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Lambert, Cori A. 2000 Subsurface meso-scale structural geology of the Kucing Liar and Amole Drifts and petrology of the heavy sulfide zone, South Grasberg Igneous Complex, Irian Jaya, M.Sc. Thesis, Department of Geological Sciences, University of Texas at Austin.

    © Cori A. Lambert, 2000. Use of any part of this thesis for any purpose must be acknowledged.


Meso-scale structures were mapped ~1 km in the subsurface south of and adjacent to the Grasberg Igneous Complex, located within the Gunung Bijih Mining District. Bedding planes, faults and veins were mapped and measured in the Kucing Liar Exploration Drift (inclusive of the Kucing Liar Dewatering Drift and the Kucing Liar north offshoot) and the Amole Drift. Structural analysis reveals the following: (1) A calculated fold axis for bedding measured in the subsurface, on the southern limb of the Yellow Valley Syncline (YVS) (311 degrees/28 degrees) shows a divergence from the calculated YVS fold axis from previous data at the surface (285 degrees/15 degrees) by Quarles van Ufford (1996) of 26 degrees clockwise and 13 degrees steeper. (2) The movement direction indicated by the average trend of slickensides on bedding-plane faults is 032 degrees, consistent with the movement direction determined for regional-scale contraction as concluded by Quarles van Ufford (1996), 210 degrees-220 degrees to 030 degrees-040 degrees for the episode of folding from ~12 to ~4 Ma. (3) Strike-slip faults are the dominant structure, but sense of shear indicators are present on very few fault planes. Nonetheless, kinematics are inferred to be similar to that found by Sapiie (1998), who concluded that the GBMD was subjected to strike-slip deformation as a left-lateral Reidel shear system from ~4 to ~2 Ma. (4) The average orientation of 463 veins measured indicates an overall NW-SE extension direction, almost exactly parallel to that observed at the surface by Sapiie (1998) and Luck (1999). (5) Extended domain boundaries connecting similar structural domains along the Kucing Liar Dewatering and Amole drifts trend roughly NE-SW, and are most likely related to the presence of the SE-corner of the pull-apart zone into which the Grasberg Igneous Complex was intruded.

Chapter 2: Petrology of the Heavy Sulfide Zone, Grasberg Igneous Complex, Irian Jaya, Indonesia

The Heavy Sulfide Zone (HSZ) is a cylindrical zone that surrounds the Grasberg Igneous Complex (GIC) in the subsurface, below approximately 3700 m elevation. For logging classification purposes, it is defined by the presence of >20% sulfide minerals per 2-3 m-segment of core. The relationships among the HSZ, the GIC, and the surrounding carbonate country rocks have been interpreted from this petrologic study of 71 samples collected at 2900 m elevation along mine drifts on the south side of and within the GIC.The mineralogical characteristics of the HSZ are: (1) Pyrite is the dominant sulfide phase and is typically fine-grained (hundreds of microns to a few millimeters in total diameter). (2) Non-sulfide phases (depending on the location of the sample) typically consist of dolomite or calcite, with trace amounts of quartz, potassium feldspar, sericite, biotite, and chlorite. No garnet, pyroxene or other calc-silicate minerals were identified. (3) Mineralogical banding (pyrite, sphalerite and galena) at the hand-sample scale occurs in samples that are outliers to the HSZ proper. (4) Sphalerite and galena are present at the margins of the HSZ, and pyrite. Minor amounts of chalcopyrite, digenite, bornite and covellite are present in the mid- to proximal-Grasberg Igneous Complex. (5) Pb-isotopic analyses of samples from the HSZ and from banded outliers reveal a magmatic signature similar to the nearby igneous rocks of the GIC.As observed along the Kucing Liar north offshoot, the limit of marbleization as defined by an average grain size of 100 to 200 Ám occurs ~130 m from the GIC. As observed along the Amole Drift, the limit is more gradational, with an average grain size of ~200 mm at ~100 m from the GIC and an average grain size of ~100 mm at ~225 m from the GIC. Mineralized Type 3A marginal breccias, described at the surface by Sapiie (1998), occur between the GIC and the carbonate country rock. These breccias are evidence for marbleization pre-dating HSZ mineralization. The preferential replacement of mud clasts over marble clasts in the most peripheral samples suggests that the dominant chemical control was one of infiltration-reaction process, triggered most likely by a change in pH when the incoming hydrothermal fluids were neutralized by carbonate host rock. Banded outliers to the HSZ represent fracture pathways away from the HSZ that reopened several times.The mineralogy implies that the temperature at which this deposit formed had an upper limit of ~500 degreesC and a lower limit of ~250 degreesC. The pressure at which the deposit formed was approximately 500 bars, at the depth of the Amole and Kucing Liar Drifts. The HSZ was most likely formed by pressure-driven infiltration of hydrothermal fluids starting at temperatures of ~800 degreesC, upwards and outwards from a pocket beneath a cupola above a cooling and crystallizing stock. Cooling of outward-migrating hydrothermal fluid and contact with the carbonate country rock caused precipitation of sulfide phases in the contact zone. Highly permeable breccia zones focused flow of the hydrothermal fluids.


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