Estimation of paleostress from pore fluid pressure of the quartz veins and its significance in the Cu-Pb-Zn mineralization (Ambaji, Aravalli-Delhi mobile belt, NW India)

Quartz veins are produced from the crystallization of the last silica enriched hydrothermal phase from granitic magma circulating along the pre-existing fracture of rock. In many instances, these hydrothermal fluid act as a carrier for the ore minerals. The intrusion of quartz veins along fractures depends upon the tectonic stress conditions in the area. Fluid pressure (P_f) of these ascending liquids should be higher than the normal compressive stress (σ_n) to dilate the fractures. We are studying the quartz vein intrusion in the Cu‒Pb‒Zn mineralization belt of Ambaji, South Delhi terrane, Aravalli- Delhi mobile belt, NW India. The host rocks include mica schist, amphibolite, calc schist, talc tremolite schist, and four phases of granite intrusion (G₀‒G₃). The age of G₀, G₁, G₂ and G₃ granite are 960, 860, 800, and 750 Ma respectively. The rocks underwent three phases of folding (F₁‒F₃) and show greenschist to amphibolite facies metamorphism. The quartz vein intrusion is related to syn to post F₃ folding and G₃ granite magmatism. This final phase hydrothermal fluid extremely altered host rock and formed biotite-tourmaline-quartz and tremolite-actinolite-talc-chlorite greisen along the contact. The greisen host chalcopyrite-pyrite-galena-sphalerite mineralization suggesting the ore minerals were transported by the quartz vein. Vein orientation, stress condition, fluid pressure fluctuation, and fluid temperature can decide the fracture dilation and mineralization processes. Therefore, this work concentrates on the geometrical distribution of the vein orientation data. From this we deduced (i) girdle distribution pattern of vein data  (ii) σ₁ = 120º/75º, σ₂ = 052º/07º, σ₃ = 323º/07º indicate maximum extension was NW-SE and σ₁σ₂ plane strikes was N52ºE, (iii) θ₂ =12º, θ₃ = 40º  and (iv) R'(driving pressure ratio) = 0.95, ϕ (tectonic stress ratio) = 0.90 indicates high value for R' leading to dilation of wide range of fractures. Further, the high ϕ value suggests uniaxial extension. Microscopic petrography of fluid inclusions shows three generations of inclusion like primary inclusion, secondary inclusion, and pseudosecondary inclusion. Most of the inclusion has aqueous and vapour phase and some inclusions show solid halite phase. We observed different types of trail bound of inclusion like intragranular inclusion, intergranular inclusion and transgranular inclusion, which suggest deformation and recrystallization in the rock. We are studying microthermometry analysis of fluid inclusion present in the quartz vein and trying to estimate the fluid pressure. With the help of fluid pressure, the 3D Mohr circle will be constructed and paleostress will be quantified. That will help in understanding the stress condition and mineralization in the rock.

Keywords: Veins, Fractures, Paleostress, 3D Mohr Circle, Mineralisation, Fluid Inclusion, Microthermometry