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The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa

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The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa minerals Article Evaluating the Changes from Endogranitic Magmatic to Magmatic-Hydrothermal Mineralization: The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa Leonidas Vonopartis 1,* , Paul Nex 1 , Judith Kinnaird 1 and Laurence Robb 1,2,3 1 School of Geosciences, University of the Witwatersrand, Private Bag 3, Johannesburg 2050, South Africa; [email protected] (P.N.); [email protected] (J.K.); [email protected] (L.R.) 2 Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK 3 Department of Geology, University of Johannesburg, P.O. Box, 524, Johannesburg 2006, South Africa * Correspondence: [email protected] Received: 24 March 2020; Accepted: 14 April 2020; Published: 23 April 2020 Abstract: The stanniferous granites of the Zaaiplaats Tin Field are part of the A-Type Lebowa Granite Suite, within the greater Bushveld Igneous Complex of northeast South Africa. The tin field comprises three granites: (1) the Nebo, a leucocratic, equigranular biotite granite; (2) The brick-red hypidiomorphic Bobbejaankop granite, which is extensively microclinized with chloritized biotite and characteristic synneusis-textured quartz; and (3) The variably altered roof facies of the Bobbejaankop granite known as the Lease microgranite. The Bobbejaankop and Lease granites were both extensively mined for cassiterite until 1989. The cassiterite is hosted in disseminations, miarolitic cavities, and within large hydrothermal, tourmalinized, and greisenized pipes and lenticular ore-bodies. An extensive petrological and whole-rock XRF and ICP-MS geochemical study, has provided new insight into the magmatic and magmatic-hydrothermal mineralization processes in these granites. Trace elements and Rayleigh Fractionation modelling suggest the sequential fractionation of the Nebo granite magma to be the origin of the Bobbejaankop granite. Incompatible elemental ratios, such as Zr/Hf and Nb/Ta, record the influence of internally derived, F-rich, hydrothermal fluid accumulation within the roof of the Bobbejaankop granite. Thus, the Lease granite resulted from alteration of the partially crystallized Bobbejaankop granite, subsequent to fluid saturation, and the accumulation of a magmatic-hydrothermal, volatile-rich fluid in the granite cupola. The ratio of Nb/Ta, proved effective in distinguishing the magmatic and magmatic-hydrothermal transition within the Bobbejaankop granite. Elemental ratios reveal the differences between pre- and post-fluid saturation in the mineralizing regimes within the same pluton. Thus highlighting the effect that the location and degree of hydrothermal alteration have had on the distribution of endogranitic tin mineralization. Keywords: XRF and ICP-MS; Zaaiplaats tin field; A-type granites; tin granites; fractionation; hydrothermal; cassiterite mineralization 1. Introduction The Paleoproterozoic Bushveld Complex, in the northeast of South Africa, is the largest known layered igneous complex on Earth. It exceeds an area of 90,000 km2 and has a magma volume estimated at 450,000 km3, hosting some of the world’s largest mineral deposits [1–3]. Despite decades of research, its origin is still debated [3,4]. Although recent work by Zeh et al. [5] on the origin of the Rustenburg Layered Suite has made a significant contribution, the origin of the granites remains poorly understood. Minerals 2020, 10, 379; doi:10.3390/min10040379 www.mdpi.com/journal/minerals MineralsMinerals 20202020,, 1010,, x 379 FOR PEER REVIEW 2 2of of 26 26 poorly understood. The Lebowa Granite Suite of the Bushveld is the largest A-Type batholith on Earth.The Lebowa It is subdivided Granite Suite into ofvarious the Bushveld facies that is theincludes largest the A-Type coarse-grained batholith Nebo, on Earth. Bobbejaankop, It is subdivided and Makhutsointo various granites, facies that and includes the fine-grained the coarse-grained Klipkloof and Nebo, Lease Bobbejaankop, granites [4,6–9]. and Makhutso granites, and the fine-grainedThe Makapansberg Klipkloof escarpment, and Lease part granites of the [4 ,Northe6–9]. rn Limb of the Bushveld Igneous Complex, is knownThe for Makapansberg tin mineralization, escarpment, particularly part of within the Northern the Zaaiplaats Limb of thetin Bushveldfields (Figure Igneous 1) [6,7,9]. Complex, The is Lebowaknown forGranite tin mineralization, Suite is host to particularly polymetallic within magmato-hydrothermal, the Zaaiplaats tin fields endo- (Figure and1 exogranitic)[ 6,7,9]. The deposits, Lebowa includingGranite Suite the isnotable, host to polymetallicworld-class, magmato-hydrothermal, Vergenoeg fluorite mine endo- [6,7]. and Cassiterite exogranitic mineralization deposits, including was discoveredthe notable, in world-class, 1908 in the Vergenoegroof of the fluoriteBobbejaankop mine [ 6and,7]. in Cassiterite the Lease mineralization granites and, wassoon discoveredafter, two minesin 1908 were in the established roof of the by Bobbejaankop the Zaaiplaats and Tin inMining the Lease Company, granites on and, the soonfarms after, of Groenfontein two mines wereand laterestablished Zaaiplaats by the (Figure Zaaiplaats 2) [7,10]. Tin Approximately Mining Company, 37,079 on the tons farms of tin of were Groenfontein produced and before later the Zaaiplaats closure of(Figure the 2Zaaiplaats)[ 7,10]. Approximately tin field in 37,0791989 [7,10]. tons of Cassiterite tin were produced mineralization before the is closure hosted of as the low-grade Zaaiplaats disseminations,tin field in 1989 pods [7,10 ].and Cassiterite unique, large, mineralization and remarkably is hosted mineralized as low-grade hydrot disseminations,hermal pipe- and pods lens- and shapedunique, ore-bodies large, and [8,11–16]. remarkably mineralized hydrothermal pipe- and lens-shaped ore-bodies [8,11–16]. FigureFigure 1. 1. SimplifiedSimplified geological geological map map of of the the Bushveld Bushveld Ig Igneousneous Complex Complex showing showing the the location location of of the the ZaaiplaatsZaaiplaats tin fieldfield (red (red star) star) with with other other occurrences occurrences of tin, of silver, tin, silv ander, fluorite and mineralization.fluorite mineralization. Adapted Adaptedfrom Hunter from [ 17Hunter]. [17]. VariousVarious petrogenetic petrogenetic models models for for the the formation formation of of the the granites granites and and hydrothermal hydrothermal features features of of the the ZaaiplaatsZaaiplaats tin tin field field ha haveve been been suggested. suggested. The The Nebo Nebo granite granite was was su suggestedggested to to be be the the result result of of fractional fractional crystallizationcrystallization from from a a bulk bulk Nebo Nebo parent [12,18– [12,18–20].20]. However, However, this this model model has has subsequently subsequently been been contested.contested. For example,example, HillHill et et al. al. [21 [21],], suggested suggeste thed formationthe formation of the of granites the granites from thefrom partial the meltingpartial meltingof the Archean of the Archean basement basement during theduring emplacement the emplacement of the mafic of the to mafic ultramafic to ultramafic Rustenburg Rustenburg Layered LayeredSuite. This Suite. resulted This resulted in multiple in multiple episodic episodic ascending ascending batches ofbatches siliceous of siliceous melt. Wilson melt. etWilson al. [22 ]et also al. [22]proposed also proposed several di severalfferent magmaticdifferent pulsesmagmatic that pu arelses petrographically, that are petrographically, geochemically, geochemically, and geophysically and geophysicallysimilar, yet distinct. similar, The yet Bobbejaankop distinct. The graniteBobbejaan waskop suggested granite towas be suggested a metasomatised to be a roofmetasomatised of the Nebo Minerals 2020, 10, 379 3 of 26 Minerals 2020, 10, x FOR PEER REVIEW 3 of 26 roofgranite of the [4 ,Nebo20–24 ],granite whereas [4,20–24], other modelswhereas maintain other models that themaintain Bobbejaankop that the Bobbejaankop granite was produced granite was by producedfractional by crystallization fractional crystallization of the underlying of the Nebo underlying granite Nebo [14,21 granite,23–25]. [14,21,23–25]. TheThe Lease Lease granite granite was was originally originally interpreted interpreted as a chilled as a chilled margin margin to the Bobbejaankop to the Bobbejaankop granite, implyinggranite, implyingthat it crystallized that it crystallized before the before latter, thealthough latter, this although concept this has concept now been has nowdiscarded been [7,13,14,26,27].discarded [7,13 Strauss,14,26,27 and]. Strauss Truter [13] and then Truter proposed [13] then that proposed the Lease thatgranite the formed Lease graniteas a post-reaction formed as producta post-reaction from the productaccumulation from of the volatiles accumulation along th ofe roof volatiles of the alongBobbejaankop the roof granite of the [7,13,14,26,27]. Bobbejaankop Alternativegranite [7,13 models,14,26, 27suggest]. Alternative that the Lease models granite suggest resulted that the from Lease quenching, granite induced resulted by from the quenching,fracturing ofinduced the volcanic by the roof fracturing rocks, ofor the that volcanic the Bobbejaan roof rocks,kop or and that Lease the Bobbejaankop granites were and produced Lease granites by the local were metasomatismproduced by the of localvarious metasomatism types of Klipkloof of various granites types [14,20,23,28]. of Klipkloof However, granites [14 most,20,23 of,28 the]. However,research conductedmost of the on research
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