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首页> 外文期刊>Economic geology and the bulletin of the Society of Economic Geologists >Platinum Group Element Geochemistry of Mineralized and Nonmineralized Komatiites and Basalts
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Platinum Group Element Geochemistry of Mineralized and Nonmineralized Komatiites and Basalts

机译：矿化和非矿化科马提岩和玄武岩的铂族元素地球化学

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Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new highprecision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores.Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion,and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion,commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiitehosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within orebearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors.Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes.
机译：铂族元素（PGE）具有强烈的亲硫性，因此可能是涉及硅酸盐岩浆中硫化物熔体分离和聚集过程的潜在敏感指示。超过500个新的高精度的科马铁矿和科马提岩玄武岩PGE数据，涵盖了广泛的沉积和结晶史，并结合了来自其他贫瘠和矿化程度不同的岩浆系统中PGE的文献数据，以检验PGE地球化学作为一种矿物的有效性。结果表明，与Fe-Ni-Cu硫化物矿化在空间和遗传上相关的S-贫钾辉石岩和Komatiite玄武岩中PGE的贫化不如预期或普遍：样品显示PGE代表来自任何位置的任何给定数据集的不到10％。数据证实，即使不是全部，大多数科马铁矿与来源分离时，硫化物仍未饱和，而在喷发时仍未饱和。某些含矿的科迈铁矿序列没有显示出可探测的损耗，而PGE损耗的程度通常小于使用实验确定的分配系数进行建模所预期的程度。 PGE富集比PGE耗竭更为普遍且在空间上广泛分布，即使在缺少异常Ni或S含量的样品的情况下，PGE富集也是岩溶化学勘探的更好方法。通过所有PGE的协方差，可以将与硫化物富集和/或偏析相关的PGE富集和/或耗尽与二次热液和/或变质过程区分开，但在某些情况下，Ir，Ru和Os的丰度可能因复杂而复杂化。富含Ir-Os的液相线存在饱和并积累。 PGE / Ti比值的变化以及矿化系统中Pt / Ti，Pd / Ti和Rh / Ti比值之间的强相关性可以区分可归因于其他岩浆作用的变化，例如橄榄石堆积和分馏。 PGE的耗竭程度与许多科马蒂特岩矿床的相对较低的R系数相一致，汤普森矿床的R系数在20至200的范围内，坎巴尔达矿床在100至500的范围内，拉格伦在300至1100的范围内，这意味着硅酸盐的体积与硫化物液体相互作用的岩浆相对较小。这也与在蠕动流序列内显示PGE损耗的比例相对较小的科马铁矿一致，因为只有成矿通道或导管中的岩浆会与硫化物相互作用。假阴性，即矿化的科马铁矿序列，没有可检测的PGE耗尽，与具有高R因子特征的系统相关。玄武岩和科马蒂玄武岩表现出更复杂的变化模式，大致可分为三类：（1）系统性PGE消耗如在MORB套件中那样，在一定范围的Mg值范围内，这与部分熔融过程中硫化物在地幔中的保留相一致; （2）在大火成岩省（LIP）相关的玄武岩中，PGE损耗增加而镁含量降低，这被解释为反映了分馏过程中硫化物饱和的实现以及随后的共晶橄榄石-硫化物的偏析; （3）在Komatiitic玄武岩（例如Raglan）中，Mg值范围内的变量PGE耗竭程度被解释为反映了成矿硫化物的结合和偏析过程。这项研究的结果证实，科马铁矿和玄武岩的PGE地球化学是硫化物饱和和成矿过程的有力指示，但必须在物理火山学和流体动力学过程的背景下加以解释。

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