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LARGE-SCALE FLUID FLOW SYSTEMS IN THE CRUST AND THEIR IMPLICATIONS FOR METALLOGENESIS OF HYDROTHERMAL ORE DEPOSITS

Ma Dong-sheng   

  1. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing, 210093
  • Received:1998-09-20 Revised:1998-09-20 Online:1998-09-20 Published:1998-09-20

Abstract: In the late 1980s, the study on geochemistry of gold and Au deposits in the northwestern Hunan, China revealed gold depletion up to more than one thousand km2 around the Woxi Au- Sb deposit in the Proterozoic strata which contro1 most Au(Sb)deposits in region. Together with other evidences, such as regional distribution patterns of Sb, Sr and Hg in the strata, it was explained as a result of large- scale fluid flow leaching the ore- forming materials from the strata and focusing them into the fossil fluid pockets and now expressed as gold deposits. Actually, the researches on the large- scale or regional scale fluid flow have made considerable progress in the world since 1980s. It has be determined, hydrogeo1gists as pioneers in this field, that groundwater and oil- gas can transfer up to several hundreds and even to thousand km, and several numeric models for genesis of hydrothermal ore deposits have be developed in which large- scale fluid flow is emphasized. Present paper reviews recent advances and tendencies in research of large- scale fluid flow through the continenta1 crust, discusses the models and mechanisms of the flow systems and their implications for metallogenesis of hydrothermal ore deposits. The reservoirs of total aqueous fluids in the crust are very close 1o the ocean in volume, and the uppermost ten km of the continental crust is ubiquitously saturated with aqueous fluids having high activity. According to driven forces, three flow systems occur in the crust: the gravity-, buoyancy-, and stress- driven system. The last two can be divided into four subtypes or subsystems, respectively, the therma1 and density- differential convection- driven, and the tectonic and compactional stress- driven. The large- scale fluid flow not only has a long distance, but also bears impressive quantities of fluids to form hydrothermal ore deposits. Via studies of Woxi Au-Sb deposit and Xikuangshan Sb deposit in Hunan, it is conservatively estimated that the mass of fluid 1o move the tonnage of gold and antimony (which equals the known reserves) to the deposit would be at lest of the order of 10^16 g. and that to form the regiona1 gold- depleted zone around Woxi Au-Sb mine be 5.2 × 10^18g. Therefore, 1arge- scale fluid flow with high flux and high water/rock ratio should play an important role to form hydrothermal ore deposits, especially for those elements with 1ow abundance and concentrated under 1ow temperature such as Au, Sb, Hg, As etc. The paper suggests that the regional geochemical background of palaeofluid field involving large- scale fluid flow might be a key link to integrally understand hydrothermal ore-forming processes, and research of large- scale fluid flow in the crust wil1 become one of the main currents of geologic fluid study in the future.