The geology and petrology of the Merelani tanzanite deposit, NE Tanzania

[摘要] Tanzanite, a blue/violet gemstone variety of zoisite (Ca2Al2O.AlOH[Si2O7][SiO4]) isonly produced in NE Tanzania. The only known locality is an approximately 7km2deposit in the Merelani area. It is one of the most sought after gemstones in the worldwith an industry sales value of between hundred and fifty and two hundred milliondollars per year. At the current production rates and estimated resources the tanzanitedeposit has a life expectancy of around 20 years.Despite the economical and scientific importance as well as the geological uniquenessof the deposit very little research has been conducted on the geology and petrology ofthe deposit and the characteristics of tanzanite.The primary aim of the research summarised in this dissertation was to gain anunderstanding of the geological conditions that led to the formation of this uniquevariety of zoisite. In order to achieve this, a variety of geological disciplines wereaddressed including the lithostratigraphic setting, the deformational history, themetamorphic history and conditions, the geochemical and isotopic composition, themineral chemistry as well as the physical and optical properties of the tanzanite.Extensive field work was conducted over a seven year period, which included surfaceand underground mapping, surface trenching, surface and underground core drilling,structural measurements and an intensive sampling programme. Various analyticaltechniques were used in order to petrologically and mineralogically investigate both thedeposit and tanzanite itself, including optical microscopy, XRF analyses, laser-ablationICP-MS, quantitative chemical analyses by means of the electron microprobe, XRDanalyses, 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 oftanzanite. The deposition of carbon-rich layers, formed during the development of asequence 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 weredeposited in a back- or fore-arc spreading basin. The organic carbon-rich mudstonelayers acted as the first phase of vanadium accumulation in the stratigraphic sequence,and are intercalated with a material with a volcanic origin (metabasites), which mostlikely 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 to1000 oC) at ca. 1000 Ma. The high-grade metamorphic history was followed bymultiphase retrograde deformation events (D2 and D3) that developed as a result ofcrustal uplift. The D2 event probably occurred between 850 and 600 Ma at P-Tconditions estimated at between 8 to 7 kbar and 700 to 650 oC. A third stage ofdeformation (D3) resulted in the formation of overprinting structures and occurredduring the later stages of the Pan-African (550 – 500Ma) event duringamphibolite/greenschist facies conditions (7 to 6 kbar and 600 to 520 oC).The deformational history of the deposit played a critical role in the mineralisationprocesses. Of the four main deformation events identified, three played a critical role inthe mineralisation process. The first event led to the development of early structuralfeatures as defined by F1, S1 and L1. This was followed by a multiphase D2 eventconsisting of three different orders of folding (F2a, F2b and F2c) and the formation ofboudinage. The association between tanzanite and boudins as well as the stacking andmultiple duplication of the boudinaged ore-zone through isoclinal folding resulted inmultiple 'ore-shoots. These ore-shoots follow the plunge of the F2c fold closures andresults in mine-able features within the ore-body. The third deformation event led tocomplex structural overprinting of the earlier fabric as observed in S3 and L3 andresulted in the deformation of the F2 structures through crosscutting F3 folds.Calc-silicate layers developed in the stratigraphic sequence as a result of metamorphicand metasomatic interaction between calcium enriched (boudinaged calc-silicates) anddepleted horizons (graphitic gneisses) during a skarn-forming episode. V-rich greengrossular garnet (tsavorite) crystallised in tension zones within and in proximity to theboudins 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 and650 ± 50 oC.Two distinctive tanzanite-forming processes are distinguished. The first involves theformation of tanzanite as a result of retrograde reaction of grossular garnet. The secondprocess involves the migration of V and Ca -enriched fluids along brittle shear zones totension sites where fluids reacted with wall rock during a drop in P-T conditions toprecipitate tanzanite.Fluid inclusion and stable-isotope studies concluded that the ore-forming fluids werederived from the dehydration of the metasedimentary sequence and consisted of amixture of H2O, CH4, H2S and N2.Mineralogical investigation of tanzanite indicated that trace concentrations of vanadiumwithin its crystals structure causes its blue / violet colour. It was proved that thevanadium originated from the abundant organically derived graphite within the deposit.Spectroscopic and EPR analyses revealed the importance of the Ti4+ / Ti3+ ratio withinthe crystal structure of tanzanite with regard to its colour characteristics. The heating oftanzanite results in a couple valence exchange reactionTi 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 thetanzanite mining and treatment activities in the Merelani area of NE Tanzania. As suchthe research contributed to the establishment of a successful tanzanite mine, based onsound geological principles, which may act as a role model for other gemstone minesworldwide.

[发布机构] Stellenbosch University