A Preliminary Study on Skarn‐Related Calc‐silicate Rocks Associated with the Batu Hijau Porphyry Copper‐Gold Deposit, Sumbawa Island, Indonesia

Abstract Batu Hijau is a world‐class gold‐rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc‐silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc‐silicate rocks occur at the contact with copper‐gold‐bearing tonalite porphyries. The rocks are fine‐grained and granular with green, reddish‐brown and white layers. The green layers consist mostly of fine‐grained clinopyroxene (diopside and hedenbergite) and the reddish‐brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc‐silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc‐silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340–360 C (high salinity 35–45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280–300 C (low salinity 1–10 NaCl wt% eqn.). Late calcite ± quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe‐rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc‐silicate rocks associated with massive magnetite‐chalcopyrite‐pyrite assemblage indicates the occurrence of calcic‐exoskarn surrounding the Batu Hijau porphyry copper‐gold deposit.