Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization
Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization
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蒙大拿州斯蒂尔沃特复合体硫化物中的铅同位素：固相线下再活化的证据
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发表时间：
1999
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通讯作者：
Bruce K. Nelson
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作者：
I. S. Mccallum;M. Thurber;Hugh O’Brien;Bruce K. Nelson
摘要 对斯蒂尔沃特杂岩体含铂 J-M 礁石的一组样品中的硫化物矿物（主要是磁黄铁矿、黄铜矿和镍黄铁矿）中的 Pb 同位素比进行了测量，以阐明硫化物与寄主硅酸盐矿物之间的时间和成因关系。结果表明，硫化物和共存的斜长石通常不处于同位素平衡状态，硫化物和长石均在 2.7Ga 处记录了高放射成因的初始比率，并且“侵位后”放射成因 Pb 的成分与硫化物中的常见 Pb 混合。涉及引入由复合体外部来源的流体携带的放射性铅的模型受到青睐。对切割复合体的静脉中硫化物的铅同位素组成的分析表明，添加到硫化物中的放射性铅的很大一部分是在广泛的热液事件期间从外部衍生出来的，与 1.7Ga 左右的元古代区域变质作用有关。不能忽视部分放射性铅可能源自低级变质作用期间改变的原生矿物的可能性。添加的放射性铅的量是可变的，并且在某些情况下可以忽略不计。铅同位素组成与次生矿物组合的性质之间存在良好的相关性。样品中的硫化物和斜长石对原生矿物几乎没有或没有改变，通常处于同位素平衡状态，并保持与 2.7Ga 岩浆结晶一致的同位素比。放射性成因硫化物最多的样品含有丰富的与绿片岩相变质作用相关的次生矿物（蛇纹石、滑石、阳起石、绿泥石和黝帘石）。硫化物中的一些放射性铅可以通过逐步浸出来去除。然而，在大多数样品中，变质过程中硫化物的重结晶在整个晶体结构中混合了普通 Pb 和放射性 Pb，因此，在这些样品中，逐步浸出无法恢复初始 Pb 同位素比率。斜长石对低温重结晶的抵抗力要强得多，并且在几乎所有情况下，逐步浸出都会揭示最初的铅同位素组成。硫化物在很宽的温度范围内的反应性增强了它们的实用性，不仅可以了解其在岩浆就位时形成的过程，还可以了解岩浆后过程，这对于矿带内铂族元素 (PGE) 的重新分布和富集非常重要。
Abstract Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement” radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals (serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone.