A 2.5 Ga porphyry Cu–Mo–Au deposit at Malanjkhand, central India: implications for Late Archean continental assembly

Abstract

Eight Re–Os ages from six molybdenite samples representative of the Cu–Mo–Au mineralization at the giant Malanjkhand deposit in Madhya Pradesh were obtained using ID-NTIMS. These data provide a clear Late Archean–Early Paleoproterozoic age for this deposit and by implication for its calc-alkaline granitoid host. Among other diverse models, the origin of Malanjkhand has been debated as orogenic lode-style associated with development of a quartz reef in a shear zone or as porphyry-style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle–ductile deformed quartz reef. Previous models included Middle Proterozoic ages for the Cu–Mo–Au mineralization based on poorly constrained Rb–Sr and K–Ar data. Direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 ± 8 Ma (2489.5 ± 1.4 Ma based on regression without uncertainty in the 187Re decay constant, MSWD = 0.5, n = 5). Additional molybdenite from the quartz reef dated at ∼2475 and ∼2450 Ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position. We suggest that the quartz reef is a high-silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north. Petrographic evidence coupled with high Re concentrations for molybdenites (400–650 ppm) support this scenario and suggest that Malanjkhand is a subduction-related stockwork-porphyry-style Cu–Mo–Au deposit involving mantle. Malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes. We suggest that the Malanjkhand Cu–Mo–Au deposit and the surrounding batholith containing multiple Cu–Mo–Au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern Indian craton at 2.5 Ga, as the southern craton began to converge with the northern Indian craton or other protocontinents now embedded or consumed within the central Indian Tectonic Zone (CITZ). This scenario could imply an Early Paleoproterozoic age for the earliest development of the CITZ, at least in the region of the Central Indian Shear/Suture (CIS), whose marked ENE trend imprints the northwestern part of the Malanjkhand batholith and the Cu–Mo–Au deposit. Based on a compilation of ∼2.5 Ga events in both the southern and northern Indian cratons, we suggest that some units in the CITZ linked an extensive and continuous Late Archean–Early Paleoproterozoic orogenic belt that may extend to the Moyar–Bavali, Bhavani, Attur, Palghat, and Cauvery shear zones in southern India, and include inboard units of the Eastern Ghats and the Aravalli–Delhi orogenic belts. Robust geochronology that can be tied to the early history within these orogenic belts is scarce, and extensive reworking during Grenvillian and Pan-African time creates a pervasive overprint at some localities. This Late Archean–Early Paleoproterozoic belt preserves sutures within a coalesced Late Archean continent that included East Antarctica (Napier and Vestfold Hills complexes) at ∼2.5 Ga.