Hydrothermal fluids and Cu-Au mineralization of the Deep Grasberg Porphyry Deposit, Papua, Indonesia
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The Grasberg porphyry Cu-Au deposit is located in the Ertsberg District in the Central Range of Papua, Indonesia. The Deep Grasberg is the deepest explored part of the Grasberg Igneous Complex (GIC) at elevations between 2450 and 3050 m, more than 1100 m below the pre-mining surface. The deposit is hosted by three quartz-monzonite to diorite units, emplaced approximately 3 Ma: the Dalam, the Main Grasberg Intrusion (MGI), and the Kali. In the Deep Grasberg, the intrusions contain abundant plagioclase phenocrysts with biotite and hornblende as the dominant mafic minerals. Plagioclase has undergone varying degrees of hydrothermal alteration to sericite. Complete alteration of plagioclase to sericite is common in the Dalam, whereas the Early and Late Kali are significantly less altered. Matrix minerals include potassium feldspar and quartz. Quartz occurs widespread stockwork veins and also is the dominant mineral in the silicified zone, near the GIC-wallrock contact in the Deep Grasberg. Chalcopyrite, with lesser bornite and covellite are the most abundant economic minerals in the Deep Grasberg and are both vein hosted and disseminated. Bornite is most abundant near the center of the Deep Grasberg, whereas covellite abundance increases toward the periphery. Hydrothermal fluids, supplied from a cupola at depth, created the Grasberg orebody. Due to its relative proximity to the cupola, the Deep Grasberg records a unique fluid evolution and constrains fluid processes during ore formation. Some changes in the temperature and composition of the hydrothermal fluids in the Deep Grasberg are recorded in quartz crystals. Textures observed using scanned luminescence of 32 samples of quartz veins from the Dalam, MGI, and Kali reveal a complex vein history in the GIC. Alternating light and dark bands forming concentric growth zone textures record changing conditions during quartz precipitation. Point analyses were unable to detect compositional differences in the quartz bands, but X-ray mapping revealed bright bands had higher Al and Ti contents. Subtle compositional changes in quartz suggest that chemical changes in fluid composition may be responsible for the observed growth zonations. Irregular textures in some quartz crystals suggest a period of quartz dissolution occurred as hydrothermal fluids cooled, followed by the resumption of quartz precipitation. Fluid inclusion populations from 27 samples from the Dalam, MGI, and Kali reveal that a range of fluid compositions were trapped during quartz precipitation. Five types of fluid inclusions are identified: liquid + vapor, vapor-rich, liquid + vapor + halite, liquid + vapor + multi-daughter crystal, and liquid + vapor + opaque. Liquid + vapor + opaque inclusions are believed to represent a supercritical fluid exsolved from a magma at depth, whereas the coexistence of vapor-rich inclusions and high salinity liquid + vapor + halite and liquid + vapor + multi-daughter crystal inclusions represent two separated phases of this fluid after changes in pressure and temperature caused the hydrothermal fluid system to move from the one-phase to the two-phase field. Homogenization temperatures recorded for liquid + vapor + halite, liquid + vapor + multi-daughter crystal, and vapor-rich inclusions are similar. The lowest homogenization temperatures recorded are about 225°C, but some inclusions did not homogenize at temperatures as high as 700°C, the maximum temperature able to be reached on the fluid inclusion stage. Two groups emerge from the data, a set of moderate temperature inclusions and a set of higher temperature inclusions. The moderate temperature inclusions have an average homogenization temperature of 368 °C. The higher temperature inclusions have an average homogenization temperature of >598 °C. Calculated salinities for liquid + vapor + halite and liquid + vapor + multi-daughter crystal inclusions indicate that the separated fluid phase from which these inclusions sourced was very saline, containing about 50 wt.% NaCl equivalent. Fluid inclusions in anhydrite from late-stage veins record a significantly cooler fluid, at approximately 250°C at a salinity near 10 wt.% NaCl equivalent. creating two fluids: a brine and a vapor. These changes in the hydrothermal fluid account for the variety of fluid inclusion types observed in the Deep Grasberg. Additionally, the repetitive growth zonation patterns in quartz suggest that the hydrothermal fluids were released from a cupola at depth into fractures in pulses during quartz precipitation. Fractures remained open, allowing for quartz growth with successive fluid pulses. Similarity in fluid inclusion compositions, temperatures, textures, and chemical zonations in quartz crystals in the veins in all three intrusive units suggest a common fluid source. It also suggests that similar fluid processes were prevalent during quartz vein development in these intrusive units. In the Deep Grasberg, vein quartz precipitated both directly from a supercritical fluid released from a cupola at depth and from a hydrothermal fluid post-boiling. The narrowing of the GIC at depth may have caused “throttling” of fluids in the Deep Grasberg, resulting in relatively increased pressure in the fractures. Decompression of the fluid was inhibited, and fluid inclusions of liquid + vapor + opaque record the earliest fluids. A decrease in pressure resulted in boiling, creating two fluids: a brine and a vapor. These changes in the hydrothermal fluid account for the variety of fluid inclusion types observed in the Deep Grasberg. Additionally, the repetitive growth zonation patterns in quartz suggest that the hydrothermal fluids were released from a cupola at depth into fractures in pulses during quartz precipitation. Fractures remained open, allowing for quartz growth with successive fluid pulses.