Abstract

Gold-mineralized skarns occur near the village of Muara Sipongi in West Sumatra and were mined for gold prior to World War II. They are developed in limestones and andesitic volcanics of Permo-Triassic age into which Late Jurassic diorites and granodiorites have been intruded. The intrusions are of I-type affinity.The skarns range from andradite-diopside rocks to grossular-idocrase-wollastonite-diop-side rocks which formed at about 450 degrees to 650 degrees C. Later retrogressive alteration caused the formation of epidote, prehnite, pumpellyite, actinolite, chlorite, calcite, and quartz. These phases record temperatures down to less than 200 degrees C. Fluids during skarnification probably contained less than 5 mole percent CO 2 . During retrogressive hydration the skarns were mineralized locally with chalcopyrite, pyrite, magnetite, hematite, bornite, and gold, followed by sphalerite, arsenopyrite, marcasite, tetrahedrite, Co-Ni sulfarsenides, and Au-Ag tellurides.Chlorite-calcite retrogressive alteration of the skarns is related to quartz veining which is hosted by the volcanic members of the country rocks. These veins are enriched in Pb + Zn compared to the skarns. Gold occurs as inclusions within arsenopyrite. Fluid inclusion data indicate the vein-forming fluids to be weakly saline and to contain minor CO 2 but also to contain significant amounts of CH 4 and N 2 . Homogenization temperatures fall in the range 180 degrees to 240 degrees C, which together with the low CO 2 correlate with the conditions inferred for chlorite-calcite skarn alteration.The composition of native gold derived from skarn and associated veins is characterized by 5 to 35 at. percent Ag and up to 0.8 at. percent Cu. A suite of alluvial golds in the area have negligible Cu and 45 to 65 at. percent Ag and can be related to a nearby suite of Tertiary epithermal quartz veins.The bulk Au-Ag-Cu pattern and gold compositions of the Muara Sipongi mineralization are discussed and compared with other examples of gold-bearing skarns and Cu porphyries with which they are often associated. The source and controls on mobilization of the gold may be significantly influenced by regional faults in the area which form part of the 1,600-km-long Sumatra fault zone. Pervasive alteration of altrabasic and basic assemblages, which are associated with this fault system, is one mechanism for liberating Au and other metals that are subsequently channeled upward along faults to be deposited in a favorable environment. The gold was most probably transported by thio and/or carbonyl complexes.

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