University of Tasmania
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The Tumpangpitu porphyry gold-copper-modlybdenum and high-sulfidation epithermal gold-silver deposit, Tujuh Bukit, Southeast Java, Indonesia

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posted on 2023-05-27, 08:55 authored by Harrison, RL
The Tumpangpitu porphyry and high- to intermediate-sulfidation epithermal deposit is the largest deposit in the Tujuh Bukit district, southeast Java, Indonesia. The porphyry resource contains 1.9 billion tonnes at 0.45 % Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion llbs of Cu. There is an additional 1.9 Moz Au and 60.3 Moz Ag in oxidized high-sulfidation epithermal deposits. Tumpangpitu is located along a NW-striking structural corridor covering an area of 12 x 5 km that hosts several Cu-Au-Mo mineralized tonalitic porphyries, each with varying degrees of metal enrichment. At least eight discrete intrusions, span the alteration-mineralization sequence at Tumpangpitu. What is unusual, however, is the presence of both a pre-mineralization, relatively dry volcanic breccia pipe (Tanjung Jahe) and a late-mineralization diatreme complex associated with a large magmatic-hydrothermal system (Tumpangpitu) in the same district. Type sections have been constructed for Tumpangpitu based on crosscutting relationships of intrusions and breccias, supported by radiometric dating. This work has provided the first comprehensive geological model for the deposit. Magmatism, alteration and mineralization at Tumpangpitu occurred in response to north-directed subduction of the Indo-Australian plate beneath the Asian continental plate margin. The Tujuh Bukit district is floored by Early to Late Miocene sedimentary and andesitic volcanic rocks. Volcanichydrothermal activity at Tujuh Bukit began with the formation of the weakly-altered Tanjung Jahe diatreme complex (U-Pb\\(_{zircon}\\) ages of 8.78 ± 0.22 to 8.52 ± 0.21 Ma). Mineralization at Tumpangpitu was preceded by the intrusion of a large, equigranular, dioritic pluton (5.81 ± 0.20 to 5.18 ± 0.27 Ma). Hydrothermal activity associated with mineralization has been constrained by U-Pb age determinations from syn- to latemineralization porphyry intrusions that were emplaced in the Early Pliocene from 5.40 ± 0.46 to 3.94 ± 0.69 Ma. Tumpangpitu is a large, complicated example of telescoped high-sulfidation mineralization and associated advanced argillic alteration (part of a district-scale lithocap) that has overprinted the top of a Cu-Au-Mo porphyry deposit. Mineralization extends from the present-day surface to depths of more than 1,000 m bsl. High-sulfidation mineralization is hosted by five main NW- and N-trending residual quartz ridges known as Zone A, Zone B, Zone B East, Zone C and Zone E, containing Au-Ag oxide ore close to surface with transitional and sulfide Au-Ag-Cu ore at depth. Porphyry mineralization extends from an upper level of 50 m asl to levels of over 1,000 m bsl and the 0.2 % Cu shell extends 1.2 - 1.5 km laterally around the tonalite intrusive complex. Early magnetite (M), A and B veins and veinlets are associated with hypogene bornite, chalcopyrite, chalcocite and covellite. Native gold occurs as refractory inclusions in these early copper sulfide minerals and B veins contain most of the molybdenite at Tumpangpitu. Rare C veins contain highgrade chalcopyrite and bornite, but are not extensively developed at Tumpangpitu. Late pyritic D veins with bleached muscovite alteration halos have overprinted all earlier porphyry stage veins. Late massive pyrite veins that lack quartz typically contain one or more of the following: enargite, luzonite, covellite, bornite, and/or tennantite-tetrahedrite with relict chalcopyrite. These high-sulfidation state epithermal veins are typically associated with high temperature alteration assemblages, including quartz - alunite, quartz - pyrophyllite ± topaz ± diaspore and quartz - dickite ± kaolinite. Late tennantite - tetrahedrite - chalcopyrite veins are associated with pyrophyllite, dickite, kaolinite and illite alteration. The general pattern of metal zoning with respect to the porphyry centres at Tujuh Bukit indicate central Au and Cu associated with deep porphyry mineralization, central and proximal Mo associated with porphyry ore and the occurrence of Mo in the base of the lithocap, highlighting the location of the underlying porphyry deposit. Enargite defines the core of both shallow and deep structurally controlled high-sulfidation mineralization. Enargite has produced a strong As anomaly, grading outwards to Au, Cu, Ag, Sb, Zn associated with tennantite - tetrahedrite. Late stage, distal galena - sphalerite veins with muscovite alteration halos form a Pb-Zn enrichment halo around the zone of advanced argillic alteration. A zonation both in space, as well as time has been recognized in porphyry-related alteration from central quartz - magnetite - K-feldspar - biotite through rare actinolite to distal epidote - chlorite and chlorite - calcite alteration of country rocks. Early ores and potassic alteration minerals were overprinted by structurally controlled pervasive advanced argillic alteration assemblages. Spectral data has been used to create an alteration model of clay mineral zonations. Quartz - alunite - dickite assemblages are dominant at shallower levels in the epithermal system and pyrophyllite ± diaspore ± topaz at deeper levels. Fluids channelled along structures are interpreted to have formed 750 x 200 m wide vuggy quartz ridges surrounded by >500 m x 1,200 m of advanced argillic envelopes containing dickite - kaolinite with decreasing quartz contents laterally. These late, structurally controlled advanced argillic alteration zones extend for over 12 km along a NW strike through the project area, overprinting all early porphyry-related alteration. The silicic and advanced argillic zones associated with enargite and tennantite are surrounded by illite - smectite and chlorite - calcite ± epidote alteration. A zone of quartz - muscovite ± pyrophyllite alteration separates and has overprinted the deep magnetite-stable potassic alteration and the shallow advanced argillic alteration domains. This muscovite zone is characterized by the alteration of magnetite to hematite. The areal extent of the late alteration zones are greater than that of the early potassic related alteration, which remains hidden at Tumpangpitu. The advanced argillic alteration provided a large exploration target that was used to vector towards porphyry mineralization at depth. Pervasive chlorite - illite - smectite ± calcite alteration of the diatremes post-dates all other styles of alteration. The highest grade porphyry mineralization is intersected in magnetic, quartz - magnetite - biotite - K-feldspar altered tonalite. This has allowed airborne magnetics to be used successfully in conjunction with mapping and soil molybdenum geochemistry to facilitate exploration targeting, which ultimately led to the discovery of the deep porphyry deposit. Fault-controlled high-sulfidation state enargite - luzonite ± covellite ± bornite mineralization and associated advanced argillic alteration (part of a district-scale lithocap) has been overprinted by intermediate-sulfidation state tetrahedrite - tennantite - chalcopyrite mineralization. Sn-sulfides and sulfosalts crosscut bornite, chalcocite and covellite and appear to have formed contemporaneously with tennantite. Both HSE and ISE mineralization have overprinted and significantly upgraded the top of the porphyry orebody. \\(^{40}\\)Ar\\(^{39}\\)Ar dating of alunite (4.385 ± 0.049 Ma) and Re-Os dating of molybdenite (4.303 ± 0.018 Ma) has defined a short time period between the high-sulfidation epithermal and porphyry mineralization events, respectively. This suggests extremely rapid rates of uplift, exhumation and erosion in the vicinity of the Sunda-Banda magmatic arc. Volcanic-hydrothermal activity associated with the Tumpangpitu diatreme occurred during epithermal mineralization (breccia matrix zircon age of 2.7 ± 1.0 Ma). Clasts of high-sulfidation state mineralized rocks are a minor, but significant, component of the diatreme, and late-stage epithermal veins cutting the diatreme, demonstrate an inter-mineralization timing with respect to epithermal activity in the district, implying that epithermal mineralization continued intermittently for 1 - 1.5 m.y. after porphyry mineralization ceased at Tumpangpitu.


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