Tsavorite and Tanzanite
Tsavorite, the vanadian variety of green grossular garnet, is a high-value semi-precious gemstone. It is hosted exclusively in metasedimentary and typically meta-evaporitic formations within the Neoproterozoic Metamorphic Mozambique Belt. Deposits are mined in Kenya, Tanzania and Madagascar, and other occurrences in the metamorphic belt are located in Pakistan and East Antarctica. All are situated within metasomatised graphitic rocks, such as graphitic gneiss and calc-silicates, intercalated with meta-evaporites. Tsavorite is found as primary deposits either in nodules (type I) or in quartz veins (type II) and placers (type III). The primary mineralisation (types I and II) are controlled by lithostratigraphy or structure (K).
Tsavorite
For the African occurrences, the protoliths of the host rocks are either anhydrite or baryte. Both hosts were deposited at the beginning of the Neoproterozoic within a saline marine coastal sabkha environment, deposited at the margin of the Congo–Kalahari cratons. Subsequently, during the East African–Antarctican orogeny, these saline sediments experienced high amphibolite to granulite facies grade metamorphism, forming the tsavorite between 650 and 550 Ma. Nodules of tsavorite formed during prograde metamorphism, with calcium supplied by metamorphosing sedimentary sulphates and carbonates, whereas the alumina, silicates, vanadium and chromium come from a clay-chlorite protolith (K). Veins formed during deformation and shearing of the metasedimentary platform were partially filled with tsavorite and other metamorphic phases. Further metasomatism occurred subsequent retrograde metamorphism with ongoing reactions with pore waters.
The presence of evaporitic minerals, such as gypsum, anhydrite, and scapolite, still characterises all the metasedimentary sequences today. Evaporites are essential to tsavorite as they provide calcium and facilitate mobilising all the chemical elements for tsavorite formation. The H2S–S8 metamorphic fluids characterised in primary fluid inclusions of tsavorites and the δ11B values of coeval dravite consistently around -20‰ confirm the fluids' marine evaporitic (high salinity) origin. The V2O3 and Cr2O3 contents of tsavorite range respectively from 0.05 to 7.5 wt.%, while their δ18O values are 9.5–21.1‰.
The genetic model proposed for tsavorite is metamorphic, based on chemical reactions developed between an initial assemblage composed of gypsum and anhydrite, carbonates and organic matter deposited in a sabkha-like sedimentary basin. Two kinds of nodules were likely precursors to tsavorite, and both were hosted in an evaporitic protolith: (i) nodules Type NI were initially anhydrite concretions within silica-rich shales; (ii) nodules Type NII were initially baryte concretions located within calcareous shales. Both types of shales contained V(–Cr)-rich clays and organic matter (K).
At the beginning of the prograde metamorphism (stage 2), the hosting shales turned into schists, and V(–Cr)-rich clays and organic matter transformed into respectively V(–Cr)-rich micas and graphite. For the crystallisation of tsavorite in nodules Type NI, Si and Al come from the schist, and V and Cr from the clays and are transported into the anhydrite core where it forms tsavorite utilising Ca from the evaporitic sulphate, following the equation;
3CaSO4 + 2Al4 + 3SiO2 + 6H2O -> Ca3Al2(SiO4)3+6O2+3H2S+6H+.
During tsavorite growth, H2S is trapped in fluid inclusion cavities, while sulphur is expelled into the schist to form pyrite. At the end of the prograde stage, the anhydrite is replaced by tsavorite, which is also present as tiny crystals scattered in the evaporitic rims. During retrograde metamorphism, these crystals are rehydrated and change into vanadian zoisite in the rims.
During the crystallisation of tsavorite in Type NII nodules, Ca and Mg came from the carbonates. Silica was transported towards the central part of the nodule, replacing baryte with diopside. A minor amount of vanadium is sourced from the micas and incorporated into the diopside core. Ba and S were expelled into the schist, the latter forming pyrite. Then, the diopside core increased in size, replacing all the baryte. Transport of Al, V and Cr from the schist into the core led to the formation of tsavorite following the reaction;
3CaMgSi2O6 + 2Al3+ --> Ca3Al2(SiO4)3 + 3SiO2+3Mg2+
Then, tsavorite and diopside interacted to form scapolite following the reaction;
2CaMgSi2O6 + 2Ca3Al2(SiO4)3 + 2SiO2+2CO2 + 4Al2O3 --> 2Ca4Al6Si6O24CO3 + 2MgO.
Hence, primary tsavorite deposits are of two types, with predictable occurrence positions in the field, which can be used to construct helpful exploration paradigms (figure above). Type I nodules occur in meta-evaporite beds, creating stratiform units, intercalated within the metamorphic series and more precisely in Ca-dominated rocks such as carbonate gneiss and calc-silicates with intercalations of marble (as in Kenya, Madagascar, part of Tanzania and Antarctic deposits). Type II associations occur where the metamorphic formations are affected by tight ductile isoclinal folding and shearing. The tsavorite occurs in quartz veins located at the hinges of the sheared isoclinal folds (Type IIA) and within ‘saddle reef’ structures (Type IIB). In this situation, deformation and shearing are associated with fluid circulation and percolation in the tectonically-opened structures.
Schematic cross-section of the different types of tsavorite deposits in the Neoproterozoic Metamorphic Mozambique Belt (after Feneyrol et al, 2013, and references therein)
The metamorphic rocks undergo metasomatic phenomena such as pyritisation, graphitisation, silicification and carbonatisation, sometimes with precipitation of tsavorite (Type IIC). This type of deposit is found in the Lelatema area and Ruangwa in Tanzania and Pakistan. Secondary tsavorite deposits (Type III) result from the erosion of the primary ones. Three secondary (reworked) sub-types are described: (i) sub-type IIIA corresponds to eluvial deposits such as those found in the Merelani area and in Kenya; (ii) sub-type IIIB is colluvial deposits such as those located in the Merelani area and in Kenya; (iii) sub-type IIIC is alluvial deposits forming the placers of Tunduru and Umba in Tanzania.
Tanzanite
During retrograde metamorphism, hydration of both tsavorite and scapolite can lead to the formation of zoisite (4[Ca2Al3(O/OH/SiO4/Si2O7)]), a member of the epidote group, which can become another gem mineral, known as Tanzanite. In the Merelani region of Tanzania, tanzanite and tsavorite-garnet veins have resulted from pegmatitic and hydrothermal fluids reacting with medium- to high-grade meta-evaporitic rocks along the crests of folds (L).
Tanzanite with graphite and laumonite
The faceted crystal form of many of the tanzanites results from their growth into dissolution cavities in the anhydrites and gypsums, fed by undersaturated mineralising solutions and free of interference from other minerals. The zone of mineralisation with such excellent gems is only a few meters wide. Still, it extends for about 9 km and includes other spectacular gem growths such as chrome tourmaline and chrome diopside. The known occurrences of gem-quality tanzanite are restricted to the Merelani area.