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 understanding conducted these on understanding granites has been these focused granites on has the been Zaaiplaats focused tin on mine, the Zaaiplaats which is on tin themine, northern which edge is on of the the northern intrusion. edge Thus, of the since intrusion. Strauss Thus, [27] and since the Strauss closure [27 of] andthe theGroenfontein closure of tin the mineGroenfontein in the early tin 1930s, mine inthe the main early body 1930s, of the the Bobbe mainjaankop body of theand Bobbejaankop Lease granites and has Leasebeen more granites or less has neglected.been more Therefore, or less neglected. this study Therefore, is focused this at study Groenfontein, is focused with at Groenfontein, the aim of defining with the the aim petrogenetic of defining relationshipthe petrogenetic between relationship the Lease between and the the Bobbejaan Lease andkop the Bobbejaankopgranites, and granites,to explain and their to explainorigin theirand mineralization.origin and mineralization.

FigureFigure 2. 2. RevisedRevised geological geological map map of of the the Zaaiplaats tin field. field. Lithological Lithological boundaries, boundaries, disseminated disseminated mineralization,mineralization, hydrothermal hydrothermal pipes pipes and and lenticular lenticular ore-bodies ore-bodies were were validated validated by by the the integration integration of of publishedpublished geological maps andand extensiveextensive fieldfield mapping mapping [ 11[11,27].,27]. Lithological Lithological nomenclature nomenclature is basedis based on onSACS SACS [29 [29].]. Cross Cross section, section, shown shown in in Figure Figure3, is 3, along is along the the line line A–A’. A–A’. The The bottom bottom right right insert insert shows shows the thelocation location of the of Zaaiplaatsthe Zaaiplaats tin field tin (redfield star) (red on star the) on Northern the Northern Limb of Limb the Bushveld of the Bushveld Igneous Complex.Igneous Complex.The lithological The lithological units are theunits same are asthe in same Figure as1 in. Figure 1. 2. Regional Geology 2. Regional Geology The Groenfontein tin mine is situated 30 km northwest of Mokopane and 2 km southeast of the The Groenfontein tin mine is situated 30 km northwest of Mokopane and 2 km southeast of the Zaaiplaats tin mine (Figure2). A sequence of sheeted granites comprise the majority of the region; Zaaiplaats tin mine (Figure 2). A sequence of sheeted granites comprise the majority of the region; the Nebo granite which is overlain by the Bobbejaankop and Lease granites, respectively. These the Nebo granite which is overlain by the Bobbejaankop and Lease granites, respectively. These granites are capped by the Rashoop Granophyre Suite and the entire sequence dips conformably granites are capped by the Rashoop Granophyre Suite and the entire sequence dips conformably between 10 and 15 westward (Figures2 and3)[ 7,13,24]. The Nebo granite is a 1.5 to 2.5 km thick between 10°◦ and 15°◦ westward (Figures 2 and 3) [7,13,24]. The Nebo granite is a 1.5 to 2.5 km thick sheeted granitic intrusion, composed of equigranular quartz, perthitic alkali-feldspar, hornblende, sheeted granitic intrusion, composed of equigranular quartz, perthitic alkali-feldspar, hornblende, and biotite [4,7,14,20,21]. The Nebo progressively evolves from a grey, coarse-grained, mesocratic

Minerals 2020, 10, 379 4 of 26 andMinerals biotite 2020 [4, ,107,, 14x FOR,20, PEER21]. REVIEW The Nebo progressively evolves from a grey, coarse-grained, mesocratic4 of 26 hornblende-bearing granite at its base, to a finer-grained, pinkish-red biotite granite in its roof facies hornblende-bearing granite at its base, to a finer-grained, pinkish-red biotite granite in its roof facies within the Zaaiplaats region [4,20,21]. The Bobbejaankop granite is characterized as a brick-red, within the Zaaiplaats region [4,20,21]. The Bobbejaankop granite is characterized as a brick-red, coarse-grained facies that is primarily composed of alkali-feldspar and synneusis-textured quartz, coarse-grained facies that is primarily composed of alkali-feldspar and synneusis-textured quartz, formingforming linked linked aggregates. aggregates. ItIt hashas variablyvariably chloritized biotite biotite and and miarolitic miarolitic cavities, cavities, which which are are occasionallyoccasionally filled filled with with cassiterite cassiterite and and minor minor tourmalinetourmaline [[4,11,14,20,23,24].4,11,14,20,23,24].

FigureFigure 3. 3.Schematic Schematic geological geological cross cross sectionssections acrossacross the Groenfontein tin tin mine. mine. (A (A) Schematic) Schematic cross cross sectionsection from from the the Rashoop Rashoop Granophyre Granophyre SuiteSuite towardstowards the the Nebo Nebo granite granite (SW (SW to to NE) NE) along along line line A–A’ A–A’ shownshown in in Figure Figure1.( 1. B(B)) Schematic Schematic representationrepresentation of a vertical cross cross section section through through the the Lease Lease and and BobbejaankopBobbejaankop granites granites (NW (NW toto SE).SE). AA transitionaltransitional zone rather than than a a sharp sharp contact contact between between the the BobbejaankopBobbejaankop and and Lease Lease granites granites isis suggested.suggested. Th Thee zones zones where where the the various various types types of of tourmaline tourmaline featuresfeatures occur occur are are represented represented by by the the bars bars on on the the rightright sideside ofof the diagram.

TheThe Lease Lease granite granite is is a a relatively relatively thinthin (120(120 mm thick),thick), lenticular sheet sheet that that is is restricted restricted to tothe the roof roof of theof the Bobbejaankop Bobbejaankop granite, granite, beneathbeneath the Rashoop Granophyre Granophyre Suite Suite (Figure (Figure 3).3 The). The Lease Lease granite granite is described as a metasomatically altered red microgranite, exhibiting pervasive sericitic alteration in is described as a metasomatically altered red microgranite, exhibiting pervasive sericitic alteration feldspars, extensive chloritization of biotite, patches of epidotization, and zones of disseminated in feldspars, extensive chloritization of biotite, patches of epidotization, and zones of disseminated cassiterite [4,7,13,14,26]. The mineralization at Groenfontein is confined to the upper 20 m of the Lease cassiterite [4,7,13,14,26]. The mineralization at Groenfontein is confined to the upper 20 m of the granite, 3 to 10 m below the contact pegmatite (Figure 3) [7,11–13,27]. These discontinuous lensoidal Lease granite, 3 to 10 m below the contact pegmatite (Figure3)[ 7,11–13,27]. These discontinuous pegmatites are 2 to 5 m in length and have been used to demarcate the otherwise cryptic contact lensoidal pegmatites are 2 to 5 m in length and have been used to demarcate the otherwise cryptic between the Lease granite and the Rashoop Granophyre Suite. The pegmatite lenses are usually contactunmineralized between the and Lease primarily granite composed and the Rashoopof pegmatitic Granophyre quartz and Suite. reddened The pegmatite feldspars. lenses Cassiterite are usually is unmineralizedtypically disseminated, and primarily although composed it does of occur pegmatitic as small quartz clusters and and reddened within lenticular feldspars. and Cassiterite pipe-like is typicallyore bodies disseminated, (Figure 3) although[7,12,13,26]. it doesThe mineralizati occur as smallon at clusters Zaaiplaats and withinoccurs lenticulardominantly and as pipe-likecassiterite ore bodieswith (Figureaccessory3)[ scheelite,7,12,13,26 sulfides,]. The mineralization fluorite, and molybdenite, at Zaaiplaats compared occurs dominantly to Groenfontein, as cassiterite which withis accessorymainly cassiterite, scheelite, sulfides,with minor fluorite, scheelite, and and molybdenite, molybdenite compared [7,12–14,26]. to Groenfontein, which is mainly cassiterite, with minor scheelite, and molybdenite [7,12–14,26].

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3. Petrography and Field Relationships The network of hydrothermal pipes and lenticular ore-bodies have been comprehensively discussed in a number of publications [7,13–16,24,27,30]. Therefore, these features are only briefly described here. The contact between the Nebo and Bobbejaankop granites is marked by an upward gradational reddening of the feldspars and increasingly aggregated quartz. This gradation is almost imperceptible, as the contact zone occurs over several meters and is marked by a subtle break in slope. The nature of the obscure contact is corroborated by observations made by Strauss and Truter [13], Strauss [27] and Coetzee [11]. The Bobbejaankop granite forms an elongated sheeted intrusion, extending from the middle of the farm Zaaiplaats 223 KR in the northwest, towards the southeast along the strike of the Northern limb (Figure2). It extends from Roodepoort 222 KR through Groenfontein 227 KR, before terminating along the boundary of Sterkwater 296 KR and Solomons Temple 230 KR. The Bobbejaankop granite is typically composed of subhedral quartz (0.5 to 3 mm, 35%) with sub- to anhedral alkali-feldspar (0.5 to 6 mm, 35%) and plagioclase (<0.1 to 0.5 mm, 25%), as well as extensively chloritized, interstitial magmatic biotite (<0.1 to 1 mm, <5%). The quartz grains are transparent, granular, hypidiomorphic and typically display synneusis-textures, for which the Bobbejaankop is well known. Small (<0.1 mm) feldspar and chlorite inclusions are a common feature within the quartz grains. The perthitic feldspars of the Bobbejaankop granite are remarkably turbid in the thin section. The perthite (0.5 to 3 mm) exhibits randomly distributed irregular patches (<0.1 to 1 mm) of exsolved plagioclase, delineated by a diffuse contact zone between the two species (Figure4A). The exsolution of plagioclase within the host alkali-feldspar exists as patch, chessboard, and veined variants (Figure4B). The plagioclase exhibits polysynthetic twinning and is selectively sericitized (Figure4). Microclinized feldspars become brick-red, which is considered a result of hematitic coating by the exclusion of Fe from the feldspar (Figure4A). This is accomplished during the crystallographic reordering of the feldspar lattice during deuteric sub-solidus microclinization, allowing the precipitation of Fe, which then lined microcavities within the feldspars as hematite [31,32]. Plagioclase is selectively sericitized within the perthites by a very fine-grained pale green/white micaceous aggregate. Sericitization has replaced feldspar, biotite, and chlorite, in the upper facies of the granite. Essentially, all magmatic biotites (<0.1 to 1 mm) are extensively chloritized and partially stained red from the exclusion of Fe along the mineral boundaries and cleavages. Relict brown magmatic biotites are visible within the dark-green/brown chlorite (Figure5A). Small anhedral grains of quartz, feldspar, and purple fluorite are intergrown with, and form inclusions within, the chlorite (Figure5B). Chloritization of the biotite and feldspars increases towards the Lease granite. Miarolitic cavities (0.5 to 5 cm) are common and typically filled with small clusters of intergrown spherulitic tourmaline and quartz (Figure6). These cavities are scattered throughout the upper facies of the Bobbejaankop granite, although concentrated proximal to larger pipes. Some miarolitic cavities display a unidirectional solidification texture of quartz, reddened feldspars, fluorite, and calcite (Figure5B). Fluorite is ubiquitous in the Bobbejaankop granite as inclusions in quartz and feldspar, within miarolitic cavities and interstitially. It is typically purple, with lesser transparent, white, and green varieties. Tourmaline within the Bobbejaankop granite primarily occurs as orbicules and in hydrothermal pipes, although it is also present in miarolitic cavities. Tourmaline-bearing miarolitic cavities comprise a central zone of calcite, subhedral cassiterite and fluorite, surrounded by a radial blue/green to brown tourmaline (Figure6). Ilmenite and sulfides (0.5 to 1.5 mm) such as pyrite and chalcopyrite with lesser arsenopyrite are uncommon, although, where they do occur, they are associated with chloritization. The paragenetic sequence of the sulfides is suggested as early arsenopyrite, followed by pyrite and later chalcopyrite. Cassiterite is uncommon in the Bobbejaankop granite at Groenfontein and is restricted to tourmaline- bearing miarolitic cavities. Minor subhedral 100 µm-sized zircons are associated with biotite. Most miarolitic cavities appear to have experienced various stages of hydrothermal alteration, with a suggested sequence of microclinization, chloritization, sericitization then hematization. Minerals 2020, 10, 379x FOR PEER REVIEW 66 of of 26 MineralsMinerals 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 66 ofof 2626

Figure 4. Photomicrographs of the Bobbejaankop granite. (A) PPL image of patch perthite. Within the FigureperthiteFigure 4. 4. there PhotomicrographsPhotomicrographsPhotomicrographs are red alkali-feldspars ofof of thethe the BobbejaankopBobbejaankop Bobbejaankop (Afs), with pa granite.granite. granite.tches of ((AA plagioclase ())A PPLPPL) PPL imageimage image (Pl) ofof patchexsolution,patch of patch perthite.perthite. perthite. which WithinWithin follows Within thethe perthitethenoperthite apparent perthite therethere crystallographic there areare redred are alkali-feldsparsalkali-feldspars red alkali-feldspars orientation. (Afs),(Afs), Chlorite (Afs), withwith (Chl) papa withtchestches is patchesseen ofof plagioclase plagioclaseselectively of plagioclase replacing (Pl)(Pl) exsolution,exsolution, (Pl) plagioclase exsolution, whichwhich within followsfollows which the nofollowsperthiteno apparentapparent nograin. apparent crystallographiccrystallographic (B) crystallographicXPL image orientation.orientation. of orientation.(A), ChloriteChlorite depicting (Chl)(Chl) Chlorite the isis seenseen poly (Chl) selectivelyselectivelysynthetic is seen selectively replacing replacingtwinning plagioclaseplagioclase replacing of the plagioclase plagioclase withinwithin thethe perthitewithinexsolutionperthite the grain.grain. perthitewithin ((BB the)) grain. XPLXPL perthite. (imageBimage) XPL ofof image (A),(A), of depictingdepicting (A), depicting thethe poly thepoly polysyntheticsyntheticsynthetic twinningtwinning twinning ofof of thethe the plagioclaseplagioclase exsolutionexsolution within within the the perthite. perthite.

Figure 5. Photomicrographs of the Bobbejaankop granite. (A) PPL image of interstitial magmatic FigurebiotiteFigure 5.5.(Bt)5. PhotomicrographsPhotomicrographsPhotomicrographs exhibiting chloritization of ofof the thethe Bobbejaankop Bobbejaankop Bobbejaankop(Chl) followed granite. granite.granite. by (Asericitization) ( PPL(AA)) PPLPPL image imageimage of(Ser). interstitial ofof ( B interstitialinterstitial) PPL magmatic image magmaticmagmatic biotiteof the biotite(Bt)intergrowthbiotite exhibiting (Bt)(Bt) exhibiting exhibitingof chloritization biotite withchloritizationchloritization fluorite (Chl) followed (Fl) (C(C hl)withinhl) byfollowedfollowed sericitization a miarolitic byby sericitizationsericitization (Ser).cavity ( Bthat) PPL was(Ser).(Ser). image subsequently ((BB) of) PPLPPL the intergrowthimageimage chloritized ofof thethe of intergrowthbiotiteandintergrowth sericitized. with fluorite ofof biotitebiotite (Fl) withwith within fluoritefluorite a miarolitic (Fl)(Fl) withinwithin cavity aa miaroliticmiarolitic that was subsequently cavitycavity thatthat waswas chloritized subsequentlysubsequently and sericitized. chloritizedchloritized andand sericitized.sericitized.

Figure 6. Photomicrographs from the Bobbejaankop granite. ( A)) PPL image of aa tourmaline-quartztourmaline-quartz FigurevugFigure exhibiting 6.6. PhotomicrographsPhotomicrographs acicular brown/green brown fromfrom/green thethe tourmaline BobbejaankopBobbejaankop (Tur) granite. granite. intergrown ((AA)) withPPLwithPPL image euhedralimageeuhedral ofof a aquartz tourmaline-quartztourmaline-quartz (Qz). Doubly vugterminatedvug exhibitingexhibiting quartzquartz acicularacicular crystals crystals brown/greenbrown/green occur occur within within tourmalinetourmaline these these vugs. (Tur)(Tur)vugs. (B) intergrown PPLintergrown(B) imagePPL image of withwith a tourmaline-quartz euhedraleuhedralof a tourmaline-quartz quartzquartz (Qz).(Qz). vug showing DoublyDoubly vug terminatedradialshowingterminated blue radial/ brownquartzquartz blue/brown tourmaline crystalscrystals touroccuroccur withmaline withinwithin euhedral with thesethese quartz.euhedral vugs.vugs. quartz. ((BB)) PPLPPL imageimage ofof aa totourmaline-quartzurmaline-quartz vugvug showingshowing radialradial blue/brownblue/brown tourtourmalinemaline withwith euhedraleuhedral quartz.quartz.

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Towards itsits upperupper contact,contact, the Bobbejaankop granite exhibits features characteristic of the Lease granite, such as an increase in microclinization, microclinization, chloritization, chloritization, seri sericitization,citization, granophyric granophyric intergrowths, intergrowths, and fluorite.fluorite. A A radial radial granophyric granophyric texture texture forms forms around around the the edges edges of of the larger feldspar grains, becoming finer-grainedfiner-grained and increasingly intergrown with perthite.perthite. The pervasive microclinization, associated withwith the the hematization hematization of feldspars,of feldspars, resulted resulted in a purplein a purple/maroon/maroon coloration coloration (Figure 7(Figure). Initially, 7). microclinizationInitially, microclinization selectively selectively affected the affected feldspars, the feldspars, however, however, towards the towards Lease granite,the Lease the granite, alteration the graduallyalteration gradually destroys the destroys synneusis-textured the synneusis-textur quartzed and quartz increased and increased granophyric granophyric intergrowths, intergrowths, resulting inresulting an overall in an reductionoverall reduction in grain in sizegrain (Figure size (Figure7). Although 7). Although the the contact contact between between the the Bobbejaankop Bobbejaankop and Lease granites has has been been described described as as sharp sharp [7,11,26,27], [7,11,26,27], it it appears appears to to be be gradational gradational over over 0.5 0.5 to to 4 4m. m. As As microclinization microclinization intensified, intensified, quartz quartz synneusis synneusis was was replaced replaced by by granophyric granophyric intergrowths and grain-size waswas reduced reduced within within the the Bobbejaankop Bobbejaankop granite granite as it as grades it grades imperceptibly imperceptibly into the into fine-grained the fine- Leasegrained granite Lease (Figure granite7 ).(Figure 7).

Figure 7.7. AA collagecollageof of images images of of core core from from the the top top of theof the Bobbejaankop Bobbejaankop granite granite into theinto transitional the transitional zone. (zone.A) Pink (A Bobbejaankop) Pink Bobbejaankop granite (Bobb) granite altering (Bobb) into altering the transitional into the zone transitional (TZ). Selective zone microclinization(TZ). Selective ofmicroclinization the feldspars is of present the feldspars as a maroon is present incipient as alterationa maroon(IA) incipient within alteration the pink Bobbejaankop(IA) within the granite. pink However,Bobbejaankop from granite. the transitional However, zone from it is presentthe transitional as pervasive zone alteration it is present (PA). as ( Bpervasive) Brick-red alteration Bobbejaankop (PA). granite(B) Brick-red exhibits Bobbejaankop patches of selective granite incipient exhibits microclinization patches of selective that become incipient pervasive microclinization towards the upper that contact.become pervasive (C) Typical towards Bobbejaankop the upper granite contact. towards (C) Typical the bottom Bobbejaankop of the core, granite showing towards minor the incipient bottom purpleof the core, patches showing of microclinization. minor incipient purple patches of microclinization.

Minerals 2020, 10, x FOR PEER REVIEW 8 of 26

The Lease granite is heterogeneous, alternating between fine- and coarse-grained, and between a dull-red, pale-green, and a maroon-purple facies with no obvious systematic control (Figure 8). MineralsOverall2020 it is, 10 typically, 379 finest-grained in its roof facies (<0.5 mm) and coarsest at the base (1 to 2 mm).8 of 26 However, differences in grain size, color, and mineralogy provide a basis for the distinction of three facies of Lease (Table 1). The transitions between these types are either sharp or gradational and The Lease granite is heterogeneous, alternating between fine- and coarse-grained, and between a occasionally meet at a nexus (Figure 8). The mineralogy varies depending on the facies of Lease dull-red, pale-green, and a maroon-purple facies with no obvious systematic control (Figure8). Overall granite, however, it is characteristically composed of hypidiomorphic quartz and red feldspars with it is typically finest-grained in its roof facies (<0.5 mm) and coarsest at the base (1 to 2 mm). However, variable amounts of chlorite, biotite and fluorite. Quartz-lined miarolitic cavities (2 to 10 cm) are differences in grain size, color, and mineralogy provide a basis for the distinction of three facies of composed of calcite, white-purple fluorite, and, occasionally, large euhedral cassiterite (0.1 to 2 cm) Lease (Table1). The transitions between these types are either sharp or gradational and occasionally with minor tourmaline. meet at a nexus (Figure8). The mineralogy varies depending on the facies of Lease granite, however, it is characteristically composed of hypidiomorphic quartz and red feldspars with variable amounts Table 1. Mineralogical variations in the different facies of Lease granite. of chlorite, biotite and fluorite. Quartz-lined miarolitic cavities (2 to 10 cm) are composed of calcite, Grain white-purpleType Color fluorite, and, occasionally,Mineralogy large euhedralAlteration cassiteriteMineralization (0.1 to 2 cm) with Description minor tourmaline. Size Quartz and Minor Table 1. Mineralogical variations in the different facies of Lease granite.Typical appearance of the reddened Microclinization cassiterite <0.2 to Lease granite. Generally 1 Red feldspars, minor and within Type Color0.5 Graincm Size Mineralogy Alteration Mineralizationcoarser-grained Description at the base and chlorite and chloritization miarolitic Quartz and Typicalfiner appearancein the roof offacies. the Lease Microclinization Minor cassiterite <0.2 to biotitereddened feldspars, cavities granite. Generally 1 Red and within miarolitic 0.5 cm Increasedminor chlorite and coarser-grained at the base and Increasedchloritization Disseminatedcavities fluorite, epidote,biotite finer in the roof facies. microclinization, cassiterite Finest-grained facies of the <0.1 to miaroliticIncreased fluorite, Increased Disseminated 2 Maroon chloritization, hosted in Lease granite. Associated with epidote, miarolitic microclinization, cassiterite Finest-grained facies of the 0.5 mm<0.1 to cavities, and 2 Maroon cavities, and andchloritization, miarolitichosted in Leasedisseminated granite. Associated cassiterite with 0.5 mm chloritization of chloritization ofepidotizationand cavitiesmiarolitic disseminated cassiterite biotitebiotite epidotization cavities Similar mineralogy and grain Minor Similar mineralogy and grain Increased Minor cassiterite sizesize to toType Type 1 1 although although hashas Increased quartz, cassiterite quartz, chlorite, GreaterGreater degree degree present in greatergreater greisenization greisenization resulting 3 3Green Green <0.2 cm<0.2 cm chlorite, sericite, present in sericite, and of greisenizationof greisenization cavities and in inthe the green green tinge. tinge. This This facies and tourmaline cavities and tourmaline dissemination usuallyusually exists exists as as an an alteration alteration dissemination halo around pipes halo around pipes

Figure 8. Surface features of the three Lease granite variants.

Type 11 is the most abundant, and can be considered the typical, “baseline” Lease granite. Typical Typical Lease is is brick-red, brick-red, fine-grained, fine-grained, and and composed composed of ofsub- sub- to anhedral to anhedral graphically graphically intergrown intergrown quartz quartz and andperthite perthite (Figure (Figure 9A).9 QuartzA). Quartz has haslost lostits synneusis its synneusis texture, texture, becoming becoming fine-grained, fine-grained, more more rounded rounded and andanhedral anhedral (0.2 to (0.2 1.2 to mm, 1.2 mm,35%) 35%)(Figure (Figure 9). The9). feldspars The feldspars become become more moreturbid turbid and redder and redder than in than the inBobbejaankop the Bobbejaankop granite, granite, with the with alkali-feldspar the alkali-feldspar (0.5 to 1 (0.5mm, to 35%) 1 mm, and 35%)plagioclase and plagioclase grains (<0.2 grains mm, (<0.2 mm, 20%) becoming smaller and increasingly altered. Biotite is larger (0.5 to 1 cm, 8%) than in the Bobbejaankop granite and typically interstitial, sub- to anhedral, and extensively chloritized with subsequent sericitization (Figure9D). The exclusion of Fe and Ti from the biotite during chloritization Minerals 2020, 10, x FOR PEER REVIEW 9 of 26

20%) becoming smaller and increasingly altered. Biotite is larger (0.5 to 1 cm, 8%) than in the MineralsBobbejaankop2020, 10, 379granite and typically interstitial, sub- to anhedral, and extensively chloritized 9with of 26 subsequent sericitization (Figure 9D). The exclusion of Fe and Ti from the biotite during chloritization results in reddening of the mineral edges and rutile crystallization within the chlorite (Figure 9D). resultsInterstitial in reddening subhedral offluorites the mineral (0.1 to edges 0.4 mm, and 1%) rutile are crystallizationmore common, within with lesser the chlorite amounts (Figure of epidote9D). Interstitialand calcite subhedral (0.1 to 0.4 fluorites mm, <1%). (0.1 to Feldspars 0.4 mm, 1%)exhi arebit morepatch, common, snowball, with and lesser chessboard amounts exsolution of epidote andtextures calcite and (0.1 are to 0.4highly mm, sericitized.<1%). Feldspars The zones exhibit of patch, granophyric snowball, intergrowth and chessboard are purely exsolution quartz textures and andfeldspar are highly with few sericitized. interstitial The or zones included of granophyric phases. Miarolitic intergrowth cavities are are purely common quartz in andthese feldspar facies of with the fewLease, interstitial with amygdales or included composed phases. of Miarolitic calcite and cavities inward-projecting are common in euhedral these facies quartz of theand Lease, feldspars. with amygdalesSmall (<10 µm) composed inclusions of calcite of chloritized and inward-projecting biotite and fluorite euhedral are quartz present and within feldspars. quartz Small and (perthite.<10 µm) inclusions(Figures 9C of and chloritized 9D). biotite and fluorite are present within quartz and perthite (Figure9C,D).

Figure 9. PhotomicrographsPhotomicrographs from the Lease granite. ( A) PPL PPL image image of of granophyric granophyric texture of the Lease granite. ( B) XPL XPL image image of of alkali-feldspar alkali-feldspar (Afs), (Afs), showing showing th thee granophyric overprinti overprintingng of larger quartz (Qz) grains,grains, which which are are optically optically continuous. continuous. The The plagioclase plagioclase (Pl) exsolution (Pl) exsolution within within the perthite the perthite exhibits selectiveexhibits selective replacement replacement by sericite. by (sericite.C) PPL image(C) PPL showing image showing veined exsolution veined exsolution of perthite of andperthite multiple and smallmultiple chlorite small (Chl) chlorite inclusions (Chl) inclusions within quartz within and quartz a miarolitic and a cavity miarolitic filled cavity withsericite filled with (Ser) sericite and fluorite (Ser) (Fl).and fluorite (D) PPL (Fl). image (D) showing PPL image chloritized, showing andchloritized, later sericitized, and later interstitial sericitized, biotite interstitial (Bt). Iron-stainingbiotite (Bt). Iron- (Fe) coatsstaining the (Fe) edges coats of thethe chloritizededges of the biotite chloritized and small biot growthsite and small of rutile growths (Rt) are of present.rutile (Rt) Small are present. fluorite inclusionsSmall fluorite are inclusions scattered within are scattere the quartzd within and the perthites. quartz and perthites.

Miarolitic cavities are common within the Type 1 Lease,Lease, particularlyparticularly proximal and parallel to the BobbejaankopBobbejaankop contact and roof. Cross-cutting calcite veins are associated with a minor sulfidesulfide assemblage of of pyrite pyrite and and lesser lesser chalcopyrite. chalcopyrite. Miarolitic Miarolitic cavities cavities are are typically typically surrounded surrounded by bya 1 ato 1 1.5 to 1.5cm cmwide wide zone zone of ofhighly highly sericitized sericitized granophyric granophyric quartz quartz and and perthite. perthite. Approximately Approximately 5 5 m m above above and parallel to the Bobbejaankop–Lease contact, there is a 1 m thick zone of circular, tourmaline orbicules (1 toto 2020 cm),cm), comprisingcomprising aa melanocraticmelanocratic tourmaline-quartztourmaline-quartz core surrounded by a halo of leucocraticleucocratic quartz-tourmaline with minor cassiterite,cassiterite, fluoritefluorite andand calcite.calcite. Type 22 LeaseLease granitegranite is a maroonmaroon/purple/purple variety, mineralogicallymineralogically similar to Type 1,1, although exhibiting a distinct decreased grain sizesize ((<0.1<0.1 to 0.5 mm), increase in microclinization, granophyric texture, and accessoryaccessory fluorite.fluorite. Granophyric Granophyric quartz quartz displays an optical continuity that delineatesdelineates much largerlarger grains grains (Figure (Figure9B). 9B). Increased Increased microclinization microclinization and hematization and hematization of the feldspars of the producedfeldspars a maroon appearance, which was subsequently overprinted by chloritization then sericitization.

Accessory ilmenite and sulfides, associated with biotite alteration, including pyrite and chalcopyrite Minerals 2020, 10, x FOR PEER REVIEW 10 of 26 producedMinerals 2020a maroon, 10, 379 appearance, which was subsequently overprinted by chloritization10 of 26 then sericitization. Accessory ilmenite and sulfides, associated with biotite alteration, including pyrite and chalcopyrite(0.2 to 0.3 (0.2 mm), to 0.3 are mm), more are abundant more abundant than in the than Bobbejaankop in the Bobbejaankop granite. Cassiterite granite. Cassiterite is red-brown, is red- brown,medium-grained medium-grained (0.5 to(0.5 0.7 to mm),0.7 mm), sub- sub- to euhedral to euhedral and and rarely rarely shows shows zonation. zonation. It occurs It occurs within within miaroliticmiarolitic cavities cavities as asinclusions, inclusions, and and it it appears appears to havehave exsolvedexsolved during during chloritization, chloritization, alongside alongside sulfides,sulfides, decorating decorating relict relict biotit biotitee cleavage. cleavage. These These exsolutions amalgamateamalgamate on on the the mineral mineral edges, edges, formingforming euhedral euhedral crystals crystals (Figure (Figure 10). 10).

FigureFigure 10. Photomicrographs 10. Photomicrographs from from the TypeType 2 Lease.2 Lease. (A )( PPLA) PPL image image of an anhedralof an anhe cassiteritedral cassiterite (Cst) grain (Cst) grainassociated associated with with extensively extensively chloritized chloritized (Chl) (Chl) biotite biotite surrounded surrounded by perthite by perthite and quartz and (Qz). quartz (B) (Qz). PPL (B) PPL imageimage showing showing the exclusionthe exclusion of Sn alongof Sn the along cleavages the ofcleavages the chloritized of the biotite, chloritized which accumulated biotite, which accumulatedinto discrete into grains discrete of cassiterite grains of along cassit theerite periphery along the of the periphery chlorite. of the chlorite. Type 3 is the pale green facies of the Lease granite that is restricted to zones of extensive sericitization,Type 3 is the particularly pale green surrounding facies of pipe-like the Lease and granite lenticular that bodies. is restricted This facies to is similarzones toof Type extensive 1, sericitization,however there particularly is a notable surrounding increase in pervasivepipe-like over-printingand lenticular of sericitizationbodies. This and,facies to ais lesser similar extent, to Type 1, howeversilicification. there Perthites is a notable are extensively increase digestedin pervasiv withe only over-printing skeletal perthites of sericitization remaining (Figure and, 11toA,B). a lesser extent,Plagioclase silicification. is replaced Perthites by very are extensively fine-grained digested white mica, with quartz, only andskeletal fluorite, perthites leaving remaining the anhedral, (Figure 11A,B).reddened Plagioclase alkali-feldspars is replaced in a fine-grainedby very fine-grained greisenized white groundmass. mica, Microcrystallinequartz, and fluorite, or fine-grained leaving the anhedral,anhedral reddened quartz appears alkali-feldspars to have replaced in a fine-gra the originalined quartz greisenized grains within groundmass. and surrounding Microcrystalline the relic or fine-grainedfeldspar grains,anhedral retaining quartz their appears graphic texture.to have Tourmaline replaced withinthe original this facies quartz is more grains prominent, within and and surroundingdisplayed the as incipientrelic feldspar tourmalinization grains, retaining of feldspars their thatgraphic has produced texture. nestsTourmaline and interlocking within this fans facies of is acicular radiating blue-green tourmalines (Figure 11C,D). These tourmaline nests are intergrown with more prominent, and displayed as incipient tourmalinization of feldspars that has produced nests white mica and quartz within miarolitic cavities and occasionally host euhedral cassiterite (Figure 11D). and interlocking fans of acicular radiating blue-green tourmalines (Figure 11C,D). These tourmaline The roof of the Lease granite is poorly exposed and demarcated by a zone of discontinuous, nestslaterally are intergrown extensive pegmatiteswith white which mica are and interfingered quartz within with Type miarolit 1 Leaseic cavities (Figure2 ).and These occasionally irregularly host euhedralshaped, cassiterite lensoidal (Figure pegmatites 11D). are 5 m below the Rashoop Granophyre Suite contact and range from 1The5 cmroof to of 0.05 the 0.7Lease m in granite size. They is poorly are composed expose ofd pegmatiticand demarcated quartz and by reda zone feldspar, of discontinuous, exhibiting × × laterallyperpendicular extensive growthpegmatites inwards which from are the interfingered roof and base, with indicative Type 1 Lease of a unidirectional (Figure 2). These silicification irregularly shaped,texture lensoidal (UST). Subhedralpegmatites biotite, are 5 variably m below colored the Rashoop fluorite, andGranophyre calcite are commonlySuite contact interstitial and range phases. from 1 × 5 cmSöhnge to 0.05 [30 ]× recorded 0.7 m in mineable size. They occurrences are composed of cassiterite of pegmatitic and scheelite, quartz but theseand red were feldspar, not observed exhibiting in perpendicularthe present study.growth inwards from the roof and base, indicative of a unidirectional silicification texture (UST). Subhedral biotite, variably colored fluorite, and calcite are commonly interstitial phases. Söhnge [30] recorded mineable occurrences of cassiterite and scheelite, but these were not observed in the present study.

Minerals 2020, 10, 379 11 of 26 Minerals 2020, 10, x FOR PEER REVIEW 11 of 26

Figure 11. PhotomicrographsPhotomicrographs from from Type Type 3 Lease granite.granite. ( (AA)) PPL PPL image image showing the complete replacement of of the the plagioclase plagioclase component component of of a aperthi perthitete by by greisenization, greisenization, leaving leaving behind behind patches patches of sericiteof sericite (Ser) (Ser) and and skeletal skeletal alkali-feldspars alkali-feldspars (Afs). (Afs). (B) (AnB) AnXPL XPL image image of (A) of (A) illustrating illustrating the the extent extent of greisenizationof greisenization experienced experienced by bythe theprimary primary mineral mineral assemblage. assemblage. The Therelict relict perthite perthite is outlined is outlined in red. in (red.C) A ( CPPL) A image PPL image of a miarolitic of a miarolitic cavity cavitywithin withinthe Lease the granite Lease granitehosting hosting tourmaline tourmaline (Tur) and (Tur) doubly and terminateddoubly terminated quartz quartz(Qz). ( (Qz).D) A (DPPL) A image PPL image from from Type Type 2 showin 2 showingg the the pervasive pervasive and and destructive tourmalinization of the alkali-feldspars resulting inin thethe growthgrowth ofof radialradial blueblue/green/green tourmaline.tourmaline.

Cassiterite Mineralization There are three styles of mineralization within the Zaaiplaats tin fields:fields: disseminated miarolitic zones, tourmalinized–greisenized tourmalinized–greisenized hydrothermal hydrothermal pipes pipes and and lenticular lenticular ore ore bodies. bodies. The The 600 600 m long, m long, 15 15to 30 to 30m mwide wide zones zones of ofdisseminated disseminated mineralization mineralization at at Groenfontein Groenfontein are are hosted hosted within within Type Type 2 Lease granite, 20 to 40 m below the contact pegmatite.pegmatite. However, at Zaaiplaats mineralization is hosted in both the upper Bobbejaankop and lower LeaseLease granites.granites. Cassiterite in the low-grade disseminated zones occurs in various coexisting styles: (1) (1) free eu euhedralhedral cassiterite crystals that appear to occur interstitially and and to have replaced quartz; (2) sub- to anhedral cassiterite which are associated with the chloritization of biotite and subsequent sericitization of the chlorite; and (3) clusters of euhedral cassiterites within miarolitic cavities (1 to 3 cm) surrounded byby euhedraleuhedral quartzquartz andand redred feldspars.feldspars. The majoritymajority of of the the cassiterite cassiterite was minedwas mined from an fr anastomosingom an anastomosing network ofnetwork high-grade of greisenized,high-grade pipe-likegreisenized, and pipe-like lenticular and ore lenticular bodies [7 ,ore11,12 bodies,26,27 ,[733,11,12,26,27,33].]. The grades of The these grad pipeses of ranged these pipes between ranged 10% betweenand 70%, 10% with and rare 70%, occurrences with rare of occurrences up to 80% cassiteriteof up to 80% in o ffcassiteriteshoots. The in offshoots. swarms of The hydrothermal swarms of pipeshydrothermal radiate upwards pipes radiate through upwards the Bobbejaankop, through the terminating Bobbejaankop, into lenticular terminating ore bodiesinto lenticular in the upper ore bodiesfacies ofin the the Lease upper granite. facies Theof the pipes Lease shrink granite. from The 25 to pipes 3 m wideshrink in thefrom Bobbejaankop 25 to 3 m wide to 5 toin 1the m withinBobbejaankop the Lease to granite.5 to 1 m The within pipe the swarms Lease stem granite. from The a single, pipe swarms larger pipe stem and from share a single, a common larger plunge pipe andtowards share the a common northwest plunge [7,13]. towards These hydrothermalthe northwest pipes[7,13]. are These composed hydrotherm of threeal pipes parts: are (1) composed a core of ofalmost three pure parts: sericite (1) a core that of contains almost uppure to sericite 60% to 80%that cassiteritecontains up [7 ,to13 60%]; (2) to an 80% annular cassiterite melanocratic [7,13]; (2) zone an annularcomposed melanocratic of radial and zone botryoidal composed tourmalines, of radial and intergrown botryoidal with tourmalines, anhedral quartzintergrown and minor with anhedral feldspar; quartzand (3) and a thin minor (1 to feldspar; 5 cm) white and quartz (3) a thin rim. (1 There to 5 cm) is a white halo (2quartz to 15 rim. cm) ofThere highly is a hematized halo (2 to 15 feldspars cm) of highlyand silicification hematized of feldspars the surrounding and silicification granite. of the surrounding granite. The lenticular ore-bodies are inaccessible as they were mined out and backfilledbackfilled but detailed descriptions cancan bebe foundfound in in Kynaston Kynaston et et al. al. [34 [34],], Söhnge Söhnge [30 [30],], Strauss Strau andss and Truter Truter [13], [13], and and Coetzee Coetzee [11]. [11]. These lenses are 5 m below and contiguous to the contact pegmatite and although similar are distinctly different from the pipes [7,27,34]. The main ore grade is recorded between 30% to 70%

Minerals 2020, 10, 379 12 of 26

These lenses are 5 m below and contiguous to the contact pegmatite and although similar are distinctly different from the pipes [7,27,34]. The main ore grade is recorded between 30% to 70% cassiterite hosted in a highly greisenized groundmass composed of sericite, cassiterite, chlorite, fluorite and pyrite similar to the pipes, although distinctly tourmaline-poor [7,13,34]. The Lease granite, below these lenses, shows an increased dissemination of fluorite and cassiterite (0.5% to 2%) that was mined alongside these bodies. In order to determine the petrogenetic relationship between the Lease and Bobbejaankop granite, and to get a better constraint on their formation and mineralization, a number of geochemical analyses were undertaken to complement the petrographic descriptions and contact relationships described.

4. Methods and Materials Whole-rock geochemical analyses were undertaken using the Norrish Fusion [35] technique with in-house correction procedures and using a Panalytical PW2404 X-ray fluorescence (XRF) spectrometer at the Earth Lab of the University of the Witwatersrand, Johannesburg (Supplementary Table S1). Samples for XRF majors and ICP-MS traces were prepared and run according to standard procedures using methodology outlined in Wilson [36]. Samples were crushed in a carbon-steel jaw crusher and milled by a carbon-steel swing mill. Major elements were fused using Johnson Matthey Spectrolflux 105 at 1100 ◦C, and raw data corrected using in-house software. Standard calibrations were made using synthetic oxide mixtures and international standard rocks as well as in-house controls. Calibration standards were primary International Reference Materials USGS series (USA) and NIM series (South Africa). Trace elements were determined using the Perkin Elmer DRC-e inductively coupled plasma mass spectrometer (ICP-MS), and analyzed against certified primary solution standards. International reference materials AGV-2, BCR-1 and BR-1 were analyzed with every run, ensuring the reliability of the data. Of the forty five samples collected in situ, five were of Nebo granite, sixteen of Bobbejaankop granite, twenty of Lease granite, and four of the Transitional Zone. Due to a large degree of overlap, the geochemical data obtained from both the Zaaiplaats and Groenfontein samples are combined under the broader category of “Zaaiplaats tin field”. The geochemical data of the samples from their respective mines can be found in Supplementary Table S1. The historical data of the Nebo granite have been compiled from various parts of the Lebowa Granite Suite, while the Bobbejaankop and Lease granites were sampled specifically from the Zaaiplaats tin mine [20,21,28,37].

5. Results

Whole Rock XRF and ICP-MS The granites of the Zaaiplaats tin field have a typical granitic composition, ranging from (tot) SiO2 = 74.22% to 77.35%, TiO2 = 0.05% to 0.14%, Al2O3 = 10.99% to 12.85%, FeO = 0.96% to 3.95%, with a relatively high alkali content of Na2O = 0.90% to 3.09%, K2O = 4.57% to 6.3%, variable CaO and low MgO (Figure 12). These ferroan alkali granites exhibit the weakly peraluminous characteristic of A-type granites, with an A/CNK between 0.95 to 1.33, plotting with other global mineralized and unmineralized A-type granites (Figure 13)[38–44]. The trace element differentiation index (TEDI, (Ba Sr)/Rb) [1,45] has proved to be useful as × a fractionation proxy and in distinguishing the various granite facies of the study area (Figure 12). The TEDI defines a progressive fractionation trend from the Nebo, through Bobbejaankop to Lease granite (Figure 12). The major element compositions of the Bobbejaankop and Lease granites are comparable, and distinct in terms of alkali and incompatible element enrichment to the Nebo granite (Figure 12E–G). Minerals 2020, 10, 379 13 of 26 Minerals 2020, 10, x FOR PEER REVIEW 13 of 26

FigureFigure 12. 12.Harker Harkerstyle style graphs graphs showing showingmajor majorand and trace trace element element distribution distributionwithin withinthe the granites granites of of thethe Zaaiplaats Zaaiplaats tin tin field. field. TheThe tracetrace elementelement didifferentiatfferentiationion index index (TEDI) (TEDI)by byWalraven Walraven [ 45[45]] is is used used in in all all

graphs.graphs. AA decreasedecrease inin TEDITEDI represents represents an an increase increase in in fractionation fractionation vs. vs. ( A(A)) SiO SiO2 2showing showing an an increase increase fromfrom the the Nebo Nebo to to the the Lease Lease granite. granite. ((BB)) TiOTiO22 showingshowing aa decreasedecrease fromfrom thethe NeboNebo toto thethe LeaseLease granite.granite. (C(C)) Al Al22OO33 exhibitingexhibiting aa slightslight decreasedecrease fromfrom NeboNebo toto Bobbejaankop,Bobbejaankop, however,however, it it does does not not vary vary between between thethe Bobbejaankop Bobbejaankop and and Lease Lease granites. granites. (D) ( FeOD) totFeOshowingtot showing a decrease a decrease from thefrom Nebo theto Nebo the Lease to the granite. Lease (Egranite.) CaO exhibiting (E) CaO exhibiting a decrease a from decrease the Nebo from to the the Nebo Bobbejaankop to the Bobbejaankop before increasing before in theincreasing Lease granite. in the

(FLease) Na2 Ogranite. showing (F) aNa slight2O showing decrease a from slight the decrease Nebo to thefrom Lease the granite.Nebo to (Gthe)K Lease2O showing granite. very (G) little K2O variation.showing very (H) Li little reveals variation. a distinct (H) increase Li reveals in thea distinct Lease granite.increase (inI) Snthe is Lease enriched granite. in the (I) Bobbejaankop Sn is enriched andin the Lease Bobbejaankop granites. (J )and Cu Lease is relatively granites. consistent (J) Cu is apart relatively from theconsistent increase apart in the from Lease the granite. increase ( Kin)W the showingLease granite. very little (K) W variation. showing (L very) Th increaseslittle variation. from the(L) NeboTh increases to the Lease from granite.the Nebo Oxides to the andLease elements granite. areOxides in wt% and and elements ppm, respectively. are in wt% and ppm, respectively.

TheThe Bobbejaankop Bobbejaankop and and Lease Lease granites granites are are enriched enriched in in Si, Si, Li, Li, Sn, Sn, Cu, Cu, and and Th Th and and depleted depleted in Ti, in Al, Ti, Fe,Al, and Fe, Naand compared Na compared to the to Nebo the graniteNebo granite (Figure (Figure 12). The 12). CaO The content CaO ofcontent the Nebo of the granite Nebo steadily granite steadily decreases with fractionation, compared to the Bobbejaankop and Lease granites, which

Minerals 2020, 10, 379 14 of 26

Minerals 2020, 10, x FOR PEER REVIEW 14 of 26 decreases with fractionation, compared to the Bobbejaankop and Lease granites, which conversely becomeconversely more become calcic withmore fractionation calcic with (Figurefractionation 12E). This(Figure is enigmatic, 12E). This as is other enigmatic, alkali elements as other doalkali not replicateelements thisdo not trend. replicate this trend. Although they they are are geochemically geochemically similar, similar, there there are are discernible discernible chemical chemical differences differences between between the theLease Lease and and Bobbejaankop Bobbejaankop granites granites based based on ontheir their lo locations,cations, from from the the mines mines of of Groenfontein or Zaaiplaats. Generally, the granites at Zaaiplaats areare more peraluminous and exhibit a greater alkali and incompatible element enrichment than their GroenfonteinGroenfontein equivalents.equivalents. Lithium in the Nebo and Bobbejaankop are similar between <<11 to to 24 ppm, althou althoughgh they increase to 30.86 ppm in the Lease granite (Figure 1212H).H). There is a sharp increase in Th from the Nebo to the Lease granite, from 10 to 95.85 ppm, respectively, however,however, samplessamples fromfrom thethe transitionaltransitional contact zone appear to be noticeably depleted (Figure 1212L).L). The Sn content of the BobbejaankopBobbejaankop and Lease granites is generally greater than in thethe Nebo,Nebo, althoughalthough certaincertain samplessamples containingcontaining disseminated cassiterite have Sn contents greater than 100 ppm and, according to published data, can be as high as as 1085 1085 ppm ppm [14] [14] (Figure (Figure 12I).12I). The Bobbejaankop granite at Zaaiplaats and the LeaseLease granite at Groenfontein have the highest Sn contents, although Sn at Zaaiplaats is significantlysignificantly higher. Similarly, Cu in the ZaaiplaatsZaaiplaats Bobbejaankop and Lease granites reaches 248 and and 2410 2410 ppm, ppm, respectively, respectively, although although comparatively, comparatively, at Groenfontein, Groenfontein, only the Lease granite exhibits a Cu enrichment of up to 400 ppm (Figure 1212J).J). The transitional zone typically plots between thethe Bobbejaankop andand LeaseLease clusters,clusters, butbut isis unusuallyunusually Th-Th- andand Li-depleted.Li-depleted.

Figure 13. Alumina saturation and ferroan classification classification diagrams comparing the Zaaiplaats tin fieldfield granites withwith literatureliterature data. data. (A ()AA) /CNKA/CNK vs. vs. A/NK A/NK granite granite discrimination discrimination diagram. diagram. The range The ofrange global of mineralizedglobal mineralized and unmineralized and unmineralized A-Type A-Type granites gran shownites shown within within the dashed the dashed ellipse, ellipse, compiled compiled from publishedfrom published data [38 data,40– 44[38,40–44].]. Circular Circular datapoints datapoin representts represent the range the of geochemicalrange of geochemical analyses taken analyses from thetaken literature; from the Nebo literature; granite Nebo (grey), granite Bobbejaankop (grey), Bo granitebbejaankop (blue), granite and Lease (blue), granite and Lease (red) granite [14,20,21 (red),37]. ([14,20,21,37].B) Ferroan classification (B) Ferroan diagramclassification after diagram Frost et al. after [46 ]Frost displaying et al. [46] the displaying ferroan nature the ferroan of these nature granites. of these granites. The normative Qz-Or-Ab ternary diagram displays an evolution of these granites from the Nebo, whichThe plots normative between Qz-Or-Ab the hydrous ternary minima diagram of 0.4 displays and 0.07 Han2 Oevolution activity of (aH2O these), towardsgranites afrom more the Qz Nebo,> Or composition (Figure 14). The most primitive Nebo granite compositions plot just above the hydrous which plots between the hydrous minima of 0.4 and 0.07 H2O activity (aH2O), towards a more Qz > Or minimumcomposition at (Figure 5 kb, and 14). cluster The most at the primitive 2 kb (0.5 Nebo aH2O granite) minimum. compositions The majority plot just of above the Bobbejaankop the hydrous and some of the Lease granite compositions cluster along the 2 kb hydrous cotectic line with an H O minimum at 5 kb, and cluster at the 2 kb (0.5 aH2O) minimum. The majority of the Bobbejaankop and2 activity below 1, and migrate towards the Qz-Or boundary (Figure 14). The Groenfontein granites are some of the Lease granite compositions cluster along the 2 kb hydrous cotectic line with an H2O moreactivity tightly below clustered 1, and migrate and are nottowards as fractionated the Qz-Or asboundary their Zaaiplaats (Figure equivalents.14). The Groenfontein granites are more tightly clustered and are not as fractionated as their Zaaiplaats equivalents.

Minerals 2020, 10, 379 15 of 26 Minerals 2020, 10, x FOR PEER REVIEW 15 of 26

Figure 14. Normative Qz-Ab-Or plot of the granites from the Zaaiplaats tin field field (Triangles). (A) White squares and solid lines represent haplogranite, hydrou hydrouss eutectic points and and cotectic cotectic lines lines respectively, respectively, whichwhich varyvary due due to to pressure pressure [47 [47–51].–51]. The The white white diamond diamond represents represents the anhydrous the anhydrous minimum minimum [52]. White [52].

circlesWhite representcircles represent the displacement the displacement of eutectic of pointseutectic as points a function as a offunction H2O activity of H2O (a activityH2O). (B (a) CircularH2O). (B) datapointsCircular datapoints represent represent the Nebo the granite Nebo (grey), granite Bobbejaankop (grey), Bobbejaankop granite (blue) granite and (blue) Lease graniteand Lease (red) granite from the(red) literature from the [14 literature,20,21,37 [14,20,21,37].].

The apparent fractionation from the Nebo to Bobbejaankop and then to the LeaseLease granite is demonstrated by ratios ratios such such as as Rb/Sr Rb/Sr and and TEDI TEDI (Figure (Figure 15A) 15A) [1,45] [1,45 and] and can can be be modelled modelled in interms terms of ofRayleigh Rayleigh Fractionation. Fractionation. A representative A representative assemblage assemblage for the for Nebo the Nebo granite granite of 50% of 50%alkali-feldspar, alkali-feldspar, 30% 30%quartz, quartz, 10% plagioclase, 10% plagioclase, and 10% and hornblende 10% hornblende was used, was used,and the and partition the partition coefficients coeffi cients(KD) for (K Rb,D) forSr, Rb,and Sr,Ba andare given Ba are in given Supplementary in Supplementary Table S2. Table The S2.conc Theentration concentration of Rb, Sr, of and Rb, Ba Sr, within and Ba the within residual the residualliquid (C liquidliquid) was (Cliquid calculated) was calculated using Equation using Equation (1). The (1). bu Thelk partition bulk partition coefficient coefficient was wascalculated calculated by bymultiplying multiplying the themineral mineral proportions proportions and andthe respective the respective KD ofK DRb,of Sr, Rb, and Sr, Ba. and The Ba. K TheD valuesKD valuesfor K- forfeldspar, K-feldspar, plagioclase, plagioclase, biotite, biotite, hornblende, hornblende, and andquartz quartz are are within within ranges ranges of of published published valuesvalues (Supplementary(Supplementary Table S2)S2) [[5353–56].–56]. The Nebo granite, in the vicinity of the Zaaiplaats tin field,field, is considered closeclose to its roof and would inherently be the most evolved Nebo granite facies. Therefore, Therefore, thethe initial composition and modal proportions werewere selectedselected toto represent thethe mostmost primitiveprimitive faciesfacies ofof thethe NeboNebo granite,granite, takentaken fromfrom thethe literatureliterature datadata [[19,21,37].19,21,37].

() 𝐶 = 𝐶 × (𝐹KD 1) (1) C = C F − (1) liquid initial × Rayleigh Fractionation modelling identifies a linear fractionation trend from the Nebo to the LeaseRayleigh granite (Figure Fractionation 15B). The modelling Nebo granite identifies clusters a linear tightly fractionation along the fractionation trend from thetrend, Nebo while to the LeaseBobbejaankop granite (Figure and Lease 15B). granite The Nebo data graniteare considerably clusters tightly more alongscattered. the fractionationModelling indicates trend, while that 10% the Bobbejaankopfractionation of and the Lease Nebogranite granite data is required are considerably to produce more the scattered.first compositions Modelling of indicatesthe Bobbejaankop that 10% fractionationgranite. After of60% the fractionation, Nebo granite the is required Nebo granite to produce ends and the firstthe Bobbejaankop compositions ofgranite the Bobbejaankop composition granite.becomes Afterdominant 60% fractionation,(Figure 15B). theAt approximately Nebo granite ends 65% andfractionation, the Bobbejaankop the composition granite compositionof the Lease becomesgranite appears dominant and (Figureboth the 15 LeaseB). At and approximately Bobbejaankop 65% granite fractionation, compositions the compositionare present before of the ending Lease graniteat 80%. appearsFractionation and both modelling the Lease of andthe Nebo Bobbejaankop granite reveals granite little compositions distinction are between present the before Lease ending and atBobbejaankop 80%. Fractionation granites. modelling However, ofelements the Nebo such granite as Rb, reveals Sr, and little Ba may distinction be problematic between as the they Lease are andsusceptible Bobbejaankop to modification granites. by However, later hydrothermal elements such interaction, as Rb, Sr, so and other Ba mayfractionation be problematic ratios are as theyconsidered. are susceptible to modification by later hydrothermal interaction, so other fractionation ratios are considered.

Minerals 2020, 10, 379 16 of 26 Minerals 2020, 10, x FOR PEER REVIEW 16 of 26

Figure 15.15. DiagramsDiagrams illustrating illustrating the the extent extent of fractionation of fractionation of the Zaaiplaatsof the Zaaiplaats tin field granites,tin field comparing granites, comparingthem to the them literature to the data. literature (A) Plot data. of TEDI(A) Plot vs. of Rb TEDI/Sr exhibiting vs. Rb/Sr theexhi fractionationbiting the fractionation trend from Nebotrend from(grey) Nebo to Lease (grey) (red). to Lease (B) Rayleigh (red). (B Fractionation) Rayleigh Fractionation of the Zaaiplaats of the tin Zaaiplaats field granites, tin field showing granites, the

showingpercentage the fractionation percentage fractionation along the purple along line the (Concentration purple line (Concentration in Liquid, C Lin). Liquid, Circular C dataL). Circular points datarepresent points the represent range of the the literature’s range of geochemical the literatur analyses;e’s geochemical Nebo granite analyses; (grey), Nebo Bobbejaankop granite granite(grey), Bobbejaankop(blue) and Lease granite granite (blue) (red) and [14 ,Lease20,21, 37granite]. (red) [14,20,21,37].

The TEDI TEDI parameter parameter is is able able to to distinguish distinguish the the Ne Nebobo and and Bobbejaankop Bobbejaankop granites, granites, but but is unable is unable to differentiateto differentiate the the Lease Lease from from the the Bobbejaankop Bobbejaankop gran granite.ite. The The inferred inferred TEDI TEDI fractionation fractionation trend trend is replicated with other, more robust, large ion lithop lithophilehile element (LILE) ratios ratios such such as as Zr/Hf Zr/Hf and and K/Rb, K/Rb, which are regarded as eeffectiveffective indicesindices ofof granitegranite fractionationfractionation [[57–61].57–61]. These fractionation indices proved moremore e ffectiveeffective in distinguishingin distinguishing between betw theeen two granites,the two althoughgranites, the although complete the diff erentiationcomplete differentiationof the Bobbejaankop of the andBobbejaankop Lease granites and wasLease similarly granites unsuccessful. was similarly unsuccessful. Incompatible element element ratios ratios such such as as Nb/Ta Nb/Ta and ZrZr/Hf/Hf are suggested to remain constant within closed, fractionatingfractionating magmatic magmatic systems systems [59, 62[59,62,63].,63]. The LeaseThe granite,Lease granite, from Groenfontein from Groenfontein and Zaaiplaats, and Zaaiplaats,has an average has Zran/ Hfaverage ratio ofZr/Hf 18.49 ratio and of 20.18, 18.49 respectively. and 20.18, Therespectively. Bobbejaankop The Bobbejaankop from Groenfontein from Groenfonteinand Zaaiplaats and shows Zaaiplaats a higher shows average a higher Zr/ Hfaverage of 23.41 Zr/Hf and of 20.92, 23.41 and respectively, 20.92, respectively, whereas the whereas Nebo thegranite Nebo displays granite a displays larger range a larger from range 19.74 tofrom 88.57 19.74 (Figure to 88.57 16A). (Figure Additionally, 16A). Additionally, comparing Nb comparing/Ta against Nb/TaZr/Hf, theagainst Bobbejaankop Zr/Hf, the and Bobbejaankop Lease granites and of ZaaiplaatsLease granites display of a Zaaiplaats significant decreaseddisplay a Nbsignificant/Ta ratio decreasedcompared Nb/Ta to the equivalentratio compared lithologies to the from equivalent Groenfontein, lithologies while from maintaining Groenfontein, an expected while maintaining decreasing anZr /expectedHf ratio (Figure decreasing 16D). Zr/Hf The Zrratio/Hf (Figure fractionation 16D). The proxy Zr/Hf shows fractionation a progression proxy from shows the Neboa progression through fromBobbejaankop the Nebo tothrough Lease granite,Bobbejaankop as does to the Lease TEDI granite, and Rayleigh as does Fractionationthe TEDI and (FiguresRayleigh 15 Fractionation and 16). (FiguresThe 15 granites and 16). of Zaaiplaats and Groenfontein, although part of the same granitic pluton, exhibit differentThe granites Nb/Ta ratios. of Zaaiplaats The Nebo and andGroenfontein, Bobbejaankop although granites part fromof the Groenfonteinsame granitic andpluton, Zaaiplaats exhibit differentdo not significantly Nb/Ta ratios. di ffTheer in Nebo Nb, and whereas Bobbejaankop the Lease granites granite isfrom considerably Groenfontein enriched and Zaaiplaats (Figure 16 A).do notMoreover, significantly the Nb differ/Ta ratios in Nb, of thewhereas Bobbejaankop the Lease and granite Lease is fromconsiderably the two mines,enriched are (Figure significantly 16A). Moreover,different (Figure the Nb/Ta 16B,C). ratios The of granites the Bobbejaankop from Groenfontein and Lease have from a comparatively the two mines, lower are Tasignificantly than their differentcounterparts (Figure at Zaaiplaats,16B,C). The however granites thefrom Lease Groenfontein granite is have generally a comparatively more enriched lower in Ta Ta than than their the counterpartsBobbejaankop at granite Zaaiplaats, (Figure however 16B,C). the Two Lease trends gran areite observed is generally within more these enriched granites; in the Ta first than trend the Bobbejaankopshows an increased granite Ta (Figure of the Groenfontein 16B,C). Two Leasetrends without are observed variation within from these the Nbgranites;/Ta ratio. the The first second trend showstrend exhibited an increased by the Ta Bobbejaankopof the Groenfontein and Lease Lease from without Zaaiplaats variation shows from both the Nb/Ta an increased ratio. The Ta contentsecond trendand a exhibited deviation by in thethe NbBobbejaankop/Ta ratio (Figure and Lease16C). from Zaaiplaats shows both an increased Ta content and aREE deviation and other in the trace Nb/Ta element ratio (Figure patterns 16C). display a slight enrichment in light rare earth elements (LREE),REE a and negative other Eu trace anomaly element and patterns a plateauing display of thea sl heavyight enrichment rare earth elementsin light rare (HREE), earth withelements high (LREE),Zr, Nb, Y,a negative U, Th, REE Eu and anomaly negative and Ba, a plateauing Nb, Sr, P, Ti of and the Euheavy anomalies, rare earth consistent elements with (HREE), typical with A-types high Zr,(Figure Nb, Y,17 U,)[20 Th,,38 REE,64– and67]. negative There is Ba, little Nb, separation Sr, P, Ti and between Eu anomalies, the granites consistent apartfrom with thetypical enrichment A-types (Figureof incompatible 17) [20,38,64–67]. and mobile There elements is little such separation as Rb, Ba,between Th, U, the Sr, granites P, Sn and apart HREEs from in the the enrichment Lease granite of incompatible and mobile elements such as Rb, Ba, Th, U, Sr, P, Sn and HREEs in the Lease granite (Figure 17B). The transitional zone, however, exhibits an atypical trend compared to the other

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(FigureMinerals 17B). 2020 The, 10, transitionalx FOR PEER REVIEW zone, however, exhibits an atypical trend compared to the other17 of granites. 26 Minerals 2020, 10, x FOR PEER REVIEW 17 of 26 Withgranites. the exception With the of one exception sample of that one plots sample similarly that plots to the similarly Bobbejaankop to the Bobbejaankop granite, the transitional granite, the zone is significantlygranites.transitional With REE-,zone the is Ce-, exceptionsignificantly La-, Th-, of REE-,one Sm- sample andCe-, U-depletedLa-, that Th-, plots Sm- (Figure similarlyand U-depleted 17 toA,B). the (FigureBobbejaankop 17A,B). granite, the transitional zone is significantly REE-, Ce-, La-, Th-, Sm- and U-depleted (Figure 17A,B).

FigureFigure 16. Diagrams16. Diagrams illustrating illustrating the the LILEs LILEs and and di differentfferent indexes indexesof of fractionationfractionation from the the Zaaiplaats Zaaiplaats tin fieldFiguretin granites field 16. granites(A Diagrams) Graph (A) illustratingGraph of Zr/Hf of vs.Zr/Hf the Nb LILEs vs. indicating Nb and indicating different an increasing anindexes increasing of Nb fractionation withNb with decreased decreased from the Zr / ZaaiplaatsHf.Zr/Hf. (B )(B Plot) of tin field granites (A) Graph of Zr/Hf vs. Nb indicating an increasing Nb with decreased Zr/Hf. (B) Ta vs.Plot Rb of that Ta representsvs. Rb that therepresents geochemical the geochemica signaturel ofsignature internally of internally derived magmatic derived magmatic fluids, displaying fluids, a Plotdisplaying of Ta vs.a spectrum Rb that representsof increasi ngthe hydrothermalgeochemical signatureactivity of of th internallye Bobbejaankop derived and magmatic Lease granites fluids, spectrum of increasing hydrothermal activity of the Bobbejaankop and Lease granites from Groenfontein displayingfrom Groenfontein a spectrum and of Zaaiplaats. increasing ( Chydrothermal) The Nb/Ta vs.activity Ta graph of th eshows Bobbejaankop two trends: and (1) Lease increased granites Ta and Zaaiplaats. (C) The Nb/Ta vs. Ta graph shows two trends: (1) increased Ta without variation fromwithout Groenfontein variation in and the Zaaiplaats.Nb/Ta ratio; ( Cand) The (2) Nb/Taincrease vs. in Ta Ta graph with deviation shows two in thetrends: Nb/Ta (1) ratio.increased (D) The Ta in the Nb/Ta ratio; and (2) increase in Ta with deviation in the Nb/Ta ratio. (D) The Zr/Hf vs. Nb/Ta withoutZr/Hf vs. variation Nb/Ta graph in the highlightsNb/Ta ratio; the and decoupling (2) increa seof inthe Ta Nb/Ta with deviation magmatic in signature the Nb/Ta producing ratio. (D) Thetwo graphZr/Hfdifferent highlights vs. ratiosNb/Ta thefor graph the decoupling Bobbejaankop highlights of the the and decoupling Nb Lease/Ta granit magmatic of thees (dashed Nb/Ta signature magmaticline) caused producing signature by F-rich two producing hydrothermal different two ratios for thedifferentfluids Bobbejaankop (HTF). ratios Circular for the and Bobbejaankopdatapoints Lease granites represent and (dashedLease the granit range line)es of (dashed caused the literature line) by F-rich caused geochemical hydrothermal by F-rich analyses; hydrothermal fluids Nebo (HTF). Circularfluidsgranite datapoints (HTF). (grey), Circular Bobbejaankop represent datapoints the granit range represente (blue) of the and the literature Leaserange graniteof geochemicalthe literature(red) [14,20,21,37]. geochemical analyses; Nebo analyses; granite Nebo (grey), Bobbejaankopgranite (grey), granite Bobbejaankop (blue) and granit Leasee (blue) granite and (red) Lease [14 granite,20,21 (red,37].) [14,20,21,37].

Figure 17. Rare earth element (REE) and multi-element diagrams of the granites from the Zaaiplaats FigureFiguretin 17.field Rare17. (A Rare) earthChondrite-normalized earth element element (REE) (REE) and andREE multi-element multi-elementpatterns for diagramsth dieagrams Nebo, ofBobbejaankopof thethe granites andfrom from Lease the the ZaaiplaatsZaaiplaats granites. tin fieldtinValues (A )field Chondrite-normalized normalized (A) Chondrite-normalized using data REE from patterns REE McDonough patterns for the for and Nebo, th eSun Nebo, Bobbejaankop [68]. Bobbejaankop (B) Primitive and and Leasemantle-normalized Lease granites. granites. Values Values normalized using data from McDonough and Sun [68]. (B) Primitive mantle-normalized normalizedmultielement using patterns data from for McDonoughthe Nebo, Bobbejaankop and Sun [ 68an].d Lease (B) Primitive granites. mantle-normalizedNormalized using values multielement from multielementSun and McDonough patterns [69].for the Nebo, Bobbejaankop and Lease granites. Normalized using values from patterns for the Nebo, Bobbejaankop and Lease granites. Normalized using values from Sun and Sun and McDonough [69]. McDonough [69].

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6. Discussion The mineralogical evolution of the Nebo granite, from its hornblende-bearing base to the biotite-bearing roof, suggests in situ fractionation of the granite [4,20–24]. Therefore, it is sensible to suggest that the gradual evolution from the Nebo to the Bobbejaankop granite would represent a continuation of this fractionation trend. Trace element fractionation proxies and Rayleigh Fractionation display a linear trend from the Nebo to the Bobbejaankop and Lease granites, supporting the origin of the Bobbejaankop by fractionation of the Nebo granite (Figure 15). Rayleigh Fractionation shows the Lease and Bobbejaankop granites to be the upper limit of what can be considered a reasonable degree of fractionation (Figure 15B). However, taking the relative scales of these plutonic bodies into account, the degree of fractionation is not as unrealistic as initially thought. The Nebo granite has an approximate volume of 100,000 km3 and is internally fractionated [70]. Thus, as the Bobbejaankop granite is only several hundred meters thick, approximately 65% fractionation of the Nebo granite is probable [20,70]. However, the inability to geochemically distinguish between the Bobbejaankop and Lease granites, using fractionation ratios suggests that fractionation alone is not responsible for the formation of Lease granite. Petrographically, the differences between the Bobbejaankop and the Lease granite are granophyric grain-size reduction and increased microclinization, chloritization, and sericitization (Figures7 and 13A). Geochemically the differences are more subtle, mainly pertaining to increased incompatible and large ion lithophile elements, such as Ce, Cs, Cu, Li, Lu, Nb, Rb, Sn Ta, Th, U, and Yb (Figure 17). These elements are readily mobilized by hydrothermal fluids and, as there was no exogranitic fluid escape [28], can be inferred to represent the accumulation of late-stage fractionated hydrothermal fluids. The increased calcium content of the Lease compared to the Bobbejaankop granite is seen by the increased presence of fluorite and epidotization of the plagioclase. Chloritization, the dominant alteration within the Bobbejaankop granite, becomes increasingly subordinate to microclinization, sericitization, and occasionally epidotization within the Lease granite. A geochemical representation of this shift in alteration is proxied by (K2O + CaO)/Na2O, where an increased ratio represents a corresponding increase in microclinization (K) and epidotization (Ca) and the loss of Na-plagioclase. The Na depletion, is the result of the breakdown of plagioclase and the loss of Na, which resulted from more intense greisenization particularly apparent within the Type 3 Lease (Figures 11B and 12F) [71–73]. Generally, (K2O + CaO)/Na2O increases from the Nebo to the Bobbejaankop granite, however, the Lease granite at Zaaiplaats is significantly more altered (Figure 18A). Similarly the migration of the data for the Lease granites, on the Qz-Ab-Or diagram, away from the Nebo and Bobbejaankop clusters suggests that the Lease experienced a greater degree of microclinization and greisenization (Figure 14)[18]. Furthermore, by combining the feldspar alteration proxy with a biotite chloritization (tot) proxy TiO2/FeO [73], a shift in the dominant style of hydrothermal alteration is identified (Figure 18B). Additionally, combining alteration with Sn content reveals a concentration of Sn at the transition from chloritization to microclinization and epidotization as the dominant styles of alteration (Figure 18B). Therefore, the highest Sn grade, at both mines coincides with the change in alteration from chloritization to microclinization and epidotization (Figure 18B). Mineralized granites from both mines occur at this transition between alterations, suggesting a “goldilocks” region of alteration. Moreover, it appears that the disseminated mineralization is not associated with specific lithologies, but is rather dependent on the dominant style of hydrothermal alteration. The contact between the Bobbejaankop and Lease granites is not as sharp as previously described [7,11,26,27]. The shift from an unaltered Bobbejaankop through a transitional contact zone is defined by the incipient and selective alteration of feldspars and granophyric grain-size reduction. This develops a texture that is intermediate between that of the Bobbejaankop and Lease granites (Figure7). Geochemically, this transition zone is marked by a distinct REE and incompatible element depletion, suggesting that before being depleted and incompletely altered to Lease granite, this zone was originally typical Bobbejaankop granite (Figures 12 and 17). Minerals 2020, 10, 379 19 of 26 Minerals 2020, 10, x FOR PEER REVIEW 19 of 26

Figure 18. DiagramsDiagrams showing showing the the various various alteration alteration pr proxiesoxies and and their their correlation with Sn in the

Zaaiplaats tin fieldfield granitesgranites ((AA)) GraphGraph ofof ZrZr/Hf/Hfvs. vs (K2OO + CaO)/NaCaO)/Na22OO indicate indicate the the increase in microclinization andand epidotization epidotization in in the the highly highly fractionated fractionated granite granite phases. phases. (B) Graph (B) Graph of (K2 Oof +(KCaO)2O +/ (tot) (tot) CaO)/NaNa2O vs.2 TiOO vs2/ FeOTiO2/FeOdescribing describing the dominant the dominant style of style hydrothermal of hydrothermal alteration. alteration. Scaling ofScaling the symbols of the symbolsrepresents represents the concentration the concentration of Sn (ppm) of Sn within (ppm) the wi samples.thin the Circularsamples.datapoints Circular datapoints represent represent the range theof the range literature’s of the literature’s geochemical geochemical analyses; Neboanalyses granite; Nebo (grey), granite Bobbejaankop (grey), Bobbejaankop granite (blue) granite and (blue) Lease andgranite Lease (red) granite [14,20 (red),21,37 [14,20,21,37].].

Where present, the transitional zone at Groenfontein is several centimetres thick, however at Zaaiplaats the transitional zone reaches up to 4 m wide. This This is is explained explained by by a a combination combination of of factors; factors; firstfirst the downward migrating of a dynamic alteration front which, as as fluid fluid accumulated accumulated in in the the cupola, cupola, prograded downwards into the Bobbejaankop magma, stripping stripping mobile mobile elemen elementsts before before regressing. regressing. The transitional zone represents a region where thethe alteration front regressed before completing the alteration ofof the the Bobbejaankop Bobbejaankop granite. granite. Secondly, Secondly the variability, the variability in the permeability in the permeability of the Bobbejaankop of the Bobbejaankopmush, controlled mush, by thecontrolled degree by of crystallization,the degree of crysta wouldllization, have variably would have facilitated variably or inhibitedfacilitated the or inhibitedinfiltration the of infiltration the hydrothermal of the hy fluids.drothermal This fluids. would This have would resulted have in resulted gradational in gradational or sharp contacts,or sharp wherecontacts, the where permeability the permeability was either was high either or low, high respectively. or low, respectively. The geochemical decoupling of incompatible elements between the Lease and Bobbejaankop granites from the twotwo minesmines isis duedue toto anan increasedincreased hydrothermalhydrothermal signature signature at at Zaaiplaats, Zaaiplaats, suggesting suggesting a agreater greater accumulation accumulation of of hydrothermal hydrothermal fluids fluids towards towards the the Zaaiplaats Zaaiplaats end end of of the the pluton pluton (Figure (Figure 16 16).). Incompatible Zr Zr and Hf are concentrated in a silicicsilicic magmas at a constant ratio, although the crystallization ofof zircon zircon decreases decreases the the Zr/ HfZr/Hf ratio ratio and whichand which then canthen be can used be to monitorused to themonitor evolution the evolutionof granitic of magmas granitic [61 magmas,74–76]. This[61,74–76]. is evident This in is the evident consistent in the Zr /consistentHf decrease Zr/Hf from decrease the Nebo from through the NeboBobbejaankop through toBobbejaankop the Lease granite to the at Lease both granite Groenfontein at both and Groenfontein Zaaiplaats. and A decrease Zaaiplaats. in Zr A/Hf decrease below 30in Zr/Hfhas also below been 30 linked has also to the been enrichment linked to ofthe LILE enrichment and HFSE of LILE such asand Sn, HFSE W, Mo, such Ta, as Nb, Sn, Li, W, Zr Mo, and Ta, Hf, Nb, to Li,the Zr point and of Hf, economic to the point viability of economic [61,74–76]. viability The Lease [61,74–76]. and Bobbejaankop The Lease granites, and Bobbejaankop unsurprisingly, granites, have unsurprisingly,Zr/Hf ratios between have Zr/Hf 15 and ratios 30, which between correspond 15 and 30, to which greisen correspond related Sn-W-Mo-Be to greisen related deposits, Sn-W-Mo- whereas Bethe deposits, unmineralized whereas Nebo the graniteunmineralized has a Zr Nebo/Hf ratio granite higher has than a Zr/Hf 30 [61 ratio,74, 76higher]. than 30 [61,74,76]. Similarly, the NbNb/Ta/Ta parameterparameter is usedused as anan indexindex of fractionationfractionation as the ratios should remain broadly similar in an evolving magmatic system [5 [599–63,74–77].–63,74–77]. However, However, an an increase increase in in Ta Ta (decrease in NbNb/Ta)/Ta) isis attributedattributed toto thethe involvementinvolvement ofof fluorine-rich,fluorine-rich, autometasomatic,autometasomatic, mineralizing and greisenizing fluids fluids [59–63,77]. [59–63,77]. The The Bobbejaankop Bobbejaankop an andd Lease Lease granites granites deviate deviate from from the the Nb/Ta Nb/ Taratio ratio of theof the Nebo, Nebo, displaying displaying an enrichment an enrichment of Ta, of particularly Ta, particularly at Zaaiplaats at Zaaiplaats (Figure (Figure 16). The16). deviation The deviation from thefrom magmatic the magmatic Nb/Ta Nb ratio/Ta ratio is attributed is attributed to toincrease increasedd interaction interaction with with internally internally derived derived magmatic fluids.fluids. The The correlation correlation of of incompatible incompatible elements elements such such as Ta as Tavs. vs.Rb, suggest Rb, suggest the involvement the involvement of such of internallysuch internally derived derived magmatic magmatic fluid prior fluid to prior greisenization to greisenization (Figure (Figure16B) [61,78]. 16B) The [61, 78deviation]. The deviation from the magmaticfrom the magmatic Nb/Ta ratio Nb highlights/Ta ratio highlights a transition a transitionfrom a magmatically from a magmatically dominated, dominated, to an F-rich, to magmatic- an F-rich, hydrothermal dominant environment (Figure 16D). Therefore, the main difference between the granites of the two mines is the increased accumulation and influence of pre-greisenization, F-rich magmatic-hydrothermal fluids at Zaaiplaats.

Minerals 2020, 10, 379 20 of 26 magmatic-hydrothermal dominant environment (Figure 16D). Therefore, the main difference between the granites of the two mines is the increased accumulation and influence of pre-greisenization, F-rich magmatic-hydrothermal fluids at Zaaiplaats. These fluids have been shown to be CO2-bearing and hypersaline, containing KCl, NaCl, and FeCl [28]. Thus, coupled with the abundance of hematite and fluorite, it is reasonable to suggest these granites were altered by an extremely, F−, Cl−, and Fe-rich fluid [28,79]. The presence of Cl− and CO2 within granitic magmas enhanced the complexation and mobility of HREEs in the magmatically-derived hydrothermal fluids, as well as reducing the activity of H2O, as evident in the low H2O activity shown in Figure 14B. However, the anomalous Cu enrichment within the Lease granite may be related to the exsolution of an aqueous fluid phase. The mobility and concentration of Cu in hydrothermal fluids is largely a function of the salinity of the aqueous fluid because the - solubility of a likely CuCl2 complex is proportional to the Cl concentration of the fluid [80,81]. As the granites of the Zaaiplaats tin field are thought to have intruded at approximately 4 to 5 km depth, it is very likely that the magmas underwent fluid saturation as evident in the abundant manifestations of magmatic-hydrothermal alteration and mineralization described herein [15,28,82]. This would have occurred at around 1 to 2 kb of pressure, as shown in Figure 14 and assuming a 0.3 kb/km pressure gradient in the continental crust (after Winter [83]). Evidence for fluid saturation, by decompression of the crystallizing magma, is provided by the abundant miarolitic cavities. Fluid saturation may have been caused by continuous crystallization and/or decompression of the magma. The decreasing volume, caused by crystallization, may have led to shrinking of the volume occupied by the magma and partial detachment from the roof, allowing the formation of the marginal pegmatitic zone. The abundance of hematized feldspars, suggests a relatively oxidizing environment, which could have been achieved by fluid mixing or boiling [81,84,85]. Pollard et al. [28] showed that these granites evolved in a closed system with little to no external influence, thus oxidation via boiling is the most likely mechanism. Furthermore, the abundance of fluorite and particularly tourmaline within the Lease granite would suggest a reasonable amount of F and B within the fluid. During the exsolution of a vapor-rich fluid, these volatiles would have preferentially partitioned into the vapor phase, therefore, together with the coexisting brine, they were transported and accumulated into the granite cupola. The solubility of metals improves with increasing volatile concentration and acidity, facilitating their transportation within these magmatic-hydrothermal fluids [81,86–89]. Thus, internally derived magmatic fluid, produced by the crystallizing Bobbejaankop granite, would have become enriched in various metals, such as Fe, Al, and Sn. Such fluids would have had the ability to concentrate and transport significant concentrations of Sn, however they would not necessarily have precipitated cassiterite. During fluid saturation, Sn preferentially partitions into the aqueous phase, possibly as a SnCl2 complex in a relatively saline, acidic fluid [85,87,90]. Therefore, the loss of H, F, and B into a vapor phase, would have simultaneously buffered and oxidized the aqueous phase, precipitating SnO2 [85,87,90]. The process of neutralization-induced greisenization of the surrounding granite caused the decomposition of feldspars and precipitation of cassiterite. Cassiterite mineralization, where present within the hydrothermal pipes, is concentrated within the sericitized core, and partially within the surrounding melanocratic tourmaline-rich layer. Similarly, miarolitic and pod-like mineralization within the Lease granite is associated with the greisenized decomposition of feldspars. The disseminated mineralization within the Bobbejaankop granite was the result of the neutralization of the Sn-rich aqueous phase, while the B- and F-rich fluid accumulated in the roof. Thus, the Lease granite is proposed to be the altered product of a semi-crystallized Bobbejaankop magma, by the saturation and accumulation of a magmatic-hydrothermal acidic, saline-rich fluid. Therefore, the Lease granite represents the transition from a magmatic to a magmatic-hydrothermal dominant system. Minerals 2020, 10, 379 21 of 26

Formation of the Lease and Bobbejaankop Granites The results of this study strongly support the formation of the Bobbejaankop granite by fractionation of the Nebo granite. The linear trend displayed by the normative Qz-Ab-Or diagram suggests that the Lease granite is neither a separate intrusion nor a chilled margin of the Bobbejaankop granite (Figure 14). Rather, it suggests that the Lease granite gradually evolved in the cupola of the Bobbejaankop mush by fluid saturation and subsequent hydrothermal alteration. The extensive microclinization and reordering of the Bobbejaankop feldspars increased their porosity, allowing pervasive fluid movement [32,91]. The inward crystallization of the Bobbejaankop granite would have concentrated LILE and incompatibles into a late-stage, acidic, and F-rich autometasomatic magmatic-hydrothermal fluid. Eventual fluid exsolution and the porosity of the feldspars facilitated the upwards migration and accumulation of the fluids in the upper facies of the Bobbejaankop mush. The gradual effervescence and fluid saturation produced ubiquitous miarolitic cavities and the subsequent precipitation of cassiterite within them. The migration and accumulation of these volatile-rich bubbles would have concentrated B, F, H, and Cu in the granite cupola. The process of devolatilization and bubble migration is referred to as the “bubble laden plume theory” and was proposed by Candela [92]. This model describes the formation of boron, alkali-, and metal-rich bubbles that migrated upwards through a crystallizing granitic magma, accumulating in the roof. At low degrees of crystallization, these bubbles would have coalesced and formed large, bubble-laden plumes that streamed through conduits in the semi-crystalline mush. Granitic systems, such as Zaaiplaats, that have significant concentrations of Li, B, F, and H will have a higher diffusivity, greater H2O solubility, and lower viscosity, facilitating the migration of bubbles and increasing permeability for pervasively altering fluids [92–94]. Some of the hydrothermal pipes have large central cavities or voids, surrounded by a melanocratic zone of tourmaline-quartz. Thus, they may represent large bubble-laden plumes, formed by the accumulation and channeling of these exsolved greisenizing volatile and metal-rich fluids. The alumina required for the formation of the aluminosilicates within the pipes and Lease granite were sourced from the decomposition of feldspars by greisenization [87,94–96]. Thus, the neutralization of Sn-bearing fluids within the pipes explains the high-grade deposition of cassiterite within the sericitized cores. The accumulation of these fluids resulted in the complete alteration of the Bobbejaankop granite and the formation of various types of Lease granite. The transitional zone, regarded as the contact between the granites, represents a region of incomplete alteration of the Bobbejaankop granite. A possible combination of the topographic variations in the roof of the Bobbejaankop magma chamber, convection, porosity, and microstructure, channeled the fluids towards Zaaiplaats. Therefore, the peripheral Lease microgranite is the result of a magmatically derived hydrothermal fluid alteration in the semi-crystallized Bobbejaankop granite cupola.

7. Conclusions The meta- and peraluminous, ferroan granites of the Zaaiplaats tin field, exhibit the typical characteristics of A-Type granitoids. Trace element geochemistry indicates that the Bobbejaankop granite resulted from extensive fractionation of the Nebo granite, whereas the Lease granite represents the hydrothermal alteration of the semi-crystallized Bobbejaankop magmatic mush by an incompatible element rich, acidic, magmatically derived hydrothermal fluid. Incipient microclinization and chloritization, followed by a pervasive and destructive style of alteration, resulted in the fine-grained Lease in the roof of the Bobbejaankop granite. The textural and mineralogical variants in the Lease granite were dependent on the extent and dominant style of hydrothermal alteration. Fluid saturation mobilized and concentrated volatiles and incompatible elements, resulting in the ubiquitous tourmalinization and greisenization within the Lease granite. The coalescence and migration of the magmatically-derived hydrothermal fluids resulted in the formation of large tourmaline-quartz hydrothermal pipes. Neutralization of the Sn-bearing hydrothermal fluids precipitated cassiterite within miarolitic cavities, and hydrothermal pipes within the Lease and upper portions of the Minerals 2020, 10, 379 22 of 26

Bobbejaankop granite. Therefore, the Lease microgranite represents the alteration in the cupola of the semi-crystalline Bobbejaankop granite, by the accumulation of internally derived metal and volatile-rich magmatic-hydrothermal fluids.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/4/379/s1, Table S1: Complete list of whole rock XRF and ICP-MS results of the sampled Zaaiplaats tin field granites; Table S2: Partition coefficients (KD) and representative modal proportions of the Nebo granite used in Rayleigh Fractionation calculations. Author Contributions: Conceptualization, Investigation, and Writing—original draft, L.V.; Project administration, Supervision, Writing—review and editing, and assistance in field investigations, P.N., J.K. and L.R. All authors have read and agree to the published version of the manuscript. Funding: This research was funding by THRIP (Technology and Human Resources for Industry Programme) (001.410.8755101.5121105) under project TP14082193395—Mineralisation Processes in the Bushveld Complex. Acknowledgments: We would also like to acknowledge the support of VM Investment Co. and Simon Nethononda of Bushveld Minerals who assisted in field expeditions, provided access to the various mines, and for graciously providing accommodation for the duration of the field seasons. The Earth Lab at the University of the Witwatersrand is thanked for analyzing whole-rock major and trace elements by XRF and ICP-MS. Conflicts of Interest: The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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