The geology and petrology of the Merelani tanzanite deposit, NE Tanzania
Date
2006-12
Authors
Olivier, Bernard
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
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 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.
Description
Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2008.
Keywords
Dissertations -- Earth sciences, Theses -- Earth sciences