RE-OS DATING OF THE MALANJKHAND CU(MO) DEPOSIT IN CENTRAL INDIA: TRACKING FORMATION AND DEFORMATION DURING THE LATE ARCHEAN-EARLY PROTEROZOIC

The origin of the large Malanjkhand Cu(Mo) ore deposit (100 mt @ >1% Cu) has been attributed to several different models (porphyry, lode type, sedex), and its age debated as Archean or Proterozoic (Sarkar et al. 1996, Mineralium Deposita 31: 419-431). The deposit is situated in a highly deformed composite granodiorite-tonalite batholith with reported ages (Rb-Sr, K-Ar) spanning the Early Proterozoic. Much of the deposit is contained in a sheeted “quartz reef” with subordinate mineralization occurring in small veinlets, as disseminations, and as rims on dynamically recrystallized quartz and feldspar grains within altered granitoid rocks. Based on geologic relationships, mineralization in the quartz reef is younger, as patches of host granitoid occur within the quartz reef. Re-Os dating of molybdenite reveals the age of mineralization and documents a 40 m.y. period of deformation recorded in the deposit. Two different hand samples of granitoid-hosted rim/disseminated molybdenite both yielded 2493 ± 8 Ma (two-sigma). One sample of quartz reef molybdenite provided an age of 2494 ± 8 Ma and a late molybdenite vein cutting both granitoid and quartz reef provided an age of 2492 ± 8 Ma. These age data, together with the geologic relationships, lead us to conclude that the dated mineralization was part of a large porphyry Cu(Mo) event that included mineralized siliceous zones and vein stockwork. Furthermore, high Re concentrations in the Malanjkhand system (400-650 ppm) support a subduction-related porphyry model that includes mantle involvement (Stein et al. 2001, Terra Nova 13: 479-486). Two additional samples of molybdenite from the quartz reef, however, yield two distinctly younger ages. Both younger ages were reproduced with a second molybdenite separate from each sample. One reef molybdenite sample yielded 2479 ± 8 and 2474 ± 9 Ma and the other 2449 ± 8 and 2455 ± 8 Ma. We suggest that roughly coincident with the formation of the Malanjkhand porphyry Cu(Mo) deposit at about 2493 Ma, brittle-ductile deformation of the deposit commenced, configuring much of the present quartz reef. Protracted deformation was recorded by the continued (episodic?) deposition of new molybdenite from local Mo-enriched fluids. Re-Os dating may be an essential tool for identifying and dating highly deformed Proterozoic porphyry Cu(Mo) deposits.