铬铁矿床母岩浆含水性的岩石矿物学探讨

Petrological and Mineralogical Investigations on Hydrous Properties of Parental Magmas of Chromite Deposits

为了厘清铬铁矿床母岩浆的含水性及水在铬铁矿成矿中的作用,本文从岩石矿物学角度对与铬铁矿床有关的三类岩体进行了对比。蛇绿岩和大型层状镁铁-超镁铁岩体是铬铁矿床的主要载体,而弧岩浆堆晶成因的阿拉斯加型岩体则以铬铁矿化为主,少有经济价值的铬铁矿床。前两者均缺少独立产出的含水矿物,表明为典型的贫水体系,与其铬铁矿中含水矿物包裹体和流体包裹体所推测的母岩浆富水特征相悖;贫矿的阿拉斯加型岩体产出大量的含水矿物,与其产出于富水-高氧逸度的俯冲构造背景相一致。因此,岩浆的高含水量并不是铬铁矿成矿的关键。结合实验岩石学结果,我们认为铬铁矿的结晶应与岩浆演化过程中的水饱和及流体不混溶有关,铬铁矿的表面吸附水对铬铁矿的运移富集起到了重要作用。这些表面吸附水最主要是造成了铬铁矿周边橄榄石的蛇纹石化,形成铬铁矿体的选择性蚀变,有时可以进入橄榄石甚至是铬铁矿晶格,发生矿物出溶现象,亦可与演化的熔体混合形成结晶粒间矿物(以辉石为主),也有可能被生长的铬铁矿所包裹形成包裹体。

Petrological and mineralogical investigations on three types of mafic-ultramafic complexes have been conducted to reveal the hydrous properties of parental magmas of chromite deposits and the role of water in process of chromite mineralization in this paper.Global chromite deposits are mainly hosted in ophiolites and large layered intrusions,while economic chromite deposits are rarely observed in the Alaskan-type complex originated by the accumulation of arc magma though chromite mineralization occurred in the complex.Both the chromite-bearing ophiolites and layered intrusions are featured by absence of independent hydrous minerals,indicating they were from typical water-poor or "dry"magmatie systems. This is opposite to the opinion,inferred on the basis of mineral and fluid inclusions of the hydrous minerals in the chromite deposits,that their parental magma should be water enriched magma.However,the barren Alaskan-type complexes have considerably high contents of hydrous minerals.This is in accordance with the feature of enriched water and high oxygen fugaeity as they occurred in the subduction tectonic environment.As a consequence,the high water content in parental magmas could not be critical to the formation of chromite deposits.Taking into account of experimental petrological results, we have emphasized that the chromite crystallization should be associated with the water-saturation and fluid immiscibility of the evolved magma in the evolution process and the surfieial adsorbed water of chromite could have played important role for the transportation and enrichment of chromite.These surficial adsorbed water of chromite mainly resulted in the serpen- tinization of the olivine surrounding chromite,generating "selective alteration"of chromite orebodies.In some cases,the water might enter into the crystal lattices of olivine and even chromite crystals to generate the exsolution minerals,or it might be mixed with evolved melt to form inter-crystal minerals (pyroxene dominated),or it might be enclosed by growing chromite to form fluid inclusions.