Shergaon is one of the key base metal projects of the AIR Group. Shergaon PL block covers an area of 24 sq km within Proterozoic metasedimentary sequence of Arunachal Pradesh. The lithological sequence exposed in the project area is made up of quartzite, phyllites, conglomerate and dolomites, which belong to Rupa Group. These sedimentary rocks occur as a thrust sheet between lower Proterozoic Bomdila Group gneisses and Bichom Group metasediments. The mineralized dolomite, which belongs Chillepam Formation, occurs as interbedded unit within phyllites, quartz-biotite schist and quartzites. The P.L. blocks covers the two major folded and dislocated dolomite bands located in the mountainous ridge northeast of Shergaon. Among the two bands the lower or western band is mineralized and constitutes the target of exploration for its Pb-Zn mineralization. Geological Survey of India carried out preliminary exploration which includes few drill holes, which yielded 1-12.6% Zn, 1-17.6% Pb and ~70 g/t of Ag. GSI’s exploration revealed the presence of 7km long mineralized dolomite band with thickness varying from 50 to 150m.
TMRPL carried out geological mapping, rock and channel sampling from east of Shergaon to south of Mashaksing, over a strike length of over 10 km. The mineralized dolomite is smoky grey, massive, fine- to medium- grained, hard and compact in nature. It is folded, fractured and traversed by sulphide-rich quartz stringers. The two sub-horizontal mineralized dolomite bands exhibit a synformal closure to the east of Shergaon, enclosing a broad basin of barren dolomite in the middle.
The folded mineralized dolomite band has thickened closure regions (up to 150 m) with richer concentration of mineralisation and thinned (20 m) barren or poorly mineralized limbs. A few fresh samples studied under microscope have revealed the presence of galena, sphalerite, boulangerite, cerussite and Anglesite. Minor argentite also occurs as exsolution lamellae in galena crystals. Paragenetic sequence proposed for sulphides is:
Samples collected from various parts of the 50-150 m thick mineralized dolomite band contain significant zinc and lead mineralization ranging from <1% to 45.34% and <1 to 22.88% respectively. The Pb-Zn mineralization is associated with 3-49 ppm of Ag. However, distribution of mineralization is quite erratic and patchy – indicating that though it is strata-bound, but not stratiform. The localization of lead or zinc in certain fracture planes or contact zones reveals that structurally controlled remobilization of sulphides in Shergaon area, wherein fine- grained proto-ore is possibly recrystallised during metamorphism into localized pockets of stout crystal aggregates. A couple of gossanized dolomite samples from south of Mashaksing have indicated 62.40 & 60.49 ppm silver content with depleted Pb-Zn contents.
Further detailed channel sampling and drilling to evaluate the overall average grades and depth persistence is planned.
Google image with mineralized dolomite layers