Textural and spatial relationship between platinum-group elements and alteration assemblages in the Afton porphyry system, Kamloops, British Columbia

Abstract

The Afton Cu-Au porphyry system is located near of Kamloops, B.C, Canada, and contains significant platinum-group element (PGE) enrichment. The alteration assemblages at Afton are consistent with a typical porphyry systems (phyllic, propylitic, potassic and argillic), with phyllic and propylitic being the most wide-spread and pervasive. Samples demonstrating elevated grades of Pd and Au show phyllic alteration is characterized by phengitic and paragonitic end-member dioctahedral micas (identified through NIR-IR spectroscopy and petrography). Two distinct mineralizing fluids are proposed, as phengitic micas require fluid-mobile Mg, whereas paragonitic micas will only stabilize in Mg-poor/alkali-rich fluids. However, both minerals require a fluid with a high pH. The samples demonstrate propyllitic alteration, characterized by Mg-chlorite, FeMg-chlorite, epidote and actinolite. Relative Au enrichment can show a positive correlation with Mg-chlorite. Mg-chlorite, similar to phengitic mica, requires a high pH and high Mg fluid to stabilize. Using SEM-MLA, Pd was found to be hosted within temagamite, and mertieite-II, most commonly in textural association with phengitic micas, and Mg-chlorite. 3D modeling (using Leapfrog and assay data) revealed a deposit-scale negative spatial correlation between Pd and primary Cu and Au, but a secondary, structurally-controlled relationship between Pd and Au. Preliminary IR fluid inclusion and LA-ICPMS studies revealed that primary fluid inclusions hosted with hematite are coeval with Pd deposition since Pd was found in trace amounts as a dissolved species in hematite at concentrations in the 100-200 ppb range. The results of the study demonstrate that Pd deposition occurred in response to secondary hydrothermal events involving oxidizing, high pH, Mg- or alkali-rich, and chloride-rich fluids (based on mineral chemistry) operating at conditions of ~400-500[degree]C and above 0.5 kbar (based on phengite-biotite stability in absence of talc). Degassing of high Mg basaltic subvolcanics, now spatially associated with the porphyry stock, may have been the source of these Pd-rich fluids.