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 and epithermal gold deposits. We then discuss the (MSP), i.e intra-oceanic subduction and the formation of applicability of these results to exploration in older island arcs. geological terranes, and suggest clues to distinguishing In Northern Indonesia, dextral escape tectonics productive from non-productive geological structures. related to India-Asia collision resulted in the southward 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 sedimentary prism to northern Borneo. The cessation of subduction changed motion of the PSP, and collision of Southeast Asia is a collage of ophiolitic, island and the AP with the Sunda Arc resulted in counter clockwise oceanic arc terranes that formed as part of a complex rotation of Borneo. In Sulawesi, northward subduction series of intra-oceanic and oceanic-continental of a trapped fragment of the IP and final assembly of the subduction (Figure 1). These tectonic processes Sulawesi terranes gave way to southward subduction of involved three oceanic plates (the Philippine Sea (PSP) the Celebes Sea Plate (CSP) at about 7 Ma. At about the and Caroline (CP) Plate in the east, and the oceanic part same time, trapped IP between Sulawesi and Halmahera, of the Indian-Australian Plate (IP) in the west) and, the the Molucca Sea Plate (MSP) was subducted westward margins of the continental Eurasian Plate and beneath the northern and eastern arms of Sulawesi. continental part of the Indian-Australian Plate (AP). In Earthquake data seem to suggest that the angle of the this paper, we have used the Cenozoic tectonic westward subduction of the MSP is steeper than the reconstructions of Hall (2002) and a database of mineral angle of southward subduction of CSP, making it deposits first published by Rak (1999) but enhanced plausible that subducted CSP is above the actively with more recent information, as a basis for examining subducting MSP in this region. Further east, the the tectonic context of mineral deposits. Halmahera arcs are the result of the eastward subduction According to Hall (2002), the tectonics in the region of the eastern margin of the MSP below the advancing are punctuated by the following main events: (a) 45 Ma, CP. The MSP fragment was largely consumed by 1 Ma, dextral tectonic escape of blocks in Indochina and resulting in the two subduction zones merging to a subduction of the proto-South China Sea Plate (PSCSP), single west-dipping zone. with development of a spreading ridge in the PSP; (b) Deposits in western and central Borneo are 25 Ma, collision of Australia with the Philippines- dominated by low sulphidation epithermal and porphyry Halmahera Arc and of the Ontong Java Plateau with the copper-gold deposits, which were developed at ca. 20 Melanesian arcs. Closure of the PSCSP, and opening of Ma, at the time when subduction of the PSCSP stopped the South China Sea; thickening of crust in North (e.g. Mount Muro and Mirah). Other deposits developed Borneo through underthrusting of continental crust from between 13 and 9 Million Years ago and include one the northwest. Commencement of the clock-wise large porphyry copper-gold deposit (Mamut) and several rotation of the PSP; (c) 5 Ma, collision of the low sulphidation epithermal deposits (e.g. Bau District). Philippines and the SE Asian continental margin and a Deposits in northern Sulawesi include porphyry resulting jump of the subduction zone from the west to copper-gold systems (e.g. Sungai Mak) as well as the east of the Philippines. epithermal systems (e.g. Gunung Pani) emplaced The Sunda Arc in Western Indonesia has been the between 5 and 1 Ma. The deposits are potentially linked site of ongoing subduction of the IP below thin to the cessation of subduction of the MSP and the continental crust of SE Asia (Sundaland) during the complex interaction of the west-dipping and shallower entire Cenozoic period. No large porphyry systems are south-dipping subduction zones beneath northern known in the continental arc section of Indonesia Sulawesi (CSP and MSP). (Sumatra and Java), and only few epithermal gold In the Halmahera arcs, the Gosowong (epithermal) occurrences, such as the Pungkut deposit. Most gold and Kaputusan (porphyry) deposits are most likely occurrences are located along the dextral Sumatran related to the cessation of eastward subduction and Fault, which initiated sometime in the Middle Miocene subsequent start of westward subduction during the past due to the oblique collision of the IP with SE Asia. To 10 Million Years. Proceedings of the Tenth Biennial SGA Meeting, Townsville, 2009 933 Papua New Guinea comprises the northern margin Guinea; and the juxtaposition of a series of oceanic arcs of the Australian Craton in the south, and a series of and final accretion of those to SE Asian crust in the oceanic fragments and oceanic arc terranes in the north. Philippines. Active oceanic arcs are still developing east of the main In western Indonesia, subduction has been going on island in New Britain and New Ireland, and further continuously, with the consumption of the Indian plate southeast along the interface between the Solomon Sea below thin crust of SE Asia. The oblique vector of this Plate and the Pacific Plate. subduction results in the formation of an important The younger oceanic (arc) fragments of the main dextral shear zone on Sumatra, the Sumatran Fault Zone. island were accreted in the late Miocene during oblique No significant porphyry systems are known in western convergence in relation to the rotation of the PSP and Indonesia. The subducting oceanic plate in this region convergence of the CP. The main island of Papua New comprises cold Eocene crust, while the thin lithosphere Guinea hosts some world-class porphyry and epithermal of SE Asia is also relatively cold. These factors suggest gold deposits including Grasberg, Ok Tedi, Frieda River, that the subducting oceanic plate never had time to heat Porgera, Wafi, Hidden Valley and Mt Kare. Most of up sufficiently to significantly contribute to the melts these deposits are either located on or near the remnants within the arc and could explain the apparent paucity of of the longitudinal suture zones, most notably the economic porphyry systems. Ramu-Markham and Bismarck fault zones (RMB, In Eastern Indonesia the tectonic drivers become Figure 1), or along transverse faults, such as the significantly more varied. In Borneo, earlier southward Mapenduma (Grasberg), Ok Tedi and Porgera faults. subduction of the PSCSP was immediately followed by The Philippines comprises a series of island arc rotation of Borneo and formation of crustal scale terranes that have been accreted during the Pliocene. structures, providing conditions in which the slab may These terranes comprise a variety of oceanic arcs that have been trapped and heated sufficiently to induce were translated from the south since the Middle partial melting. It is worth noting that the older systems Miocene through the rotation of the PSP, and juxtaposed (e.g. Mount Muro) appear to be located along the in their present position sometime in the Pliocene. The southeastward extension of faults which accommodated westward movement of the Pacific Plate since that time the dextral escape tectonics and lateral ramp architecture has resulted in the final accretion of the Philippine arcs, related to collision of India with Asia during the and resulted in a shift of the subduction from the Manila Miocene. In northern Sulawesi, the earthquake data Trench (west) to the present Philippine Trench (east). support a lower angle of subduction of the CSP These drastic changes in tectonic regimes over the past compared to the westward subducting MSP between 5 Million Years lie at the basis of the significant gold Sulawesi and Halmahera. Devolatilisation of the hot endowment of the Philippines. The main porphyry MSP and additional heat from trapped metasomatised copper-gold deposits are located east of the crustal-scale mantle wedge, together with shallow subduction of the sinistral Philippine Fault Zone (FFZ, Figure 1) but do CSP, would facilitate higher degrees of partial melting of not appear to be primarily controlled by this crustal the subducted CSP, resulting in low sulphidation feature (e.g. the Baguio and Mankayan districts). In fact oxidised melts, suitable for the generation of porphyry most of the deposits lie along the axes of Pliocene to copper-gold systems in the region. Quaternary volcanic arcs. Extensional jogs are believed In Papua New Guinea, the main tectonic driver was to play a role in the mineralisation of vein-type copper- the transverse accretion of young oceanic arcs to the gold. Australian Craton. This transverse movement induced extensional jogs which facilitated the emplacement of 2 Plate tectonic models and mineral some of the porphyry systems. In contrast to the IP endowment below western Indonesia, the oceanic crust of the PSP and CP is relatively young and hot, while subduction It has been generally accepted that not all arc systems was very oblique with respect to the Australian craton.