Abstract The Merensky Reef of the Bushveld Complex represents a magmatic unconformity that some researchers attribute to chamber replenishment by relatively primitive magma. It is propounded that cumulate rocks in this chamber reacted with replenishing melt, as part of the process that ultimately produced chromitite stringers and reef-style platinum-group element mineralization. This study investigates as to whether chamber replenishment contributed to the formation of the Merensky Reef and its underlying anorthosite at the Rustenburg Platinum Mine in the western lobe of the Bushveld Complex. At this location, the Merensky Reef is a coarse-grained pyroxenite bracketed by millimeter-scale chromitite stringers. This sequence is underlain by a centimeter-scale anorthosite which in turn is underlain by leuconorite. The leuconorite comprises normally zoned cumulus orthopyroxene with poikilitic rims (Mg80-79) and cumulus plagioclase (An80-58), where the latter defines a magmatic fabric indicative of gravitational settling of tabular crystals in a quiescent melt. The contact between leuconorite and anorthosite is marked by an increased abundance of late-stage accessory minerals, and the composition of poikilitic orthopyroxene at this horizon is consistent with trapped liquid shift. Plagioclase crystals in the anorthosite are variably zoned (An79-64) and record a magmatic fabric that strengthens with proximity to the reef. This unit is traversed by sinuous networks of sulfides, pyroxenes, quartz, and very fine-grained chromite that terminate at the contact with the leuconorite. The lower chromitite hosts both amoeboidal and blocky chromite crystals that are enclosed by complexly zoned plagioclase oikocrysts in the lower two-thirds and by orthopyroxene oikocrysts in the upper third. The upper chromitite hosts only blocky crystals, similar to those in the upper portion of the lower chromitite. Microtextural characteristics of the amoeboidal crystals coupled with their propensity to host polymineralic inclusions, suggests that these were initially skeletal crystals that subsequently underwent dissolution-reprecipitation. There is no discernible chemical difference between amoeboidal and blocky crystals; however, accessory mineralogy and chromite chemistry imply that the upper portion of the lower chromitite and the upper chromitite experienced post-cumulus re-equilibration with evolved intercumulus silicate melt. Our observations are consistent with the anorthosite being a restite of partially molten leuconoritic cumulates. This theory is supported by thermodynamic modelling that demonstrates that under certain conditions, replenishing melts can reconstitute noritic cumulates to anorthosite, troctolite, or feldspathic orthopyroxenite restites. The porosity generated during this process was exploited by downward percolating sulfide melt that displaced a proportionate amount of intercumulus silicate melt upward to the level of the nascent reef. Initially, these partial melts were likely relatively volatile-rich, triggering Cr-supersaturation at the cumulate-melt interface, and later became Cr-bearing with the consumption of poikilitic orthopyroxene and very fine-grained chromite.
Abstract Twenty-six new 40 Ar/ 39 Ar plateau ages for 23 lavas and domes from the Uturuncu volcano in the Altiplano of SW Bolivia reveal a protracted eruptive history from 1050±5 to 250±5 ka. Eruptions have been exclusively effusive, producing some 50 km 3 of high-K dacites and silicic andesites. Bimodal mineral compositions, complex mineral textures, the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at Uturuncu. Post-458 ka, distinct high and low MgO–Cr magmas are resolved. These magmas erupt during similar times, suggesting that eruptions are tapping different parts of the magma system, albeit from the same vent system. Volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath Uturuncu, and calculated buoyancy forces are sufficient to drive effusive eruptions. Eruptive activity is episodic, with six eruptive periods separated by hiatuses of >50 kyr. Cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka. The episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying Altiplano–Puno Magma Body. Supplementary material: Detailed 40 Ar/ 39 Ar data, and lava flow and dome areas, volumes and stratigraphic ages where absolute ages are lacking are available at www.geolsoc.org.uk/SUP18815
Abstract The ca. 3.0 Ga Ni sulfide mineralisation at Maniitsoq, SW Greenland, is hosted by a cluster of relatively small, irregularly shaped mafic-ultramafic intrusions, typically 10s of m to a few km across, that are lodged within broadly coeval gneiss. Many of the intrusions are fault bounded and fragmented so that their original sizes remain unknown. The sulfides form disseminations and sulfide matrix breccia veins displaying sharp contacts to the host intrusives. The mineralisation has relatively high Ni/Cu, with 4–10% Ni and 1–2% Cu. Correlations between Ni and Cu with sulfide content are strong, consistent with a magmatic origin of the mineralisation. PGE contents are mostly below 0.5 ppm, and Cu/Pd is typically above primitive mantle levels, interpreted to reflect equilibration of the parent magma with segregating sulfide melt prior to final magma emplacement. Sulfide segregation was likely triggered by assimilation of crustal sulfur, as suggested by whole rock S/Se ratios of 7000–9000. The sulfide melt underwent extensive fractionation after final emplacement, caused by downward percolation of Cu-rich sulfide melt through incompletely solidified cumulates. We suggest that the exposed Maniitsoq intrusions represent the Ni-rich upper portions of magma conduits implying that there is potential for Cu-rich sulfides in unexposed deeper portions of the belt.
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