Characteristics of the Kematu epithermal gold deposit at Tboli, Mindanao, Philippines

Abstract

AbstractThe Kematu Au‐Ag epithermal deposit at Tboli area, Province of South Cotabato, Mindanao, Philippines, contains an inferred mineral resource of 3.8 million tonnes, at 8.7 g/t Au and 27 g/t Ag in 2012. The deposit is spatially and genetically linked to the volcanism related to the Parker Volcanic Complex of Pliocene to Holocene age. Local extension of the northwest‐trending splays of the faults in the Tboli area are associated with epithermal mineralization, including the Kematu Au‐Ag deposit. The study aimed to provide a detailed description of the characteristics and physicochemical processes involved in the formation of the Kematu Au‐Ag deposit on the basis of field occurrence, mineralogy, ore composition, mineral chemistry, and sulfur isotope data. The deposit is characterized by Au‐Ag bearing quartz‐adularia‐clay‐sulfide veins hosted in argillic‐altered andesite porphyry and coarse tuff of the Parker Volcanic Complex. The mineralization in the Kematu deposit is in the form of veins as well as open space fill and dissemination. Based on the observation of the ore bodies and petrographic analysis, three paragenetic stages with different vein characteristics have been identified. Stage 1 is dominantly quartz, adularia, and clay mineral veins, with electrum, sphalerite, pyrite, chalcopyrite, tetrahedrite‐tennantite, and galena. Stage 2 is coarse‐grained sulfide‐quartz‐adularia veins and in‐fill of vugs, consisting of sphalerite, pyrite, chalcopyrite, galena, tetrahedrite‐tennantite, and electrum. Stage 3 is massive calcite veins, with minor amounts of electrum, sphalerite, pyrite, chalcopyrite, and galena. The alteration mineral assemblage of the host rocks is chlorite‐illite‐adularia, indicative of neutral pH conditions. The FeS content of the Fe‐poor sphalerite in the stage 2 ranges from 0.03 to 1.07 mol%. Sulfur isotope compositions (δ34SCDT) of sulfides in all the stages range from 0.1‰ to 1.9‰ for pyrite, and −5.3‰ to 0.5‰ for galena, sphalerite, and chalcopyrite, where those in the stage 2 tend to have relatively lower values compared to those in the stages 1 and 3. The relatively narrow and close to zero δ34SCDT values suggest that the sulfur was mainly derived from a magma. The mineral assemblages of the stages 1, 2, and 3, and the sulfur fugacity (log fS2 = −12.5 to −8.8) and temperature (200–280°C) of ore formation based on the FeS mol%, electrum composition (at.%), and alteration mineral assemblage suggest that the Kematu deposit is of intermediate sulfidation type.