乌拉尔南部和印度南部地区隐晶质菱镁矿的地质与地球化学对比研究(英文)

Geological and geochemical comparison of Southern Urals and South Indian cryptocrystalline magnesite

印度南部和乌拉尔南部都有隐晶质菱镁矿产出,这两处矿床的产出地质环境相似,在矿物学和地球化学上具有广泛的相似性.印度南部的菱镁矿矿化主要与超镁铁质侵入杂岩体有关,并形成了部分已受变质的火山沉积地层.超镁铁质侵入杂岩体由纯橄岩,橄榄岩,辉石岩,辉长岩及它们的变质产物组成.在乌拉尔地区,菱镁矿床位于一个蛇绿岩带上的超镁铁岩地体中.隐晶质菱镁矿就以网脉状产出于超镁铁质岩地体上部的风化带中.印度和乌拉尔两个地区的矿床中的矿物组合都有菱镁矿,石英,方解石和白云石,但在印度南部的矿区中还含有滑石和菱铁矿.两个地区的菱镁矿矿石的质量都很好,所有的样品的主要成分都为菱镁矿(73～96%),而方解石(1～3%),白云石(0～7%),菱铁矿(0～2%),石英(0～5%)和滑石(0～2%)都只是次要矿物.次生的白云石和菱铁矿使一些矿石含有较高的CaO(最高达2.6%)和FeO(最高达1.6%),石英和滑石等矿物则使矿石中的SiO2较高(5～8%).滑石指示了低温成因,它的出现说明两个矿区的菱镁矿可能都是内生或外生的成矿流体在上升或下降的过程中在开放裂隙中沉淀而成的.本文研究表明,全球性的超镁铁岩中菱镁矿成矿事件与蛇绿岩带有关,这对菱镁矿的勘探有指导意义.

South Indian and Southern Urals cryptocrystalline magnesites, deposited in similar geological milieu, are broadly comparable in mineralogy and geochemistry. Magnesite mineralisation in South India is mostly associated with the rocks of ultramafic intrusive complex, forming a part of the metamorphosed volcano-sedimentary sequence. The ultramafic intrusive complex is composed of dunite, peridotite, pyroxenite, gabbro and their metamorphic products. In the Urals, magnesite deposits are located in the ultramafic massif of an ophiolitic belt. The cryptocrystalline magnesite has developed as stockwork of veins in the upper part of weathered ultramafic rocks in different parts of the massif. Both the deposits in South Indian and Southern Urals contain analogous mineralogy with magnesite + quartz + calcite + dolomite, but the Indian deposits contain admixture of talc + siderite also. In all the investigated samples the dominant mineral is magnesite (73 similar to 96%). Calcite (1 similar to 3%), dolomite (0 similar to 7%), siderite (0 similar to 2%), quartz (0 similar to 5%), talc (0 similar to 14%), and serpetine (0 similar to 2%) are the accessories. Chemical composition of magnesite from both areas indicates good quality, and some samples show slightly high contents of CaO ( up to 2.6%) and FeO ( up to 1.6%), resulting from secondary dolomite and siderite, respectively. The high contents of SiO2 up to 5 similar to 8% are due to the accessory quartz and talc. It is suggested that magnesite, in both the regions, was formed as precipitation in open fractures by ascending/descending hypogene/exogene waters, the presence of talc indicating low temperature of formation. The present study points to the need of exploring a global magnesite-forming event in the ultramafic complexes associated with ophiolite belts.