Oxygen and Multiple S Isotope Investigation of Magma-Country Rock Interaction During Early Stages of Magmatism in the Midcontinent Rift System: Constraints From the Eagle Deposit

智力优点：Eagle存款位于密歇根州北方的Baraga盆地，由半块状和块状Fe-Ni-Cu硫化物组成，发现于岩脉状橄榄岩体中。这些岩石与元古宙中大陆裂谷系（MRS）有关，被认为是早期裂谷相关火山岩的供给者。围岩包括局部石墨质和硫化物质的元古代变质沉积岩;这些岩石似乎是火成岩中所含S的可能来源，然而，火成岩中的硫化物的特征是d34 S值小于4/mil，而元古宙岩石中的硫化物的d34 S范围为~初步的S同位素分析表明，d33 S值的范围从0.02到-0.7每密耳，这表明迄今为止仅在太古代岩石中记录的S同位素的显著质量独立分馏。因此，Eagle存款中的大部分硫必须来自太古代岩石，而不是来自富含硫化物的元古代岩石。为了验证和理解这一令人惊讶的结果，建议对Eagle存款以及元古代和太古代围岩的d33 S变化进行详细调查。多硫同位素研究已被证明是重要的，继续评估地球的早期大气。然而，最近的研究表明，质量无关分馏S同位素（MIF-S）可能并不局限于年龄大于2Ga的岩石。拟定的Eagle地层控制研究将提供元古宙（两个序列，一个为~ 2.2-2.4 Ga，另一个为~1.8-1.9 Ga，将涵盖地球历史中MIF-S的重要间隔）和太古代岩石中火成岩和相关硫化物矿化中d33 S值分布的信息。我们还计划分析位于裂谷北侧的明尼苏达州杜卢斯杂岩中发现的辉长岩和相关的铜镍硫化物矿化。在这些样品中的硫化物积累的d34 S值是类似的含碳，1.8至1.9 Ga围岩中的铁硫化物的值。该协会是非常不同的鹰，并提出了一个机会，对比多个S同位素系统的矿化系统，形成在不同的时间在MRS的发展。橄榄石和斜长石的岩石在鹰的氧同位素分析也将进行调查的相对重要性的挥发分和部分熔融的围岩转移MIF-S的岩浆系统。拟议的研究结果将增加我们对MRS中岩浆硫化物矿形成的理解，将揭示幔源岩浆和地壳岩石在发展中裂谷的不同时期相互作用的性质，并提供对地球上S质量独立分馏的见解。拟议的研究代表了学术界和矿业之间的合作。肯尼科特将资助研究生奖学金作为该项目的一部分，以及有限的地质年代学，矿物学和地球化学研究。此外，美国的镍和铂族元素储量很低，需要进行这类研究，以评价已知资源和开发勘探模式。研究生和本科生都将受益于与采矿业的合作，并将接受利用最先进的分析仪器和科学调查方法的培训。

Intellectual merit: The Eagle deposit, located in the Baraga Basin of northern Michigan, consists of semi-massive and massive Fe-Ni-Cu sulfides found within a dike-like peridotite body. The rocks are associated with the Proterozoic Midcontinent Rift System (MRS), and are thought to represent feeders to early-stage, rift-related volcanics. Country rocks include Proterozoic metasedimentary rocks that are locally graphitic and sulfidic; these rocks appeared to be likely sources for S contained in the igneous rocks, However, sulfides in the igneous rocks are characterized by d34S values less than 4 per mil, whereas sulfides in the Proterozoic rocks show a d34S range from ~10 to 40 per mil. Preliminary S isotopic analyses indicate a range of d33S values from 0.02 to -0.7 per mil, indicating significant mass independent fractionation of S isotopes that to date has only been documented in Archean rocks. Thus, most of the S contained in the Eagle deposit must be derived from Archean rocks, rather than from the sulfide-rich Proterozoic rocks. To verify and understand this surprising result, a detailed investigation of d33S variations is proposed for the Eagle deposit, as well as both Proterozoic and Archean country rocks. Multiple S isotope studies have proven to be important to the continuing evaluation of Earth's early atmosphere. However, recent studies have shown that mass independently fractionated S isotopes (MIF-S) may not be restricted to rocks greater that ~2 Ga in age. The proposed stratigraphically controlled study at Eagle will provide information on the distribution of d33S values in the igneous rocks and related sulfide mineralization, in the Proterozoic (two sequences, one of ~ 2.2-2.4 Ga and the other of ~1.8-1.9 Ga, which will cover important intervals in Earth history with respect to MIF-S), and in the Archean rocks. We also plan to analyze gabbroic rocks with associated Cu-Ni sulfide mineralization found in the Duluth Complex of Minnesota, located on the north side of the rift. In these samples d34S values of the sulfide accumulations are similar to the values of iron sulfides in carbonaceous, 1.8 to 1.9 Ga country rocks. This association is much different from that at Eagle, and presents an opportunity to contrast multiple S isotope systematics in mineralized systems that formed at different times in the development of the MRS. Oxygen isotope analyses of olivine and plagioclase in the rocks at Eagle will also be undertaken to investigate the relative importance of devolatilization and partial melting of country rocks in transferring MIF-S to the magmatic system. Results of the proposed study will increase our understanding of magmatic sulfide ore formation in the MRS, will shed light on the nature of interaction between mantle-derived magmas and crustal rocks at various times in the developing rift, and provide insight into S mass independent fractionation on Earth.Broader impacts. The proposed research represents a collaboration between academia and the minerals industry. Kennecott will fund a graduate student fellowship as part of the project, as well as limited geochronologic, mineralogical and geochemical studies. In addition, reserves of Ni and platinum-group elements in the United States are low and studies of this type are needed to evaluate known resources and to develop exploration models. Both graduate and undergraduate students will benefit from the collaboration with mining industry, and will receive training in the utilization of state-of-the-art analytical instrumentation and in methods of scientific investigation.