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Contact metamorphic effects of the basic intrusive rocks on the Proterozoic uraniferous dolostone in Cuddaph Basin, Andhra Pradesh; implications on uranium mobilisation



Contact metamorphic effects of the basic intrusive rocks on the Proterozoic uraniferous dolostone in Cuddaph Basin, Andhra Pradesh; implications on uranium mobilisation



Exploration and Research for Atomic Minerals 10: 63-78



Mafic intrusive rocks in the Vempalle Formation of the mid-Proterozoic Cuddapah basin occur as sills and dykes. These include minor bodies of gabbro, olivine gabbro, olivine norite, basalt and mainly dolerite with basaltic andesite. The metamorphic effects of these intrusive rocks on the uraniferous phosphatic siliceous dolostone are mainly mineralogical (thermal) with subordinate changes in chemistry. These are manifested by (a) formation of plagioclase-hornblende hornfels, (b) notable mineralogical changes in the dolostone leading to enrichment of magnetite, epidote, anatase and de-dolomitised calcite, (c) decrease in specific gravity of dolostone from 3.0 to 2.8 due to volatilisation reaction products of epidote and smectite, and (d) formation of wollastonite, chalcedony, and secondary uranium minerals (autunite and uranophane) at places, in the contact aureole that led to notable changes in the chemistry of the intrusive body and the host rock. Intrusive rocks at the contact show enrichment in Fe (super 2+) , Mg, Cu, Cr, Pb, Zn, Ni, and depletion in Ca and Fe (super 3+) , whereas the dolostone shows enrichment in Ti, Ca, and depletion in Si, Al, alkalies and P. Depletion of uranium in the affected parts (0.003 %U (sub 3) O (sub 8) ) of mineralised dolostone (0.062 %U (sub 3) O (sub 3) ) adjacent to the basic intrusive rocks suggests its mobilisation, due to increase in temperature, resulting in baking. This phenomenon is also manifested, at places, in the formation of secondary uranium minerals - result of remobilisation of uranium from primary phases and its subsequent precipitation.

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