Fluid Evolution at the Batu Hijau Porphyry Cu-Au Deposit, Indonesia: Hypogene Sulfide Precipitation from a Single-Phase Aqueous Magmatic Fluid During Chlorite–White-Mica Alteration

Abstract

Abstract Mineralization at the Cu-Au porphyry deposit of Batu Hijau, Indonesia, was previously reported to be associated mainly within stockwork quartz veins accompanied by pervasive biotite-magnetite alteration. We use cathodoluminescence imaging of vein textures followed by microthermometry and laser ablation-inductively coupled plasma-mass spectrometry microanalysis of fluid inclusions to decipher the spatial-temporal evolution of the hydrothermal system. Our results indicate that sulfide precipitation largely postdated the main stockwork quartz veining. Chalcopyrite and bornite were found in three textural positions: (1) within conspicuous quartzpoor veinlets (“paint veins”) that postdate quartz stockwork veins and that also appear to account for the bulk of seemingly disseminated sulfides, (2) as centerlines in B-type veins, and (3) as interstitial grains in A-type veins. In all three textural positions, the sulfides occur together with a volumetrically minor, dull-luminescent quartz generation after local dissolution of the granular quartz dominating the stockwork veins. All three positions are associated with chlorite ± variable phengitic white mica with 3–6 wt % FeO + MgO. In the barren core of the deposit, quartz veins host, almost exclusively, fluid inclusions of intermediate density (~0.6 g/cm3) and near-constant salinity of ~3.7 wt % NaCl equiv, representing the input magmatic fluid. This fluid subsequently separated into a highly saline brine1 and low-density vapor during quartz vein formation in the mineralized parts of the deposit, but we found no textural or fluid-chemical evidence that brine + vapor already reached saturation in sulfides. Within the studied samples, Cu-Fe sulfides are invariably associated with the dull-luminescent quartz hosting only low-salinity (~2–8 wt % NaCl equiv) aqueous fluid inclusions with a density of ~0.8 g/cm3 and minimum formation temperatures of 300°–360°C, in agreement with Ti-inquartz and chlorite thermometry indicating trapping conditions only slightly above the boiling pressure of these liquids. On average, this mineralizing aqueous fluid is compositionally similar to the initial magmatic fluid, suggesting a common source, but some inclusion assemblages deviate to significantly lower or higher salinities (0.5–25 wt % NaCl equiv). We propose a formation model for the Batu Hijau porphyry Cu-Au deposit in which mostly barren quartz veins formed at high temperature (>400°C) in the central part of the system, while sulfide mineralization commences to form peripheral to this zone. The economic ore shell was growing inward and downward as a zone of active sulfide precipitation at 300°–360°C shifted in response to progressive retraction of isotherms, while barren quartz vein formation continued in the system’s core at higher temperature. The aqueous ore-forming liquid is interpreted to have formed by rehomogenization of magmatic brine and vapor that previously formed by phase separation and later became miscible again after cooling over a narrow temperature interval. Vapor condensation into the highly saline brine phase at low pressure and subcritical temperature led to partial dissolution of earlier formed quartz veins and created secondary porosity for subsequent sulfide deposition. We propose that Cu-Fe sulfide precipitation by the low-temperature aqueous fluid was driven by the rehomogenization of S-rich vapor with Cu-rich brine originating from the same input fluid. The selective dissolution of earlier quartz veins in an inward- and downward-growing ore shell explains the positive correlation of ore grades with the density of earlier quartz veining in the ore shell, even though copper mineralization postdates quartz vein formation at any location in the deposit. Late-stage sulfide deposition in paint veins has been noted at other porphyry Cu-(Au-Mo) deposits worldwide, indicating that the proposed fluid evolution model may be applicable to many other porphyry systems.