Geochronology of the tumpangpitu porphyry Au-Cu-Mo and high-sulfidation epithermal Au-Ag-Cu deposit: evidence for pre- and postmineralization diatremes in the Tujuh Bukit District, Southeast Java, Indonesia

The Tumpangpitu porphyry and high-intermediate-sulfidation epithermal deposit is the largest deposit in the Tujuh Bukit district, southeast Java, Indonesia. The porphyry resource contains 1.9 billion tonnes @ 0.45% Cu and 0.45 g/t Au, for 28.1 Moz Au and 19 billion lbs of Cu. There are an additional 2.1 Moz Au and 72.9 Moz of Ag in oxidized high-sulfidation epithermal mineralization.

Tumpangpitu is located along a NW-striking structural corridor covering an area of 12 × 5 km that hosts several Cu-Au-Mo mineralized tonalitic porphyries, each with varying degrees of metal enrichment. At least eight discrete intrusions spanning the alteration-mineralization sequence have been identified at Tumpangpitu. What is unusual, however, is the presence of both a premineralization, relatively dry volcanic breccia pipe (Tanjung Jahe) and a late-mineralization diatreme complex associated with a significant, large magmatic-hydrothermal system (Tumpangpitu) in the same district.

Magmatism, mineralization, and alteration 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. Volcanic-hydrothermal activity at Tujuh Bukit began with the formation of the weakly altered Tanjung Jahe diatreme complex (U-Pb_zircon ages of 8.78 ± 0.22–8.52 ± 0.21 Ma). Mineralization at Tumpangpitu was preceded by the intrusion of a large, equigranular, dioritic batholith (5.81 ± 0.20–5.18 ± 0.27 Ma). Hydrothermal activity associated with mineralization has been constrained by U-Pb age determinations from syn- to late-mineralization porphyries that were emplaced in the early Pliocene from 5.40 ± 0.46 to 3.94 ± 0.69 Ma.

High- and intermediate-sulfidation Au-Ag ± Cu mineralization and associated advanced argillic alteration (part of a district-scale lithocap) has overprinted and significantly upgraded the top of the porphyry orebody. ⁴⁰Ar/³⁹Ar dating of alunite (4.385 ± 0.049 Ma) and Re-Os dating of molybdenite (4.303 ± 0.018 Ma) have defined a short time period between the high-sulfidation epithermal and porphyry mineralization events. This suggests extreme 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 with systematic errors). 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 intermineralization timing with respect to epithermal activity in the district, implying that epithermal mineralization continued intermittently for 1 to 1.5 m.y. after porphyry mineralization ceased at Tumpangpitu.