西藏罗布莎铬铁矿床是我国目前研究程度最高、规模最大、地幔橄榄岩相对新鲜的豆荚状铬铁矿床,主要工业矿体产于蛇绿岩壳-幔边界(即岩石莫霍面)以下方辉橄榄岩相带一定层位中,主要有块状、浸染状和豆状等矿石类型.罗布莎铬尖晶石成分变化范围大,依据铬尖晶石的化学成分与矿物学研究至少可识别出3个期次铬尖晶石:(1)成矿前期铬尖晶石,主要以熔蚀残斑晶、出溶晶及少量自形晶形式产于方辉橄榄岩中,以富Al2O3为特征,Cr#值变化范围大(17.19 -66.30),且大部分小于60,并与Mg#值呈负相关关系,由出溶晶,残斑晶到自形晶铬尖晶石,总体表现向富Cr、Fe的方向演变；(2)成矿主期铬尖晶石,可分为早、晚2个阶段.早阶段铬尖晶石主要以它形晶产于不同类型铬铁矿石中,部分呈自形-半自形晶产于铬铁矿体的纯橄岩外壳中,主要以富铬为特征,矿石中Cr#值变化范围小(70.08～87.03),均大于60,其中块状铬铁矿具有最高的Cr#和Mg#,由纯橄岩外壳中副矿物铬尖晶石向豆状、浸染状矿石以及块状矿石演变过程中,铬尖晶石化学成分总体向更富Cr、富Mg方向演变；晚阶段铬尖晶石:主要以自形-半自形晶产于具堆晶结构的纯橄岩相带中,成份上以更加富而贫Al2O3,且具有最低Mg#( 18.79～44.77)值为特征；(3)成矿晚期铬尖晶石,主要以网状集合体产于豆状-网脉状(眼眉状)矿石中,以更贫Al、富Fe为特征,具有最高的Cr#值和低的Mg#值.综合研究表明,罗布莎铬铁矿中的铬主要来自原始地幔岩本身,且主要来自于地幔橄榄岩中2种辉石的不一致熔融和对副矿物铬尖晶石的改造,原始富铬矿物可能来自地幔深部的八面体硅酸盐矿物.罗布莎豆荚状铬铁矿的成矿作用具有多期次、多成因、多种构造背景下成矿特征,成矿作用过程经历了由大洋中脊(MOR)扩张环境向岛弧体系俯冲环境的转变过程,洋内俯冲带之上(SSZ)的弧间盆地环境是形成冶金级豆荚状铬铁矿的最为有利构造环境.研究提出了罗布莎铬铁矿的“三阶段”成矿模式,即,经历了大洋中眷预富集阶段,俯冲带之上主成矿阶段及之后的构造抬升改造阶段.纯橄岩与方辉橄榄岩接触带之下的方辉橄榄岩相带是寻找较大规模铬铁矿床的有利地带.%The Luobusha chromite deposit, which is the largest podiform deposit with maximum study in China, occurred in a fresh mantle peridotite in the east part of Yarlung Zangbo ophiolite belt in South Tibet The economic ore bodies of Luobusha chromite deposits are mainly hosted in a specific lithological horizon of harzburgite facies beneath the boundary of ophiolitic crust and mantle (beneath the Moho boundary) , and the ore type mainly includes massive, disseminated and nodular chromitite ores. Luobusha Cr-spinel have a large variety in composition, based on the chemical composition and the detailed mineralogical study of Cr-spinel, at least three crystalline epochs of the Cr-spinel in the Luobusha podiform chromite deposit can be recognized. (1) Pre-mineralization Cr-spinel : This kind of Cr-spinel is mainly as accessory mineral in the harzburgite, and occurred in the form of residual phenocryst, exsolution crystal and euhedral crystal with high A12O3 content, most of the Cr value of the Cr-spinel is less than 60, and has a negative correlation ship with Mg#. The overall evolution of this kind of Cr-spinel from exsolution crystal, residual phenocryst to euhedral crystal of Cr-spinel tends to be riched in Cr and Fe; (2) Main mineralization Cr-spinel can also be divided into two stages. The early stage of Cr-spinel, has a typical characteristic of Cr-riched, is a major mineral with anhedral crystal in a variety type of chromitite ores, and a minor mineral with euhedral crystal in dunite ' envelop' crust outside of the orebodies. Most of the Cr value of the Cr-spinel in chromite ores is greater than 60, while the greatest value of Cr# and Mg# belongs to the Cr-spinel in massive ore. The evolution of this kind of Cr-spinel, from the associate Cr-spinel in dunite and dunitic crust to nodular, disseminated and massive chromitite ores, tends to be riched in Mg and Cr. The late stage of Cr-spinel, which mainly occurred in the form of euhedral and hypidiomorphic crystal in cumulate structure dunite, is riched in < FeO > and depeleted in Al2O3, and has the lowest Mg (18. 79 ~ 44. 77); (3) Late mineralization Cr-spinel: This kind of Cr-spinel is mainly occurred in the nodular-orbicular ore ( eyebrows shaped). The composition of the Cr-spinel, with the largest value of Cr# and the lowest value of Mg#, tend to depleted in Al and enriched in Fe. Comprehensive study suggested that the chromium element of chromitite coming from primitive mantle itself, and mainly coming from the regulation and reformation of accessory spinel and the incongruent melting of the two pyroxenes ( chromium diopsite and enstatite) , it was inferred that the primitive Cr-riched minerals in mantle may be the Cr-riched octahedral silicate minerals. The mineralization process of Luobusha podiform chromite deposits, which has undergone the process transformed from MOR expansion setting to the subduction setting of island arc system, have the characteristic of multi- stages, polygenesis and a variety of tectonic settings, while the inter-arc basin above the supra-subduction zone (SSZ) is the most favorable structural environment for the formation of metallurgical-grade chromite ore. Furthermore, we first put forward 'three stages' metallogenic model of Luobusha chromite deposit, which undergoing the early stage of preconcentration in MOR, the mainly metallogenic stage of the supra-subduction zone, and the late reconstruction stage in tectonic uplifting and reforming. The Luobusha harzburgite phase belt beneath the contact face with the dunite phase is the favorable target for searching large-scale chromite deposit.
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