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

摘要

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 (AMS) 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 (＞σ_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(＜σ_2). It is argued that at this low Pf, NW- SE oriented structures continued to remain susceptible to reactivation and vein emplacement, while fractures of all other orientations were inactive and remained sealed. As a consequence, the study area has a cluster of NW- SE oriented veins. R' is calculated to be 0.07 from 3-D Mohr circle analysis at low Pf, when fractures with NW-SE orientation only were susceptible to dilation. However, it is envisaged that any emplacement of veins in these fractures would have sealed them, thus reducing the permeability and initiating the next cycle of rise in P_f (＞σ_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＞σ_2 to P_f ＜σ_2.