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Harrison, Jeffrey S. 1999 Hydrothermal Alteration and Fluid Evolution of the Grasberg Porphyry Cu-Au Deposit, Irian Jaya, Indonesia, M.Sc. Thesis, Department of Geological Sciences, University of Texas at Austin.

    © Jeffrey S. Harrison, 1999. Use of any part of this thesis for any purpose must be acknowledged.

Abstract

The Grasberg is a gold-rich porphyry copper deposit located at the crest of the Central Range of Irian Jaya and is the largest orebody of the prolific Ertsberg District. The deposit is hosted within a nested suite of 3 Ma quartz monzonite to quartz monzodiorite intrusions. Currently the units of the Grasberg are divided into three groups of intrusions. From oldest to youngest, these are the Dalam, the Main Grasberg Intrusion and the Kali.

The units are all porphyritic and very similar petrographically. To date, textural criteria regarding the percentage and character of phenocryst phases serve as the best manner to differentiate between units. Major-oxide and trace-element analyses of the least altered samples available for each unit show strong genetic ties between the rocks of the Grasberg and also document an evolution to slightly more evolved/fractionated compositions with the increasingly younger units.

Two distinct types of alteration affect the rocks of the Grasberg that show consistent spatial arrangements throughout the deposit. The very center of the deposit is relatively unaltered but surrounded by an alteration type which consists of pervasive magnetite+biotite+K-spar. Towards the margins of the complex, an alteration assemblage dominated by quartz+sericite becomes increasingly well-developed and overprints K-silicate alteration. These expressions of alteration are interpreted to be similar to or variations of the K-silicate and phyllic alteration styles described in other porphyries. Other frequently recognized alteration types noted in similar deposits, such as propylitic and advanced argillic alteration, are largely absent but may have been better-developed in the eroded or excavated portions of the complex.

The history of vein formation appears to have been closely related to the development of alteration at Grasberg, as the paragenetically earliest vein phases (magnetite+biotite) correspond with the earlier alteration assemblages observed in the center of the deposit. Temporally late veins are strongly associated with sericite-rich selvages suggesting that the conditions of late vein-forming fluids were similar to the fluid conditions responsible for the phyllic alteration pervasive at the margins of the complex. If fluids responsible for vein formation and the development of pervasive wall-rock alteration can be linked, than mineralizing veins appear to occur temporally between the development of the two alteration styles identified.

Alteration at Grasberg is arranged concentrically and symmetrically around the youngest Kali intrusion. Note however that pervasive alteration and high-grade Cu-Au mineralization are observed within the Kali near contacts with other lithologies. Major-oxide and trace-element analyses of altered samples, when compared to average unaltered compositions, generally show that samples affected by K-silicate alteration are enriched in Fe 2 O 3 , MgO and K 2 O, while SiO 2 concentrations were elevated overall within phyllic-altered samples. The trace-element data generally mimicked trends observed in the major-oxide data and secondary mineral abundance is believed to exert the strongest control on compositions of altered samples. The exception to secondary mineral abundance controlling the compositions of altered rocks is Na 2 O which likely reflects the relative preservation of plagioclase phenocrysts rather than an alteration phase.

Based on the phase relations between alteration minerals and fluid inclusion measurements in quartz veins associated with specific alteration types, the K-silicate alteration preserved in the center of the complex appears to have been the result of high temperature (>700°C), iron-rich oxidizing brines (~70 wt% NaCl equiv.). The zonation observed within the K-silicate alteration at Grasberg, from magnetite in the center of the deposit to biotite outwards, may have been produced by a slightly less oxidizing fluid with decreased iron activity, perhaps also at lower temperatures (~500°C), as suggested from fluid inclusion data. Phyllic alteration appears to have been produced by fluids that were increasingly cooler (200-500°C), less saline (~30-50 wt% NaCl equiv.), and more acidic. Stable isotope measurements performed on mineral separates prepared to represent major expressions of alteration suggest that Grasberg's hydrothermal system was predominantly magmatic throughout the course of its development.


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