Neoarchean orogenic gold-only deposits in the Dharwar Craton occur in wide variety of host rocks such as metamorphosed mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. The characteristic features of these deposits are (i) greenschist to mid-amphibolite facies conditions of metamorphism; (ii) alteration and mineralization in shear-induced higher order structural splays, and (iii) low salinity aqueous-gaseous (H2O+CO2?CH4+NaCl) fluids that precipitated gold in a narrow P-T window of 0.7-2.5 kbar and 215-320 ?C. There are also reports of uncharacteristic high salinity granitic fluids from surface samples of Ramagiri. Further, propositions favoring mantle- and granite-derived fluids have been made from the calculated fluid ?18O-?D and ?13C values of quartz and calcites. Major, trace element and B-isotope composition of tourmaline, on the other hand, suggest metamorphic ore fluids, with minor magmatic involvement. Similarly, composition of scheelite, pyrite and arsenopyrite from the ore zones suggest operation of fault-valves resulting in fluid phase separation, that acted as the dominant process of gold precipitation, besides fluid-rock sulfidation reactions. Available ages of greenstone metamorphism, granitic activity and mineralization in the Hutti Belt are almost identical, making a geochronology-based resolution of the existing debate on the metamorphic vs. magmatic fluid source impossible. On the other hand, consistently observed low salinity nature of the ore fluid points to its unlikely extraction from granites, emplaced at mid to upper crust. On the contrary, fluids released by metamorphism of pyritic carbonaceous shales at the greenschist-amphibolite facies boundary are characteristically of low salinity and have elevated concentrations of S, Au and As, necessary to form orogenic gold deposits.
Gold in Dharwar Craton: Current Genetic Understanding
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Neoarchean orogenic gold-only deposits in the Dharwar Craton occur in wide variety of host rocks such as metamorphosed mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. The characteristic features of these deposits are (i) greenschist to mid-amphibolite facies conditions of metamorphism; (ii) alteration and mineralization in shear-induced higher order structural splays, and (iii) low salinity aqueous-gaseous (H2O+CO2?CH4+NaCl) fluids that precipitated gold in a narrow P-T window of 0.7-2.5 kbar and 215-320 ?C. There are also reports of uncharacteristic high salinity granitic fluids from surface samples of Ramagiri. Further, propositions favoring mantle- and granite-derived fluids have been made from the calculated fluid ?18O-?D and ?13C values of quartz and calcites. Major, trace element and B-isotope composition of tourmaline, on the other hand, suggest metamorphic ore fluids, with minor magmatic involvement. Similarly, composition of scheelite, pyrite and arsenopyrite from the ore zones suggest operation of fault-valves resulting in fluid phase separation, that acted as the dominant process of gold precipitation, besides fluid-rock sulfidation reactions. Available ages of greenstone metamorphism, granitic activity and mineralization in the Hutti Belt are almost identical, making a geochronology-based resolution of the existing debate on the metamorphic vs. magmatic fluid source impossible. On the other hand, consistently observed low salinity nature of the ore fluid points to its unlikely extraction from granites, emplaced at mid to upper crust. On the contrary, fluids released by metamorphism of pyritic carbonaceous shales at the greenschist-amphibolite facies boundary are characteristically of low salinity and have elevated concentrations of S, Au and As, necessary to form orogenic gold deposits.
Publication Mode |
Online |
---|---|
Publication Author |
Biswajit Mishra |
Publication Language |
English |
Publication Type |
Conference Paper |
Publication Year |
2021 |
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