豆荚状铬铁矿床成矿地球化学指标对比和成矿作用讨论

Geochemical Indicator for Podiform Chromite Mineralization and its Formation Mechanism

豆荚状铬铁矿主要赋存于地幔橄榄岩中,与方辉橄榄岩密切相关。在全球的分布与蛇绿岩带分布基本一致,但并非所有蛇绿岩体都赋存有铬铁矿,且其中赋存的铬铁矿体规模和分布都是很不规律的。我们对比研究了国内外9个含铬矿和4个不含铬矿蛇绿岩中地幔橄榄岩的地球化学组成,认为含铬矿地幔橄榄岩具有全岩低含量的CaO(<1.91%)和Al_2O_3(<1.76%)、方辉橄榄岩轻稀土元素富集,橄榄石高Fo值(>90),斜方辉石低Al_2O_3含量(<3.8%)以及副矿物铬尖晶石高Cr/Fe值(>1.5)等特征,可以作为该蛇绿岩体含矿评价的地球化学指标。通过这些指标可知豆荚状铬铁矿床是较高程度部分熔融和地幔交代作用的共同结果,结合前人提出的铬铁矿成矿模式,对铬铁矿的成矿过程有了进一步的认识。

Being the vital materials for metallurgical industry,podiform chromite deposits usually occur in mantle peridotites,and are associated with the ophiolitic complexes. However,chromite ores are not developed in all ophiolitic complexes,and the size and distribution of them are quite irregular. By comparing geochemical composition of nine chromite orebearing ophiolitic complexes and four barren ones,we found the mineralized ones have low CaO(< 1. 91%) and Al2O33(<1. 76%) content,and LREE-enrichment of harzburgites,high Fo(> 90) value of the olivine,low Al2O3 content(<3. 8%) of the orthopyroxene and high Cr/Fe ratio(>1. 5) of chromite,which can be the geochemical indicator of the ore-bearing ophiolitic complexes. Finally,we can conclude that the podiform chromite deposits have experienced both partial melting and metasomatism,and provide a genetic model for podiform chromite mineralization by combining with previous ideas.