Ore Fluid Characteristics of Orogenic Gold Deposits: An Overview

Abstract

Primary gold production in India is only up to 4 tons per annum, barely 0.4% of its annual gold consumption. However, India holds 9% of the available global gold reserves that is estimated at about 14,000 tons (IGCP-540 Report 2011). Currently, in India, primary gold is produced mainly from the world class orogenic-type Hutti gold deposit in the Archaean Hutti-Maski greenstone belt in the eastern Dharwar Craton (EDC). The world famous Kolar gold deposit in the Kolar greenstone belt had produced about 800 tonnes of gold before its closure in 1998. Other gold-hosting greenstone belts in the EDC include Sandur, Ramagiri, Jonnagiri, Kushtagi, Magalur, Gadag, Kadiri, and south Kolar belts. All of these deposits fall into a single type deposit called ‘orogenic’ gold deposits. These are epigenetic, structurally controlled, shear zone-hosted lode gold quartz-carbonate vein systems associated with regionally metamorphosed and deformed terranes. Orogenic gold deposits collectively account for significant amount of world gold (more than 25000 tons gold) production. There have been worldwide intense studies on these deposits, for the past three decades, on various aspects of regional-and mine-scale structures, host rock metamorphism, ore mineralogy, alteration geochemistry and physico-chemical conditions pertaining to nature and evolution of the ore fluid, and transport-precipitation mechanisms of gold. Ore fluid compositions in orogenic gold deposits are very well understood from wide range of fluid inclusion, petrological and geochemical studies. A comprehensive review shows that the nature of ore fluids responsible for Archaean lode-gold systems are generally neutral to weakly alkaline, lowmoderate-salinity aqueous-gaseous with high CO2 (+CH4) (0.050.25 XCO2) for deposits ranging from sub-greenschist to upper-amphibolite facies conditions, capable of carrying Au, Ag, As and Sb but with limited capacity to transport base metals. In spite of all these studies the source of gold-carrying fluids and controls on its composition still remain potential unknowns.