PGE-rich Podiform Chromitites in the Al ‘Ays Ophiolite Complex, Saudi Arabia: An Example of Critical Mantle Melting to Extract and Concentrate PGE
PGE-rich Podiform Chromitites in the Al ‘Ays Ophiolite Complex, Saudi Arabia: An Example of Critical Mantle Melting to Extract and Concentrate PGE
复制标题
DOI：
10.2113/gsecongeo.103.7.1507
复制
发表时间：
2008-11
期刊：
影响因子：
5.8
通讯作者：
H. Prichard;C. Neary;P. Fisher;M. O'hara
中科院分区：
文献类型：
--
作者：
H. Prichard;C. Neary;P. Fisher;M. O'hara
沙特阿拉伯的Al‘Ays蛇绿岩杂岩是一个蛇绿岩的例子，它在豆荚状铬铁矿中含有全部六种铂族元素(PGE)的异常浓度，最大值为2,570 ppb铂、6,870 ppb Pd、840 ppb Rh、5,800 ppb Ru、6,200 ppb Ir和3,300 ppb Os。平滑的球粒陨石标准化的PGE剖面指示了火成岩的PGE比率。这些结果表明，在矿物学的二次变质过程中，PGM的原位改变只引起了PGE的轻微迁移性。因此，尽管随后的蚀变引起了矿物学的变化，但仍然可以检查火成岩浓缩过程的地球化学。观察到三种主要的PGE矿化类型，由它们各自的PGE相对丰度来定义。类型1具有铂和钯的Ru&Gt；，具有负斜率球粒陨石标准化剖面。类型2具有铂或钯的Ru<；，(Pt+Pd)/Ir比值为1：5，具有向上凸起的球粒陨石标准化剖面。类型3具有Ru<；铂或钯，(Pt+Pd)/Ir比值为5~60，具有正斜率球粒陨石标准化剖面，并与铜和镍浓度升高有关。该杂岩中铬铁矿中未改变的铬铁矿中心具有异常大的组成范围；例如，Cr2O3的重量百分比从39%到69wt%不等。PGE矿化类型1、2和3与铬铁矿的组成有关。类型1出现在从39wt%到69wt%的铬铁矿组成的范围内，类型2出现在具有53wt%到61wt%Cr2O的铬铁矿中，类型3出现在具有39wt%到51wt%Cr2O3的铬铁矿中。铂族矿物组成的铂族矿物种类繁多，在三种类型的铂族矿物成矿作用中表现出一定的差异性。类型1的特征是通常包裹在铬铁矿中的正面体Os、Ir和Ru(IPGe)合金和Laurite，以及通常位于铬铁矿颗粒间隙的铁铝石霍林沃特固溶体系列和含铂IPGe的PGM。第二类PGE以含IPGe、Pt和Rh的PGm为特征。第三类PGE富集区以含Pd、Pt型PGM为主，伴生有含镍、含铜矿物。在蛇纹岩化过程中，PGM会变成合金、砷化物、锑化物和氧化物，形成不规则形状或可能形成以前PGM的假象。它们通常与含镍和含铜的矿物共生，包括鲁桑镍黄铁矿、千里石、砷化物和含PGE的阿魏特矿。地幔熔融和后续结晶作用是Al‘Ays蛇绿岩杂岩中PGE的最佳富集期。IPGE的结晶通常发生在铬铁矿结晶之前，后来伴随着一些铂和Rh，发生在铬铁矿的组成范围内。在更演化的岩浆形成的铬铁矿中，在硫饱和期间，Pd和部分剩余的铂发生了结晶。我们认为这种结晶来自富含PGE的岩浆，因为地幔熔融的程度只足以提取PGE，而不是在包括进一步地幔熔融的熔体中稀释它们。这一特征很可能适用于其他富含PGE的蛇绿岩杂岩，如英国设得兰、挪威莱卡、加拿大塞特福德、希腊平多斯、阿尔巴尼亚特罗波贾和新喀里多尼亚。如果Al‘Ays继续进行平衡的部分熔融，那么岩浆将被随后的贫PGE熔体稀释。这将阻止硫饱和，直到层序中更高的硫含量，在地壳辉绿岩和辉长岩中产生含铂和钯的贱金属硫化物，就像在塞浦路斯和阿曼蛇绿岩中所发生的那样。
The Al ‘Ays ophiolite complex in Saudi Arabia is an example of an ophiolite that contains anomalous concentrations of all six platinum group elements (PGE) in podiform chromitite with maximum values of 2,570 ppb Pt, 6,870 ppb Pd, 840 ppb Rh, 5,800 ppb Ru, 6,200 ppb Ir, and 3,300 ppb Os. Smooth chondrite-normalized PGE profiles indicate igneous PGE ratios. These suggest that in situ alteration of the PGM caused only minor mobility of PGE during secondary modification of the mineralogy. Thus the geochemistry of the igneous concentration processes can be examined despite the mineralogical changes caused by subsequent alteration. Three main types of PGE mineralization are observed that are defined by their relative abundances of individual PGE. Type 1 has Ru > both Pt and Pd, with negative-slope chondrite-normalized profiles. Type 2 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 1 to 5, and convex upward chondrite-normalized profiles. Type 3 has Ru < either Pt or Pd, (Pt+Pd)/Ir ratios of 5 to 60, positive-slope chondrite-normalized profiles and is associated with elevated Cu and Ni concentrations. The unaltered centers of chromite grains in the chromitite within this complex have an unusually large range of composition; for example, Cr2O3 varies from 39 to 69 wt percent. PGE mineralization types 1, 2, and 3 are related to the composition of the chromite. Type 1 occurs across the range of chromitite compositions from 39 to 69 wt percent Cr2O3, type 2 occurs in chromitite having a range of 53 to 61 wt percent Cr2O3 , and type 3 occurs in chromitite having a range of 39 to 51 wt percent Cr2O3. The PGE form a great variety of platinum group minerals (PGM) and they differ among the three types of PGE mineralization. Type 1 is characterized by euhedral Os, Ir, and Ru (IPGE) alloys and laurite, both commonly enclosed in chromite, as well as members of the irarsite hollingworthite solid-solution series and Pt-IPGE-bearing PGM, both commonly interstitial to the chromite grains. Type 2 PGE enrichment is characterized by IPGE-, Pt- and Rh-bearing PGM. Type 3 PGE enrichment hosts predominantly Pd- and Pt-bearing PGM associated with Ni- and Cu-bearing minerals. Where exposed to the serpentinization process, the PGM are altered to alloys, arsenides, antimonides, and oxides that form irregular shapes or may form pseudomorphs of former PGM. They are commonly associated with Ni- and Cu-bearing minerals, including ruthanian pentlandite, millerite, arsenides, and PGE-bearing awaruite. Mantle melting and subsequent crystallization were at an optimum to concentrate PGE in the Al ‘Ays ophiolite complex. Crystallization of IPGE, commonly prior to chromite crystallization and latterly with some Pt and Rh, occurred across the range of chromitite composition. Crystallization of Pd with some remaining Pt occurred during sulfur saturation in chromitite formed from a more evolved magma. We propose that this crystallization was from a magma that was enriched in PGE because the degree of mantle melting was just sufficient to extract the PGE, but not dilute them in a melt that includes further mantle melting. This feature is likely to be common to other PGE-rich ophiolite complexes such as in Shetland in the United Kingdom, Leka in Norway, Thetford in Canada, Pindos in Greece, Tropoja in Albania and in New Caledonia. If equilibrium partial melting had continued in Al ‘Ays, then the magma would have been diluted by subsequent PGE-poor melt. This would have prevented sulfur saturation until much higher in the sequence, producing Pt- and Pd-bearing base metal sulfides in the crustal wehrlite and gabbro, as has occurred in the Cyprus and Oman ophiolites.