The Ni-Cu-PGE sulfide ores of the komatiite-hosted Fortaleza de Minas deposit, Brazil: Evidence of hydrothermal remobilization
The Ni-Cu-PGE sulfide ores of the komatiite-hosted Fortaleza de Minas deposit, Brazil: Evidence of hydrothermal remobilization
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DOI：
10.2113/gscanmin.45.4.751
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发表时间：
2007-08
影响因子：
0.9
通讯作者：
C. Almeida;G. Olivo;Sebastiao Gomes de Carvalho
中科院分区：
文献类型：
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作者：
C. Almeida;G. Olivo;Sebastiao Gomes de Carvalho
福塔莱萨德米纳斯镍铜铂族元素硫化物存款是由太古代科马提岩的Morro do Ferro绿岩带，在西南边缘附近的圣弗朗西斯科克拉通，米纳斯吉拉斯州，巴西。该存款含有600万吨矿石，平均品位为2.2重量% Ni、0.4% Cu、0.05% Co和1.2 ppm PGE+Au，并包括（i）主矿体，其沿着区域剪切带变质、变形和移位，主要由浸染状、角砾状和细脉状硫化物矿石组成，被解释为早期岩浆成因，富含铂族元素的不整合脉体，赋存于横切主矿体的N-S向和NE-SW向晚期断裂中。不整合的富含铂族元素的矿石（铂族元素总量高达4 ppm）的特征是由蛇纹岩和滑石片岩承载的块状硫化物的薄、不连续和不规则的矿脉和透镜体，与早期类型的矿石相比，相对未变形。它主要由磁黄铁矿、镍黄铁矿、黄铜矿、磁铁矿、碳酸盐和角闪石组成，少量的钴矿-锗硅铁矿、闪锌矿、钛铁矿和石英，以及少量的红柱石（Ni 11 As 8）、碲化物和铂族矿物（PGM）。铂族金属主要为奥镁石、铁砷石、闪锌矿和含镍的镁矾，其次为少量的铁钯矿和含Ru、Te和As的未知相。铂族金属的存在要么包括在硫化物、硫砷化物、硅酸盐和氧化物中，要么位于硫化物、硫砷化物、硅酸盐和氧化物的边缘，要么填充磁黄铁矿、镍黄铁矿和黄铜矿中的裂缝，这表明它们开始与这些矿物一起沉淀，并在硫化物形成后继续沉淀。两个不整合脉样品的地幔标准化金属分布显示出不同的模式：一个富含Ni-Pd-Ir-Rh-Ru-Os，另一个具有较高的Cu-Pt-Bi含量。如果与该存款的变质岩浆矿石相比，两者都强烈贫Cr，该矿床遵循一般的Kambalda型岩浆趋势。根据构造、矿物学和地球化学证据，我们提出，富含PGE的不整合矿石可能是由变形变质岩浆矿体（这些元素呈亏损模式）中的金属通过还原（磁黄铁矿-镍黄铁矿-黄铁矿是稳定的）、中性至碱性和碳质流体（碳酸盐稳定的）再活化而形成的。PGE可能是以二硫化物络合物的形式迁移，并在晚期的N-S向和NE-SW向断裂中以碲化物（主要是Pd）和砷化物（Pt、Rh、Ru、Os、Ir）的形式沉淀，这是由于硫化物在脉中沉淀导致S的活度降低所致。
The Fortaleza de Minas Ni–Cu–PGE sulfide deposit is hosted by Archean komatiitic rocks of the Morro do Ferro greenstone belt, near the southwestern margin of the Sao Francisco craton, Minas Gerais state, Brazil. The deposit contains 6 million tonnes of ore with an average grade of 2.2 wt% Ni, 0.4% Cu, 0.05% Co and 1.2 ppm PGE+Au, and comprises (i) a main orebody, which is metamorphosed, deformed and transposed along a regional shear zone, consisting mainly of disseminated, brecciated and stringer sulfide ores that are interpreted to be of early magmatic origin, and (ii) PGE-rich discordant veins that are hosted in N–S- and NE–SW-trending late faults that cross-cut the main orebody. The discordant PGE-rich ore (up to 4 ppm total PGE) is characterized by thin, discontinuous and irregular veins and lenses of massive sulfides hosted by serpentinite and talc schist, and is relatively undeformed if compared with the early types of ore. It is composed mainly of pyrrhotite, pentlandite, chalcopyrite, magnetite, carbonates, and amphiboles, with minor cobaltite–gersdorffite, sphalerite, ilmenite, and quartz, and rarely maucherite (Ni11As8), tellurides and platinum-group minerals (PGM). Omeeite, irarsite, sperrylite, and Ni-bearing merenskyite are the main PGM, followed by minor amounts of testibiopalladite and an unknown phase containing Ru, Te, and As. The PGM occur either included in, or at the margins of, sulfides, sulfarsenides, silicates and oxides, or filling fractures in pyrrhotite, pentlandite, and chalcopyrite, suggesting that they started to precipitate with these minerals and continued to precipitate after the sulfides were formed. The mantle-normalized metal distribution of the two samples of discordant veins shows distinct patterns: one richer in Ni–Pd–Ir–Rh–Ru–Os and another with higher amounts of Cu–Pt–Bi. Both are strongly depleted in Cr if compared with the metamorphosed magmatic ore of this deposit, which follows the general Kambalda-type magmatic trend. On the basis of structural, mineralogical and geochemical evidence, we propose that the PGE-rich discordant ore may have formed by remobilization of metals from the deformed, metamorphosed magmatic orebody (which shows a depleted pattern in these elements) by reduced (pyrrhotite – pentlandite – pyrite are stable), neutral to alkaline and carbonic fluids (carbonate-stable). The PGE may have been transported as bisulfide complexes, and precipitated as tellurides (mainly Pd) and arsenides (Pt, Rh, Ru, Os, Ir) in the late N–S and NE–SW-trending faults owing to a decrease in the activity of S caused by the precipitation of sulfides in the veins.