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THE GEOLOGY and PETROLOGY of the MERELANI TANZANITE DEPOSIT, NE TANZANIA Bernard Olivier Dissertation Presented for the Degree O

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THE GEOLOGY and PETROLOGY of the MERELANI TANZANITE DEPOSIT, NE TANZANIA Bernard Olivier Dissertation Presented for the Degree O THE GEOLOGY AND PETROLOGY OF THE MERELANI TANZANITE DEPOSIT, NE TANZANIA Bernard Olivier Dissertation presented for the degree of Doctor of Philosophy at the University of Stellenbosch, South Africa. Promoter: Prof R. Scheepers December 2006 Declaration I, the undersigned, hereby declare that the work contained in this dissertation is my own original work and that I have not previously in its entirety or in part submitted it at any university for a degree. Bernard Olivier Date: 21/04/06 ii Abstract Tanzanite, a blue/violet gemstone variety of zoisite (Ca2Al2O.AlOH[Si2O7][SiO4]) is only produced in NE Tanzania. The only known locality is an approximately 7km2 deposit in the Merelani area. It is one of the most sought after gemstones in the world with an industry sales value of between hundred and fifty and two hundred million dollars per year. At the current production rates and estimated resources the tanzanite deposit has a life expectancy of around 20 years. Despite the economical and scientific importance as well as the geological uniqueness of the deposit very little research has been conducted on the geology and petrology of the deposit and the characteristics of tanzanite. The primary aim of the research summarised in this dissertation was to gain an understanding of the geological conditions that led to the formation of this unique variety of zoisite. In order to achieve this, a variety of geological disciplines were addressed including the lithostratigraphic setting, the deformational history, the metamorphic history and conditions, the geochemical and isotopic composition, the mineral chemistry as well as the physical and optical properties of the tanzanite. Extensive field work was conducted over a seven year period, which included surface and underground mapping, surface trenching, surface and underground core drilling, structural measurements and an intensive sampling programme. Various analytical techniques were used in order to petrologically and mineralogically investigate both the deposit and tanzanite itself, including optical microscopy, XRF analyses, laser-ablation ICP-MS, quantitative chemical analyses by means of the electron microprobe, XRD analyses, back-scattered electron microscopy, isotope analyses, fluid-inclusion studies, Vis/UV/NIR spectroscopy, IR spectroscopy, and EPR studies. This study revealed an intricate and complex geological history for the formation of tanzanite. The deposition of carbon-rich layers, formed during the development of a sequence of shallow shelf sediments consisting primarily of various organic carbon (δ13C between –22,85 ‰ and –26,74 ‰) -rich mudstone horizons and limestone beds is iii seen as the first phase of the mineralisation process. These Archaean sediments were deposited in a back- or fore-arc spreading basin. The organic carbon-rich mudstone layers acted as the first phase of vanadium accumulation in the stratigraphic sequence, and are intercalated with a material with a volcanic origin (metabasites), which most likely contributed to the enrichment of amongst others V. Early diagenetic processes were followed by an extended metamorphic and deformation (D1) history, reaching upper granulite facies conditions (10 – 12 kbar and 850 oC to 1000 oC) at ca. 1000 Ma. The high-grade metamorphic history was followed by multiphase retrograde deformation events (D2 and D3) that developed as a result of crustal uplift. The D2 event probably occurred between 850 and 600 Ma at P-T conditions estimated at between 8 to 7 kbar and 700 to 650 oC. A third stage of deformation (D3) resulted in the formation of overprinting structures and occurred during the later stages of the Pan-African (550 – 500Ma) event during amphibolite/greenschist facies conditions (7 to 6 kbar and 600 to 520 oC). The deformational history of the deposit played a critical role in the mineralisation processes. Of the four main deformation events identified, three played a critical role in the mineralisation process. The first event led to the development of early structural features as defined by F1, S1 and L1. This was followed by a multiphase D2 event consisting of three different orders of folding (F2a, F2b and F2c) and the formation of boudinage. The association between tanzanite and boudins as well as the stacking and multiple duplication of the boudinaged ore-zone through isoclinal folding resulted in multiple “ore-shoots”. These ore-shoots follow the plunge of the F2c fold closures and results in mine-able features within the ore-body. The third deformation event led to complex structural overprinting of the earlier fabric as observed in S3 and L3 and resulted in the deformation of the F2 structures through crosscutting F3 folds. Calc-silicate layers developed in the stratigraphic sequence as a result of metamorphic and metasomatic interaction between calcium enriched (boudinaged calc-silicates) and depleted horizons (graphitic gneisses) during a skarn-forming episode. V-rich green grossular garnet (tsavorite) crystallised in tension zones within and in proximity to the boudins during prograde metamorphism. Tanzanite mineralisation occurred during the iv retrograde stages at ca. 585 ± 28 Ma with P-T conditions estimated at ca. 5 to 6 kbar and 650 ± 50 oC. Two distinctive tanzanite-forming processes are distinguished. The first involves the formation of tanzanite as a result of retrograde reaction of grossular garnet. The second process involves the migration of V and Ca -enriched fluids along brittle shear zones to tension sites where fluids reacted with wall rock during a drop in P-T conditions to precipitate tanzanite. Fluid inclusion and stable-isotope studies concluded that the ore-forming fluids were derived from the dehydration of the metasedimentary sequence and consisted of a mixture of H2O, CH4, H2S and N2. Mineralogical investigation of tanzanite indicated that trace concentrations of vanadium within its crystals structure causes its blue / violet colour. It was proved that the vanadium originated from the abundant organically derived graphite within the deposit. Spectroscopic and EPR analyses revealed the importance of the Ti4+ / Ti3+ ratio within the crystal structure of tanzanite with regard to its colour characteristics. The heating of tanzanite results in a couple valence exchange reaction Ti 3+ + V 4+ → Ti 4+ + V 3+ which causes an increase the blue / violet colour of tanzanite. The research conducted led to the development of a successful geological model for the tanzanite mining and treatment activities in the Merelani area of NE Tanzania. As such the research contributed to the establishment of a successful tanzanite mine, based on sound geological principles, which may act as a role model for other gemstone mines worldwide. v Uittreksel Tanzaniet is ‘n blou edelsteen variëteit van die zoisiet groep van minerale (Ca2Al2O.AlOH[Si2O7][SiO4]) en word slegs gemyn in die Merelani gebied in noordoos Tanzanië. Tanzaniet is een van die mees gesogte gekleurde edelstene in die wêreld met ‘n industriële verkoopswaarde van tussen honderd en vyftig en twee honderd miljoen Amerikaanse dollar per jaar. Die Merelani myngebied beslaan ‘n gebied van ongeveer 7 km2 en die afsetting het ‘n lewensverwagting van ongeveer 20 jaar teen die huidige tempo van ontginning en beraamde reserwes. Ten spyte van die ekonomiese en wetenskaplike waarde asook die uniekheid van die afsetting is daar baie min navorising gedoen op die geologie en petrologie van die afsetting en die mineralogiese eienskappe van die mineraal. Die hoof doel van die navorsing vervat in die proefskrif was om die geologiese toestande wat gelei het tot die vorming van tanzaniet te verstaan. Om die doel te bereik is ‘n verskeidenheid van geologiese dissiplines nagevors insluitende die litostratigrafiese opeenvolging, die vervormingsgeskiedenis, die metamorfe toestande en geskiedenis, die geochemie en isotoop samestelling, die mineraalchemie en ook die fisiese en optiese eienskappe van tanzaniet. Laasgenoemde is spesifiek toegepas om die oorsake van kleur en kleur verandering van tanzaniet a.g.v verhitting te bestudeer. Uitgebreide veldwerk is oor ‘n tydperk van sewe jaar onderneem insluitende oppervlak en ondergrondse kartering, oppervlak en ondergrondse kern-boorwerk, sloot-karterings, strukturele data versameling en rotsmonstering. ‘n Verskeidenheid analitiese tegnieke is toegepas om beide die afsetting en die tanzaniet petrologies en mineralogies te ondersoek. Die tegnieke het ingesluit; optiese mikroskopie, XSF (X-Straal Fluorisensie) analise, LA IGP-MS (Laser ablasie induktief gekoppelde massa spektrometrie), kwantitatiewe en semi-kwantitatiewe chemiese analises d.m.v elektron mikrosondes (golflengte dispersie, energie dispersie en teruggekaatste elektron analise), XSD (X- straal diffraksie analises), stabiele isotoop analises, vloeistofinsluitsel mikrotermometriese studies, Raman – sonde analise, spektroskopiese analise (sigbare, vi ultraviolet en naby-infrarooi golflengtes) en EPR (elektron paramagnetiese resonans) studies. Die navorsing het ‘n ingewikkelde en komplekse geologiese geskiedenis vir die vorming van tanzaniet aan die lig gebring. Koolstofryke moddersteen en kalksteen lae is afgeset tydens die ontwikkeling van ‘n opeenvolging van sedimente in ‘n vlak see omgewing. Die isotopiese samestelling van die koolstof (δ13C tussen –22,85 ‰ and – 26,74 ‰) dui aan dat organiese koolstof dominant was in die opeenvolging. Die Argaiëse sedimente was afgeset tydens die ontwikkeling van agter-eiland kom of ‘n kom-omgewing voor ‘n subduksiesone.
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