Abstract

The Cirotan gold deposit, dated at 1.7 + or - 0.1 Ma, is a mineralized fault with ore shoots contained within a system of fractures cutting a Miocene volcano-sedimentary series (9.5 Ma) intruded by Pliocene microdiorite (4.5 Ma). The mineralization, hosted by a right-lateral strike-slip fault that evolved into a normal fault, has been partly eroded.The deposit is of the adularia-sericite type with strong vertical ore zoning (Ag/Au = 64 at the top and only 7 in depth) and resulted from a single mineralizing event with five stages of deposition: silicification (stage a), siliceous breccias (stage b), cockade breccias with polymetallic sulfide ores and local rhodochrosite-rhodonite (stage c), precious brecciated ore including bonanzas (stage d), and late vuggy quartz (stage e). This evolution was accompanied by an increase in W, Sn, Au, and Bi reflected in the late appearance of abundant ferberite, scheelite, gold, cassiterite, and rare minerals such as Te-bearing canfieldite and lillianite. Fluid inclusion studies indicate a slight increase in temperature (235 degrees -255 degrees C) and salinity (2.89-7.15 wt % NaCl equiv) from the initial silicification to the bonanza ore, and an absence of volatiles other than H 2 O; that this could reflect an increased participation of magmatic fluids is supported by the sulfur isotope compositions of pyrite and galena (avg delta 34 S = 2.2 and 1.0ppm, respectively) close to the 0 per mil isotopic value of magmatic sulfur. Lead isotope results establish a common origin for both the Cirotan gold deposit ( 206 Pb/ 204 Pb = 18.77 and 207 Pb/ 204 Pb = 15.67) and the Pliocene andesitic-dacitic magmas to which the gold is related, and indicate a remobilization of Precambrian continental basement.Although Cirotan shows strong affinities with classic epithermal gold deposits, the late geochemical and mineralogical evolution with W, Sn, and Bi is extremely unusual, if not unique. In the context of the Malaysia-Sumatra tin belt, this is considered to reflect a genetic link with porphyry tin deposition and is explained by a progressive upward migration of Pliocene magma favored by strike-slip faulting. The occurrence of the two types of mineralization within a single structure thus makes Cirotan a hybrid deposit.

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