Mineralogical and ore-fluid characteristics of the Onto Porphyry-HSE transition in Hu’u district, West Nusa Tenggara, Indonesia

Abstract

Abstract Onto prospect is located in the eastern part of Sumbawa Island in West Nusa Tenggara Province, Indonesia. The island is situated within the Sunda-Banda magmatic arc, which extends from west to east, consists of volcanic rocks from the early Miocene to the Holocene. Onto is one of the highly potential prospects and holds significant economic value among ore deposits in Indonesia. The aim of this research is to explain the geological setting and determine the genesis of mineralization based on petrographic, ore microscopic, and fluid inclusion analyses. Eight lithological units have been identified, namely early andesites, polymictic diatreme breccia, upper sedimentary package, caping andesite, quartz-phyric andesite sill, early porphyry, early intra-mineral porphyry, and late intra-mineral porphyry. Nine samples were analyzed for petrography to identify the alteration types consisting of illite-smectite, quartz-dickite, residual vuggy quartz, silicic, quartz-alunite, quartz-pyrophyllite±diaspore, potassic, phyllic-chloritic, and propylitic alterations. Additionally, pyrite, chalcopyrite, and covellite are predominantly present in the study area. Meanwhile, native gold is found as an inclusion in covellite and pyrite. From micro thermometric analysis of fluid inclusions in quartz veins associated with multiphase intrusive rocks, it is shown that the homogenization temperature of early porphyry ranges on average from 279°C to 377°C, early inter-mineral porphyry ranges from 263°C to 361°C, and late intra-mineral porphyry ranges from 275°C to 336°C. The salinity level of the measurements ranges from 5.71 to 10.27% by weight NaCl equivalent, with an average of approximately 8.15% by weight NaCl equivalent. The mineralization formation during the early or prograde phase is associated with the EDM vein system with a mineralization set of biotite-magnetite, digenite, chalcocite, and coevellite, type A veins with quartz-sulfides + anhydrite, M veins type associated with quartz-magnetite-chalcopyrite-bornite mineralization, AB veins type associated with quartz-sulfides + anhydrite textures, and B veins type with sulfide centerline characteristics. The mineralization system during the early stage is generally associated with potassic alteration (K-feldspar-magnetite-secondary biotite). Whereas the retrograde phase, the vein system is associated C veins type with sulfide mineralization groups consisting of massive chalcopyrite + quartz with phyllic alteration (quartz-muscovite-chlorite ± dickite), D veins type with quartz-pyrite+chalcopyrite associated with advanced argillic alteration (quartz-alunite-pyrophyllite ± diasporite), E veins representing the epithermal stage associated with minerals such as quartz-carbonates, galena, sphalerite, chalcopyrite, tetrahedrite-tennatite in the alteration stage of kaolinite-illite-smektite-sericite.