ボリビア,アンデス地帯の多金属熱水鉱床産鉱石の硫黄同位体組成

A number of tin-polymetallic deposits are distributed in the Eastern Cordillera of the Bolivian Andes. Ores produced from these deposits are divided into three main types from their age and mineral paragenesis: (A) tin and tungsten ore formed by the meso- to hypothermal mineralization of Mesozoic; (B) tin and base metal ore formed by the polymetallic mineralization in Miocene; (C) lead and zinc ore at the Matilde mine.<BR>The sulfur isotopic compositions obtained from these ores have a wide range from -9.3 to 15.6 per mil. In the La Paz district (northern portion of the Eastern Cordillera), the range of δ³⁴S is from 0.0 to 15.6 per mil. Among them, the A-type ore has δ³⁴S values of 0.6 to 13.8 per mil, while δ³⁴S values obtained from the B-type ore are 0.0 to 15.6 per mil. The C-type ore from the Matilde mine shows δ³⁴S values between 10.9 and 14.6 per mil. The δ³⁴S obtained from the B-type ore in the Oruro-Potosi district (central portion of the Cordillera) is between -9.3 and 7.9 per mil. These values show remarkable distinction at the northern area (-9.3 to 0.4 per mil) and at the southern area (-2.0 to 7.9 per mil). The B-type ores from the Quechisla-Sur Lipez district (southern portion of the Cordillera) have δ³⁴S values between -7.9 and 3.9 per mil, which are similar to those obtained from the southern portion of the Oruro-Potosi district.<BR>The isotopic geothermometry using sphalerite-galena pairs shows rather good agreement with homogenization temperature of the fluid inclusions. Thermochemical caluculation shows that sulfide minerals of these ore deposits had precipitated from the solution in which dominant sulfur species is H₂S. The δ³⁴S values of sulfide minerals correspond to those of bulk isotopic composition in the ore-forming fluid. The wide range of δ³⁴S value can not be explained that the sulfur was derived from a single source. Regional characteristic of sulfur isotopic composition suggests that a part of sulfur was derived from the country rocks. Besides the sulfur of deep seated origin, the biogenic sulfide sulfur in sedimentary rocks, and sulfur which was derived from sulfate minerals near the paleo-surface were responsible for the ore formation.