Abstract
Most of the world’s economically-viable platinum deposits occur as ‘reefs’ in layered intrusions – thin layers of silicate rocks that contain sulphides enriched in noble metals. There are two contrasting magmatic hypotheses for their formation. The first suggests accumulation through gravity-induced settling of crystals onto the magma chamber floor. The alternative argues for in situ crystallization, i.e. upward growth from the floor. Here we report on our discovery of the Merensky Reef in the Bushveld Complex that occurs on subvertical to overturned margins of depressions in a temporary chamber floor. Such relationships preclude crystal settling and demonstrate that the reef crystallized in situ. This finding indicates that platinum deposits can grow directly at the chamber floor, with immiscible sulfide droplets sequestering ore-forming noble metals from strongly convecting silicate magmas. Our model also provides evidence for the paradigm that argues for magma chambers being masses of nearly crystal-free melt, which gradually loses heat and crystallizes from the margins inward.
Highlights
Merensky-type platinum deposits in mafic-ultramafic layered intrusions (Fig. 1a) – named after the classical Merensky Reef (MR) of the Bushveld Complex in South Africa (Fig. 1b) – are key repositories of strategically important metals (Fig. 1c), which are essential for the sustainable development of modern human society and a major petrological challenge for almost a century[1,2,3,4,5,6,7]
The MR usually occurs as a package of sulphide-bearing silicate rocks of broadly melanoritic to orthopyroxenitic composition that are extremely enriched in the platinum group elements (PGE)
This evidence is inconsistent with the hypothesis that the MR formed by crystal settling on the chamber floor – an idea that is deeply entrenched in explanations of platinum reefs in layered intrusions1–5,8–16 – and indicate, instead, that in situ growth of the MR on irregular surface of the chamber floor appears to be the only explanation for these geometries
Summary
Merensky-type platinum deposits in mafic-ultramafic layered intrusions (Fig. 1a) – named after the classical Merensky Reef (MR) of the Bushveld Complex in South Africa (Fig. 1b) – are key repositories of strategically important metals (Fig. 1c), which are essential for the sustainable development of modern human society and a major petrological challenge for almost a century[1,2,3,4,5,6,7]. Our principal finding is that, in potholes, the entire MR package may locally develop along steeply dipping portions of potholes and even the undersides of overhanging sidewalls This evidence is inconsistent with the hypothesis that the MR formed by crystal settling on the chamber floor – an idea that is deeply entrenched in explanations of platinum reefs in layered intrusions1–5,8–16 – and indicate, instead, that in situ growth of the MR on irregular surface of the chamber floor appears to be the only explanation for these geometries. This implies that the chemical evolution of melt in magma chambers occurs through physical separation of evolved liquids from crystals growing in situ by compositional convection[41,42,47,48,58]
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