Geochemical Signature of PGE Mineralization in the Torappadi Ultramafic–Mafic Complex, Southern India

The Torappadi Ultramafic–Mafic Complex (TUC) in the Southern Granulite Terrain (SGT) is mainly composed of pyroxenite (websterite) and gabbro along with their variants—orthopyroxenite, diopsidite, gabbroic anorthosite and anorthositic gabbro was investigated for PGE mineralization. Orthopyroxene (enstatite), clinopyroxene (chrome diopside), plagioclase feldspar (An57–73%) and primary hornblende (magnesio-hornblende) are the major minerals largely present in TUC in the order of decreasing abundance. The MgO values in enstatite rich pyroxenite ranges between 17.15–30.78 wt%, it varies from 25.21 to 32.19 wt% for chrome diopside rich pyroxenite while the MgO wt% in gabbro ranges between 10.5 and 25.5. A total of 145 rock samples from pyroxenite (81), ortho-pyroxenite (13), diopsidite (7) and gabbro (44) were analysed for Platinum Group Elements (PGE). The total PGE (ΣPGE) in the pyroxenite (N = 94) varies from 16.2 to 380.5 ppb, it ranges between 14.1 ppb and 117.4 ppb in gabbro (N = 44) while the diopsidite (N = 7) analyzed from 54.7 to 272.7 ppb. The PGE mineralization in TUC appears to be lithologically controlled. However, the elevated ΣPGE content around fold hinges (F1 and F2) provides evidence limited re-distribution of PGE during deformational event. Wide range of ΣREE (8–1021 ppm), LaN/SmN (0.58–6.10), and GdN/YbN (0.08–6.69) ratios, indicate significant post magmatic deformation assisted fluid–rock interaction. The primitive mantle-normalized Ni–Cu–PGE patterns of the pyroxenites and gabbro of TUC indicate that these rocks were formed from an evolved mafic magma that experienced early silicate fractionation and sulfide saturation. The ratios of Pd vs. Cu/Pd, and Cu vs. Pd, suggest that the TUC sequence might have been evolved through mantle derived, episodic magmatic pulses with early-stage sulphur fractionation.