A multi-isotope (S-Sr-Nd) investigation of the Flatreef, Northern Limb, Bushveld Complex: Petrogenetic implications and comparison with the Merensky Reef

Abstract

Historically, research on the Bushveld Complex (BC) in the northern limb mainly focused on the near surface Platreef. The Platreef is well-known for being complicated and erratic due to significant degrees of country rock assimilation and contamination along strike. A correlation of the disrupted magmatic stratigraphy of the Platreef to the Upper Critical Zone of the western and eastern limbs has thus proven difficult. The discovery of the Flatreef, the down-dip, sub-horizontal extension of the Platreef, on the farms Turfspruit and Macalacaskop, opened new avenues for enquiry. The proportion of assimilated country rock within the Flatreef is significantly less than in the near surface Platreef, with a stratigraphy that is less disrupted and affected by footwall interaction than the Platreef, such that Bastard Reef and Merensky Reef correlates may be identified in the Flatreef stratigraphy. In this study Sr, Nd and S isotopic compositions across the stratigraphic units of the Flatreef as intersected by drill hole UMT-393 on the farm Macalacaskop, and to a lesser extent, drill hole UMT-276 on the farm Turfspruit is reported. The initial ⁸⁷Sr/⁸⁶Sr ratio (Sri) (also indicated as ⁸⁷Sr/⁸⁶Sri) results show a significant shift from about 0.706 to 0.707 in the immediate footwall, to values > 0.709 near the top of the Main Reef. This isotopic shift matches the isotopic shift described for the Merensky Reef of the eastern and western BC. The mineralized units of the Flatreef, namely the Main Reef and Upper Reef, have 𝛿³‏⁴S values ranging between -0.2 to 1.5‰ (with the exception of three outliers), and 0.52% and 11.2‰ in UMT-393, respectively. The S-isotope data for UMT-276 are lighter with 𝛿³‏⁴S values ranging between -0.96 and 2.24‰ for the Main Reef and 3.19‰ was recorded for an Upper Reef sample. These near-mantle 𝛿³‏⁴S values fall within the range of about 0 to 3‰ reported for the Merensky Reef in the eastern and western BC.

In die verlede het navorsing oor die Bosveldkompleks in die noordelike uitbreiding hoofsaaklik gefokus op die naby-oppervlak van die Platinumrif (“Platreef”). Die Platinumrif is bekend daarvoor dat dit ingewikkeld en onreëlmatig is as gevolg van beduidende grade van assimilasie en kontaminasie van omliggende gesteentes langs dié strekking. Dit was dus moeilik om ’n korrelasie van die versteurde magmatiese stratigrafie van die Platinumrif met die boonste kritieke sone/strook van die westelike en oostelike uitbreidings te bewys. Die ontdekking van die Platrif (“Flatreef”) – die afwaartse, subhorisontale uitbreiding van die Platinumrif op die plase Turfspruit en Macalacaskop – het nuwe navorsingsmoontlikhede geopen. Die verhouding van geassimileerde gesteentes binne die Platrif is aansienlik minder as in die naby-oppervlak van die Platinumrif met ’n stratigrafie wat minder versteur en minder beïnvloed is deur interaksie met die onderliggende gesteentes (oppervlak). Dit maak dit moontlik om korrelasies met die Basterrif (“Bastard Reef”) en Merenskyrif binne die Platrif-stratigrafie te identifiseer. In hierdie studie word Sr-, Nd- en S-isotoopsamestellings oor die stratigrafiese eenhede van die Platrif, soos gekruis deur boorgat UMT-393 op die plaas Macalacaskop en in ’n mindere mate deur boorgat UMT-276 op die plaas Turfspruit, gerapporteer. Die aanvanklike ⁸⁷Sr/⁸⁶Sr-verhoudingsgetal (Sri) toon ’n beduidende verskuiwing van ongeveer 0,706 tot 0,707 in die onmiddellike onderliggende gesteentes (oppervlak), tot waardes > 0,709 naby die bokant van die hoofrif. Hierdie isotopiese verskuiwing stem ooreen met dié wat beskryf is vir die Merenskyrif van die oostelike en westelike Bosveldkompleks. Die gemineraliseerde eenhede van die Platrif, naamlik die hoofrif en die boonste rif, het δ³⁴S-waardes wat wissel tussen -0,2 en 1,5‰ (met drie uitskieters) en onderskeidelik 0,52‰ en 11,2‰ in UMT-393. Die S-isotoopdata vir UMT-276 is ligter met δ³⁴S-waardes tussen -0,96 en 2,24‰ vir die hoofrif en ’n waarde van 3,19‰ vir ’n monster van die boonste rif. Hierdie nouverwante δ³⁴S-waardes val binne die reeks van ongeveer 0 tot 3‰ wat vir die Merenskyrif in die oostelike en westelike Bosveldkompleks gerapporteer is.

Ho latela nalane, dipatlisiso mabapi le Bushveld Complex (BC) karolong e ka Leboya e ne e shebane haholo le sebaka se haufi sa Platreef. Platreef e tsebahala haholo ka ho rarahana le ho ba botjhopotjhopo ha mafika ka nqeng ena ya naha le ho silafala karolong e moeding. Kgokahano ya magmatic stratigraphy e senyehileng ya Platreef ho ya Upper Critical Zone ya maoto le matsoho a Bophirima le Botjhabela e ipakile e le thata. Ho sibollwa ha Flatreef, sebaka se tlasetlase, se katoloso se sephara sa Platreef, mapolasing a Turfspruit le Macalacaskop, ho butse mekgwa e metjha ya ho etsa dipatlisiso. Karolo ya majwe a naha e tshwaretsweng ka hara Flatreef e fokotsehile haholo ho feta sebakeng se haufi sa Platreef, se nang le mealo e fokolang haholo le e amehang ke ho sebedisana ha maoto ho feta Platreef, e le hore Bastard Reef le Merensky Reef correlates di ka khethollwa ho Flatreef stratigraphy. Thutong ena, Sr, Nd le S diqapi tsa isotopic ho pholletsa le dikarolo tsa mealo tsa Flatreef ha di kopane ka lesoba la ho phunya UMT-393 polasing ya Macalacaskop, mme ho isa ho bohole bo itseng, ho tlalehwa lesoba la UMT-276 polasing ya Turfspruit. Sephetho sa pele sa ⁸⁷Sr/⁸⁶Sr (Sri) (hape se bontshitswe (⁸⁷Sr/⁸⁶Sri) diphetho di bontsha phetoho e kgolo ho tloha ho 0.706 ho isa ho 0.707 leboteng la maoto, ho ya ho bohole > 0.709 haufi le bokahodimo ba Reef. Phetoho ena ya isotopic e tsamaisana le phetoho ya isotopic e hlalositsweng bakeng sa Merensky Reef ya Botjhabela le Bophirimela ba BC. Diyuniti tse nang le diminerale tsa Flatreef, e leng Main Reef le Upper Reef, di na le 𝛿³‏⁴S boleng bo pakeng tsa -0.2 ho 1.5‰ (ntle le tse tharo tse kantle), le 0.52% le 11.2 ‰ ho UMT-393, ka ho latellana. Dintlha tsa S-isotope bakeng sa UMT-276 di bobebe ka 𝛿³‏⁴S boleng bo pakeng tsa -0.96 le 2.24‰ bakeng sa Main Reef le 3.19‰ e ile ya rekotwa bakeng sa sampole ya Upper Reef. Ditekanyetso tsena tse haufi le lera la lefatshe 𝛿³‏⁴S di wela ka hara sebaka se ka bang 0 ho isa ho 3‰ se tlalehilweng bakeng sa Merensky Reef ka Botjhabela le Bophirimela ba BC.

Ngokomlando, ucwaningo ngeBushveld Complex (BC) engxenyeni esenyakatho lugxile kakhulu endaweni eseduze ne-Platreef. I-Platreef yaziwa kakhulu ngokuba inkimbinkimbi futhi iyaguquguquka ngenxa yamazinga abalulekile okulinganisa kwamadwala ezwe kanye nokungcoliswa ngesikhathi sesiteleka. Ukuhlotshaniswa kwe-magmatic stratigraphy ephazamisekile ye-Platreef kuya ku-Upper Critical Zone yamalungu asentshonalanga nasempumalanga kubonakale kunzima. Ukutholakala kwe-Flatreef, i-down-dip, isandiso esivundlile se-Platreef, emapulazini i-Turfspruit ne-Macalacaskop, kwavula izindlela ezintsha zophenyo. Ingxenye yedwala lezwe elishunyekiwe ngaphakathi kwe-Flatreef incane kakhulu kunase-Platreef eseduze, ene-stratigraphy engaphazanyiswa kancane futhi ethintwa ukusebenzisana kwe-footwall kune-Platreef, kangangokuthi ukuhambisana kwe-Bastard Reef ne-Merensky Reef kungase kubonakale ku-Flatreef stratigraphy. Kulolu cwaningo izingoma ze-isotopic zika-Sr, Nd kanye no-S kuwo wonke amayunithi e-stratigraphic e-Flatreef njengoba ephambana nembobo yomgodi i-UMT-393 epulazini i-Macalacaskop, futhi kancane, abhoboza imbobo i-UMT-276 epulazini i-Turfspruit kubikiwe. Isilinganiso sokuqala sika-⁸⁷Sr/⁸⁶Sr (Sri) (esiphinde siboniswe njengokuthi ⁸⁷Sr/⁸⁶Sri) imiphumela ibonisa ushintsho olubalulekile ukusuka cishe ku-0.706 ukuya ku-0.707 odongeni oluseduze, ukuya kumanani > 0.709 eduze kwaphezulu ne-Main Reef. Lokhu kushintsha kwe-isotopic kufana nokushintsha kwe-isotopic okuchazwe ku-Merensky Reef yasempumalanga nasentshonalanga BC. Amayunithi anamaminerali e-Flatreef, okuyi-Main Reef kanye ne-Upper Reef, anamanani angu-𝛿³‏⁴S aphakathi kuka -0.2 kuya ku-1.5‰ (ngaphandle kwama-outliers amathathu), kanye no-0.52% no-11.2‰ ku-UMT-393, ngokulandelana. Idatha ye-S-isotope ye-UMT-276 ilula ngamavelu angu-𝛿³‏⁴S asukela phakathi kuka--0.96 no-2.24‰ we-Main Reef kanye no-3.19‰ yarekhodwa ngesampula ye-Upper Reef. Lawa manani e-Near-mantle 𝛿³‏⁴S awela phakathi kwebanga elingaba ngu-0 kuye ku-3‰ abikwe e-Merensky Reef empumalanga nasentshonalanga yeBC.