MAGMATIC-HYDROTHERMAL FLUID EVOLUTION IN THE PORPHYRY-EPITHERMAL CU-AU RANDU KUNING PROSPECT, SELOGIRI, CENTRAL JAVA, INDONESIA: INSIGHTS FROM FLUID INCLUSION STUDIES

EVOLUSI FLUIDA HIDROTERMAL-MAGMATIK PADA PROSPEK PORFIRI-EPITERMAL Cu-Au RANDU KUNING, SELOGIRI, JAWA TENGAH, INDONESIA:KAJIAN BERDASARKAN STUDI INKLUSI FLUIDA

Abstract

The Randu Kuning Prospect, located in the Southern Mountains Zone of East Java, Indonesia, represents a magmatic-hydrothermal mineralization system that evolved from a porphyry environment to a shallow-level epithermal system. The prospect is hosted by volcaniclastic, siliciclastic, and carbonate rocks of the Mandalika and Semilir Formations, which are intruded by numerous dioritic bodies related to Late Eocene–Early Miocene magmatism. This study aims to reconstruct the evolution of ore-forming fluids based on fluid inclusion characteristics and their relationship to hydrothermal alteration and mineralization. Fluid inclusion analyses were conducted on ten vein samples representing both porphyry and epithermal environments using a Linkam heating-freezing stage. To support the interpretation, petrographic, X-ray diffraction (XRD), QEMSCAN, X-ray fluorescence (XRF), and ICP-MS analyses were also performed. The fluid inclusion data indicate that the hydrothermal system evolved through three major stages. The early porphyry stage was characterized by high-temperature and high-salinity fluids, with homogenization temperatures of approximately 600°C and salinities ranging from 16 to 72 wt.% NaCl equivalent. These fluids were associated with A-type and M-type quartz veins and potassic alteration assemblages. During the intermediate porphyry stage, fluid temperatures decreased to approximately 300–350°C, while salinities ranged from 16 to 48 wt.% NaCl equivalent. This stage was associated with sulfide-bearing quartz veins (AB-type and C-type veins) and phyllic as well as chlorite–magnetite–sericite alteration assemblages. The late porphyry to epithermal stage was characterized by lower temperatures (200–350°C) and lower salinities (2.31–36.74 wt.% NaCl equivalent), resulting in the formation of D-type veins, carbonate–pyrite–sphalerite–chalcopyrite veins, and other epithermal-style veins accompanied by propylitic, intermediate argillic, and locally developed advanced argillic alteration. The progressive decrease in temperature and salinity indicates continuous evolution of the hydrothermal fluids from porphyry c to a shallower epithermal environment, thought to be primarily controlled by cooling and mixing processes with meteoric water.