Cenozoic magmatism in Kalimantan and its related geodynamic evolution
The NE-SW Tertiary magmatic belt of central Kalimantan is related to two separate periods of subduction; during the Eocene-Oligocene and Late Oligocene-the earlier belt. This magmatic belt is characterized chiefly by Late Oligocene-Miocene volcanic products, among which limited exposures of the Eocene volcanics have also been mapped by previous investigators. This calc-alkaline magmatic belt has become known as the 'gold belt' of Central West Kalimantan on account of a number of discoveries of Neogene epithermal gold mineralization. This mineralization is found in central to proximal volcanic settings and occurred at relatively shallow depths. The earliest known subduction-related magmatism took place in the Eocene-Early Oligocene with the emplacement of calc-alkaline silicic pyroclastics, followed by a period of continental collision. Subsequent subduction-related magmatism continued from Late Oligocene-Pleistocene, during which time the magma evolved from calc-alkaline to potassic calc-alkaline. Plio-Pleistocene magmatism resulted in the formation of basalt flows. The present available K-Ar ages of the Cenozoic volcanics range from 51 to 1 Ma.
The Ongkang area!!!!!!!!!
Alteration and epithermal mineralization in the Masupa Ria volcanic center, Central Kalimantan, Indonesia
The most economically important mineralized veins are located on the eroded southern margins of the Masupa Ria alteration zone in the Ongkang area.
The upper part of the Ongkang vein cross-cuts a zone of intense silicification that forms major cliffs in the lower part of the exposed Masupa Ria alteration zone. Advanced argillic alteration (quartz-pyrophyllite-diaspore-kaolinite�alunite) occurs beneath the silification and is overprinted by mineralized quartz veins and related quartz-illite alteration. These veins have been subsequently overprinted by later advanced argillic alteration. Similar relationships are present in the Menyawang area, but are not present in mineralized occurrences (Kiwi and Tosa-Ucang) to the east of the regional alteration zone. Alteration mineralogy, supported by preliminary fluid inclusion data, from the Ongkang vein area suggest temperatures of formation between 250 and 300�C. This implies a minimum depth of approximately 400 meters, assuming relatively dilute waters, boiling, and hydrostatic conditions. Approximately 300 meters of acid alteration and silicification have been removed by subsequent erosion.
Epithermal precious metal bearing veins occur throughout the district. These vary from narrow discontinuous veinlets to major veins up to ten meters across. The major veins are hosted by steep structures, some of which are regional in extent (strike lengths in excess of 10 km), and the predominant strike directions are 060�, 290� and 330�. Precious metal mineralization is typically associated with pyrite-sphalerite-galena-tetrahedrite bands within complex quartz veins. Economic grades are restricted to zones of a few tens of meters in length. Wall rock alteration is dominated by a quartz-illite assemblage and adularia is rarely observed in any of the mineralized zones in the district.
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