Late Eocene to Pliocene magmatic evolution and its association with gold mineralization in central western Sulawesi, Indonesia

Abstract

Late Eocene and late Miocene–Pliocene magmatism in central western (CW) Sulawesi were defined by the U-Pb ages of magmatic zircon from the intrusive rocks. The late Eocene magmatism formed diorite and granitoid with hornblende-rich, calc-alkaline, and medium- to high-K compositions, derived from hydrous (av. 5.6 wt% H₂O) and oxidized I-type intermediate to felsic magmas. The late Miocene–Pliocene magmatism formed gabbro and granitoid. The granitoid is biotite-rich, calc-alkaline, and high-K composition, derived from hydrous (av. 8.4 wt% H₂O) and reduced I-type felsic magma. Negative anomalies of Nb, Ta, and Ti, and positive anomalies of K and Pb in the primitive mantle normalized trace element plots of the late Eocene hornblende-rich diorite and granitoid suggest formation in a subduction-related setting. The late Miocene–Pliocene gabbro formed by partial melting of upper mantle with minimal crustal contribution, whereas the granitoid formed through the reworking of previous subduction-related rocks during post-subduction regional extension. These are suggested by the occurrence of late Eocene inherited zircon grains in the late Miocene–Pliocene gabbro and granitoid and by similar trace element anomalies between biotite-rich granitoids and late Eocene diorite–granitoid. We observed that the oxidation state of magma strongly influences associated Au mineralization. The intrusion-related Fe-oxide-Au mineralization in the Awak Mas deposit is genetically linked to either late Eocene or previously reported middle Miocene oxidized I-type dioritic and granitic intrusions. Conversely, the intrusion-related base metal-Au-type mineralization in the Palopo and Esang prospects is associated with the late Miocene–Pliocene reduced I-type granitic intrusions.