Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

Magmatic fluids, both vapour and hypersaline liquid, are a primary source of many components in hydrothermal ore deposits formed in volcanic arcs. These components, including metals and their ligands, become concentrated in magmas in various ways from various sources, including subducted oceanic crust. Leaching of rocks also contributes components to the hydrothermal fluid—a process enhanced where acid magmatic vapours are absorbed by deeply circulating meteoric waters. Advances in understanding the hydrothermal systems that formed these ore deposits have come from the study of their active equivalents, represented at the surface by hot springs and volcanic fumaroles.

The role of magmas in the formation of hydrothermal ore deposits

Origin and History of the South China Sea Basin

Late Jurassic–Cenozoic reconstructions of the Indonesian region and the Indian Ocean

Abstract Reconstructions of SE Asia at 5 Ma intervals for the past 50 Ma are presented. They are constrained by new data from the Philippine Sea plate, which forms the eastern boundary of the region, by recent interpretations of the South China Sea and Eurasian continental margin, forming the western boundary, and by the known motions of the Indian-Australian plate to the south. An attempt is made to satisfy geological and palaeomagnetic data from the region. The implications of these reconstructions for the Tertiary evolution of SE Asia are discussed in the light of other new data from the region. There are two regionally important periods of change during the past 50 Ma. Both appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc caused major effects which propagated westwards through the region. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan. This appears to be a key to the recent tectonics of the region. Principal features of the model include the following interpretations. Middle Tertiary counter-clockwise rotation of Borneo closed a large proto-South China Sea and led to the development and destruction of marginal basins north of the Celebes Sea. The rotation implies that much of the north Borneo margin was not a subduction, but a strike-slip boundary for most of this period. It also suggests that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin which opened between late Eocene and mid Oligocene, and narrowed westwards like the present South China Sea. Luzon is suggested to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin, whereas most of the other Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the Early Miocene. Arc-continent collision in the early Miocene caused plate boundaries to change and initiated the clockwise rotation of the Philippine Sea plate. Since then the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, with significant strike-slip motion of fragments at the plate margin. Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Molucca Sea was a very wide area which formed part of the Philippine Sea plate before c. 15 Ma and originated as trapped Indian ocean lithosphere. It has been eliminated by subduction on its east and west sides. The present-day double subduction system never extended north of the present Molucca Sea into the Philippines. The Sulawesi ophiolite has an Indian ocean origin and was emplaced on the west Sulawesi continental margin at the end of the Oligocene. The major change in plate boundaries at the beginning of the Miocene following arc-continent collision of the Australian margin with the Philippine Sea plate arc caused initiation of the Sorong Fault system and led to westward movement of continental fragments which were accreted to Sulawesi during the late Neogene. The Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird’s Head before c. 15 Ma. They moved to their present positions after slicing of fragments from this microcontinent at different times and each was attached to the Philippine Sea plate for a few million years before collision. Most of the Banda Sea is interpreted to have an extensional origin and to have opened during the late Neogene. The reconstructions imply that there has been little convergence at the north Australian margin in Irian Jaya since the early Miocene and most convergence has occurred during the last c. 5 Ma. Movement of Philippine Sea arc fragments within the northern New Guinea margin along strikeslip zones probably accounts for the terrane character of this orogenic belt.

Reconstructing Cenozoic SE Asia

25 years of mineral exploration and discovery in Indonesia

Petrologic, isotopic, and radiometric age constraints on the origin and tectonic history of the Malino Metamorphic Complex, NW Sulawesi, Indonesia

Stratigraphy and tectonic setting of the Cretaceous and Paleogene volcanic-sedimentary successions in northwest Sulawesi, Indonesia: implications for the Cenozoic evolution of Western and Northern Sulawesi

Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia

Mineralized hydrothermal breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal brecciation. Explosive phreatic brecciation was followed by in situ hydraulic brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia- and/or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (∼0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ∼4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ 34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal breccia bodies. © 2008 Society of Economic Geologists, Inc.

Theo van Leeuwen

Papers

25 years of mineral exploration and discovery in Indonesia

Petrologic, isotopic, and radiometric age constraints on the origin and tectonic history of the Malino Metamorphic Complex, NW Sulawesi, Indonesia

Stratigraphy and tectonic setting of the Cretaceous and Paleogene volcanic-sedimentary successions in northwest Sulawesi, Indonesia: implications for the Cenozoic evolution of Western and Northern Sulawesi

Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia

Hydrothermal Breccias and Veins at the Kelian Gold Mine, Kalimantan, Indonesia: Genesis of a Large Epithermal Gold Deposit