Brecciated unit and Th-REE mineralization in the Se-Chahun ore deposit, Bafq mining district, Central Iran

The Bafq region hosts the most important magnetite-apatite deposits of Iran. The geology of this region has been studied by many researchers (e.g., Haghipour, 1977). The ore deposits are mainly hosted by a volcano-sedimentary unit. The presence of a brecciated unit at the margin of the magnetite -apatite ore deposits is discussed by several authors. This unit contains remarkable concentration of Th and REE minerals paragenetically associated with magnetite, actinolite, calcite and albite. Mineralogical properties of the brecciated unit as one of the most important geological events in the magnetite-apatite ore deposits of the Bafq region, and Th-REE mineralization hosted by this zone at the Se-Chahun ore deposit is discussed. The present study has been carried out in four stages including: field work, microscopic studies, ICP-MS and ICP-OEA analysis as well as EPMA analysis. The field work included observations, investigations, radiometry, spectrometry and sampling from different lithologies in both open pits and drilled cores. The microscopic studies were carried out in order to identify the minerals and examine the textural properties of these minerals found in the brecciated unit. The ICP-MS and ICP-OEA analysis were carried out on the samples taken from the ore bodies and the radioactive parts of the mine. The EPMA analysis was also carried out to achieve a more precise hint at the occurrences of the Th and REE minrals and also to investigate the paragenetic relationships between the minerals probed. The brecciated unit is generally formed at the margin of or within the ore deposits mentioned. The matrix of the brecciated unit at the Se-Chahun ore deposit is composed of different minerals including magnetite, titanomagnetite, actinolite, albite, apatite, titanite, calcite, epidote, chlorite and Th silicates. The coarse rock fragments are mainly of the rhyolitic rocks and metasomatic fragments. Based on the mineralogical studies, the brecciated unit is the host of Th-REE minerals. The Th-silicates are formed in two crystallized forms including monoclinic (huttonite) and tetragonal (thorite). Thorium occurrence is found in three types: granular, massive and veinlet. The geological investigations indicate the role of solutions derived from magmatic arc originated in calc-alkaline magmas as a source for Th(-REE) in the brecciated unit. Based on the field, mineralogical and geochemical evidence, a remarkable part of Th has been transported by carbonate complexes in basic and reduced solutions. Apatite and monazite show a notable concentration within the brecciated unit. Monazites are found mostly as single crystals not always hosted by apatite crystals. Two types of actinolite are recognized, 1. Older than Th mineralization within the magnetite ore and 2. A younger generation paragenetically associated with Th silicate. Two types of albites are recognized: an early (white) albite found within the magnetite ore; a late (red) albite also found within the brecciated zone in association with Th occurrences.
Metals such as Th and REE, at the Se-Chahun magnetite-apatite ore deposit are thought to be predominantly derived from the associated magmas, via magmatic–hydrothermal fluids exsolved upon emplacement into the crust. Two main sources exist for the origin of the metals (Th and REE): 1: sediments on the downgoing slab subducted into the mantle wedge (located between the downgoing slab and the overriding plate); 2: assimilation of crustal rocks within the magma chamber and also during ascending of the magmas. Th-REE have been transported mainly by carbonate complexes in alkaline and reduced environments. The presence of a reduced environment during Th-REE mineralization is evidenced by paragenetic association of magnetite and pyrite (and minor chalcopyrite) supported by negative Eu anomaly. Presence of an alkaline environment is also supported by the presence of calcite crystals, veins and veinlets paragenetically associated with Th-REE minerals. A limited number of models have been suggested to explain the provenance of the brecciated unit. Mohseni and Aftabi (2012), among others, suggested that this zone is a proximal zone of magnetite-bearing keratophyres formed in submarine environments. By contrast, no clear source for thorium silicate is suggested. Recently, Khoshnoodi (2016) discussed the subject in one of the largest iron-apatite ore deposits in the region, the Choghart. According to his suggestions, the solutions derived from the calc-alkaline magmas are the source of thorium.
According to our suggestions, the lower continental crust and also the continental derived sediments on the sea floor adjacent to the subduction zones can be proposed as one of the most important sources for limited amounts of thorium found within the magmatic arc magmas. It is proposed that these magmas and associated mineralization are not limited to the margin of the magnetite-apatite ore deposits. Until now, the importance of the Bafq mining district has been due to its discovered magnetite-apatite resources. Further exploration programs supported by mineralogical and geochemical studies may lead to opening new ways in exploration of uncovered ore deposits in the Bafq district containing more economical resources.