Summary:
Mining of the ruby mineralisation by Gemfields has been primarily focused on three main clusters of production pits in separate areas, termed by Gemfields as Mugloto (the western portion of the deposit), Maninge Nice (the north-eastern portion of the deposit) and Glass (the south-eastern portion of the deposit).
The Montepuez ruby deposit is hosted by the Montepuez Complex, a strongly ductile-deformed, wedge-shaped, metamorphic terrane. The Montepuez Complex is composed of orthogneisses ranging from granitic to amphibolitic in composition, and para gneisses comprising quartzite, meta-arkose, marble lenses, quartz-feldspar gneiss, and biotite gneiss. These metamorphosed sedimentary rocks have been intruded by granite, granodiorite, and tonalite.
Intense deformation has resulted in a highly complex structural framework, with the local units folded into tight and isoclinal folds dissected by a suite of mainly northeast to southwest trending shear zones.
The Montepuez Complex is bounded by thrust faults to the north by the Nairoto Complex, the oldest rocks in the region composed of ductile-deformed metamorphosed intrusives, and to the west by volcano-sedimentary meta-suites of the Xixano Complex.
The Montepuez Ruby Mine primarily exploits secondary deposits, which consist of alluvial and colluvial deposits formed from the weathering and transport of primary ruby-bearing rocks. Primary rubies from amphibolite sources are typically tabular and hexagonal crystals with a strong basal plane and a pinkish red to light pink colour – often classified as sapphires, usually highly fractured with specific inclusions and produce a relatively large volume of stones of low-quality.
Ruby and corundum mineralisation is found in two styles: primary amphibolite, and a secondary gravel bed. The main source of rubies and corundum is secondary mineralisation, although mining has also occurred from the primary mineralisation. The secondary gravel bed horizon comprises variably rounded quartz and clastic fragments, forming a semi-continuous horizon, at or near the basement contact. The gravel bed horizon is generally less than 2 m thick, with an average thickness of 0.45 m. The primary mineralisation is associated with a variably weathered amphibolite unit, which is being mined in the Maninge Nice area.
Within the gravel bed unit, the quality and quantity of ruby gemstones varies significantly across the deposit. The grade and type of ruby encountered also varies across the secondary mineralisation. MRM attributes this to the variability of the primary host lithology, the geomorphology of the area, and the nature of the physical and chemical weathering during deposition of the secondary mineralisation.
Differences in the grade and quality of stones recovered from Mugloto and Glass are attributed to different primary sources. Geochemical analyses and XRF studies indicate that the ruby/corundum recovered from Glass are typically higher in Cr and V, and lower in Fe than stones from Mugloto.
The Maninge Nice secondary deposits lie very close to the underlying primary amphibolite deposits, resulting in a higher number of carats per ton being recovered. These are associated with a high incidence of ruby/corundum, when compared to the secondary deposits at Mugloto and Glass. The ruby/corundum at Maninge Nice tend to be more platy in shape, indicating reduced transportation distances. The stones at Maninge Nice are typically tabular hexagonal crystals, highly fractured and included with a strong basal cleavage.
The stones recovered from Glass are similar to those at Maninge Nice, except the secondary mineralisation does not overly the primary source. The stones indicate a higher transportation distance, are more rounded, meaning the number of stones recovered is reduced. The stones recovered from the Glass area typically have a better pink colour than those in Mugloto.
Stones recovered from Mugloto are relatively high in Fe content. The primary source for these stones is thought to lie outside the area currently delineated by exploration drilling and pitting. The stones are typically dark red in colour, more transparent with fewer inclusions, and often rounded or tumbled in shape.
MRM has put in place a classification system to record the quality of the rubies, in order to reflect this variation.