Mineral Paragenesis, Fluid Inclusions and Sulfur Isotope Systematics of the Lepanto Far Southeast Porphyry Cu‐Au Deposit, Mankayan, Philippines

Abstract

Abstract: The Lepanto Far Southeast porphyry Cu‐Au deposit is located beneath and to the southeast of the Lepanto enargite‐luzonite Cu–Au deposit in Mankayan, Benguet Province, Philippines.The principal orebody consists of potassic alteration subjected to partial retrograde chlorite alteration that rims stock‐work of quartz‐anhydrite veinlets. Fluid inclusions found in stockwork quartz and anhydrite in the biotitized orebody center are dominated by polyphase inclusions that homogenize at temperatures of >500C. Sulfur isotopic thermometry applied to the sulfides‐anhydrite pairs suggests around 500C. The principal ore minerals associated with quartz‐anhydrite stockworks are chalcopyrite and pyrite with minor bornite and Bi–Te–bearing tennantite, with trace of native gold. Rounded pyrite grains appear fractured and corroded and are interpreted as remnants of primary intermediate solid solution + pyrite assemblage.A breccia pipe truncates the deposit. Mineralization in the breccia pipe is brought by quartz‐anhydrite veinlets and infilling in the interstices between clasts. Chalcopyrite‐Au mineralization associated with molybdenite is recognized in the deeper zone in the breccia pipe. Fluid inclusion microthermometry on polyphase inclusions in veinlet quartz as well as sulfur isotope thermometry applied for the pair of anhydrite and sulfides suggests >450C.Fluid inclusions in veinlet quartz and anhydrite in the fringe advanced argillic alteration are chiefly composed of coexisting liquid‐rich inclusions and gas‐rich inclusions, in addition to coexisting polyphase inclusions and gas‐rich inclusions. These inclusions exhibit a wide range of homogenization temperatures, suggesting heterogeneous entrapping in the two‐fluid unmixing region.Sulfur isotopes of aqueous sulfide and sulfate exhibit a general trend from the smallest fractionation pairs (about 11%) in the biotitized orebody center to the largest fractionation (about 25%) pairs in the fringe advanced argillic alteration, suggesting a simple evolution of hydrothermal system. The slopes of arbitrary regression lines in δ34S versus 34S[SO4 = –H2S] diagram suggest that the abundance ratio of aqueous sulfate to sulfide in the hydrothermal fluid has been broadly constant at about 1:3 through temperature decrease. The intersection of these two regression lines at the δ34S axis indicates that the bulk δ34S is about +6%. Thus, the Lepanto FSE deposit is a further example which confirms enrichment in 34S in the hydrous intermediate to silicic magmas and associated magmatic hydrothermal deposits in the western Luzon arc.