The use of sulfide mineral chemistry to understand PGE mineralization processes in the Northern Limb of the Bushveld Igneous Complex
Date
2020-01-10
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Abstract
The Platreef is a complex lithological unit defining the base of the Northern Limb of the Bushveld
Igneous Complex (BIC) and typically characterized by complex magma-footwall interaction. It is of
interest primarily because it contains significant PGE-Ni-Cu mineralization and there is no consensus
regarding the mineralization processes, particularly on the role of footwall assimilation. One puzzling
aspect is the apparent lack of correlation between sulfide abundance and PGE-Ni-Cu contents.
Another aspect is the possible relationship between PGE-Ni-Cu mineralization in the Northern Limb
and PGE-Ni-Cu mineralization elsewhere in the BIC (e.g. the Merensky Reef). This study focuses on
the mineral chemistry (major and trace elements) of major sulfides: pyrrhotite (Po), pentlandite (Pn),
and chalcopyrite (Ccp) in Platreef samples intersected by Ivanplats during deep drilling in the
Turfspruit area. A drill core (UMT094), for which Sr and S isotope stratigraphy was collected
previously, was used as reference because the isotopic data helped define the degree of footwall
assimilation. Whole rock geochemistry (Cu, Ni, S, PGE) showed the existence of at least two distinct
sample populations in UMT094: one defined by relatively low S/Ni (with no evidence of footwall
assimilation and stratigraphically associated with PGE mineralization), the other defined by relatively
high S/Ni (and stratigraphically associated with footwall assimilation and with low PGE grades). The
project had three objectives. The primary goal was to assess whether the differences identified in
whole rock geochemistry were also expressed in the composition of major sulfides (pyrrhotite,
pentlandite, chalcopyrite) in UMT094. Having established those parameters, the second goal was to
use the same approach (and assess the degree of footwall assimilation) on samples (from drill cores
UMT231 and UMT233) considered representative of the main PGE-Ni-Cu mineralization (i.e. the
Flatreef) in the Turfspruit area. Finally, the data obtained in this study were compared to similar data
from the Merensky Reef (from published peer-reviewed literature) to evaluate possible matches
between sulfide chemistry in assemblages associated with PGE mineralization in the Northern Limb
and the Western Limb of the Bushveld Igneous Complex (to assess a possible common origin). After
careful petrographic observations and documentation, major and trace element contents in sulfides
were acquired using electron probe microanalysis (EPMA) and laser ablation inductively coupled
plasma mass spectrometry (LA-ICP-MS). The results showed that there are no significant differences
in major element chemistry (e.g. Fe/Ni in pentlandite, Fe/S in pyrrhotite) between the two populations
identified. However, the trace element data validate the hypothesis tested and show that in UMT094:
(a) Selenium contents in all sulfides associated with PGE-Ni-Cu mineralization are higher than in
sulfides associated with footwall assimilation; and (b) Other trace elements (such as Co and Ru in
pyrrhotite; Co, Pd and Ru in pentlandite; Cd and Sn in chalcopyrite) are also useful in distinguishing
between sulfides in intervals with no evidence of footwall assimilation and sulfides (lower in the
stratigraphy) associated with footwall assimilation. When applied to the Flatreef samples (UMT231
and UMT233) the results show that, in general, trace element content in sulfides from the Flatreef also
varies with stratigraphy: sulfides from Flatreef intervals with high PGE grades (and intervals above)
have trace element contents that are closer match to sulfides from the unaltered (magmatic) intervals
in UMT094. Finally, when data from this study is compared to similar data from the Merensky Reef
the results show significant overlap. This is interpreted as compelling evidence to conclude that the
mechanisms that operated in the formation of PGE-Ni-Cu mineralization in the Flatreef were similar
to those involved in the formation of the Merensky Reef (for which local footwall assimilation is not
considered a significant factor). Thus, footwall assimilation was not the primary process in PGE-NiCu mineralization. These results show that trace element data in major sulfides can be an additional
and useful tool to distinguish among different sulfide mineralization events
Description
Keywords
Bushveld Igneous Complex, Northern Limb, Platreef, Flatreef, sulfides