Scaling the 3-D Mohr circle and quantification of paleostress during fluid pressure fluctuation - Application to understand gold mineralization in quartz veins of Gadag (southern India)

In this study, orientations of 157 quartz veins occurring in metabasalts of the Gadag region (Dharwar craton, southern India) are used to plot the 3-D Mohr stress circle, which provides information about relative stress/fluid pressure (P-f) conditions, as well as stress state during P-f fluctuation. To scale the 3-D Mohr circle, vein orientation data are integrated with (a) available estimates from fluid inclusions of highest recorded P-f (390 MPa) and lowest recorded P-f (50 MPa) and (b) intrinsic rupture criterion that empirically quantify rock properties. Based on the scaled 3-D Mohr circle, the absolute magnitudes of the three principal stresses are quantified for high and low P-f. Of 157 veins investigated here, 14 veins are identified as having favourable orientation for dilation at high as well as low P-f. These 14 veins have a mean strike of 150 degrees, which is similar to the orientation of the gold-bearing quartz lodes reported in the region. The effective normal stress (sigma(')(n) = sigma(n)-P-f) prevalent during dilation of fracture/fabric anisotropy with 150 degrees strike is calculated to be -11.5 MPa at high P-f, and -1.0 MPa at low P-f. Thus, it is interpreted that in the Gadag region, a change in sigma(')(n) of 10.5 MPa prevailed during P-f fluctuation and associated separation of gold from the fluid. (C) 2016 Published by Elsevier Ltd.