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TitleDistribution of siderophile elements during sulphide fractionation within magmatic Ni-Cu-PGE systems: a LA-ICP-MS investigation of the Crystal Lake Intrusion, 1.1 Ga Midcontinent rift
AuthorSmith-Holder, JORCID logo; Bleeker, WORCID logo; Petts, D CORCID logo
SourceOre Geology Reviews vol. 148, 105002, 2022 p. 1-23, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20220088
Mediapaper; digital; on-line
File formatpdf; html
AreaLake Superior
Lat/Long WENS -89.6833 -89.2333 48.1667 47.9997
SubjectsScience and Technology; Nature and Environment; general geology; Crystal Lake Intrusion
Illustrationslocation maps; diagrams; photographs; tables
ProgramTargeted Geoscience Initiative (TGI-5) Volcanogenic massive sulphide ore systems - base metal sources and processes
Released2022 09 01
AbstractWithin the magmatic pyrrhotite-pentlandite-chalcopyrite assemblage, which is ubiquitous of many Ni-Cu-PGE deposits, the spatial distribution of PGEs and semi-metals during both sulphide fractionation and low temperature alteration is not well documented. In-situ characterisation of sulphides provides a mechanism to trace the behaviour of elements during ore formation, with trace element distribution patterns providing valuable insights at the microscale into the genesis of magmatic sulphide deposits. Here we report trace element data determined by quantitative LA-ICP-MS mapping for a suite of variably altered samples from the taxitic rocks of the undeformed Crystal Lake Intrusion, within the 1.1 Ga Midcontinent Rift. Combining LA-ICP-MS data with detailed petrography and multivariate statistics helps identity the processes responsible for elevated metal concentrations and observed trace element distribution patterns within Ni-Cu-PGE systems. Such data can also improves our understanding of the effects fluid interaction and assimilation of volatile rich rocks has on the metal contents of sulphides. Element maps reveal complex distribution patterns throughout the disseminated sulphides of the Crystal Lake Intrusion, which are notably enriched in Pd relative to the massive sulphides developed locally along the basal contact. Replacement of the primary sulphides by low temperature phases is restricted to the northern limb of the intrusion and results in heterogeneous distributions and elevated concentrations of As, Pd, Ag and Bi along fractures. Zonation of As, Mo, Ru, Rh, Re and Bi is observed in both pentlandite and chalcopyrite. Within the globular ores, petrographic observations suggest the pyrrhotite-chalcopyrite contact with which 'contact' pentlandite resides along, may not be indicative of the initial boundary between monosulphide solid solution (MSS) and the residual sulphide liquid. This has implications for the interpretation of trace element zonation patterns, which in the case of this study are not considered to be indicative of formation by peritectic reaction. We suggest much of the trace element zonation observed in the Crystal Lake Intrusion relates to inheritance from previous grain boundaries and as such, the possible presence of high temperature pentlandite can only be identified through statistical clustering of the LA-ICP-MS data. With multivariate statistical analyses revealing subtle geochemical features within each phase, discrete mineralising events and sulphide generations are recognised. Within the locally vapour saturated taxites of the southern limb, a new microfabric is documented within the globular ores, revealing a close spatial relationship between enhanced trace element concentrations and microstructures within undeformed magmatic sulphides. The microfabric, which is defined by V, Cr, As, Pb and Bi and less frequently by Mo, Rh, Pd and Re, is prevalent in unaltered coarse pyrrhotite and in contact, granular and fan textured pentlandite, and is developed in association with both silicate-capped and uncapped sulphide globules. Regardless of the exact mechanism by which the microfabric formed (e.g., fluid interaction, exsolution, gravitational compaction), this study highlights that regional deformation is not required to drive the preferential enrichment of select elements along discrete planes. Only as more high resolution element map data become available will it become apparent if the microfabric is a common feature of magmatic Ni-Cu-PGE deposits or a unique characteristic of the Crystal Lake Intrusion.
Summary(Plain Language Summary, not published)
The manuscript presents new high resolution spatial distribution patterns for trace elements in magmatic sulphides and discusses key observations and their implications for Ni-Cu-PGE systems. The study provides insights into the behaviour of elements during ore formation. The manuscript also documents a new trace element distribution pattern through combining detailed petrography with LA-ICP-MS mapping. This new microfabric has implications for our understanding of how certain metals reside within magmatic sulphides and could suggest some elements are more susceptible for subsequent remobilisation by fluids.

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