A preliminary study on micro-disseminated sediment-hosted gold in the Youjiang basin, South China.

Abstract

In the Youjiang basin, South China, numerous sediment-hosted micro-disseminated gold (SMG) deposits were found in Upper-Paleozoic to Triassic calcareous siltstone, mudstone, sandstone and chert breccias, and usually regarded as Carlin-type gold. Ore bodies show remarkable stratabound features, and occur often in transitional zone between shallow water facies and deep-water facies of the basin, particularly on marginal slope of submarine carbonate highs constrained by syndepositional faults. In ores and in hostrocks there are abundant synsedimentary-syndiagenetic fabrics of sulfides such as lamination, convolute bedding, slumping texture, diagenetic crack, soft deformation etc, indicating syndepositional faulting and diagenetic compaction. Abundant fluid-escape and liquefaction fabrics in ores and hostrocks imply strong fluid migration during sediment basin evolution. High content of organic matters and various kinds of biogenic-organogenic fabrics in ores imply a probable genetic connection between ores and sedimentary organic matters. Mineral associations (pyrite, arsenopyrite, realgar, cinnabar, stibnite, chalcedony) as well as alteration assemblages are typically low-temperature. Ore and its sedimentary hostrock shows similar trace element, REE and S isotope features implying that both of them are probably product of similar processes, with no evidence for a strong later (epigenetic) material-introduction after sediments were lithified. C-O isotope study reveals that CO2 in ore-forming fluids might be mainly produced by dissolution of carbonates in surrounding sedimentary rocks. All these imply a close relationship between SMG ore-formation and basin evolution, and are inconsistent with the popular genetic models of Carlin-type gold, because Carlin-type gold is regarded as a typical epigenetic hydrothermal deposit with little genetic connection to basin evolution.