Geochemistry and geochronology of the igneous suite associated with the Kelian epithermal gold deposit, Indonesia

The Kelian gold deposit, located 250 km west of the provincial capital of Samarinda, East Kalimantan, is Indonesia's principal gold producer. The deposit is an intrusive-related low sulphidation system, situated within the Central Kalimantan Continental Arc, which consists of andesitic to rhyolitic volcanics and intrusives of Miocene age. Hydrothermal activity produced extensive brecciation, porphyry- to epithermal-style alteration and gold and base metals mineralisation. The nature of genetic relations is the main aspect of this study and is approached through the geochemical evolution of the calc-alkaline suites in relation to the metallic mineralisation. Geochemical evolution in the Miocence calc-alkaline suites from the Kalimantan volcanic arc exhibit two distinctive trends of magmatic differentiation The first trend is defined by a series of "productive" igneous suites such as Kelian, Muyup and Ritan, and is a "typical" calc-alkaline series characterised by low Mg, moderate K, relatively high Ti and Al and depletion in Cr and Sc. The second trend is defined by the chemical variations of the Magerang-Imang and Nakan suites which have remarkably high concentrations of MgO. Major and trace element geochemistry of the high Mg andesites from MagerangImang and Nakan is comparable with that of low-Ca type-2 boninites. The Kelian Igneous Complex is characterised by positive Zr and Hf anomalies in the trace element patterns which is uncommon for calc-alkaline subduction zone magmas. The chemical diversity in the Magerang-Imang and Nakan suite might have been generated by a combined wallrock assimilation and fractional crystallisation process involving a parental basaltic magma and a Zr-rich cumulate. It is suggested that the Magerang-lmang and Nakan high Mg andesites were fed by magma chambers that formed deep in the crust, and were emplaced into pre-existing intrusions of felsic composition that formed as part of the Kelian Igneous Complex cycle. The shallow level stocks at Magerang-1.mang and Nakan were generated by intrusions that melted the walls and roofs of related, but pre-existing intrusions, and extracted abundant xenocrystic zircons during the assimilation process. This study represents the first Platinum Group Element data for a fractionated suite of calc-alkaline andesite. The technique developed in this study represents a breakthrough in our ability to monitor important ore elements in felsic igneous system. The PGE distribution patterns in the Magerang-lmang hornblende andesite are subparallel to each other over a range of concentrations that vary by about a factor of 20. All the Magerang-lmang samples are depleted in Ru, Ir and Os concentrations relative to Re, Pd, Pt and Rh concentrations and have Pd/Ir values of 15 to 54 and Ru/Ir - 1. The PGE concentrations decrease with increasing Si02, showing that they are depleted by fractional crystallisation. Gold is depleted by an order of magnitude and relative to Re and Pd. The low concentration of gold in the igneous rocks associated with the Kelian gold deposit is unexpected. Most metal deposits are found in association with rocks that are already enriched in the metal of interest. It is therefore surprising to find a major gold deposit in host rocks that are depleted in Au. It is also interesting that Au and PGE ratios change little during fractionation. This is surprising because it implies either that the partition coefficients for the PGEs into the sulphides are similar, which seems unlikely, or that Au and the PGEs are not being depleted by simple equilibriwn fractional crystallisation of sulphide. Alternatively, the gold and PGE fractionation are due to the assimilation of crustal material. This appears to be the most plausible process for the gradual depletion of Au and all of the PGE at Kellan. It is suggested that simple dilution with crustal material that contains no Au or PGE is the most likely process that will decrease the abundance of all of the PGE equally. Zircon U-Th-Pb isotope dates were determined in situ using excimer laser ablation ICP-MS. The two different bodies of the Magerang hornblende andesite yielded a single age of 19.38 ± 0.12 Ma and 19.62 ± 0.21 Ma, while the Nakan andesite gave an age of 20.01± 0.15 Ma. The Central Andesite porphyry at Kelian gave 3 populations of U-Pb zircon dates: 21.2 ± 0.32 Ma, 20.5 ± 0.12 Ma and 19.7 ± 0.12 Ma. The youngest date (19.7 Ma) is interpreted as the emplacement age and the two older zircon populations represent the age of inherited zircons coming from the previous thermal event that affected the source region of the andesite. The U-Pb zircon dating for the Runcing Rhyolite porphyry also yielded 3 distinctive date populations: the youngest date of zircon population (19.3 ± 0.1 Ma) is interpreted as the emplacement age and the other two populations (20.0 ± 0.2 Ma and 20.8 ± 0.1 Ma) represent the ages of inherited zircons. The emplacement age of the Magerang-Imang andesite implies that the highsulphidation Cu-Au mineralisation at Magerang is younger than the low-sulphidation Au deposit at Kelian. The Kelian and Magerang andesites have a relatively short interval of emplacement ages suggesting that the duration of magmatism and related epitbermal mineralisation in the larger Kelian region was between 0.5 - 1 Ma. During this period, the magmatic-hydrothermal system has produced 2 distinctive types of epithermal mineralisation: firstly, low-sulphidation Au deposit at Kelian and secondly highsulphidation Cu-Au mineralisation at Magerang-Imang. Detrital zircons from the Mahakam and Kelian rivers were dated to obtain the overall duration of volcanism in the region. These zircons are dominated by Pliocene, Miocene, Cretaceous, Triassic, Permian and Carboniferous zircons. The youngest detrital zircon from the Kelian river gave an age of 1.7 ± 0.1 Ma and the oldest one gave an age of 373 Ma. Within the Tertiary zircon population, there are age spectra peaks at Pliocene (from 1.7 Ma to 2.8 Ma) and Miocene (from 15.8 Ma to 21.7 Ma). The Cretaceous zircon population ranges from 67 .6 to 126.3 Ma and peaks at l 05 Ma. The gold mineralisation at Kelian occurs toward the end of the Miocene volcanism and took place locally within the Kelian region as this Miocene volcanism is not recorded in the zircon component from the larger Mahakam river. The two large inheritance populations in both the Central Andesite and Runcing Rhyolite lie within the time range of the Kelian igneous complex as defined by the KeJian River detrital zircons. They must be derived from crustal intrusions that formed as part of the Kelian cycle. It is suggested that both the Kelian Andesite and Runcing Rhyolite were fed by 2 magma chambers that formed deep in the crust, each of which were long lived. The magma chambers that fed the Kelian Andesite and Runcing Rhyolite were emplaced into pre-existing intrusions of similar composition that formed as part of the Kelian igneous complex. The abundance of xenocrystic zircons in both units suggests that these earlier intrusions were still hot, or perhaps even partially molten, at the time of magma emplacement. That is the shallow level stocks and diatremes at Kelian were fed by nested, cannibalistic intrusions deep in the crust that melted the walls and roofs of related, but pre-existing intrusions, and inherited abundant xenocrystic zircons in the process. Both the Kelian Andesite and the Runcing Rhyolite have two populations of inherited zircons, which indicate that the pre-existing intrusions formed in two distinct episodes, 0.7 to 0.8 m.y. apart. The difference between the emplacement age and the age of the oldest of the inherited zircon populations shows that this cannibalistic activity took place over 1.5 m.y. The interval of magmatic activity in these chambers corresponds to the period of peak activity in the Kelian igneous complex as defined by the detrital zircons