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Coupled andesitic and rhyolitic magmatism in the Kelian gold deposit, Indonesia

Bambang Setiabudi, Michael Palin and Ian Campbell

The Kelian gold deposit is located within the central volcanic arc of the island of Kalimantan in Indonesia. Hydrothermal activity produced extensive brecciation, alteration and gold and base metal mineralisation predominantly hosted by a body of andesite porphyry. The absolute ages of igneous rocks in the mine area are not known, nor has it been established whether the gold mineralisation was formed by a hydrothermal system driven by the andesitic magmatism or by processes related to phreatomagmatic activity associated with a nearby maar-diatreme complex .

The andesite porphyry at Kelian displays 3 zircon age populations as measured by ELA-ICP-MS: 21.2 ± 0.3 Ma, 20.5 ± 0.1 Ma and 19.7 ± 0.1 Ma. The youngest of these is interpreted to be the emplacement age and the older ages to represent earlier episodes of zircon crystallisation in the andesite source region. The maar-diatreme complex is intruded by rhyolite porphyry that also yielded 3 age populations that are shifted slightly relative to those of the andesite: 20.8 ± 0.1 Ma, 20.0 ± 0.2 Ma and 19.3 ± 0.1 Ma (Fig. 6a). By the same line of reasoning, the youngest zircon population is interpreted as the emplacement age of the rhyolite and the older two populations to represent prior episodes of zircon growth. Taken together, the zircon dates indicate alternating cycles of andesitic and rhyolitic magmatism. Trace element compositions of the andesitic and rhyolitic zircons do not overlap (Fig. 6b), limiting mixing during 2+ m.y. of magmatic activity. These observations, combined with the spatial proximity of the volcanic centers, require physically distinct long-lived andesite and rhyolite magma source regions, perhaps coupled in their responses to periodic emplacement of primitive magma into the underlying arc crust.

Figure 6: Age spectra (A) and trace element compositions (B) of zircons in andesite and rhyolite porphyries from the Kelian gold deposit.