Geological and Geochemical Characteristics and Genesis of Langwashan Iron Deposit in Beishan, Gansu Province

Langwashan iron deposit locates in the south of Hongshishan ophiolite belt. The ore bodies occur in the volcanic tuff of Baishan formation of Lower Carboniferous. According to the received classification, this iron mine belongs to volcanic iron deposit. Because few basic geological researches on this iron deposit have been done before, we faced certain difficulties in exploring and finding the ore deposit. Beginning with the facts of basic geology, we studied the characteristics of trace element, rare earth elements and sulfur isotope of wall rocks, altered rocks and ore body, and discussed the mineralization process. Results show that mantle trace elements (Sc, Co, Ni, Cu) tend to enrich significantly in the ore body. However, the relative crust trace elements (Zr, Nb, Ta, Th) are more inclined to enrich in the wall rocks and altered rocks. According to the REE content of wall rocks, altered rocks and ore bodies as well as the spider diagram, wall rocks and altered rocks are rich in LREEs with obvious negative Eu anomaly, whereas ore bodies especially lack LREEs with significant positive Eu anomaly. The sulfur isotope results of the ore indicate that the value of δ³⁴S varies from +0.6 ‰ to +3.4 ‰, with an average of +1.82 ‰. There have been large-scale magmatic events in Early Carboniferous in Beishan area when it is in island-arc tectonic setting. These results show that the ore-forming materials are derived from ore-bearing magma produced by partial melting of upper depleted mantle. Then the ore-forming materials intruded the Baishan Formation of Lower Carboniferous and deposited to mineralize near surface. In conclusion, Langwashan iron mine can be regarded as a volcanic type iron deposit.