Volcanic Setting and Characteristics of the Onto Lithocap, Sumbawa, Indonesia

ABSTRACT The Hu'u volcano is an eroded, medium K calc‐alkaline basaltic andesite strato‐volcano 15 km in diameter with a maximum elevation of about 850 m, with a central caldera about 6 km in diameter. Intra‐caldera andesitic ash flow over 300 m thick fills the caldera and overlies older basaltic andesite to andesite flows and breccia. Overlying the ash flow tuff is fine laminar bedded tuff with accretionary lapilli, up to 50 m thick, formed in the caldera volcanic lake. This sequence is overlain by a relatively coarse crystal‐crowded hornblende andesite‐dacite dome 5.6 × 3.5 km in area, maximum thickness of 530 m, of low K calc‐alkaline composition. This dome overlies and is likely co‐magmatic with the Onto porphyry, which was intruded to near the base of the dome. The Onto porphyry system was formed at < 0.5 Ma and is remarkable for very deep overprinting by advanced argillic alteration that formed essentially together with high temperature porphyry alteration. The ore body comprises mainly gold–pyrite–covellite and minor enargite. From below the Onto dome the lithocap extends vertically over 1.8 km and is mushroom‐shaped, centered directly on a composite porphyritic micro quartz diorite to granodiorite stock about 1.4 km in diameter. The upper part of the lithocap exhibits a maximum outflow of 2.5 km SE from the center of Onto. The lithocap exhibits a vertical zonation from (1) residual quartz–dickite–kaolinite, (2) alunite with minor pyrophyllite, (3) alunite–pyrophyllite–diaspore–zunyite, with pyrophyllite increasing relative to alunite with depth, and (4) andalusite–diaspore–anhydrite, with minor pyrophyllite‐alunite, starting from 1400 m from below the top of the lithocap. At moderate to deep level the advanced argillic zone is enclosed by paragonite–albite–chlorite–high Al‐smectite (beidellite), followed outward by epidote‐chlorite. At deepest drilled levels, fluid inclusions are vapor‐rich and multiphase, typical for porphyry systems. They are hosted by quartz–andalusite–diaspore, which is cut by narrow (cm) pyrite–enargite feeder veins, with pyrophyllite selvages.