Evolution of HF and HCl activity in magmatic volatiles of the gold-mineralized Emerald Lake pluton, Yukon Territory, Canada

Abstract

We examine the halogen contents of biotite, amphibole, and apatite from four magmatic phases of the Emerald Lake pluton to document the evolution in composition of magmatic volatiles in this system, and to determine if this evolution is consistent with the nature and distribution of weak gold mineralization in the pluton. In general, the variations in halogen compositions of hydrous minerals are greater between magmatic phases than within single magma batches, and the abundance of chlorine decreases and fluorine increases in hydrous minerals during the evolution of the Emerald Lake pluton. By applying the internally consistent thermodynamic database and mineral activity–composition relationships that account for coupled site-substitution, we further employ biotite and apatite compositions as independent monitors of fluid composition. Most analyzed biotite exhibits textural and/or compositional features characteristic of late, subsolidus halogen exchange with fluid. However, we have identified biotite that appears to retain halogen contents indicative of magmatic conditions. Fluorine contents of these biotites range from around 1.5 (syenite and monzonite) to greater than 2.2 wt% (in a late-stage pegmatite dike). Chlorine, by contrast, shows a more restricted range of values, and typically falls between 1.0 (syenite and monzonite) and 0.6 wt% (pegmatite). Apatite inclusions within anhydrous phenocrysts have fluorine contents of around 3.3 (syenite) to greater than 3.7 wt% (granite), while chlorine contents fall from 0.5 to 0.1 wt% with evolution from syenite to granite phases of the pluton. The fluid aHCl/aHF conditions recorded by biotite and apatite are in close agreement. These two independent measures of HCl and HF both show a modest decrease in HCl/HF as the system evolves. The highest log aHCl/aHF values (0.6 to 0.3) are associated with magmatic volatiles of syenite, the first phase to intrude at Emerald Lake. Activity ratios recorded from the second phase (monzonite) show a slight fall to ~0.4 to 0.1. A similar drop is recorded in the transition to hydrothermal activity recorded in biotite from a late pegmatite dike (~0.1 to –0.1). A decrease in log aHCl/aHF is consistent with a loss of chlorine and residual enrichment of fluorine in the magma, due to progressive release of a chlorine-bearing (metalliferous?) fluid. The relatively small variation in inferred fluid HCl and HF activity suggests that the magmatic volatiles present during crystallization were relatively homogeneous, and argues against both the incursion of external fluids, and of substantial temporal variation in fluid composition from the magma source. Furthermore, if magmatic volatiles are the source of the gold mineralization found across all phases of the Emerald Lake pluton, as advocated by other workers, our data would suggest that the mineralizing fluids were remarkably homogeneous and distributed throughout the pluton. The disseminated and low-grade nature of mineralization may be, at least in part, a consequence.