Previous Cu isotope work has documented a clear disparity between δ⁶⁵Cu values of sheet-style (–0.5 to 0.5‰) and conduit-style (0.5–2.0‰) intrusions associated with the Midcontinent rift system. The application of metal isotopes to the study of magmatic Ni-Cu-platinum group element (PGE) deposits is in an early stage, and very little is known regarding isotope distributions and mechanisms of fractionation at high temperatures. In order to resolve the previously mentioned Cu isotope disparity, to determine metal sources for the intrusions, and to assess sources of high-temperature metal isotope fractionation, we have measured Cu and Ni isotope ratios from a suite of exceptionally well characterized Ni-Cu-PGE massive sulfides that occur in sedimentary country rocks near intrusions within the Midcontinent rift system in Michigan and Minnesota. Previous mineralogic and S, Pb, and Os isotope measurements indicate that the massive sulfides are of magmatic origin and provide a framework for the interpretation of the Cu and Ni isotope data in terms of magmatic processes, including assimilation of Proterozoic country rocks.Copper and Ni isotope ratios were determined for massive sulfides as well as local sedimentary sulfides, and these results were compared with available Cu and Ni isotope results of magmatic and sedimentary sulfides in the Midcontinent Rift and elsewhere. Nickel isotope ratios of the sulfides have been modeled in terms of the effects of variable silicate/sulfide ratios, or R-factors (the mass ratio of silicate magma to sulfide magma), crustal contamination, and olivine fractional crystallization. The near-zero and slightly negative δ⁶⁰Ni values of country rock-hosted magmatic sulfides (–0.45 to 0.17‰) near Tamarack and Eagle can be explained by minor degrees of crustal contamination of a mantle-derived melt at variable R-factors. Igneous-sourced, Cu-poor sulfides from below the Partridge River intrusion generally have lower δ⁶⁰Ni values (−0.77 to –0.52‰) that require substantially more contamination from a low δ⁶⁰Ni source, similar to some of the local sedimentary rocks. Copper isotope ratios of country rock-hosted massive sulfides near Eagle and Tamarack are lower than those reported by previous workers and are mostly about 0‰, similar to those expected for unaltered mantle. Tamarack samples require very little crustal contamination to explain their isotope ratios, whereas Eagle samples require no contamination from local sedimentary rocks to explain their Cu isotope compositions. Copper isotope ratios of samples beneath the Partridge River intrusion are similar to those from previous analyses of igneous-hosted sulfides, supporting their origin from a magmatic sulfide liquid. The low δ⁶⁵Cu values (–1.14 to 0.25‰) from samples below the Partridge River intrusion cannot be explained by contamination from the Virginia and Thomson Formations, which are characterized by mostly positive δ⁶⁵Cu values (–0.33 to 3.12‰), unless a lighter reservoir remains to be discovered in the local Proterozoic rocks. We suggest these values may have been produced by a combination of incomplete sulfide melting during partial melt generation and fractionation related to sulfide segregation at variable R-factors.

中文翻译：

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位于大陆中部裂谷侵入体附近的以乡村岩石为主体的块状硫化物的铜和镍同位素变化

以前的铜同位素工作记录了δ ⁶⁵之间的明显差异与中大陆裂谷系统相关的片状（–0.5 至 0.5‰）和导管式（0.5–2.0‰）侵入体的铜值。金属同位素在岩浆镍-铜-铂族元素（PGE）矿床研究中的应用尚处于早期阶段，对同位素分布和高温分馏机制知之甚少。为了解决前面提到的 Cu 同位素差异，确定侵入体的金属来源，并评估高温金属同位素分馏的来源，我们测量了一系列特征非常好的 Ni-Cu- PGE 块状硫化物发生在密歇根州和明尼苏达州中部大陆裂谷系统内侵入体附近的沉积岩中。以前的矿物学和 S、Pb、和 Os 同位素测量表明块状硫化物是岩浆成因，并为从岩浆过程（包括元古界围岩同化）方面解释 Cu 和 Ni 同位素数据提供了框架。 确定了块状硫化物的铜和 Ni 同位素比以及当地的沉积硫化物，并将这些结果与中大陆裂谷和其他地方的岩浆和沉积硫化物的可用铜和镍同位素结果进行比较。硫化物的镍同位素比率已根据可变硅酸盐/硫化物比率或 R 因子（硅酸盐岩浆与硫化物岩浆的质量比）、地壳污染和橄榄石分馏的影响进行建模。接近零且略为负的 δ 包括元古代围岩的同化作用。测定块状硫化物和局部沉积硫化物的铜和镍同位素比，并将这些结果与中大陆裂谷和其他地方岩浆和沉积硫化物的可用铜和镍同位素结果进行比较。硫化物的镍同位素比率已根据可变硅酸盐/硫化物比率或 R 因子（硅酸盐岩浆与硫化物岩浆的质量比）、地壳污染和橄榄石分馏的影响进行建模。接近零且略为负的 δ 包括元古代围岩的同化作用。测定块状硫化物和局部沉积硫化物的铜和镍同位素比，并将这些结果与中大陆裂谷和其他地方岩浆和沉积硫化物的可用铜和镍同位素结果进行比较。硫化物的镍同位素比率已根据可变硅酸盐/硫化物比率或 R 因子（硅酸盐岩浆与硫化物岩浆的质量比）、地壳污染和橄榄石分馏的影响进行建模。接近零且略为负的 δ 并将这些结果与中大陆裂谷和其他地方的岩浆和沉积硫化物的可用铜和镍同位素结果进行比较。硫化物的镍同位素比率已根据可变硅酸盐/硫化物比率或 R 因子（硅酸盐岩浆与硫化物岩浆的质量比）、地壳污染和橄榄石分馏的影响进行建模。接近零且略为负的 δ 并将这些结果与中大陆裂谷和其他地方的岩浆和沉积硫化物的可用铜和镍同位素结果进行比较。硫化物的镍同位素比率已根据可变硅酸盐/硫化物比率或 R 因子（硅酸盐岩浆与硫化物岩浆的质量比）、地壳污染和橄榄石分馏的影响进行建模。接近零且略为负的 δTamarack 和 Eagle 附近的岩浆硫化物（–0.45 到 0.17‰）的⁶⁰ Ni 值可以用可变 R 因子下地幔衍生熔体的轻微地壳污染来解释。来自鹧鸪河侵入体下方的火成岩来源的贫铜硫化物通常具有较低的 δ ⁶⁰ Ni 值（-0.77 至 –0.52‰），这需要来自低 δ ^{60 的}更多污染镍源，类似于当地的一些沉积岩。Eagle 和Tamarack 附近的围岩块状硫化物的铜同位素比低于以前的工作人员报告的铜同位素比，大多在0‰左右，与未改变地幔的预期相似。Tamarack 样品需要很少的地壳污染来解释它们的同位素比率，而 Eagle 样品不需要来自当地沉积岩的污染来解释它们的 Cu 同位素组成。鹧鸪河侵入体下方样品的铜同位素比率与之前对火成岩中的硫化物的分析相似，支持它们来自岩浆硫化物液体。低δ ⁶⁵鹧鸪河侵入体下方样品的铜值（–1.14 至 0.25‰）无法用来自弗吉尼亚和汤姆森地层的污染来解释，其特征是大部分为正 δ ⁶⁵铜值（–0.33 至 3.12‰），除非储层较轻在当地的元古界岩石中仍有待发现。我们认为这些值可能是由部分熔体生成过程中不完全硫化物熔化和与可变 R 因子下硫化物分离相关的分馏相结合产生的。