Polygenetic origin of coticules and tourmalinites associated with the Sakoli VMS deposits, Central India

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Bandyopadhyay, Bimal Kumar
Holding Date 07 October 2008

Coticules (spessartine-quartz rocks) and tourmalinites occur as minor rocks in the volcano-sedimentary Palaeoproterozoic Sakoli Group in central India. They either host metamorphosed sulphide ores or occur within, above or beneath them. Three types of coticules, namely C1 (massive, stratabound associated with stringer-type copper ores in the felsic metavolcanics), C2 (bedded, hosting the distal facies stratiform zinc deposit) and C3 occuring as small pods and tectonic boudins in the hangingwall tuff-carbon phyllite units of the stratiform zinc deposit) are found. Minor intervals of stratabound and stratiform tourmalinites also occur in the pelitic schists, felsic and mafic metavolcanic rocks, and locally are associated with stratabound volcanogenic sulphide deposits. The tourmalinites are folded by the earliest fold set (F1), and in hand specimen locally display small-scale isoclinal folds and a well-developed crenulation cleavage. Microprobe analyses of garnet grains in the coticules indicate that their composition varies between spessartine and almandine with general mol. percent composition of Alm44-53 Gro14-19 Py1-3 Sp30-43 and Alm33-40 Gro6-12 Py1-2 Sp46-56 for C1 and C2 respectively. Both C1 and C2 garnets are chemically zoned with respect to Mn and Fe and less so for Ca and Mg. Whole-rock chemical analyses of the coticules show high but variable contents of SiO2, A12 O3, Fe2O3T , MnO (upto 19.08 wt% ) and CaO and low contents of alkalies and MgO with Fe/Mn ratios ranging from 0.8 to 12.25. The Cu + Co + Ni contents of these rocks are low and suggest a hydrothermal origin; plots of A12 O3 vs. SiO2 and Fe/Ti vs. Al/(Al+Mn+Fe) indicate the presence of terrigenous or pelagic components in the source rocks. Ternary plots of A12 O3 - TiO2-Mn, Fe and Si and A12 O3 - Fe2O3T -Mn, Ti and Si in these rocks suggest incorporation of pyroclastic materials into the coticule precursors. Chondrite-normalised REE plots of these rocks are generally enriched in LREE and the sample/chondrite ratio of most elements is less than 150. C1 shows distinct negative Eu anomaly, relatively low content of LREE and a small positive Ce anomaly. C2, on the other hand, lacks both Eu and Ce anomalies. Whole-rock chemical analyses of the tourmalinites show contents of SiO2, Al2O3, Fe2O3T. MgO, and TiO2 very similar to those of the pelitic schists. Boron isotope compositions of tourmaline separates suggest a boron source dominantly from the sedimentary precursors of the local pelitic schists.
The geological setting, rock types, sulphide mineralisation, chemical and isotope data are most consistent with a mixed polygenetic origin of the precursors of the coticules. They also suggest that the tourmalinites formed by selective replacement of footwall volcano-sedimentary package from the same submarine hydrothermal systems that deposited the massive sulphide deposits, coeval with sedimentation and volcanism of the Sakoli Group