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

Abstract

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°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 δ13CPDB (−3.9 to −4.1‰) and δ18OSMOW (6.6–7.0‰) support a magmatic source for the ore‐forming fluids. Actinolite δD (5.55–6.72‰) and δ18O (−100.8 to −82.4‰) 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.

Mineralogy of various types of Th-U-REE mineralisation in the iron oxide – apatite deposits of the Bafq district, Central Iran

“…In the first stage associated with the continental arc, felsic calc-alkaline magmatism, that leads to the formation of the iron oxide-apatite mineralisation and related sodic-calcic alteration (albite, actinolite, diopside/ augite) (Stosch et al 2011; Heidarian et al 2017; Khoshnoodi et al 2017). In the later stage, late magmatic derived fluids (Deymar et al 2018; Mirzababaei et al 2021; Ziapour et al 2021; Khoshnoodi et al 2022) enriched in Th/U and rare-earth elements (especially LREE) affected mainly the sodic-calcic alteration surrounding the iron oxide-apatite ore bodies and sometimes the iron oxide-apatite ore bodies, that lead to the formation of the Th/U-REE and U-Th-REE mineralisation in the Bafq IOA deposits. Generally, the host rocks are extensively brecciated simultaneous with Th-REE and Th mineralisation (Bonyadi et al 2011; Khoshnoodi et al 2017; Mirzababaei et al 2021) and this type of mineralisation occurs in hydrothermal brecciated zone.…”

Intensive geological alteration and gas accumulation in Longmaxi Formation shales at moderate and great depths: A case study of the Luzhou area in the southern Sichuan fold zone, China

“…The homogenization temperatures of fluid inclusions were determined by determining those of primary mineral inclusions formed during the same period as fracture fillers. The distribution intervals of homogenization temperatures provide important information that can be used to infer the times when fractures and fractures were formed during the paleotectonic period (Khoshnoodi et al, 2022; Sharma et al, 2022). Thin‐section rock samples were prepared for this test.…”