• Economic and Environmental Geology
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• v.48 no.1
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• pp.15-24
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• 2015

Extensive base-metal and/or gold bearing ore mineralizations occur in the Pacitan mineralized district of the south western portions in the East Java, Indonesia. Metallic ore bodies in the Pacitan mineralized district are classified into two major types: 1) skarn type replacement ore bodies, 2) fissure filling hydrothermal ore bodies. Skarn type replacement ore bodies are developed typically along bedding planes of limestone as wall rock around the quartz porphyry and are composed mineralogically of skarn minerals, magnetite, and base metal sulfides. Hydrothermal ore bodies differ mineralogically in relation to distance from the quartz porphyry as source igneous rock. Hydrothermal ore bodies in the district are porphyry style Cu-Zn-bearing stockworks as proximal ore mineralization and Pb-Zn(-Au)-bearing fissure filling hydrothermal veins as distal ore mineralization. Sulfur isotope compositions in the sulfides from skarn and hydrothermal ore bodies range from 6.7 to 8.2‰ and from 0.1 to 7.9‰, respectively. The calculated ${\delta}^{34}S$ values of $H_2S$ in skarn-forming and hydrothermal fluids are 0.9 to 7.1‰ (5.6-7.1‰ for skarn-hosted sulfides and 0.9-6.8‰ for sulfides from hydrothermal deposits). The change from skarn to hydrothermal mineralization would have resulted in increased $SO_4/H_2S$ ratios and corresponding decreases in ${\delta}^{34}S$ values of $H_2S$. The calculated ${\delta}^{18}O$ water values are: skarn magnetite, 9.6 and 9.7‰; skarn quartz, 6.3-9.6‰; skarn calcite, 4.7 and 5.8‰; stockwork quartz, 3.0-7.7‰; stockwork calcite, 1.2 and 2.0‰; vein quartz, -3.9 - 6.7‰. The calculated ${\delta}^{18}O_{water}$ values decrease progressively with variety of deposit types (from skarn through stockwork to vein), increasing paragenetic time and decreasing temperature. This indicates the progressively increasing involvement of isotopically less-evolved meteoric waters in the Pacitan hydrothermal system. The ranges of ${\delta}D_{water}$ values are from -65 to -88‰: skarn, -67 to -84‰; stockwork, -65 and -76‰; vein, -66 to -88‰. The isotopic compositions of fluids in the Pacitan hydrothermal system show a progressive shift from magmatic hydrothermal dominance in the skarn and early hydrothermal ore mineralization periods toward meteoric hydrothermal dominance in the late ore mineralization periods.

• Economic and Environmental Geology
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• v.28 no.2
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• pp.109-121
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• 1995

The hydrothermal vein type deposits which comprise the Kasihan, Jompong and Gempol mineralized areas are primarily copper and zinc deposits, but they are also associated with lead and/or gold mineralization. The deposits occur within the Tertiary sedimentary and volcanic rocks in the Southern Mountain zone of the eastern Java island, Indonesia. Mineralization can be separated into two or three distinct stages (pre-and/or post- ore mineralization stages and main ore mineralization stage) which took place mainly along pre-existing fault breccia zones. The main phase of mineralization (the main ore stage) can be usually classified into three substages (early, middle and late) according to ore mineral assemblages, paragenesis, textures and their chemical compositions. Ore mineralogy and paragenesis of the three areas in the district are different from each other. Pyrite, pyrrhotite (/arsenopyrite), iron-rich (up to 20.5 mole % FeS) sphalerite and (Cu-)Pb-Bi sulfosalts are characteristic of the deposits in the Kasihan (/Jompong) area. On the other hand, pyrite + hematite + magnetite + iron-poor (2.7 to 3.6 mole % FeS) sphalerite assemblage is restricted to the Gempol area. Fluid inclusion data suggest that fluids of the main ore stage evolved from initial high temperatures (near $350^{\circ}C$) to later lower temperatures (near $200^{\circ}C$) with salinities ranging from 0.8 to 10.1 equiv. wt. percent NaCl. Each area represents a separate hydrothermal system: the mineralization at Kasihan and Jompong were largely due to early fluid boiling coupled with later cooling and dilution, whereas the mineralization at Gempol was mainly resulted from cooling and dilution by an influx of cooler meteoric waters. Fluid inclusion evidence of boiling indicates that pressures of ${\geq}95$ to 255 bars (${\geq}95$ bars for the Gempol area: $\approx$ 120 to 170 bars for the Jompong area: $\approx$ 140 to 255 bars for the Kasihan area) during portions of main ore stage mineralization. Equilibrium thermodynamic interpretation indicates that the evolution trends of the temperature versus fS2 variation of ore stage fluids in the Pacitan district follow two fashions: ore fluids at Kasihan and Jompong changed from the pyrite-pyrrhotite sulfidation stage towards pyritehematite- magnetite state, whereas those at Gempol evolved nearly along pyrite-hematite-magnetite reaction curve with decreasing temperature. The sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a ${\delta}^{34}S_{{\Sigma}s}$ value of about 3.3 per mil. The oxygen and hydrogen isotopic compositions of the fluids in each area indicate a progressive shift from the dominance of highly exchanged meteoric water at early hydrothermal systems towards an un- or less-exchanged meteoric water at later hydrothermal systems.