Formation and evolution of Th-REE mineralizing fluids at the Kiruna-type Choghart iron oxide-apatite deposit, Central Iran: Insights from fluid inclusions and H-C-O isotopes

The Choghart iron oxide-apatite (IOA) deposit is one of the largest iron mines of the Bafq district in Central Iran. This deposit is hosted by the Early Cambrian rhyolites, and diabase dikes crosscut both host rocks and the ore bodies. The Choghart rhyolites erupted in a continental arc setting, while the Choghart diabase dikes formed in a back-arc basin environment. Thorite, minor titanite, and REE-oxide are the main hosts of Th and REEs. The mineralogy and geochemistry support that the Th-REE mineralization formed at relatively reduced conditions. The presence of calcite accompanied by thorite and titanite suggests that Th and REE probably migrated as carbonate complexes in the mineralizing hydrothermal fluids. Microthermometric data of calcite associated with thorite indicate that the salinity of the ore-forming fluids varies from 20 to 30 wt% NaCl equivalent with temperature estimates between 300 and 370 degrees C. The narrow range of homogenization temperature and low salinities of the fluid inclusions imply the presence of only one fluid phase in the Th-REE mineralization. The calcite delta C-13(PDB) (-3.9 to -4.1 parts per thousand) and delta O-18(SMOW) (6.6-7.0 parts per thousand) support a magmatic source for the ore-forming fluids. Actinolite delta D (5.55-6.72 parts per thousand) and delta O-18 (-100.8 to -82.4 parts per thousand) also suggest a magmatic source of the ore-forming fluids. Microthermometric data imply that fluid-rock interaction and cooling were critical factors triggering the thorite precipitation at Choghart IOA deposit. Stable isotopic data indicate that post-magmatic fluids derived from rhyolitic magmas played a significant role in the Th-REE mineralization.