甘肃北山狼娃山铁矿地质地球化学特征及成因浅析

狼娃山铁矿位于红石山蛇绿岩带南侧,矿体产于下石炭统白山组火山凝灰岩中,属火山岩型矿床.因前人对于该矿床的基础地质研究少之又少,因此对于矿床的勘探和进一步找矿带来了一定的难度.笔者从矿区的基础地质入手,通过对矿区围岩、蚀变岩和矿体的微量元素、稀土元素和S同位素的研究,探讨其矿床成因.结果表明,从围岩到蚀变岩,再到矿体,较之于地壳更加富集于地幔的微量元素(Sc、Co、Ni、Cu)均数倍乃至于数百倍的富集于矿石中,而相对富集于地壳的元素(Zr、Nb、Ta、Th)则更富集于围岩和蚀变岩.根据围岩、蚀变岩和矿体的稀土元素含量及其配分图,发现围岩和蚀变岩为右倾型,并具有明显负Eu异常,而矿石较特殊亏损轻稀土并具有显著的正Eu异常,S同位素结果显示δ34S值的变化范围为+0.6‰~+3.4‰,平均值为+1.82‰.北山地区在早石炭世时曾发生大规模的岩浆事件,而同时期区域上为岛弧环境,综上结果表明,成矿物质来源于亏损上地幔部分熔融的含矿岩浆,在近地表侵入下石炭统白山组沉积成矿,据此可认为,狼娃山铁矿为火山岩型铁矿.

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.