VARIATIONS OF SULFIDE SULFUR ISOTOPE COMPOSITION AT VOLCANO-SEDIMENTARY ORE FORMATION AND ORE METAMORPHISM OF OZERNINSK ORE FIELD, WESTERN TRANSBAIKAL REGION

The Ozerninsk ore field is a unique object with which to study the thr ee-dimensional distribution of sulfur isotope compositions of ore mine rals, and associated variations, in order to solve the problems of hyd rothermal-sedimentary ore formation. The Ozerninsk field is located in a remnant of the Lower Cambrian volcanosedimentary rocks over an area of 200 km(2) in the Middle Proterozoic granitoids within the western Transbaikal region and combines more than 25 deposits and occurrences of hydrothermal-sedimentary sulfide polymetallic and sulfide-iron oxid e ores, including the large Ozernoe massive sulfide polymetallic depos it. The remnant encloses an intra-island are trough area, in which loc al basin multilevel stratiform deposits of rhythmically layered fine-g rained ores were formed. The rocks and ores are slightly metamorphosed to the lower greenschist facies. The processes of contact and hydroth ermal metamorphism are locally displayed. Within the ore field, the re gular distribution of sulfide sulfur isotopes (270 analyses), both in the section of the ore-bearing sediments about 1500-m-thick and over t he lateral, has been identified. The sulfur isotope composition weight increases from the lower ore levels to the upper most productive hori zons, and from the flanks to central zone of the intra-island are trou gh. Sulfur from the lower ore stratified levels is close to the meteor ite standard, or reveals a wide range of isotopic values (delta(34)S f rom -7.4 to +21.2 parts per thousand), testifying to the processes of biogenic sulfate reduction. Economic ore bodies of the Ozeminsk deposi t confined to the upper level are characterized by homogeneous heavy s ulfide sulfur with a mean value of delta(34)S + 14 parts per thousand. The sulfide-iron oxide ores of proximal facies on the flanks of the i ntra-island are trough have a sulfur isotope composition approaching t he meteorite standard, whereas for sulfide ores of the axial zone host ed by tuffaceous-carbonate rocks, heavy sulfur is common. Variations o f sulfur isotope composition in space and time are due to the particip ation of different sulfur sources in the process of ore formation: (1) endogenous sources (magmatogenous) and (2) biogenic sources, which do minated at the early stages of ore deposition, as well as (3) the sulf ate sulfur of seawater, which was involved in the ore-forming system a t the late and most productive stage (abiogenic sulfate reduction). Th e increase of the sulfide sulfur isotope value is recorded in the proc esses of local contact metamorphism and hydrothermal recrystallization . The sulfide sulfur of hydrothermal-sedimentary ores with delta(34)S around +14 parts per thousand during the contact-thermal and hydrother mal metamorphism becomes heavier (to 20-25 parts per thousand) and, wi th the primary isotope composition close to the meteorite standard, ap proximates at 5-10 parts per thousand at contact-metasomatic alteratio ns.