Petrology of the neoproterozoic Cape Granite Suite hosted W-Mo-REE Riviera endoskarn deposit with special reference to allanite characteristics

Santana, Monica Manuela (2015-03)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: The Riviera W-Mo deposit is located near Piketberg in the Western Cape and is hosted by I- and A-type granitoids of the Neoproterozoic to Palaeozoic Cape Granite Suite that intruded the meta-volcano-sedimentary Malmesbury Group. Scheelite and molybdenite are the principal ore minerals and are associated with endoskarn- and vein-type alteration of the granitoids. It is the 6th largest deposit of its kind in the world with a grade of 0.216% tungsten (WO3) and 0.02% molybdenum (Mo), and a tonnage of 46 Mt. The discovery of rare earth elements in addition to W and Mo in this deposit makes it attractive to the global market as there is currently a demand for tungsten and rare earth elements in the West. This study comprises a mineralogical and geochemical investigation of a representative suite of drill core samples to identify the various mineral phases, their spatial distribution and textural features. This allowed definition of the relationship between hydrothermal alteration facies and enrichment. In addition this knowledge contributes to unravelling the genesis of this unusual deposit. Results were obtained by means of optical microscopy, scanning electron microscopy, as well as whole rock geochemical XRF and LA-ICP-MS analyses. The deposit consist of primary and secondary mineral assemblages; the latter as a result of superimposed skarnification and hydrothermal alteration. Primary minerals include quartz, feldspars (plagioclase and alkali), micas (biotite) and accessory titanite. The process of hydrothermal alteration formed skarn assemblages which include prograde metasomatic minerals like bastnaesite, scheelite, garnet, pyroxene, epidote, titanite, vesuvianite, apatite, and allanite. Other prograde minerals include secondary biotite, white mica, secondary albite and alkali feldspar. A later phase of pervasive hydrothermal alteration of the granite formed retrograde minerals like chlorite, carbonates, amphiboles, sericite (white mica), goethite and clay. Enrichment is mainly in the roof of the Riviera granite cupola. Three rock types have been identified based on the modal mineralogy, they include firstly and closest to the granite wall rock contact a quartz porphyry monzogranite, followed by a biotite granite to monzogranite and lastly an aphanitic granite to monzogranite at depth in the pluton. Whole rock geochemical data is generally unreliable due to the effect of hydrothermal alteration, although it is speculated that this pluton is largely A-type, metaluminous to peraluminous in composition and subduction-related. The different hydrothermal alteration types that were identified include argillic and advanced argillic alteration, phyllic and advanced phyllic alteration, and potassic alteration. The spatial relationship between these alteration types appear to be interlayered and follows the contour of the pluton. The entire pluton has been affected by phyllic alteration and potassic alteration occurs mainly towards the roof of the granite cupola. The other types of alteration occur sporadically. The vertical distribution of various elements, based on geochemical data, allowed for the identification of a mineralized zone, close to the granite wall-rock contact, and a non-mineralized zone at depth. The mineralized zone constitutes the bulk of the enrichment and is rich in calcic skarn minerals and potassic alteration products. Although altered, the non-mineralized zone reflects the original rock assemblage more clearly with sporadic occurrences of enrichment as skarnified granite patches. Significant concentrations of allanite, a light rare earth element enriched mineral of the epidote group, were recently discovered in the endoskarn part of the Riviera pluton and could constitute an economically important by-product. Allanite is a complex mineral with poikilitic textures, irregular zonation patterns, metamictization, medium- to coarse grain sizes and no apparent consistent association with other minerals. The two most common types are allanite (Ce) and ferriallanite (Ce). Chondrite-normalized REE patterns for the whole rock analyses are similar to those of the single grains and suggest that most of the rare earth elements are hosted by allanite. The steep slope of the pattern demonstrates LREE-enrichment and Eu-anomalies vary from slightly negative, neutral to slightly positive. A high ΣLREE content of allanite correlates with a positive Eu-anomaly whereas negative anomalies are associated with low concentrations. This suggests that the partitioning of REE in allanite is redox-sensitive. Various evolutionary stages gave rise to this complex deposit; from prograde skarn formation to retrograde alteration. The main influence is due to the variability of metasomatizing fluids that evolved from early prograde, high temperature and reduced, to late retrograde, low temperature, and oxidized. A similar study of zoned scheelite from this deposit supports these observations and indicates genetically early scheelite and late stage allanite.

AFRIKAANSE OPSOMMING: Die Riviera W-Mo skarn afsetting is naby Piketberg geleë in die Wes-Kaap, en word deur die I- and A-tipe granitoïedes van die Neo-Proterosoïese tot die Paleosoïese Kaapse Graniet Suite gehuisves. Hierdie graniete het die meta-vulkanies-sedimentêre gesteentes van die Malmesbury Groep ingedring. Scheeliet en molibdeniet is the hoof erts minerale en word hoofsaaklik geassosieer met die endoskarn- en aar-tipe fases van die graniete. Die Riviera afsetting is die grootste afsetting van sy soort in die wêreld met ‛n tonnemaat van 46 Mt en ‛n graad van 0.216% WO3 en 0.02% Mo. Die afsetting is sedert die ontdekking van skaars aardes baie meer aantreklik vir die globale mark omdat daar „n groot tekort en aanvraag is vir wolfram en skaars aardes in die Weste. Hierdie projek beoog om „n mineralogiese en geochemiese studie te maak van „n verteenwoordige reeks kern monsters. Dit word gedoen om die aantal en verskeie mineraal fases te identifiseer, hul ruimtelike verspreiding af te baken asook hul teksturele eienskappe uit te ken. Dit sal lei tot die definisie van die verhouding tussen die verskeie hidrotermale verandering fasies asook mineralisasie. Benewens sal hierdie inligting bydra tot die begrip van die genese van die afsetting. Resultate is verkry deur middel van optiese mikroskopie, skandeer elektronmikroskopie, asook geochemiese heelrots XRF en mineraal LA-ICP-MS analises. Die afsetting bestaan uit primêre en sekondêre minerale samestellings; die laasgenoemde as gevolg van gesuperponeerde skarn-vorming en hidrotermale verandering. Primêre minerale sluit kwarts, veldspate (plagioklaas en alkali), mika (biotiet) en bykomende sfeen in. Die proses van hidrotermale verandering het skarn minerale tot gevolg gehad wat onder andere prograad metasomatiese minerale soos bastnaesiet, scheeliet, granaat, pirokseen, epidoot, sfeen, vesuvianiet, apatiet, en allaniet insluit. Ander prograad minerals sluit biotiet, wit mika, sekondêre albiet en alkali veldspaat in. Die hele graniet is verander deur deurdringende hidrotermale verandering en het retrograad minerale soos chloriet, karbonate, amfibole, serisiet (wit mika), goethiet en klei tot gevolg gehad. Mineralisasie is hoofsaaklik in die dak van die Riviera graniet koepel. Die gasheer rots bestaan uit drie tipes gesteentes waarvan die klassifikasie gebaseer is op die modale mineralogie. Dit sluit in die eerste plek en die naaste aan die dak van die graniet cupola 'n kwarts porfier monzograniet in, gevolg deur 'n biotiet graniet en monzogranite en laastens 'n afanitiese graniet en monzograniet. Heelrots geochemiesie data is oor die algemeen onbetroubaar as gevolg van die effek van hidrotermal verandering, alhoewel dit voorstel dat die pluton „n A-tipe graniet is, met „n metalumineuse tot peralumineuse komposisie en subduksie-verwante aard. Die verskillende tipes hidrotermale verandering wat geïdentifiseer is, sluit argilliese en gevorderde argilliese verandering, fillietiese en gevorderde fillietiese verandering, asook veldspatiese verandering. Die ruimtelike verhouding tussen hierdie verandering tipes blyk asof dit tussengelaagd is, en die lae volg ook die kontoer van die pluton. Die hele pluton is verander deur fillietiese verandering. Veldspatiese verandering kom meestal in die dak van die graniet koepel voor. Die ander vorme van verandering kom sporadies voor. Die vertikale verspreiding van verskeie elemente (gebaseer op heelrots geochemiese data), het die identifisering van „n geminaraliseerde sone, wat naby die kontak van die graniet en wandgesteentes geleë is, en „n nie-geminilariseerde sone dieper in die pluton, moontlik gemaak. Die geminaraliseerde sone verteenwoordig die oorgrote meerderheid van die mineralisasie, is ryk in kalsium skarn minerale en produkte van veldspatiese verandering. Die nie-geminilariseerde sone weerspieël die oorspronklike rots duideliker alhoewel dit steeds hidrotermaal verander is, en het sporadiese voorvalle van mineralisasie wat bekend staan as geskarnifiseerde graniet kolle. Beduidende konsentrasies van allaniet, 'n ligte skaars aarde-verrykte mineraal van die epidoot groep is onlangs in die endoskarn deel van die Riviera Pluton ontdek en kan beskou word as ʼn ekonomies belangrike by-produk. Allaniet is 'n komplekse mineraal met poikilitiese teksture, onreëlmatige sonerings patrone, medium tot growwe korrel groottes en geen oënskynlike konsekwente verwantskap met ander minerale in die pluton. Die twee mees algemene tipes is allaniet (Ce) en ferri-allaniet (Ce). Chondriet genormaliseerde SAE patrone vir die heel rots is soortgelyk aan dié van enkele allaniet korrels en dui dus daarop aan dat die meeste van die skaars aarde elemente in allaniet gehuisves word. Die steil helling van die patroon toon LSAE verryking en die Eu-afwykings wissel van effens negatief, neutraal tot effens positief. 'n Hoë ΣLSAE inhoud van allanitet stem ooreen met 'n positiewe Eu-afwyking, terwyl „n negatiewe afwyking geassosieer word met lae konsentrasies. Dit dui daarop dat die verdeling van SAE in allanite redoks sensitief. Verskeie evolusionêre fases het aanleiding gegee tot hierdie komplekse afsetting, vanaf „n prograad skarn-vorming stadium tot retrograad verandering. Die belangrikste invloed is as gevolg die wisselvalligheid van die metasomatiserende vloeistowwe wat ontwikkel het vanaf „n vroeë prograad, hoë temperatuur en gereduseerde, tot „n laat retrograad, lae temperatuur en geoksideërde vloeistof. 'n Soortgelyke studie van die sonering van scheeliet in hierdie afsetting ondersteun hierdie waarnemings en dui dus op geneties vroeë scheeliet en laat stadium allaniet aan.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96600
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