Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: The aim of the study is to ascertain whether geochemical exploration techniques can be used in the search for tanzanite deposits in the Merelani area, NE Tanzania. Previous studies have successfully demonstrated a partial extraction method (in situ soil leaching) in identifying prospective ultramafic bodies at the Rockland ruby mine in the Mangare area, Kenya, thereby demonstrating the usefulness of geochemical methods in gemstone exploration. In this study, a partial extraction as well as a whole-rock geochemical method was used to determine the applicability of these methods in prospecting for tanzanite mineralisation using different sampling media, such as soil, stream sediment and calcrete. It is possible that this geochemical approach may not be as effective as physical methods such as the separation and examination of heavy mineral suites. However, its viability needs to be evaluated due to the potential efficiency and relative logistic ease of the method. In essence the scientific method employed is to compare overburden (soils, stream sediments and calcrete) chemistry with known underlying geology, the latter having been established via diamond core drilling. A positive correlation would allow the prediction of overburden covered tanzanite mineralisation. Soil samples were collected from a trench dug perpendicular to regional lithological strike over both barren and tanzanite-bearing horizons. XRF trace element data for the soils was compared to the chemistry of the underlying lithologies. ICP-AE data derived from 1 molar HCL soil leachate (12 hour leach) and soil XRF data, from the same samples, was compared, using a mass balance index, to discern any hydromorphic dispersion of selected trace elements and to evaluate the leachate as a viable alternative to XRF analysis. In general, a good correlation exists between the soil and rock trace element data profiles over the length of the section. However, Ti- and Zr-normalised mass balance calculations show some down-hill drift, but this does not disrupt the overall pattern. The ICP-AE acid leach data show that hydromorphic dispersion is low, that the trace elements of interest (V, Cr, Ni and Cu) are hosted within non-soluble phases. Consequently, the leach technique is not a viable alternative to XRF analysis of the soils. XRF analysis of the soils was shown to be potentially useful in identifying new areas of mineralisation as the soils overlying a graphitic calc-silicate schist, that always occurs adjacent to the tanzanite mineralisation in the Merelani area, was found to be easily identifiable based on anomalous concentrations of V. An exploration concession was chosen for stream sediment sampling on the basis of the presence of large streams, of a few tsavorite mines indicating high prospectivity for tanzanite, and because of a variation in geology on the property. Tanzanite and tsavorite are cogenetic in the known tanzanite deposits. In this case the aim was to investigate the possible occurrence of tanzanite-like geochemical anomolies (i.e. the anomalous V observed in the soil chemistry investigation) could be detected in the vicinity of the tsavorite mines. Tsavorite, the gem variety of grossular garnet, also contains high concentrations of V. The samples were analysed by XRF whole-rock methods for trace element content. The data shows a number of clear positive V anomalies in the study area. The data also shows that each of the existing or abandoned mines in the area is marked by a positive V anomaly. This section of the study also demonstrated a relatively low degree of stream sediment dispersion of the trace elements of interest – most likely a function of the semi-arid climate. The fine fraction (<90μm), however was shown to be mobilised to a relatively larger degree than the coarse (180μm – 300μm) and medium (90μm - 180μm) fractions. As is predictable from the leachate analysis, factor analysis of the data shows that the trace elements are dominated by heavy mineral geochemistry and that a study in heavy mineral exploration might provide a cheaper and more viable option to those explored in this study. Calcrete samples were taken from an abandoned, 10m deep mine shaft, which was sunk through the calcrete to reach the tanzanite deposit. The shaft was sampled from the bottom, closest to the tanzanite mineralisation, to the surface to investigate the association between trace element geochemistry and proximity to the deposit. There was no vertical association between the trace element geochemistry of the calcrete and proximity to the tanzanite deposit. There was also no clear indication in the geochemistry of the calcrete of the existence of the tanzanite deposit beneath it. This further indicates the immobility of the elements of interest in this environment. This study has demonstrated that properly constrained soil and stream sediment geochemical studies may be of use in tanzanite exploration. However, it must be stressed that this is only the case if the geochemical signature of the lithological package associated with the mineralisation is unique and well known. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie is om te bepaal of geochemiese eksplorasie tegnieke vir die soek na tanzaniet afsettings in die Merelani area, noord-oos Tanzanië, gebruik kan word. Voorige studies het gewys dat ‘n gedeeltelike ekstraksie metode (in situ grond looging) gebruik kon word om prospektiewe ultramafiese liggame by the Rockland rubyn myn in die Mangare area, Kenia te identifiseer. Hierby is gedemonstreer dat geochemiese eksplorasie metodes suksesvol in edelsteen eksplorasie toegepas kan word. In hierdie studie is ‘n gedeeltelike ekstrasksie en heel-rots geochemiese metodes gebruik om die toepaslikheid van hierdie metodes op tanzaniet eksplorasie te toets. Verskillende geologiese materiale is gemonster, naamlik grond, stroom sedimente en kalkreet. Dit is moontlik dat hierdie geochemiese benadering nie so effektief soos fisiese metodes soos swaar mineraal skeidings mag wees nie. Dit is nogtans belangrik om die toepaslikheid van hierdie metodes op tanzanite eksplorasie te toests, as gevolg van die potensiële effektiwiteit en relatiewe logistiese gemak van die metodes. Die essensie van die wetenskaplike metodiek wat in hierdie studie gebruik is, is om die geochemie van die grond, stroom sedimente en kalkreet te vergelyk met die geochemie van die onderliggende geologie wat deur middel van diamant boorwerk vasgestel is. ‘n Positiewe korrelasie sou dan dui op ‘n bedekte tanzaniet afsetting. Grond monsters is van ‘n sloot geneem wat loodreg op die strekking van die tanzaniet gemineraliseerde en ongemineraliseerde horisonne gegrawe is. XRF spoor element data van die gronde is vergelyk met die chemie van die onderliggende gesteentes. IGP-AE data wat bekom is deur die monsters met 1 molaar HCl te loog (12 uur loging) is vergelyk met XRF data van dieselfde monsters deur middel van ‘n massa balans indeks om te bepaal of daar enige hidromorfiese dispersie van sekere spoor elemente is en om die toepaslikheid van loging as ‘n alternatief tot die heel-rots metode te bepaal. In die algemeen is daar ‘n goeie korrelasie tussen die grond en rots spoor element data profiele oor die lengte van die seksie. Alhoewel, Ti- en Zr-genormaliseerde massa balans data profiele wys dat daar ‘n mate van afwaartse beweging van grond na die voet van die heuwel is, maar dat hierdie ‘n breuk in die algemene patroon vorm nie. Die IGP-AE data dui daarop dat die hidromorfiese verspreiding van spoor elemente laag is en dat die spoor elemente wat van belang is (V, Cr, Ni en Cu) in nie-oplosbare fases gesetel is. Gevolglik is die logings metode nie ‘n toepaslike alternatief tot die heel-rots XRF metode op gronde nie. XRF analises op die gronde het gewys dat die XRF metode moontlik nuttig kan wees om nuwe areas van tanzanite mineralisasie aan te dui, omdat die gronde wat ‘n grafietiese kalk-silikaat skis oorlê, wat altyd langs die tanzaniet draende horisonne voorkom, is op grond van anomale konsentrasies van V geïdentifiseer. ‘n Eksplorasie konsessie is op die basis van die teenwoordigheid van groot strome, ‘n paar tsavoriet myne wat aanduidend is van hoë prospektiwiteit vir tanzaniet is en as gevolg van ‘n variasie in geologie in die area vir stroom sediment monstering gekies. Tanzaniet en tsavoriet is kogeneties in bekende tanzaniet afsettings. In hierdie geval was die doel om te ondersoek of tanzanietagtige anomalieë (nl. die anomale konsentrasies van V wat in die ondersoek van die grond chemie opgemerk is) in die omgewing van die tsavoriet myne geïdentifiseer kan word. Tsavoriet, die edelsteen variëteit van grossulaar granaat, bevat hoë konsentrasies V. Die monsters is deur middel van die XRF heel-rots metode vir spoor elemente geanaliseer. Die data dui op ‘n paar monsters met hoë V konsentrasies in die ondersoek area. Hierdie studie het ook gedui op ‘n lae stroom sediment verspreiding van die spoor elemente van belang, heel waarskynlik is dit ‘n funksie van die semi-ariede klimaat. Die fyn fraksie (< 90μm) blyk tot ‘n groter mate as die growwer (90μm tot 180μm en 180μm - 300μm) fraksies gemobiliseer te word. Soos voorspel kan word deur die loogings analise het faktor analise gewys dat die spoor elemente deur swaar mineraal geochemie gedomineer word en dat ‘n studie op swaar minerale moontlik ‘n goedkoper en meer toepaslike eksploraise metode is as die wat in hierdie studie ondersoek is. Kalkreet monsters is van ‘n ongebruikte, 10m diep myn skag wat deur die kalkreet gesink is om by die tanzaniet gemineraliseerde horison uit te kom geneem. Monsters is van die bodem van die skag, naaste aan die tanzaniet mineralisasie, tot die oppervlak geneem om die assosiasie tussen die spoor element geochemie en afstand van die tanzaniet mineralisasie te ondersoek. Geen vertikale assosiasie tussen spoor element geochemie en die nabyheid tot die tanzaniet afsetting kon vasgestel word nie. Daar was geen duidelike aanduiding in die geochemie van die kalkreet op die onderliggende tanzanite afsetting nie. Hierdie is ‘n verdere annduiding op die nie-mobiele toestand van spoor elemente in hierdie omgewing. Hierdie studie het suksesvol gedemonstreer dat goed gedefinieerde grond en stroom sediment geochemiese studies moontlik in geochemiese eksplorasie vir tanzaniet bruikbaar kan wees. Dit is belangrik om in gedagte te hou dat dit slegs die geval is as die geochemie van die litologiese paket wat met die mineralisasie geassosieer is uniek en goed bekend is.

Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2008. / 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 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 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.