Browsing by Subject "Stillwater Complex"

Layered igneous intrusions such as the Stillwater Complex in Montana contain the most economic concentrations of platinum-group elements (PGE) in the world, yet the processes involved in the enrichment of these PGE remain unclear. Some researchers propose that the PGE were enriched into sulfide phases through purely magmatic processes, while others postulate that late-stage, high-temperature fluids caused remobilization of the more soluble elements upwards from the base of the crystal pile. Although much work has been carried out on the economic PGE-enriched ore zone (J-M reef), the silicate mineralogy and the bulk geochemistry of the Complex, the detailed petrographic trends have not been investigated. This dissertation comprises a detailed petrographic study into the assemblages associated with sulfide and other trace minerals throughout the stratigraphy.

Sampling was carried out from both surface outcrops and drill cores over four consecutive field seasons. Polished thin sections were produced which were then examined by petrographic microscope and electron microprobe. In addition, bulk rock analysis was carried out by x-ray fluorescence spectrometry (XRF).

In brief, the sulfide and trace mineral assemblage studies described below reveal a number of interesting observations. An upwards trend from pentlandite-rich to pyrrhotite-rich to chalcopyrite + pyrite-rich assemblages is observed below the reef, and the same trend occurs above the reef with the transition occurring just below the reef, in upper GN-I. Trace element analysis shows that Cu levels are higher above the reef than below it, and that although Zn and Cu contents are correlated below the reef, a restricted range of Zn contents occurs above the reef, while Cu is highly variable. As all `low-temperature' assemblages (those associated with extensive silicate alteration or the presence of greenschist facies minerals such as chlorite, clinozoisite and epidote) were discounted, the majority of sulfide assemblages present were either pristine(multiphase, often globular in shape, with no associated silicate alteration) or high-temperature (multiphase, with high-temperature minerals such as biotite, hornblende, carbonates, etc, and with little associated silicate alteration) in occurrence. Some differences were observed between the hanging-wall and footwall rocks, including the presence of native copper, sphalerite in a calcite-hornblende vein, and high-temperature carbonates in footwall and not hanging-wall rocks. The high-temperature carbonates observed comprise dolomite with exsolved patches of calcite. The textural relationships and Fe-Mn compositions of the Stillwater carbonates are similar to those of mantle carbonates. High-temperature desulfidation is also observed both above and below the reef, in the form of pyrite being converted to magnetite, and chalcopyrite to a Cu-Fe-oxide (delafossite). Both sets of assemblages are associated with little to no silicate alteration. When taken together, the upwards increase in Cu and S, the variable Cu contents above the reef, the native copper, high-temperature carbonates and high-temperature sphalerite-bearing veins below the reef, and the evidence for desulfidation are all most readily explained by the remobilization of selected phases by a high-temperature fluid. This dissertation provides evidence that the fluid present in the latter stages of Stillwater formation had a carbonic as well as a Cl-rich component, and would therefore have been efficient in PGE remobilization.

The genesis of layered intrusions has been the focus of countless studies. Layered intrusions have historically been viewed as natural laboratories to understand the evolution of a single large magma chamber. Many contain platinum- and palladium-rich reef-type deposits, making layered intrusions particularly important economically. Further, layered intrusions may be a useful analogue for understanding subduction zone hydrothermal systems.This dissertation investigates layered intrusion genesis, specifically in relation to the suggested hydrothermal model of layered intrusion formation, which suggests migrating fluids may have remobilized economically important elements, creating the deposits observed in these intrusions today. The work is divided into eight chapters that explore three layered intrusions: the Bushveld Complex, South Africa, the Stillwater Complex, Montana, and the Skaergaard Intrusion, Greenland. New samples collected from the Stillwater Complex were analyzed for major and trace element compositions and radiogenic and stable isotopes. Investigations into the Bushveld Complex and Skaergaard Intrusion were based on previously published data. The second chapter examines evidence for fluid circulation in the Bushveld Complex, South Africa, as responsible for some of the geochemical and isotopic signatures present in the complex. Previous isotopic studies of Bushveld are combined with numerical modeling of footwall dehydration to suggest that diapir-like structures injected fluids into the Main Zone of the intrusion. This chapter further details the similarities between diapiric structures in the Bushveld Complex and those that have been modeled in subduction zone hydrothermal systems. The third chapter expands upon the Bushveld model, specifically in relation to the formation of iron-rich ultramafic pegmatoids and dunite pipes, which the work presented here suggests to be fluid-related. In the fourth chapter, strontium, neodymium, and lead isotopes are analyzed for rocks from the Stillwater Complex, Montana, to compare with the isotopic mixing model results of the Bushveld Complex. Initial isotopic ratios are used to explore various proposed models of complex formation. Results suggest isotopic heterogeneity during complex formation, whether due to heterogeneous source regions or crustal/fluid contamination. In the fifth chapter, stable isotope analyses (oxygen, hydrogen, and lithium) are used to better understand the formation of the pegmatoidal bodies thought to be related to fluids at Stillwater. Some evidence of fluid circulation may be observed in hydrogen and lithium isotopes. Geothermometry using oxygen isotopes is suggestive of lower cooling temperatures in the pegmatoids, and may provide evidence of mineral-scale disequilibrium attributable to fluid circulation. The sixth and seventh chapters utilize the thermodynamic modeling program MELTS to explore problems of layered intrusion evolution. Understanding the evolution of the liquids that formed various layered intrusions, and identifying magmas parental to layered intrusions, can pose a challenge. Using MELTS, bulk rocks can be synthetically remelted, and the evolution of the complex can be examined through analysis of estimated trapped liquid contents. The sixth chapter applies this method to the Stillwater Complex, while the seventh chapter extends this work to the Skaergaard intrusion in East Greenland. These investigations allow for examination of the magmatic processes operating alongside hydromagmatic processes in layered intrusions.