Tectonic controls on the distribution of large copper and go

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Tectonic controls on the distribution of large copper and gold deposits in Southeast Asia to identify productive and non-productive structures

Bert De Waele, Peter Williams, Gavin Chan

SRK Consulting, 10, Richardson Street, West Perth, WA6005, Western Australia

Abstract. Ore deposits occur in a wide range of tectonic the east, however, on Sumbawa, several porphyry settings, and are highly variable in their local systems are known, of which the largest is the 5 Million characteristics. In this paper, we focus on the distribution year-old Batu Hijau deposit. Tectonics in that region, of copper and gold deposits in the Southeast Asia region, east of longitude 115°E, consists of the consumption of and investigate large-scale plate-tectonic controls on the

the IP below the remnants of the Molucca Sea Plate distribution and characteristics of these porphyry copper

(MSP), i.e intra-oceanic subduction and the formation of and epithermal gold deposits. We then discuss the

island arcs. applicability of these results to exploration in older

In Northern Indonesia, dextral escape tectonics geological terranes, and suggest clues to distinguishing

related to India-Asia collision resulted in the southward productive from non-productive geological structures.

subduction of the proto-South China Sea Plate (PSCSP) Keywords: Porphyry Copper-Gold, Tectonics below Borneo. This subduction ceased at about 24 Ma, with continent-continent collision and accretion of the

1 Tectonics and mineralisation in SE Asia

Southeast Asia is a collage of ophiolitic, island and oceanic arc terranes that formed as part of a complex series of intra-oceanic and oceanic-continental subduction (Figure 1). These tectonic processes involved three oceanic plates (the Philippine Sea (PSP) and Caroline (CP) Plate in the east, and the oceanic part of the Indian-Australian Plate (IP) in the west) and, the margins of the continental Eurasian Plate and continental part of the Indian-Australian Plate (AP). In this paper, we have used the Cenozoic tectonic reconstructions of Hall (2002) and a database of mineral deposits first published by Rak (1999) but enhanced with more recent information, as a basis for examining the tectonic context of mineral deposits.

According to Hall (2002), the tectonics in the region are punctuated by the following main events: (a) 45 Ma, dextral tectonic escape of blocks in Indochina and subduction of the proto-South China Sea Plate (PSCSP), with development of a spreading ridge in the PSP; (b) 25 Ma, collision of Australia with the Philippines-Halmahera Arc and of the Ontong Java Plateau with the Melanesian arcs. Closure of the PSCSP, and opening of the South China Sea; thickening of crust in North Borneo through underthrusting of continental crust from the northwest. Commencement of the clock-wise rotation of the PSP; (c) 5 Ma, collision of the Philippines and the SE Asian continental margin and a resulting jump of the subduction zone from the west to the east of the Philippines.

The Sunda Arc in Western Indonesia has been the site of ongoing subduction of the IP below thin continental crust of SE Asia (Sundaland) during the entire Cenozoic period. No large porphyry systems are known in the continental arc section of Indonesia (Sumatra and Java), and only few epithermal gold occurrences, such as the Pungkut deposit. Most gold occurrences are located along the dextral Sumatran Fault, which initiated sometime in the Middle Miocene due to the oblique collision of the IP with SE Asia. To

Proceedings of the Tenth Biennial SGA Meeting, Townsville, 2009

sedimentary prism to northern Borneo. The cessation of subduction changed motion of the PSP, and collision of the AP with the Sunda Arc resulted in counter clockwise rotation of Borneo. In Sulawesi, northward subduction of a trapped fragment of the IP and final assembly of the Sulawesi terranes gave way to southward subduction of the Celebes Sea Plate (CSP) at about 7 Ma. At about the same time, trapped IP between Sulawesi and Halmahera, the Molucca Sea Plate (MSP) was subducted westward beneath the northern and eastern arms of Sulawesi. Earthquake data seem to suggest that the angle of the westward subduction of the MSP is steeper than the angle of southward subduction of CSP, making it plausible that subducted CSP is above the actively subducting MSP in this region. Further east, the Halmahera arcs are the result of the eastward subduction of the eastern margin of the MSP below the advancing CP. The MSP fragment was largely consumed by 1 Ma, resulting in the two subduction zones merging to a single west-dipping zone.

Deposits in western and central Borneo are dominated by low sulphidation epithermal and porphyry copper-gold deposits, which were developed at ca. 20 Ma, at the time when subduction of the PSCSP stopped (e.g. Mount Muro and Mirah). Other deposits developed between 13 and 9 Million Years ago and include one large porphyry copper-gold deposit (Mamut) and several low sulphidation epithermal deposits (e.g. Bau District). Deposits in northern Sulawesi include porphyry copper-gold systems (e.g. Sungai Mak) as well as epithermal systems (e.g. Gunung Pani) emplaced between 5 and 1 Ma. The deposits are potentially linked to the cessation of subduction of the MSP and the complex interaction of the west-dipping and shallower south-dipping subduction zones beneath northern Sulawesi (CSP and MSP).

In the Halmahera arcs, the Gosowong (epithermal) and Kaputusan (porphyry) deposits are most likely related to the cessation of eastward subduction and subsequent start of westward subduction during the past 10 Million Years.

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