| dc.description.abstract |
<p>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.</p><p>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).</p><p>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.</p>
|
|
