Veins and alteration envelopes in the Grasberg Igneous Complex, Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia




Penniston-Dorland, Sarah C.

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The Grasberg Igneous Complex (GIC) consists of three main phases of igneous activity: the Dalam Igneous Complex, the Main Grasberg Intrusion, and the Kali Intrusion. Each contains veins revealing a history of fluid flow that has concentrated minerals of economic value. A generalized sequence of early magnetite ± quartz veins followed by quartz ± sulfide/oxide veins followed by late chalcopyrite/pyrite veins is observed in the Dalam Igneous Complex and Main Grasberg Intrusion. The youngest igneous body, the Late Kali Intrusion, cross-cuts the older igneous bodies as well as their veins, and has biotite ± quartz, quartz ± pyrite and pyrite ± quartz veins. Pyrite ± quartz veins with alteration envelopes up to 14 cm total width are found in regions of the complex that are higher in elevation and distant from the center of copper mineralization. Geochemical analyses of wall rock and alteration envelopes from eleven samples are compared to determine which components were added to the altered samples and which were removed by the fluids. Most major components were removed by fluids (Na₂O, MgO, SiO₂, CaO, FeO, and TiO₂) along with many trace elements (Cu, Cl, Ga, Rb, Sr, Nb, Ba, Y, Zr, Eu, Yb, Tl, Au, La, Th, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Tm, and Lu). Gain or loss of K₂O and P₂O₅ vary depending on the sample. H₂O and S were added to the altered wall rock. Typical host-rock mineral assemblages include plagioclase, biotite, quartz, alkali feldspar, pyrite, chalcopyrite, and magnetite. Typical alteration envelope mineral assemblages include muscovite, alkali feldspar, pyrite, and quartz. Balanced reactions between wall rock minerals and fluid to produce alteration minerals typically involve the consumption of HCI, indicating that the altering fluids had a low pH. The alteration envelopes are believed to be the result of changes as fluids flowed through the complex, including decreasing temperature, generation of HCI by the precipitation of pyrite and chalcopyrite from copper and iron chlorides in the lower and central parts of the complex, and/or the decrease in the fluid prewall rock fluid pressure surrounding veins. Scanned cathodoluminescence of quartz in quartz-sulfide veins reveals detailed textures on the scale of tens to hundreds of microns including concentric growth zoning and fractures. Growth textures indicate that the quartz grew into open space, so these veins remained open during infilling. Vein growth is believed to have occurred from fluids that flowed through the veins. Microfracturing occurred after the veins began to close. Experimental studies of Cline and Bodnar (1991) applied to fluid exsolution from magma chambers are used as a basis to explain the sequence of veining. Fluid separating from a magma at low pressure (<1 kilobar) has initially low concentrations of copper, whereas fluid separating from a magma at high pressure (≥2 kilobars) has initially high concentrations of copper. Crystallization from a deep batholithic magma chamber at depths greater than 6 km with a molten stock reaching up to shallower depths (less than 3 km) can account for the changes in copper precipitation observed in the GIC system over time. Early crystallization in a stock at shallow depths led to exsolution of an early fluid that was relatively copper-poor. This resulted in early magnetite ± quartz veins. Deeper-seated crystallization eventually generated copper-rich fluids forming chalcopyrite/pyrite veins. Finally, the latest stages of veining following the Late Kali Intrusion were relatively copper-poor due to the last fluids exsolving from the deeper copper-depleted part of the magma chamber


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