Other Names: Tujuh Bukit
Commodities : Copper, Gold
The Tujuh Bukit group of epithermal porphyry copper-gold-silver deposits are located in the province of East Java in Indonesia, approximately 205 km southeast of Surabaya, and 120 km west of Denpasar, Bali. Tujuh Bukit comprises the deposits Tumpangpitu, Candrian, Katak and Gunung Manis, all within a 2.5 km radius. The deposits lie with the east-west trending eroded volcanic remnants of the older Oligocene to Miocene Sunda-Banda magmatic arc, above the north dipping subduction of the Australian-Indian plate below the Eurasian plate. The arc magmatism that produced the cluster of Tujuh Bukit deposits migrated northward during the Pliocene to its present position as defined by the axis of juvenile Quaternary volcanic centres.
The Tumpangpitu deposit consists of two components, a large gold-rich porphyry Cu-Au-Mo system at depth, which extends into a high-sulphidation Cu-Au-Ag epithermal system nearer the surface.
The main mineralization is hosted by the deep, magnetic tonalitic intrusive system that has intruded into an older and more extensive feldspar-hornblende diorite stock. The older dioritic intrusion was emplaced within a cover sequence of volcaniclastic tuffs and breccias that lie at shallow levels of the deposit. Encompassing the coarse-grained tonalitic intrusion is a shell of magnetite, quartz-magnetite and quartz vein stockwork which contains the porphyry Cu-Au-Mo mineralisation. This alteration envelope overprints the outer margins of the tonalitic intrusive as well as its host rock. This mineralization occurs mainly within areas of phyllic overprinting of potassic alteration, and to a lesser extent, areas of potassic alteration within the tonalite intrusion.
The argillic alteration halo of the shallow, high-sulphidation epithermal system is represented by intense silica-clay-alunite-pyrite hydrothermal alteration of the andesitic lithic volcanic breccias, diatreme breccias, hydrothermal breccias and diorite rocks. This alteration signature covers an area of 4 km by 2.5 km and is cross cut by several northwest-trending and potentially structurally controlled zones of later hydrothermal breccias displaying advanced argillic alteration, and are typically more silicious.
Within Tumpangpitu there are five zones of near surface gold-silver oxide mineralization and deeper sulphide copper-gold mineralization. As of December 2009 the Inferred Resource for Zone A, B and C was estimated at approximately 100 million tonnes at 1.0 g/t gold equivalent at a 0.5 g/t gold equivalent cut-off. The Inferred Resource estimate contributes over two million ounces of gold and almost eighty million ounces of silver (3.3 million ounces of gold equivalent at a 0.5 g/t gold equivalent cut-off). (* Based on 80 x 80 metre grids over the three zones).
Source: Intrepid Mines Technical Report 2011