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)

Abstract

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 (Pf) conditions, as well as stress state during Pf fluctuation. To scale the 3-D Mohr circle, vein orientation data are integrated with (a) available estimates from fluid inclusions of highest recorded Pf (390 MPa) and lowest recorded Pf (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 Pf. Of 157 veins investigated here, 14 veins are identified as having favourable orientation for dilation at high as well as low Pf. These 14 veins have a mean strike of 150°, which is similar to the orientation of the gold-bearing quartz lodes reported in the region. The effective normal stress (σ′n = σn−Pf) prevalent during dilation of fracture/fabric anisotropy with 150° strike is calculated to be −11.5 MPa at high Pf, and −1.0 MPa at low Pf. Thus, it is interpreted that in the Gadag region, a change in σ′n of 10.5 MPa prevailed during Pf fluctuation and associated separation of gold from the fluid.