ORE-FORMING PROCESSES IN ALKALIC PORPHYRIES: A FLUID AND MELT INCLUSION STUDY OF THE MOUNT POLLEY DEPOSIT, BC, CANADA

Alkalic porphyries are Cu-Au high-grade deposits also known to host important PGE resources. Alkalic porphyries are less common than calc-alkalic porphyries, but constitute a major resource in the geologic provinces where they occur. Due to the localized nature of alkalic porphyries, a comprehensive characterization of the melts and fluids related to their formation is yet to be achieved.

This objective of this project is to constrain the nature of melts and fluids related with mineralization in alkalic porphyries towards better understanding their differences with respect to calc-alkalic systems. This project will use melt and fluid inclusions from the Mount Polley deposit to (1) identify the likely source of the mineralizing fluids in alkalic porphyries and (2) characterize the physicochemical properties of the fluids over the evolution these systems.

Mount Polley is a silica-undersaturated Late Triassic Cu-Au porphyry deposit located in south-central British Columbia. It is one of the shallowest deposits of this kind, also representative of well mineralized alkalic porphyry systems. Mineralization in Mount Polley occurs in a series of discrete Cu-Au (-Ag) ore zones related to diorite – monzonite bodies of hydrothermal breccia. Unaltered intrusive and volcanic units peripheral to the system commonly contain melt inclusions. These inclusions are hosted in diopside crystals and occur mainly as primary assemblages. The mineralogy of crystallized melt inclusions in several volcanic units has been determined through Raman spectroscopy.

Aqueous fluid inclusions are preserved in apatite crystals within extensively altered and mineralized zones. Most fluid inclusions are secondary two-phase, high-density inclusions. Intermediate-density and hypersaline fluid inclusions, which are common in calc-alkalic porphyry systems, have not been observed so far.