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Geology, Geochemistry and Geophysics of Sedimentary Rock-Hosted Au Deposits in P.R. China
Edited by Stephen G. Peters1
Open-File Report 02�131
Version 1.0
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This is the second report concerning results of a joint project between the U.S. Geological Survey and the Tianjin Geological Academy to study sedimentary rock-hosted Au deposits in P.R. China. Since the 1980s, Chinese geologists have devoted a large-scale exploration and research effort to the deposits.
As a result, there are more than 20 million oz of proven Au reserves in sedimentary rock-hosted Au deposits in P.R. China. Additional estimated and inferred resources are present in over 160 deposits and occurrences, which are undergoing exploration. This makes China second to Nevada in contained ounces of Au in Carlin-type deposits.
It is likely that many of the Carlin-type Au ore districts in China, when fully developed, could have resource potential comparable to the multi-1,000-tonne Au resource in northern Nevada.
Click here - To read more about the USGS' report (Open-File Report 02�131 ) on the Carlin-type Gold Deposit Potential of Southern China
Introduction to and Classification of Sedimentary Rock-Hosted Au Deposits in P.R. China
By Stephen G. Peters, Huang Jiazhan, Li Zhiping and Jing Chenggui
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Chinese sedimentary rock-hosted Au deposits are located along margins of the Precambrian Yangtze craton in the Qinling fold belt area in the north and northwest, the Dian-Qian-Gui area in the southwest, and the Middle-Lower Yangtze River area in the east. Regional-scale faults control their distribution, whereas secondary structures, such as short-axial anticlines (domes), district-scale high-angle faults, stratabound breccia bodies, and unconformity surfaces, control ore at the district and orebody scales. Deposits are hosted in Paleozoic to Mesozoic sedimentary rocks composed mainly of impure limestone, siltstone, and argillite. Alteration types are silicification, decalcification, argillization, carbonization, and locally albitization. Igneous intrusions usually are not present near most Chinese sedimentary rock-hosted Au deposits, except for local lamprophyre, granodioritic, and siliceous dikes. However, the Middle-Lower Yangtze River area contains many pluton-related Au deposits.
The main deposit types are Carlin-type, pluton-related, syndeformational, unconformity-hosted, and red earth and laterite Au deposits. Gold mainly is present as,disseminations in most sedimentary rock-hosted Au deposits, although local accumulations of Au-bearing massive sulfide are present, especially in polymetallic replacement mantos in the Middle-Lower Yangtze River area. The main opaque minerals include Au, electrum, pyrite, arsenopyrite, stibnite, orpiment, realgar, and cinnabar. Gangue minerals are quartz, barite, organic carbon, carbonate and clay minerals, as well as some albite. Elements associated with Au in Nevada deposits, such as As, Sb, Tl, and Hg, also are closely associated with many Chinese deposits, but U, Sr, and some PGE elements also are associated with the Au deposits.
What is a Carlin-type Ore Body?
Taken from: Comparison of Carlin-type Au Deposits in the United States, China, and Indonesia: Implications for genetic models and exploration
By Albert H. Hofstra and Odin D. Christensen
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What is a Carlin-type deposit? Those in Nevada, USA, are epigenetic disseminated auriferous pyrite deposits that are typically hosted in calcareous sedimentary rocks. Gold is present as submicron inclusions or solid solution in As-rich pyrite. The Au deposits formed where ore fluids moved up crustal structures, met less permeable cap rocks, and moved laterally into more permeable and reactive carbonate rocks. The relative importance of structural and stratigraphic controls on ore vary considerably from deposit to deposit. The roots of the ore fluid systems are difficult to trace and somewhat cryptic, suggesting that we have more to learn about their hydrology. Their common alteration, mineralogy, and geochemical signatures are a direct expression of the physical and chemical environment in which they formed. General absence of boiling and presence of mineralized rock over vertical intervals of up to 1 km argues against a shallow depth of mineralization. Geologic reconstructions and fluid-inclusion data suggest that most Carlin-type Au deposits formed at depths of 2 to 5 km and at temperatures of about 200 �C. They formed from low salinity, CO 2 and H 2 S-rich fluids that were moderately acidic and reduced. These fluids dissolved carbonate minerals, argillized silicate minerals , and sulfidized Fe-bearing minerals in the host rocks. Where there was sufficient cooling, the rocks also were silicified. Gold, As and other sulfide-complexed trace elements precipitated in pyrite as H 2 S was consumed by reaction with Fe-bearing minerals. As a consequence, Fe generally is not introduced. Late ore stage quartz, calcite, barite, orpiment, realgar, and stibnite precipitated in open fractures and pores as the systems cooled and ore fluids mixed with local ground water.
Why is there so much confusion as to what a Carlin-type deposit is? There are four main reasons: (1) they are present in regions that also contain sedex, pluton-related, and epithermal sedimentary rock-hosted Au deposits ; (2) Carlin-type Au deposits yield conflicting indications as to the source of ore fluids; (3) in several districts, they lie in or adjacent to long-lived, deep-penetrating crustal structures that localized ore deposits of different types and ages. In the Carlin trend, Nevada each of these events deposited some Au, and in places, different types of Au mineralization are superimposed. This association is probably telling us something very important about their genesis; and (4) primary relationships are obscured by supergene weathering and oxidation. Unfortunately, most early studies did not recognize this complexity, which resulted in spurious interpretations as to the age and genesis of the deposits.
Click here - To read more about the USGS' report (OP-98-466) on the Carlin-type Gold Deposit Potential of Southern China (314 KB with links to 7.3 MB, Zhiping Li and Stephen Peters)
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