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Geochemistry and mineralogy of platinum group element mineralization in the river valley intrusion, Ontario, Canada : A model for early-stage sulfur saturation and multistage emplacement and the implications for "contact-type" Ni-Cu-PGE sulfide mineralization
journal contribution
posted on 2014-07-31, 11:15 authored by David A. Holwell, R. R. Keays, E. A. Firth, J. FindlayThe River Valley Intrusion (RVI) within the ~2.48 Ga East Bull Lake Intrusive Suite in
Ontario, Canada, is an example of a mafic igneous intrusion with 'contact-type' Ni-Cu-
PGE sulfide mineralization along its base. Whereas many 'contact-type' deposits are
thought to form from in situ contamination of the magma by the addition of crustal S
during emplacement, there are some intrusions, including the RVI, which appear to
have a much more complex history where the timing of S saturation, and thus the
critical ore genesis processes, may have occurred much earlier, prior to emplacement.
The RVI is made up of a basal ~100 m of unlayered, inclusion-bearing units, overlain
by layered cumulates. The basal units contain autoliths of gabbroic rocks and
inclusions of footwall gneiss and amphibolites, all within a gabbroic matrix. Platinumgroup
element-rich magmatic sulfide mineralization occurs throughout both the
inclusions and the matrix as blebby and disseminated sulfides. The matrix and
inclusions can be separated into two distinct textural types: hydrothermally altered
greenschist assemblages and unaltered metamorphic amphibolite assemblages. The
platinum-group mineral (PGM) assemblages vary only between textural types, and not
between inclusions and matrix, being dominated tellurides in all rock types. The
hydrothermally altered rocks, however, have fewer tellurides, and an increased amount
of Sb- and As-bearing PGM, indicative of minor fluid interaction, although the PGM
have not been mobilised significantly away from the base metal sulfides. Precious and
base metal geochemistry shows all rock types to have an excellent correlation between
all the PGE, indicating the presence of a single, well homogenised, PGE-rich sulfide
liquid. However, Au and Cu appear to be decoupled from the PGE at low
concentrations, although correlate well with each other, which is interpreted to be due
to minor fluid redistribution and alteration of sulfide bleb margins. The overlying
Layered Units above the mineralized units are not PGE depleted. Trace element data,
including (Th/Yb)PM and (Nb/Th)PM ratios, demonstrate that all River Valley rocks
were formed from crustally contaminated magmas following interaction with local
country rocks in a deeper subchamber; although some samples have S/Se ratiosindicative of crustal S, most have S/Se ratios lower than the mantle range, indicative of
S loss.
We propose a multi-stage model for the formation of the mineralization in the RVI with
a major contamination event at depth with the addition of S from local crustal rocks,
inducing sulfide saturation. Sulfide droplets were then enriched in PGE within a conduit
system with possible further upgrading of sulfide metal tenors (and reduction of S/Se
ratios) via partial dissolution of sulfide. The PGE-enriched sulfide liquid then settled in a
staging chamber and partially crystallised before a major pulse of magma entrained
sulfide liquid, eroded blocks of pre-crystallised and mineralized gabbro and footwall
rocks and emplaced an inclusion-bearing package as the lower 100m or so of the RVI.
Later emplacement of main RV magma was from a S-undersaturated, PGE-fertile
magma.
The RVI is thus an example whereby 'contact-type' mineralization is purely a function
of the earliest magma intruded containing pre-formed sulfide mineralization, rather than
contamination triggering sulfide saturation in situ. In such cases, processes at depth
determine the generation and subsequent tenor of the mineralization. In particular,
dissolution of the sulfide can upgrade metal tenor, but subsequently will reduce S/Se
ratios, masking the signature of crustal contamination. In addition, a multi stage
emplacement such as this will not necessarily preserve the characteristic increase in
Cu/Pd ratios in the overlying cumulates that is often used in exploration for PGE
deposits in mafic intrusions. Thus, a full understanding of all the field, geochemical and
mineralogical factors is required to construct genetic models for such deposits, and
especially in the interpretation of S/Se and Cu/Pd ratios as an indicator of crustal
contamination and the presence of PGE mineralisation.
History
Citation
Economic Geology, 2014, 109 (3), pp. 689-712Author affiliation
/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of GeologyVersion
- AM (Accepted Manuscript)