3-D Mohr circle construction using vein orientation data from Gadag (southern India) - Implications to recognize fluid pressure fluctuation

In this paper orientations of quartz veins from the Archaean age lode-gold bearing region of Gadag (southern India) are used to determine the relative stress and fluid pressure (P-f) conditions by constructing 3-D Mohr circle. Anisotropy of magnetic susceptibility CAMS) analysis of the host massive metabasalt reveals that the magnetic foliation is NW SE striking, which is related to early NE-SW compression (D1/D2 deformation) that affected the region. The quartz veins have a wide range of orientations, with NW SE striking veins (steep northeasterly dips) being the most prominent. Vein emplacement is inferred to have taken place under NW-SE compression that is known to have caused late deformation (D3) in the region. It is argued that the NW-SE fabric defined the pre-existing anisotropy and channelized fluid flow during D3. The permeability was initially low, which resulted in high P-f (>sigma(2)). 3-D Mohr circle analysis indicates that the driving pressure ratio (R') was 0.94, a condition that favoured fracturing and reactivation of fabric elements (foliations and fractures) having a wide range of orientations. This led to an increase in permeability and fluid flowed (burped) into the fractures. Resulting vein emplacement and sealing of fractures led to a reduction of P-f (sigma(2)). Thus, it is concluded that the quartz veins in the Gadag region are a consequence of an interplay between conditions that fluctuated from P-f > sigma(2) to P-f < sigma(2). (C) 2013 Elsevier Ltd. All rights reserved.