Mineralization at the Kelly Gold Mine, Baguio District, Philippines: fluid-inclusion and wall-rock alteration studies

Abstract

Mineralogic and fluid-inclusion studies on the gold-quartz veins at the Baco-Kelly Mine indicate the existence of two types of hydrothermal fluids, each of which is characterized by distinct fluid inclusions and alteration assemblages. These two hydrothermal fluids include an earlier neutral-pH chloride fluid which is recorded in low-salinity two-phase fluid inclusions essentially of the liquid-rich type that homogenize within the temperature range of 175 to 230°C; and a later, significantly more saline, acid fluid that overprinted earlier veins. Fluid-inclusion types representing the later fluid consist of two-phase, liquid-rich and vapor-rich inclusions, as well as polyphase halite ± sylvite-bearing inclusions. Liquid-rich inclusions homogenize at a temperature range of 215 to 270°C while polyphase inclusions homogenize at temperatures greater than 350°C. Co-existing vapor-rich inclusions in both liquid-rich and polyphase types homogenize at temperatures from 260 to 385°C. The two fluid types formed two distinct alteration assemblages in the wall rock: illitic alteration (illite + muscovite) resulting from the reaction with neutral-pH fluid; and advanced argillic alteration (pyrophyllite + diaspore + alunite), a product of reaction with acid fluid. Gold and base-metal sulfides (chalcopyrite-sphalerite-galena) were deposited with quartz ± calcite in veins averaging 2–3 g/mt Au from the earlier low-temperature, less saline hydrothermal fluid responsible for the development of the illitic alteration assemblage. The high-temperature, high-salinity, boiling hydrothermal fluid also carried significant amounts of metals and overprinted and enriched the gold content of certain portions of earlier veins. This later phase is also responsible for the precipitation of gold-bearing copper sulfosalts (tetrahedrite-tennantite) and bornite in some parts of the mine. Finally, the genetic affiliation of this later high-sulfidation system with porphyry copper deposits, in general, suggests the presence of potential porphyry copper-gold mineralization at depth.