REE-Th mineralization in the Se-Chahun magnetite-apatite ore deposit, central Iran: Interplay of magmatic and metasomatic processes

Abstract

The Se-Chahun Fe-P ore deposit is located in the Bafq mining district in the Central Iranian tectono-magmatic zone and contains REE and Th mineralization. The ore deposit consists of a magnetite-apatite massive ore that is significantly modified by late-stage metasomatic fluids that finally formed brecciated rocks within and at the margin of the ore deposit. Geochemical data from the rhyolitic rocks, Fe-P ore body and breccia as well as isotopic data from the breccia within the Se-Chahun Fe-P ore bodies, have been used to deduce the magmatic origin of the Fe-P ore zone and to assess the significance of post-magmatic processes with late-stage metasomatic fluids and the effects of these processes on remobilization of magmatic originated REE and concentration and deposition of Th. The chondrite-normalized REE patterns of the mineralized rocks in both the Fe-P ore and the Th mineralization zone demonstrate LREE enrichment and strongly negative Eu anomalies (Eu/Eu* = 0.25–0.91 for Th mineralization). δD and δ18O for actinolite paragenetic to Th-bearing silicates are estimated to be between −73.29 to −42.04 and 6.65 to 7.71, respectively, which lie within the both fields of magmatic and metamorphic fluids. Na-Ca metasomatism (which is evidenced by brecciation) has been a continuous process affecting the host rock, and the Fe-P ore body. Apatite, monazite, and Ti-La-Ce-Y-Nd-oxides are the main REE-bearing phases in the Fe-P ore body (REE≫Th) that are precipitated during magnetite crystallization whereas Th silicates (huttonite and thorite) occur within the breccia (Th ≫ REE) and were deposited from a late Th- and carbonate-rich fluid. Th mineralization occurred with late sodic-calcic fluids and Th-bearing minerals are paragenetic to actinolite, red albite, magnetite, titanite, calcite, and pyrite. Paragenetic association of Th minerals and actinolite, titanite, and calcite in carbonate veins and veinlets point to transportation of Th by carbonate complexes. The pH, fluid/rock ratio, Ca/Na ratio, and higher capability of Th (with respect to REE) in formation of carbonate complexes and lower concentration of Th (with respect to REE) in the source region are the most important geochemical parameters that triggered Th (Th ≫ REE) mineralization at the end of alkali metasomatism.