Genesis of the late-stage base metal sulfide mineralisation and its relationship with the regional deformation in Ambaji-Deri, South Delhi Terrane, North-West India

Proterozoic stratiform VMS-type Ambaji-Deri Zn-Pb-Cu deposits in the Aravalli-Delhi mobile belt of North-West India are hosted within meta-sedimentary sequences of the South Delhi Terrane. Several fracture-filled discordant late-stage base metal mineralised quartz veins have intruded along with the fractures and foliation at very high P-f (sigma(2) < P-f < sigma(1)) with typical hydrothermal alteration zones. Temperatures estimated from hydrothermal chlorite vary from 226 to 293 degrees C (mean. 260 degrees C). Ar-40-Ar-39 dating of hydrothermal muscovite yielded a plateau age of 625.5 +/- 3.3 Ma. Detailed fluid inclusion (FI) analysis reveals three types of primary and four types of secondary FI. Primary H2O-NaCl-CO2 +/- N-2 type 1a FI has medium to high salinity (11.21-21.96 wt% NaCl equivalent) and high density (0.90-1.04 g/cm(3)), while the secondary type 1b FI with same compositions are characterized by medium salinity (similar to 12.6 wt% NaCl equivalent) and lower density (0.89-0.90 g/cm(3)). Phase separation/fluid immiscibility is the primary mechanism of mineralisation in the Ambaji-Deri that caused the original fluid to evolve separately as gaseous (CO2) type 2a, b and aqueous type 3a, b (H2O-NaCl) with similar salinity and density for the aqueous inclusions and low density for the gaseous inclusions (lowering up to 0.58 g/cm(3)). The ore-forming fluid with magmatic signatures possibly originated from the cooling G3 granites, remobilized the pre-existing metals, and precipitated at similar to 265 degrees C and similar to 1 kbar at the epizonal depths in the dilated fractures. Later these fluids are mixed with meteoric water and evolved into low saline aqueous fluids that produced supergene alterations.