Some mafic–ultramafic intrusions in the North American Midcontinent Rift System host disseminated to massive sulfides of magmatic origin. Massive sulfides are also present in the immediate sedimentary country rocks to some of these intrusions, such as Partridge River, Tamarack, and Eagle. Our working hypothesis is that the country rock-hosted massive sulfides are also of magmatic origin. To test this hypothesis, we have carried out an integrated mineralogical, chalcophile elements, and isotopic (S-Os-Pb) study of the country rock-hosted massive sulfide samples from Partridge River, Tamarack, and Eagle. Data for the intrusion-hosted sulfides from previous studies are used for comparison. Like the intrusion-hosted massive sulfides, the country rock-hosted massive sulfides are mainly composed of pyrrhotite, pentlandite, chalcopyrite, and cubanite and have high Ni, Cu, and PGE tenors, consistent with the crystallization products of magmatic sulfide liquids. These two different types of sulfide occurrences at Partridge River are different in some chalcophile element ratios and S-Os-Pb isotopes, but such differences can be explained by different parental magmas with different degrees of crustal contamination and different R-factors during sulfide segregation. At Tamarack and Eagle, these two different types of sulfide occurrences have similar S-Os-Pb isotope compositions, but the similarity in chalcophile element compositions between them is restricted to only some of the samples. Negative Pt anomalies are more common for the country rock-hosted massive sulfide than the intrusion-hosted sulfide ores. Positive Pt anomalies are not observed in the country rock-hosted massive sulfide samples but are present in some of the intrusion-hosted sulfide ore samples. Our modeling results show that the observed similarities and differences between these two different types of sulfide occurrences in each of the deposits can be explained by a common parental magma, variable R-factors during sulfide-liquid segregation, and variable degrees of fractional crystallization of monosulfide solid solution from sulfide liquids. Given the fact that positive Pt anomalies are present in some of the intrusion-hosted sulfides ores, we suggest that the negative Pt anomalies in the country rock-hosted magmatic sulfides are due to a nugget effect or removal of early-crystallized platinum group minerals, such as sperrylite (PtAs2), from the sulfide liquids prior to their infiltration into the surrounding country rocks.

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.

The organization of natural materials into hierarchical structures accounts for the amazing properties of many biological systems; however, translating the structural motifs present in such natural materials to synthetic systems remains difficult. Inspired by how nature creates materials, this work demonstrates that kinetically controlled sequential seeded growth is a general bottom-up strategy to prepare hierarchical inorganic crystals with distinct compositions and nanostructured forms. Specifically, 85 distinct hierarchical crystals with different shape-controlled features, compositions, and overall symmetries were readily achieved by altering the kinetics of metal deposition in sequential rounds of seeded growth. These modifications in the deposition kinetics were achieved through simple changes to the reaction conditions (e.g., pH or halide concentration) and dictate whether concave or convex features are produced at specific seed locations, much in the manner that the changing atmospheric conditions account for the hierarchical and symmetrical structures of snow crystals. As such, this work provides a general paradigm for the bottom-up synthesis of hierarchical crystals regardless of inorganic material class.

The Upper Cretaceous Fort Crittenden Formation exposed in the Santa Rita and Huachuca Mountains of southeastern Arizona is a syntectonic deposit that has been associated with Laramide tectonic activity. However, the spatio-temporal relationships among Cretaceous sedimentation, magmatism, basement exhumation, and possible flat slab-related processes in the southern Laramide region remain poorly understood. Age controls for uplift and erosion of local topography and syntectonic deposition in response to deformation remain particularly poor. The Fort Crittenden Formation comprises 800-2500 m of locally derived fluvial to alluvial fan sedimentary rocks and records paleodrainage reorganization in response to active tectonics. Changes in sedimentary facies, provenance, and paleoflow suggest deposition in a tectonically partitioned intraforeland basin. New detrital zircon data constrain the timing of deposition of the Fort Crittenden Formation between ca. 86 Ma and ca. 76 Ma. The lack of depositional age zircons throughout the majority of the Fort Crittenden Formation is consistent with a magmatic lull in the Cordilleran arc between ca. 90 Ma and ca. 76 Ma. The overlying Salero Formation and Late Cretaceous intrusions are expressions of renewed magmatism in southeastern Arizona at ca. 75 Ma. New Lu-Hf data indicate that magmas evolved from contamination of old juvenile crust. When interpreted in conjunction with other regional data sets, our study indicates that the Laramide deformation front migrated eastward into southwestern New Mexico by 75 Ma. Thermal modeling of apatite fission track and (U-Th)/He data from granitic clasts are consistent with Late Cretaceous-Paleocene (ca. 76-55 Ma) heating related to magmatism and cooling and exhumation during the Eocene and Oligocene.

The J-M Reef in the Stillwater Complex of Montana is the highest grade Pd deposit in the world, averaging approximately 14ppm Pd. One suggestion for the origin of the enrichment centers on the sequestration of metals in staging chambers via immiscible sulfide liquids that formed in response to the addition of crustal S. Because δ34S values alone are often insufficient to document in igneous rocks the assimilation of S from Archean sedimentary rocks, a combined study of both δ33S and δ34S values, and computed Δ33S values, was undertaken. δ34S and Δ33S values of cordierite-bearing hornfels, as well as massive sulfides in the metasedimentary rocks of the contact aureole can be divided into two populations. One is characterized by anomalous positive Δ33S values from 0 to 0.32‰ and δ34S values from 0.5 to −1.6‰, whereas the other is characterized by anomalous negative Δ33S values from 0 to −0.22‰ and δ34S values from 1.5 to 5.8‰. δ34S and Δ33S values for rocks from the Basal Series of the Complex range from −0.9 to 3.0‰ and 0.01 to 0.18‰, respectively; the anomalous S isotope ratios indicate that for some samples at least 50% of the their S was of crustal origin. In contrast, most of the rocks from the J-M Reef are characterized by δ34S and Δ33S values between −1.3 and 1.8‰ and −0.06 to 0.06‰, respectively. These values deviate only slightly, if at all, from those thought to be representative of mantle S, and suggest that (i) contamination was not an important process for PGE enrichment in the Complex, (ii) the contaminant was also characterized by near 0 Δ33S values, or (iii) S isotopic exchange in the staging chambers was an important process. Upgrading of PGE values via reaction between sulfide in the chamber and uncontaminated mantle-derived magma may also have resulted in the conversion of S isotopic ratios that may have been indicative of crustal S addition, to mantle-like values. Rare δ34S values up to 7.2‰ in the J-M Reef and 8.2‰ in chromitite from the Ultramafic Series provide strong evidence for localized assimilation of crustal S which did not experience isotopic homogenization. The S isotopic ratios of the J-M Reef are non-unique in terms of a specific genesis as they may be interpreted in either a downward, sulfide collection model, or an upward fluid transport model.

Fe-Ni-Cu-(PGE) sulphides are commonly found within mafic and ultramafic intrusions. However, many massive sulphides occur in sedimentary rocks, far removed from the intrusions, and are relatively underexploited, despite some significant metal endowments. Because these mineral occurrences are relatively underutilized, informed genetic models are currently lacking. Massive sulphides near the Tamarack Intrusive Complex, the Eagle Intrusion, and the Duluth Complex consist primarily of pyrrhotite, pentlandite, and chalcopyrite in variable proportions, but massive sulphides below the Stillwater complex are primarily pyrrhotite and magnetite. Major and minor elements in sulphides (Ni, Co, and Se) show significant compositional differences based on locality. Samples from Stillwater and Duluth have much higher S/Se ratios than found in the mantle, whereas Tamarack and Eagle have S/Se values near mantle values. Ni/Co at Tamarack and Eagle far exceed mantle ranges, but can potentially be explained by coupled Ni and Co scavenging in olivine -crystallizing magmas. Mantle normalized PGE patterns at Eagle, Tamarack, and Duluth are nearly identical to PGE profiles in the associated igneous -hosted sulphides. At Stillwater, however, PGE patterns are quite different in the country rock -hosted massive sulphides compared to igneous -hosted sulphides.