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Luck, Roy Burvill 1999 Structural Geology of the Grasberg Lime Quarry and Amole Drift : Implications for Emplacement of the Grasberg Igneous Complex, Irian Jaya, Indonesia, M.Sc. Thesis, Department of Geological Sciences, University of Texas at Austin.

    © Roy Burvill Luck, 1999. Use of any part of this thesis for any purpose must be acknowledged.

Abstract

The Grasberg Igneous Complex (GIC) is one of the world’s foremost copper-gold porphyry-type systems. The system is Pliocene in age and is situated at an elevation of 4000 m in the Central Range of Irian Jaya, Indonesia. Two structural regimes are noted within the Gunung Bijih Mining District - a 100 km2 Contract of Work (COW) area operated by PT Freeport Indonesia. The most apparent is a Miocene contractional episode that resulted in kilometer-scale folding in the Central Ranges, concurrent with arc-continent collision in a north-dipping subduction zone. The second, more subtle structural regime, was discovered by Sapiie (1998). It is one of strike-slip faulting, evidenced by five major strike-slip fault zones with tens to hundreds of meters of displacement, as well as ubiquitous mesoscale strike-slip faults throughout the district, including the GIC itself. This strike-slip regime lasted from ~4 Ma to ~2 Ma, and has been categorized as a left-lateral Riedel shear system.

Fieldwork was conducted in two locations - the Grasberg lime operation to the north of the Grasberg pit, and the Amole Drift. Structural analysis at the Grasberg lime operation extended the area mapped by Sapiie (1998). New data in this area is similar to that compiled elsewhere within the district. Strike-slip faulting is predominant. The pattern of north-south striking right-lateral faults and northeast-southwest striking left-lateral faults is essentially identical to that described by Sapiie along the HEAT (Heavy Equipment Access Trail) and Grasberg access roads. Similarly, the orientation of veins at the Grasberg lime operation indicates extension along an axis trending 300o/120o. Slight variations in the strike of veins and left-lateral faults observed within the Gunung Bijih Mining District are likely due to the mechanical anisotropy due to intrusion of the GIC.

Structural analyses were compiled within the GIC in the Amole Drift, at the 2900 m level. An effort was made to correlate structures in the subsurface to previously mapped structures at the surface, at 1000 m higher elevation. Strike-slip faulting is predominant in the subsurface. Dip-slip faults are concentrated in the northern portion of the GIC, north of the Late Kali Intrusion (LKI). Sense-of-slip indicators are rare in the GIC, with the result that faults are categorized only on the basis of orientation and slip direction. As is the case at surface levels within the GIC, the orientations of faults in the subsurface are domainal in character. Four fault domains (A-D, classified from south to north in accordance with Grasberg Domains 1-3 of Sapiie, 1998) are observed in the subsurface. Domain A extends within carbonate host rock from crosscut 17 to beyond crosscut 18, and features faults striking northeast/southwest (C. Lambert, 1999, personal commun.) - an orientation consistent with left-lateral faults within the district. Domain B extends from crosscut 19 to 22 (outside the margins of the GIC) and has a single population of faults striking north/south (C. Lambert, 1999, personal commun.). Between crosscuts 23 and 27, a bimodal fault population with two diverging fault strike orientations is observed (Domain C). This most complex zone is believed correlable to Grasberg Domain 2 (GD2) and coincides with the most intensive mineralization in the Grasberg. Domain D, north of crosscut 27, has a single fault population that shifts gradationally from a northwest/southeast orientation to a north/south orientation farther to the north. This domain is correlated to GD3 as mapped by Sapiie (1998), and is interpreted as a zone of faults with trajectories refracted toward the center of the GIC.

Interestingly, domain boundaries in the subsurface are correlable to the surface along planes projected parallel to the Late Kali Intrusive (LKI). The LKI is the youngest phase of the GIC. By contrast to the earlier two phases of the GIC, the LKI is tabular and dike-like. The LKI strikes 140o and dips 75o - parallel to the average orientation of faults in GD2, measured in the center of the GIC by Sapiie (1998). It is likely that this fault trend facilitated intrusion of the LKI. The boundary between domains C and D is correlated to the boundary mapped by Sapiie (1998) between GD2 and GD3 at the surface, when this boundary is shifted 100 m further to the south and projected along a plane parallel to the LKI. The boundary between domains B and C is correlable to the boundary between GD1 and GD2 when it is projected along a plane parallel to the LKI. No structural geology work has been completed in the part of the Grasberg pit overlying subsurface domains A and B because this was an area of rugged terrain prior to 1995.

Pt.2

The Grasberg Igneous Complex (GIC) contains one of the world’s foremost copper porphyry deposits. The GIC has a diameter of 1.7 km at the 4000 m level and tapers to a stock with a diameter of ~500 m at the 2500 m level. The GIC has three intrusive phases - the Dalam unit, the Main Grasberg Intrusion (MGI, age 3.06 Ma) and the Late Kali Intrusion (LKI, age 2.94 Ma). Prior to this study, a comprehensive reconstruction of the events leading to and enabling intrusion of the GIC had not been undertaken. Based on local and regional structural data, geochemical studies and petrology, a model is proposed for emplacement of the GIC - one that is consistent with the local structural geology and mineralization history. Breccia exposed along the northern and southern margins of the GIC provides important clues as to why the GIC widens towards the surface.

The GIC emanated from a parent magma chamber at 10-15 km depth, near the top of the crystalline basement of the Australian continent. Five extension mechanisms contributing to intrusion of magmas forming the GIC include: 1) elastic deformation of wallrock, 2) ductile deformation of wallrock, 3) brittle deformation of wallrock accomplished via normal faulting and extension fracturing, 4) metamorphic recrystallization of wallrock and consequent volume loss related to reduction in porosity, and 5) dissolution of carbonate host rock. The relative contributions of each extension mechanism vary temporally throughout the intrusive process, as well as vertically along the GIC.

The driving force for emplacement of the GIC is the buoyancy caused by the difference in densities between the deformed sedimentary sequence and the underlying felsic melt at a depth of 10-15 km. Extension within a dilational jog between two strike-slip faults provided space for intrusion of the GIC. At depths greater than ~5 km, closure of extension fractures and ductile distortion of wallrock account for most of the 500 m diameter of the GIC. Metamorphic recrystallization, which resulted in diminution of porosity and consequent volume loss, accounts for only tens of meters of radial accommodation space around the margins of the GIC. Nearer the surface, the decreased component of ductile wallrock deformation of wallrock is compensated by an increase in normal faulting (shear fracturing).

Sediments of the Kembelangan and New Guinea Limestone Groups were thinned by normal faulting in a pull-apart structure within a strike-slip environment. Buoyancy of magma in a parent chamber drove initial ascent of the Dalam phase of the GIC to within 1 km of the surface. As the Dalam phase ascended to the surface at shallow levels, the extended host rock was then ‘shouldered aside’ and structurally rethickened. At depths less than 1-2 km, water saturation and volatile release generated the monomict Dalam Fragmental unit, and eruptive activity resulted in formation of the polymict Dalam Volcanic unit. The Dalam conduit vented to the surface, where volcanic activity in a maar caldera setting resulted in the extrusion of volcaniclastic sediment and andesite flows. This material was mixed with Dalam Fragmental material by caldera collapse to form the Dalam Volcanic unit.

The Main Grasberg Intrusion (MGI) rose through the plastic interior of the previously intruded Dalam unit. Subsequently, collapse of the cupola roof and escape of magmatic fluids formed a chalcopyrite stockwork zone hosted by the MGI, the highest-grade ore in the GIC. Outward fluid infiltration formed the Heavy Sulfide Zone lining the margins of the GIC. Precipitation of sulfide minerals at depth increased the acidity of magmatic fluid, and fluids rising along the margins caused extensive dissolution of fractured carbonate host rock at shallower depths. Dissolution of wallrock is primarily responsible for the flaring-upward profile of the GIC to the north and south. As wallrock dissolved, the Dalam and MGI phases extended and overlying sequences of volcaniclastic sediment and andesite flows collapsed into their current, steeply-dipping profile, as opposed to their previous sub-horizontal attitude. The final phase of the GIC, the LKI, intruded the cooled center of the GIC along a fault zone trending 300°/120°.



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