Abstract
A large proportion of in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide between with up to about five modal percent sulfides, called and (or matrix) containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of patchy net-textured ores containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.
Highlights
42 Magmatic sulfide ore deposits account for some of the world’s most valuable metal43 accumulations, currently accounting for ~56% of the world’s nickel production and over 96%44 of supply of platinum, palladium and the other platinum group elements (Mudd and Jowitt, 45 2014; Zientek et al, 2014; Peck and Huminicki, 2016)
The results presented here are the culmination of an extended body of work using a variety of characterization techniques to investigate sulfide-silicate ore textures, with the core technology being x-ray computed tomography for investigating microtextures in 3D. Combining this methodology with other newly-available techniques, such as high-resolution microbeam XRF mapping, opens a range of observations impossible to obtain using conventional petrographic techniques, on the size, morphology and connectivity of phases and grain aggregates
We further suggest that the complex textures here may reflect an origin of the cumulus silicates as crescumulate dendritic phases, which underwent partial textural equilibration before displacement of the interstitial silicate melt by percolating sulfide
Summary
of supply of platinum, palladium and the other platinum group elements (Mudd and Jowitt, 2014; Zientek et al, 2014; Peck and Huminicki, 2016) They form by the accumulation of 46 immiscible sulfide liquid that has scavenged chalcophile elements from a coexisting silicate. Accumulations of widely varying proportions of sulfide in small mafic or maficultramafic intrusions, usually identifiable as magma conduits (Barnes et al, 2016a; Lightfoot and Evans-Lamswood, 2015); 3. Accumulations of widely varying proportions of sulfide in komatiite (Lesher, 1989; Lesher and Keays, 2002; Barnes, 2006) or ferropicrite (Hanski, 1992; Keays, 1995; Hanski et al, 2001) lava flows or associated shallow subvolcanic intrusions, commonly identifiable as magma conduits or feeder tubes; 4. Sulfide accumulation from an impact-generated crustal melt sheet: the unique example of Sudbury (Keays and Lightfoot, 2004; Naldrett, 2004)
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