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

智力优势：Eagle矿床位于密歇根州北部的巴拉加盆地，由在岩脉状橄榄岩体内发现的半块状和块状Fe-Ni-Cu硫化物组成。这些岩石与元古界中大陆裂谷系统(MRS)有关，被认为是早期裂谷相关火山的供给者。围岩包括元古界变质沉积岩，局部为石墨质和硫化物，这些岩石似乎是火成岩中S的可能来源，但火成岩中的硫化物具有d34S值小于4/mil的特征，而元古界岩石中的硫化物的d34S值在~10~40/mil之间。初步的S同位素分析表明，d33S值在0.02~-0.7‰之间，表明S同位素具有明显的质量无关分馏作用，迄今仅在太古宙岩石中有记载。因此，鹰型矿床中的S主要来自太古宙岩石，而不是富含硫化物的元古界岩石。为了验证和理解这一令人惊讶的结果，建议对Eagle矿床以及元古界和太古界围岩进行详细的d33S变化调查。多项S同位素研究已被证明对继续评估地球早期大气非常重要。然而，最近的研究表明，质量独立分馏S同位素(MIF-S)可能并不局限于年龄大于~2Ga的岩石。EAGLE拟议的地层控制研究将提供元古界(两个序列，一个为~2.2-2.4Ga，另一个为~1.8Ga-1.9Ga，将涵盖地球历史上关于MIF-S的重要时段)以及太古宙岩石中d33S值的分布信息。我们还计划分析明尼苏达州德卢斯杂岩中发现的辉长岩及其伴生的铜镍硫化物矿化，该杂岩位于裂谷北侧。在这些样品中，硫化物堆积的d34S值与1.8~1.9Ga碳质围岩中的铁硫化物的值相似。这种组合与EAGLE的组合有很大的不同，并提供了一个对比MIF-S在MERS形成过程中不同时期形成的成矿系统中多个MIF-S同位素系统的机会。还将进行EAGLE岩石中橄榄石和斜长石的氧同位素分析，以研究围岩的挥发分和部分熔融在MIF-S向岩浆系统转移中的相对重要性。这一研究成果将加深我们对MRS中岩浆硫化物成矿作用的认识，有助于揭示裂谷发育过程中不同时期地幔岩浆与地壳岩石相互作用的本质，也将对S质量独立分馏作用在地球上的作用提供新的认识。这项拟议的研究代表了学术界和矿业之间的合作。肯纳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，美国镍和铂族元素的储量很低，需要进行这类研究来评估已知资源并开发勘探模型。研究生和本科生都将从与采矿业的合作中受益，并将接受使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.