elements suggest that syenite, monzonite, and pyroxenite were derived from the same magma source, but the abrupt variation of rock types, copper grade, and mingling texture suggest that syenite was separated from pyroxenite melt through fluid immiscibility. Syenites with pervasive biotite±magnetite±K-feldspar and sulfides are interpreted to be the product of late magmatic fluids rather than alteration due to the absence of overprinting features. This study suggests that the Lorraine deposit is not consistent with porphyry deposit models due to the presence of orthomagmatic sulfides and late magmatic fluids-derived sulfides in syenite, which requires a new deposit model to be developed that is consistent with all results and datasets.

Text; Dataset

2024-12-16

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/89874

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/