Study of Thorium Mineralization in Choghart Iron Oxide-Apatite Deposit, Bafq District, Central Iran

Early Cambrian rhyolite hosts the Choghart iron oxide-apatite (IOA) deposit. The ground radiometric and spectrometric surveys of alteration zone on the margin of the magnetite-apatite ore body show that the radioactive anomaly of thorium occurs in the breccia zone. The chemical analyses of the breccia zone samples by ICP–MS show thorium mineralization. The mineralogical studies by transmitted- and reflected-light microscopey and EPMA indicate that the main thorium minerals in thorium mineralization zone are thorite and sphene. The alteration mineral assemblages related to thorium mineralization of Choghart is consisted of albite ± orthoclase + calc-silicate (actinolite- augite- diopside) + carbonate (calcite) + magnetite + pyrite ± chalcopyrite ± galena + sphene + rutile ± microcline ± apatite (Na-Ca-Fe alteration). The occurrence of paragentic magnetite, pyrite and chalcopyrite with thorite and negative Eu anomaly in the thorium mineralization zone indicate a reduced condition for thorium mineralizing fluids. The similarity in chondrite- and mantle-normalized REE patterns of host rhyolite and the thorium mineralization zone suggests that thorium is originated from continental-arc rhyolitic magma.