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TitleNew constraints on the age of ore at Black Mountain mine, Bushmanland Ore District, South Africa
AuthorCornell, D; Moses, A; Cawood, TORCID logo; Richter, M
SourceSouth African Journal of Geology vol. 125, issue 3-4, 2022 p. 337-344,
Alt SeriesNatural Resources Canada, Contribution Series 20220513
PublisherGeological Society of South Africa
Mediapaper; digital; on-line
File formatpdf
AreaSouth Africa
Lat/Long WENS 18.0000 22.0000 -27.0000 -31.0000
Subjectsmineralogy; Science and Technology; mineral deposits; chronostratigraphy; dykes; relative ages; boreholes; core analysis; cores; zircon; zircon dates
Illustrationsstratigraphic columns; photomicrographs; graphs; location maps; stratigraphic charts
Released2022 12 01
AbstractThe chronostratigraphy of the Bushmanland Ore District and the Namaqua-Natal Province has long been debated, but recent microbeam dating has resolved several issues. An important aspect is the precise age of the sedimentary-exhalative ores and their tectonostratigraphic context. Published constraints on the maximum age of the ores from detrital zircon dating are 1 285 ± 14 Ma (n=4, Gamsberg ore), 1 215 ± 18 Ma (n=6, Wortel Formation) and a tentative 1 118 ± 33 Ma (n=3, Hotson Formation at Black Mountain). The ore is older than the 1 130 ± 35 Ma Koeris Formation metabasalt which unconformably overlies it. Aplite dykes, which intrude the ore of the Black Mountain deposit, provide another potential minimum age constraint on the ore. A sample was dated at 1 175 ± 15 Ma by ion probe U-Pb zircon dating. This shows that the aplite dykes belong to the late-collisional Springputs Suite of granitoids which includes the 1 163 ± 11 Ma Achab and 1 149 ± 15 Ma Hoogoor Gneisses, for which the field relationship with the ores had not been established. The regional M2 metamorphism was recorded in aplite zircon rims at 1 027 ± 9 Ma and at 1 030 ± 6 Ma in monazite and xenotime in the Hotson Formation host rock schists. Detrital zircons, dated by Laser Ablation ICPMS in a host rock schist sample, reflect a dominant Palaeoproterozoic provenance with major age group at 2 003 ± 17 Ma and minor groups at 1 847 and 2 105 Ma. Only 16 analyses were made, which probably accounts for the absence of minor Mesoproterozoic provenance components found in other published datasets. The age of the Black Mountain ore is now constrained between 1 215 ± 18 Ma and 1 175 ± 15 Ma. The tentative 1 118 ± 33 Ma detrital zircon maximum age is shown to be unreliable in view of two younger magmatic rocks with older dates (1 175 and 1 130 Ma). The SEDEX ores thus formed during or just before the ~1 210 Ma assembly of Namaqua terranes and before the ~1 150 Ma syntectonic Springputs Suite granitoid magmatism.
Summary(Plain Language Summary, not published)
The Bushmanland Ore District in South Africa hosts several Cu-Pb-Zn-Ag deposits, comprising sulfides hosted by sediments that were laid down millions of years ago on an ocean floor. Despite their economic importance, the timing of ore deposit formation remains poorly constrained. The age of detrital zircons found within the host sediments can give a minimum age for the deposits (which, based on previous studies, formed at approximately the same time that the sediments were laid down on the seafloor): earlier studies obtained detrital zircon ages of 1285 ± 14 Ma, 1215 ± 18 Ma, and 1118 ± 33 Ma from different parts of the district. Furthermore, the ore deposits must be older than 1130 ± 35 Ma volcanic rocks that overlie them. Another age constraint can be obtained from intrusive magmatic dykes, formed when molten magma filled fractures below the earth's surface. Some of these dykes cross-cut the Black Mountain ore deposit, and therefore must have formed after the ore deposit. This study dated zircons in one of these dykes, obtaining an age of 1175 ± 15 Ma. This shows that the ore deposits must have formed after ~1215 Ma (the youngest zircons in sediments that must be older than the ore) but before ~1175 Ma (zircons in dykes that must be younger than the ore). The 1118 ± 33 Ma detrital zircon age must be unreliable, because it seems to be younger than magmatic rocks that cross-cut the sediments. The timing of burial and metamorphism, deep in the earth's crust, can also be constrained, because new minerals or mineral zones may grow at the higher temperatures experienced during metamorphism. This study dated rims that grew on zircons within the dykes, as well as new monazite and xenotime crystals that grew in the host rocks, and obtained ages of 1027 ± 9 Ma and 1030 ± 6 Ma respectively. This supports previous studies. Furthermore, detrital zircons in the host sediments show that these sediments were mostly derived from regions with rocks of ~2003, 1847, and 2105 Ma ages.

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