Early Miocene volcanism associated with a maar-diatreme breccia complex preceded main-stage epithermal gold mineralization at the Kelian gold mine, Kalimantan, Indonesia. Prior to brecciation, andesite intrusions (19.7 ± 0.06 Ma) were emplaced into a package of felsic volcaniclastic rocks and overlying carbonaceous sandstones and mudstones, and a weakly mineralized geothermal system was established. Intrusion of quartz-phyric (19.8 ± 0.1 Ma) and quartz-feldspar-phyric rhyolite (19.5 ± 0.1 Ma) into the active geothermal system triggered widespread fragmentation and formation of the maar-diatreme complex.

Subsurface phreatomagmatic and phreatic explosions disrupted the preexisting hydrothermal system, producing three composite diatreme breccia bodies (the Tepu, Runcing, and Burung Breccias). The diatremes consist of polymict breccias and sandstones that contain abundant carbonaceous matrix. A distinctive facies association comprising coherent rhyolite, jigsaw-fit rhyolite breccia, and matrix-rich breccias that contain wispy to blocky juvenile rhyolite clasts define the root zones of the diatremes.

The surficial products of maar-diatreme volcanic activity at Kelian are preserved as large blocks of well-stratified breccias. They contain accretionary lapilli and were deposited by a combination of wet, pyroclastic base-surge, fallout, and cosurge fallout processes. Evidence for syneruptive resedimentation of the pyroclastic deposits is preserved in poorly stratified breccia beds. Megablocks of phreatomagmatic base-surge deposits were dropped down several hundred meters from the maar environment into the underlying diatremes.

Volcanism in the Kelian maar-diatreme complex was dominated by a combination of phreatomagmatic and phreatic processes, with subordinate hydraulic, tectonic, and dry magmatic fragmentation. The carbonaceous matrix-rich diatreme breccias acted as aquicludes during subsequent hydrothermal activity, focusing fluid flow into the wall rocks adjacent to the diatremes, where epithermal gold mineralization and hydrothermal brecciation occurred.

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