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Mineralized hydrothermal breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal brecciation. Explosive phreatic brecciation was followed by in situ hydraulic brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia-and/ or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 mad 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (~0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ~4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal breccia bodies. [ABSTRACT FROM AUTHOR]