Hydrothermal Alteration and Mineralization

Bensaman et al/ICG 2015

Hydrothermal Alteration and Mineralization Characteristics of Gajah Tidur Prospect, Ertsberg Mining District, Papua, Indonesia Benny Bensaman1,2, Reza Al Furqan1, Mega F. Rosana2, Euis T.Yuningsih2 1

PT Eksplorasi Nusa Jaya/ Affiliate of Freeport-McMoran Copper and Gold, Indonesia 2

Faculty of Geology, Padjadjaran University, Indonesia Email:[email protected]

Abstract Gajah Tidur prospect is the deepest explored part of the Grasberg Igneous Complex (GIC) located at the elevation between 1,600 – 3,000m, almost 2.5 Km below the pre-mining surface. The Grasberg Porphyry Cu-Au deposit is hosted by three main monzonitic to dioritic intrusive units, emplaced approximately 3 Ma which consists of the Dalam, the Main Grasberg Intrusion (MGI) and the Kali. Deep drilling have been carried out and sited at 3,026 m elevation of Kucing Liar Exploration Drift and drill down to approximately to 2,050m elevation at holes KL98-10-21 and KL98-1022. The objective is to explore the vertical extension of Deep Grasberg porphyry Cu-Au deposit. KL98-10-21 drilled through the center of the Grasberg Intrusive Complex while KL98-10-22 has drilled outward into the carbonate and sedimentary wall rock and both drilled up to 1,700m in length. The drill section provides important new data on the evolution of the Grasberg igneous Complex (GIC). The results of study of this drill section are integrated with previous works to characterize especially on advance argillic –high sulfidation type of silica-sericite-pyrite-covelite-enargite and alunite-pyrophyllite-andalusite-halloysite alteration and mineralization assemblages which observed in both intrusions and the wall rock. Based on the drill core logging, petrography and geochemical analysis of selected samples of the two drill holes mentioned earlier, the bottom of the Grasberg Cu-Au porphyry ore body seems to decreased and terminated at about 2,750m elevation probably due to overprinted by intense silica-sericite alteration locally approaching 95100% by volume and observed phyllic and advanced argillic of high sulfidation alteration in the top 165m of the underlying Kucing Liar porphyry. The advanced argillic alteration is hypogene and composed of pyrophyllite, kaolinite-dickite, with sporadic zunyite, diaspore, alunite and svanbergite, and late sulfur, gypsum and anhydrite. Intense silica, phyllic, advanced argillic and acid-sulfate alterations which are occur beneath the higher level porphyry copper zone is anomalous compared to other porphyry systems. Some high temperature clay minerals above indicate that the magmatic hydrothermal fluid is more play significant role than the meteoric fluid. Keywords : Grasberg Igneous Complex, advance argillic, high sulphidation, Gajah Tidur mineralization.

Introduction The Gajah Tidur prospect is located in the Ertsberg Mining District in the Central Range Mobile Belt, western half of the island of the New Guinea, the Indonesia province of Papua (Figure 1). Grasberg Igneous Complex, which formed at ~3Ma (Figure 2), is host to one of the largest copper and gold porphyry-type ore deposits discovered in the world. Three main phases of intrusion at the level of the open pit mine; the Dalam, subdivided into the Dalam Andesite, Dalam Volcanic and

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Dalam Fragmental, the Main Grasberg Intrusion (MGI) and the Kali (Early and Late). In both areal extent and volume the GIC is limited, with pre-mine surficial foot print of less than 3 km². Ore grade mineralization extends to a depth of more than 2 km. Notable about Grasberg is the quantity of hypogene ore, both disseminated and vein hosted and lack of significant supergene enrichment (MacDonald and Arnold, 1994). Currently producing roughly 6% of the world’s copper supply, as well as containing significant quantities of gold, this copper-gold porphyry system is one of the most


extraordinary mineral system on Earth. Using a noneconomic cutoff grade of 0.1% Cu, the Grasbergrelated system contains 7.5 billion tonnes (grading 0.70% Cu and 0.64 ppm Au) in two deposits, the Grasberg porphyry system and the Kucing Liar skarn (Leys et al, 2012). Producing since 1989 occurs as open pit mining with underground operation by block caving expected to begin at 2016. The projected mine life is at least 45 years.

ore body have play the main reasons to conduct the deep exploration drilling into deeper part from existing Grasberg porphyry Cu-Au deposit. Two deep holes have been drilled from 3,100m elevation at the edge of the Grasberg ore body to delineate the Cu-Au mineralization extension to deep and below the Deep Grasberg ore body. KL9810-21 hole has design to drill the center of the Grasberg intrusive complex and the second hole, KL98-10-22 drilled the edge and the wall rock. Both drill holes have intersected an interesting hydrothermal alteration and mineralization. Silicasericite alteration is the dominant alteration occurs which hosted at both the intrusives and the sediments wall rock.

Geological Background

Figure 1. The Grasberg porphyry in the Ertsberg Mining District, Papua, Western part of New Guinea.

Figure 2. Looking to the West, Grasberg photo showing Yellow Valley Syncline and Marren Valley (PT. FI Internal Report).

Deep drilling had been successfully discovered high sulfidation type alteration and mineralization at the bottom part of Deep Grasberg ore body which shown intense silica-sericite alteration and stockwork veins and Cu-Au mineralization. The commitment to keep exploring and discover a new

The study area is included in PT Freeport Indonesia CoW Block "A", located in the southern part of the Central Range Mobile Belt, Papua. Regional geology of this area is dominated by a complex system of subparalel structure corresponding to the dominant West-Northwest tectonic trending. Stratigraphic unit ranged from Triassic-Miocene age clastic sedimentary and carbonates rock unit, dominated by shallow marine to shelf facies sedimentary rocks, which consists of shale, siltstone, mudstone, sandstone, limestone and dolomite (Figure 3). These sedimentary rocks sequence have intruded by Plio-Pleistocene age, intermediate compositon of sills and dikes, associated with the Cu-Au deposits in the region which are now mined in the Ertsberg and Grasberg Districts within the CoW Block "A". The Ertsberg mining district is located in the axis of the Central Range with Puncak Jaya the highest peak in Indonesia at the edge. Oblique convergence ~12 Ma (Cloos et al, 2005) subducted the Australian Plate beneath the Pacific Plate. Resulting in thin-skinned folding and thrusting and stratigraphic repetition in the Foreland and a left-stepping en-echelon arranged synclinorium in the Central Range. The initial thinskinned mechanism developed into a thick-skinned basement involved deformation with deep seated

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regional NW-SE thrust and NE-SW strike-slip faults. GIC is a product of young intrusive bodies emplaced shallow depths along the axis of the kilometer-scale syncline. Although there are at least sixteen individual intrusions in the district (McMahon, 1994a), only the Dalam phase of the Grasberg system has clear evidence for contemporaneous volcanic activity. Weiland and Cloos (1996) have shown through apatite fission track studies that unroofing of the GIC was less than 2 Km. Structurally, the district contains hundreds of strike-slip faults (Sapiie, 1998). Most of them have only minor offset (cm scale), but five northwesttrending strike-slip zones are ”major” with at least tens of meters to perhaps a few hundred meters of cummulative offset (Sapiie and Cloos, 2004). The CoW Block ”A” regional geology map shown in Figure 3.

Stratigraphy Regionally, the stratigraphic sequence consists of a series of Mesozoic and Cenozoic passive strata that were deposited on the northern flank of the Autralian continental shelf (Quarless van Ufford, 1996). Within the Ertsberg District two stratigraphic succession are exposed: the Kembelangan Group and the New Guinea Limestone Group (Figure 4.). The Kembelangan Group is a suite of Cretaceous silisiclastic units and includes four formations: the Kopai Formation, a argillaceous, glauconitic and pyritic quartz arenite with minor siltstone and mudstone at the top of the formation; the Piniya Mudstone, consisting of interlayered carbonaceous siltstones and mudstones; and the Ekmai Sandstone, a fine grained glauconitic sandstone (Quarles van Ufford, 1996). The Kembelangan Group is overlain by the Tertiary new Guinea Limestone Group. A series of carbonates and sandstone (Quarles van Ufford, 1996). The New Guinea Limestone Group has been divided into four formations: (1) Waripi Formation; a Paleocene to Eocene fossiliferous dolostone and sandstone, (2) Faumai Formation; an Eocene foraminifera-bearing limestone and dolostone, (3)

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Sirga Formation; an Oligocene coarse to medium grained sandstone and siltstone, and (4) Kais Formation; an Oligocene to Miocene foraminiferabearing limestone and marl.

Methodology The study is mainly based on the drill core observation of holes KL98-10-21 and KL98-10-22. The drill core then photographed and sampled for detailed petrological analyses. Drill core samples collected from both the intrusive and wall rocks as well as quartz veins which presumably contain the representative minerals and inclusions to be analysis to determine intrusive rock types, alteration zoning and mineralization styles. Thin and polished sections have been prepared from selected samples for petrographic study. The mineralogical data of the intrusive rock were obtain to determine the type and composition as well as secondary data of clay mineral identification by X-ray Diffraction (XRD), Near Infra Red (NIR), and whole rock analysis are used to determine for any clay minerals and chemical composition respectively. In addition, sulfides, oxides and alteration minerals were analyzed in polished thin sections by electron microprobe.

Alteration and Ore Mineralogy The research are based from drill core observation and detailed geological logging of two deep holes drilled from Kucing Liar Exploration Drift, located at 3,026 m elevation, which consists of KL98-10-21 and KL98-10-22 (Figure 5 and 6). KL98-10-22 hole collaring on replacement suphide alteration consist of fine grained pyritecovellite, locally base metal sphalerite-galena hosted on dolomitic marble and limestone of Faumai Formation, overlying Waripi Skarn comprised of magnetite-pyrite-chalcopyrite-serpentine and hornfels-exoskarn altered of Kembelangan shale, limestone, and sandstone respectively. Skarn alteration minerals are consists of calc-silicate garnet-diospide-epidote-chlorite assemblages with locally K-feldspar association.


Figure 3. Regional geology map of the Ertsberg Mining District, showing the sediments, intusions, and ore deposits have been discovered (PT Freeport Indonesia).

Figure 4. CoW “A” stratigraphic column section A-B (PT Freeport Indonesia).

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Kembelangan feldspatic quartz sandstone (Kkes) is mostly strongly altered to quartz-sericite cut by quartz±pyrite-chalcopyrite cross-cutting veins-veinlets (Trautman, 2013), pyrite-chalcopyrite are occurs as disseminated. The sedimentary units above have intruded by 4-25m thickness, fine to medium grained plagioclase-hornblende porphyry Dalam andesitic-dioritic dykes which have undergone silica-pale brown secondary biotitesericite-pyrite (potassic-phyllic), in-places endoskarn (epidote-phlogopite-chlorite-garnetpyrite) alteration cut by minor quartz-anhydritepyrite veins are observed. Other intrusion were also mapped at deeper part of the drill hole consists of up to 500m thickness, coarse grained plagioclasehornblende-K-feldspar-biotite phenocrysts, potassicphyllic alteration then overprint by up to 60% to abundant light gray-milky white silica alteration as well as intense quartz-pyrite stockwork veins and jigsaw, contain late pyrite-bornite-covellitemolybdenite as disseminated and blebs, and also veinlets. Finer grained feldspar-biotite-hornblende porphyry diorite dikes (2-75m thickness) also mapped at depth, weakly phyllic-propylitic alteration. The drill hole ended by Kembelangan quartz sandstone, strongly silica-brown secondary biotite-K-feldspar (potassic) and locally brown garnet skarn altered which contain pyrite and minor chalcopyrite up to the end of hole at 1700.30m depth.

fine-medium grained monzodiorite which possibly similar to Main Grasberg Intrusion, potassicpropylitic alteration and still contain pyritechalcopyrite as disseminated and few veinlets and coarse grained monzodiorite, with plagioclasehornblende-biotite porphyry, fine needle –shaped hornblende with weaker potassic and propylitic alteration and pyrite-chalcopyrite also noted as fractures filling. Based on overall appearance in the drillcore, there are two main alteration styles or groups in the KL98 section intrusions as: (1) Potassic alteration in the core of the Kucing Liar intrusion (monzonite porphyry), which characterized

Figure 5. Deep Grasberg Geology Map (plan view at 3,026 m elevation) showing projected KL98-10-21 and KL98-10-22 drill holes (PT Freeport Indonesia).

Phyllic and advanced argillic alteration, and massive quartz veining, are focussed underneath and form a floor to copper-gold porphyry mineralization in the pit and block cave levels. It is not know whether this extreme telescoping is due to high erosion rates during the life of the main GIC hydrothermal system, or to much younger, overprinting intrusive-hydrothermal activity. Drill hole KL98-10-21 drilled toward the center of Grasberg Intrusive Complex (GIC) were intersected lesser wall rocks and dioritic intrusions with almost similar intensity of silica-sericitesecondary biotite-pyrite-chalcopyrite±covellitemolybdenite (phyllic-potassic) alteration and mineralisation. At 1,266m depth, the hole intersected two others intrusive rock consists of

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Figure 6. KL98-10-21 and KL98-10-22 drill cross section showing alteration zones by 250m window (PT Freeport Indonesia).


by pale-redish brown secondary biotite (Figure 7AC-E). Primary textures are well preserved, and recognizable in the drill core and thin section (Figure 8A and 8B), and (2) Phyllic and advanced argillic alteration concentrated around the margins of the Kucing Liar intrusion, which dirty white to yellow in colour, primary texture generally destructive and not recognizable in the core, but may be preserved in section by textures in the alteration minerals, or in relict primary phase. The sedimentary and carbonates wall rocks adjacent to the GIC have been affected by intrusive and hydrothermal activity that occurred over the life of the system (Figure 8E and 8F), and potentially retain a record of this as dykes, veins and alteration of various ages and styles. In contrast, the intrusions can only host hydrothermal features that are younger than themselves.

Mineralization The main copper minerals identified in the KL98-10-21 and KL98-10-22 drill sections are consists of chalcopyrite, bornite and covellite (Figure 9A-F). Their occurrence is associated with with potassic and phyllic-advanced argillic altered rocks respectively which could be as a disseminated and cross cutting veinlets. Covellite in the potassic zone are occurs in narrow intervals of phyllic alteration. While chalcopyrite±bornite mineralization are occurs in potassic altered Kucing Liar porphyry and syenite. They are also occurs in sandstone-hornfels marginal to the the Kucing Liar porphyry. The porphyry Cu-Au mineralization at Deep Grasberg Lower Prospect area can divided into the following types: (1) Quartz-biotite veins that probably predate the quartz vein stockwork, (2) disseminated in potassic-phyllic altered porphyry syenite and sandstone-hornfels, (3) in biotite (potassic) altered porphyry and syenite around brecciated quartz veins, and (4) in chalcopyrite±anhydrite veins that cuts the quartz stockwork in the Kucing Liar intrusion.

In the Kali dyke, chalcopyrite has accompanied by anhydrite with no quartz since the intrusion is not quartz-veined. Chalcopyrite mineralization therefore occurred over a period of time, from potassic alteration and veining, through stockwork quartz veining and ceased after intrusion of the Kali dykes.

Discussion KL98-10-21 and KL98-10-22 drill holes have intersected at least four intrusions of monzonitic to syenitic composition. Two of three intrusions are unaltered to weakly altered with lack of quartz veins. The two other intrusions of coarse hornblende monzonite and syenite appear to be restricted to intermediate to deep levels of the Grasberg Intrusive Complex and probably do not extend to the shallow Grasberg levels. The intrusive rocks above then cut by quartz-anhydrite stockwork veined throughout, with moderately to strong K-silicate (biotite) alteration and weakly chalcopyrite-molybdenite mineralization which occurs as disseminated and fractures filling or veinlets.

Figure 7. Core samples of altered dioritic intrusions, and also carbonate and sedimentary wall rocks with quartz vein and veinlets cross-cutting the phyllic-potassic alterations. Note: Cpy = chalcopyrite, Py = pyrite, Qz = quartz.

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Figure 8. Photomicrographs showing mineralogy developed in altered intrusive rock (A-D) and altered wall rocks (E-F). Note: amf = amphibole, bio = biotite, car = carbonate, K-f = alkali feldspar, mus = muscovite, pla = plagioclase, qtz = quartz, ser = sericite.

Figure 9. Photomicrographs showing association ore minerals from different level of KL98-10-21 and KL9810-22 drill cores. Note: bn = bornite, cpy = chalcopyrite, cv = covellite, py = pyrite, sp = sphalerite.

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The potassic alteration overprinted by quartz flooding and stockwork veining and phyllic and advanced argillic alteration which formed as a cap on the top and southern margin of the intrusions. The alteration along with and accompanied by low grade high sulfidation type mineralisation consists of enargite-covellite±molybdenite. Porphyry Cu-Au mineralization in the Deep Grasberg is effectively terminated at its base which indicated by quartz flooding (massive silica) and advanced argillic alteration which formed at the top of the hornblende porphyry intrusions (Allen, 2012).

 Early quartz-biotite veins are inferred to have formed during potassic alteration, and are cut by and predate the quartz vein stockwork (Pollard and Taylor, 2005) that potassic alteration to biotite closely follows intrusion and crystallization.

Conclusions

 Late gypsum and sulfur occur in veins, fractures and cavities throughout the phyllic and advanced argillic zone, with anhydrite extending to the greater depths into the potassic zone.

 The porphyry alteration type of the shallow Grasberg deposit has extended and remains developed through the lower elevation beneath the Deep Grasberg ore body.  Potassic alteration with mostly comprised of pale brown secondary biotite has overprinted by intense quartz-sericite (phyllic) and advance argillic alteration, and commonly mapped at both KL98-10-21 and KL98-10-22 drill hole.  The quartz stockwork veining at shallow levels is centered in and around the MGI, and decreases in intensity outwards which is not extend to deep Grasberg.  Kucing Liar porphyry intrusion have fairly uniform 10-20% quartz stockwork veins by volume in both potassic and phyllic altered zones. The thickest silicic zone at the top of the porphyry on the KL98 section forms the base to the Deep Grasberg ore body.  Alteration in deep Grasberg section KL98 has a different pattern compared to the alteration at the shallow level while the younger phases of alteration are related to the highly quartz-veined at the top and margins of the Kucing Liar porphyry.  Potassic alteration of the Kucing Liar porphyry comprised of secondary biotite and K-feldspar assemblages. Biotite replaces hornblende, and pervades the groundmass of K-feldspar pseudomorphs calcic plagioclase.

 Phyllic alteration is intense within and halo in the quartz veinstockwork at the top of and along the southern margin of the Kucing Liar porphyry.  Hypogene advanced argillic alteration is also mapped sporadic within the phyllic zone which comprised of alunite-pyrophyllite and kaolinitedickite.

 The Idenberg Fault and tectonic breccia zone are clearly structurally controlled hydrothermal fluids causing the alterations and intense quartz veining at Deep Grasberg Lower area. The hydrothermal fluid inferred to have flowed up through the fault zone, along the southern margin of the Kucing Liar porphyry and blossomed at the top of the intrusion just below the coppergold deposit in the block cave, to form a silicaphyllic-advanced argillic cap. The hydrothermal fluids have also diverted along Idenberg fault into the Kucing Liar orebody and been responsible for conversion of chalcopyrite to covellite ore.  The zone of most intense quartz veining and late phyllic and advanced argillic occurs underneath the copper porphyry ore deposit, suggesting extreme telescoping during evolution of the complex.

Acknowledgements This is in regard with first author Master study of economic geology at Faculty of Geology, Padjadjaran University, Indonesia. The research has supported by PT Freeport Indonesia and PT Eksplorasi Nusa Jaya, affiliated of Freeport McMoran Inc. We would like to thanks to George

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MacDonald, Vice President of Freeport McMoran Exploration, Joseph Macpherson, Managing Director of PT. Eksplorasi Nusa Jaya, and Wahyu Sunyoto, Senior Vice Precident of Geo&Technical Services, PT Freeport Indonesia, for permission to collect data and makes this paper published.

Trautman, M. C. (2013) Hidden Intrusions and Molybdenite Mineralization beneath the Kucing Liar skarn, ErtsbergGrasberg Mining District, Papua, Indonesia. Unpub. MSc thesis, University of Texas Austin, 336 p. Weiland, R. J., and Cloos, M. (1996) Pliocene-Pleistocene asymmetric unroofing of the Irian fold belt, Irian Jaya, Indonesia:

References Allen J. M. (2012) Deep Grasberg exploration: The KL98 drill section and evolution of the GIC. Unpublished internal report to PT Eksplorasi Nusa Jaya. Cloos, M., Sapiie, B., Quarles van Ufford, A., Weiland, R. J., Warren, P. Q., McMahon, T. P. (2005) Collisional delamination in New Guinea: The geotectonics of subducting slab breakoff. Geological Society of America Special Paper 400, 51 p. Leys, C. A., Cloos, M., New, B. T. E., MacDonald, G. D. (2012) Copper-Gold ± Molybdenum Deposits of the Ertsberg-Grasberg District, Papua, Indonesia. Economic Geology, Special Publication 16, 215-235. MacDonald, G. D., and Arnold, L, C. (1994) Geological and geochemical zoning of the Grasberg Igneous Complex, Irian Jaya, Indonesia. Journal of Geochemical Exploration, 50, 143-178. McMahon, T. P. (1994a) Pliocene intrusions in the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia; Major and trace element chemistry. International Geology Review, 36, 925-946. Quarles van Ufford, A. I. (1996) Stratigraphy, structural geology, and tectonics of a young forearc-continent collision, western Central Range, Irian Jaya (western Papua). Unpub. Ph.D thesis, University of Texas Austin, 421 p. Pollard, P. J., and Taylor, R. G. (1998) Paragenesis, mineralogy association, sites and characteristics of gold within selected areas of the Grasberg Cu-Au deposit, Irian Jaya. Unpub. Report to PT. Freeport Indonesia, 56p. Sapiie, B. (1998) Strike-slip faulting, breccia formation and porphyry Cu-Au mineralization in the Gunung Bijih (Ertsberg) mining district, Irian Jaya, Indonesia. Unpub. Ph.D thesis, University of Texas Austin, 304 p. Sapiie, B., and Cloos, M. (2004) Strike-slip faulting in the core of the Central Range of west New Guinea: Ertsberg Mining District, Indonesia. Geological Society of American Bulletin, 116, 227-293.

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Apatite

fission-track

thermochronology.

Geological Society of America Bulletin, 108, 1438-1449.