津巴布韦大岩墙南段两翼铬铁矿床特征及成因探讨

Discussion on geological characteristics and genesis of chromite deposits on both limb of the southern Great Dyke in Zimbabwe

津巴布韦大岩墙蕴含丰富的铬铁矿资源,不同部位成矿特征存在差异。钙碱性铁质基性超基性岩浆结晶分异过程中伴随着铬铁矿的成矿作用,南部岩浆房两翼300 m深度范围内共赋存7个不连续的铬铁矿层,矿层垂向间距30~50 m,南部矿层发育优于北部;铬铁矿层厚度多为0.2~1 m,铬铁矿层发育于辉橄岩系和辉石岩系的接触部位,三者呈渐变式过渡接触,共同构成典型的韵律结构;两翼矿层产状呈对称状倾向于岩墙的轴部,倾角约为10°~20°;矿石多为自形半自形微细粒结构,稀疏浸染状及中等浸染状构造,矿石为需选贫铬矿石,w(Cr2O3)=5%~30%,且矿体内Pt+Pd含量可观。大岩墙南段两翼铬铁矿为典型的早期岩浆结晶分异矿床,后期构造运动加剧了矿层的不连续性,并对局部岩矿层产状产生改造,蛇纹石化蚀变形成了指示矿体的标志性斑杂构造岩。

The Great Dyke of Zimbabwe here is abundant with Cr resources and mineralization characteristics are varied at its different parts.Crystal differentiation of calc-alkaline iron basic-ultrabasic magma is accompanied by Cr mineralization.At a depth of 300 m 7 discontinuous chromite layers occur at the two limbs of the southern magmatic chamber.Vertical interval of each two ore layers is 30~50 m.The layers are 0.2~1 m thick developed in the contact of augite peridotite and pyroxenite.From augite peridotite thyough chromite ore to pyroxenite is transitional with rhythmic structure.The development of the chromite layers in the southern part is better than that in the northern.The ore layers on the two limbs are distributed symmetrically and dip to the axis of the dyke at angle about 10°~20°.The ore is poor with Cr2O35%~30%and considerable Pt+Pd value and euhedral-subhedral fine-micro grain texture and sparse-medium dissemination structure.In conclusion,the chromite deposit is the typical early magmatic crystal differentiation deposit.Late tectonics intensifies the discontinuity of the ore layer,and reworks locally the ore layer.The serpentinized taxitic tectonite is the indicative mark of ore body.