鄂尔多斯盆地巴音青格利铀矿床黄铁矿矿物特征及化学成分研究

Study on Pyrite Mineral Characteristics and Chemical Composition of Bayinqinggeli Uranium Deposit in Ordos Basin

巴音青格利铀矿床是近年来鄂尔多斯盆地北部发现的特大型砂岩铀矿床。文章通过对该矿床含矿层中黄铁矿的野外观察和镜下显微研究,发现黄铁矿在宏观上呈条带状、浸染状和结核状3种,微观上有胶状、草莓状、粒状与脉状等多种产状;依据矿物共生组合关系及化学成分特征可将其划分为PyⅠ、PyⅡ、PyⅢ3种类型。PyⅠ型黄铁矿发育在铀矿化带,常与铀矿物和钛铁矿共生产出,具有亏Fe、S,富含Co、Ni的特征,形成于成矿期;PyⅡ型黄铁矿发育在二次还原带、矿化带和原生带,与钛铁矿共生产出,Fe、S亏损程度较PyⅠ型黄铁矿弱,w(Co)、w(Ni)值相对较小,可能主要形成于成矿后期;PyⅢ型黄铁矿主要发育在原生带,Fe、S质量分数比接近理论值,几乎不含Co和Ni,形成于沉积成岩期。导致成矿期黄铁矿(PyⅠ型)Fe亏损的原因一方面可能是其在铀成矿过程中形成了四方硫铁矿并驱散结构中的一部分Fe,也可能与黄铁矿形成时Fe与成矿流体中Co、Ni发生类质同象替换有关。而成矿过程中H2S还原高价铁离子及UO22+的过程中释放一部分S,导致成矿流体中S逸度下降,这可能是造成S亏损的重要原因。

Bayinqinggeli uranium deposit is a extra-large sandstone uranium deposit discovered in the northern part of Ordos Basin in recent years.Through field observation and microscopic identification of pyrite in the ore-bearing bed of the deposit,pyrite can be divided into three types:striped,dessiminated and nodular macroscopically,and in shape of colloid,strawberry,grain and vein microscopically.By mineral assemblage and chemical composition,the pyrite can be divided into three types:PyI、PyII and PyIII.PyI occurs in the mineralized zone,often as the co-produced with uranium minerals and ilmenite,with the characteristics of depleted in Fe and S,rich in Co and Ni,it was formed in the mineralization period;PyII occurs in the primary zone,mineralized zone and secondary reduction zone,and is co-produced with ilmenite,with less depletion of Fe and S than that of PyI,and small w(Co)、w(Ni),which may be formed in the late stage of mineralization;PyIII is in the primary zone,with the Fe and S contents close to the theoretical values,almost no Co and Ni,and were formed in the sedimentary period.The loss of Fe in pyrite during the mineralization period may be caused by the crystal structure disperse of the iron during the formation of the tetragonal pyrite or the isomorphism substitution of Fe by Co,Ni in mineralizing fluids.The depletion of S in pyrite was due to the release of S in the process of reduction of high-valence iron by H2S resulting in the S fugittance decrease in the fluid during the mineralization process.