Abstract
Pilbara Craton, Western Australia, consists of two different tectonic components; an older Archean granite-greenstone basement formed 3.6 and 2.8 Ga, occupying the east Pilbara and an unconformably overlying Archean-Proterizoic volcano sedimentary sequence called the Mount Bruce Supergroup occupying the Hamasley Basin. Pilbara Craton is one of the most important regions in the world to understand the evolution of early Earth, because the geological history is well preserved through the metamorphism of low grade. In the east Pilbara, the most conspicuous structures are broad domal granitoid complex separated by narrow synformal greenstone belts, and a model of continuous lithostratigraphy in the greenstones in which the dominant structures were produced by multi-stage granitoid diapirism. The Marble Bar greenstone belt is distributed around the Mount Edgar of granitoid pluton, and numerous hydrothermal gold veins are distributed in the greenstone near the boundary of pluton. Also base-metal veins and volcanogenic sedimentary type deposits are located in the same area. In this study, we examined the hydrothermal mineralization observed in the core samples of the Marble Bar greenstone belt, drilled at the Salgash area by the Archean Biosphere Drilling Project (ABDP).The Salgash drill hole is composed of tuff breccias with numerous fragments of black shale of 100 m in thickness, alternation of sandstone and shale of 40 m in thickness, basaltic lava and tuff of 30 m in thickness, and shale and sandstone of 110m in thickness with some sills of basalt and ultramafic rock. The rocks had been metamorphosed, and the grade is near the boundary between green schist and amphibolite facies. Low REE content (43 to 88 ppm), low La/Yb ratio (6.3 to 14.3), and high Eu/Eu* ratio (0.9 to 1.3) of the volcanic rocks are ordinal as the basaltic rock in Archean greenstone belts. On the other hand, these rocks show extremely high values of Cr (1500ppm), Ni (700ppm), Co (70 ppm), and Zn (600 ppm). The C isotopic ratios of carbonate in the volcanic rocks are around –3.8 permil. The clastic sediment sandstone and black shale show similar features, although the absolute values are slightly different. These data suggest that the clastic sediments had drived mainly from the volcanic rocks. The C isotopic ratios of organic carbon in the black shale are from –26 to –30 permil.Horizontal veins are often recognized in the clastic sediments, and are composed of quartz, calcite, sphalerite, pyrite, arsenopyrite, pyrrhotite and chalcopyrite. The C isotopic ratios of vein carbonates are from –4.6 to –5.2 permil. The S isotopic ratios of sulfide minerals are concentrated from –1.5 to +0.9 permil. The homogenization temperature of fluid inclusion in quartz vein ranges from 150 to 250 degree with the average of about 200 degree centigrade. And the fluids are composed of H2O, CO2, CH4 and C.
