Summary:
The deposits in both Sahamamy and Vatomina projects are saprolitic type.
Sahamamy
The geology in the Sahamamy block is dominated by saprolite at the top (underlying the alluvium/topsoil) with varying thickness of between 15–20m, as can be observed at several locations in the mine pits and road cuttings. The graphitic mineralization is present within the saprolite unit. The saprolite is a resultant of deep weathering of the gneiss, ‘Graphitic Gneiss’, comprises quartz, feldspar, biotite, graphite flakes disseminated with minor mica and amphiboles. Being a weathered product (soft rock), saprolite allows for ease of extraction with mechanised digging equipment.
Graphitic zones (defined by melanosome of the gneissic bands) are generally sharp in contact with the waste rock (defined by leucosome of the gneissic bands). At places though, due to the complexity and tightness of the folding, variable alteration and leaching observed within the saprolitic nature of the host formation, and the sharpness of contact between ore and waste rock is obscured. Graphitic gneiss shows sharp contact with the dolerite dyke and porphyritic granite gneiss. Contact with the dyke trends N-S and dips due East at 45° dip amount whereas contact with porphyritic granite gneiss is steeper with 65° towards East.
Quartzo-feldspathic Gneiss is exposed all around the graphitic gneiss in both the Sahamamy and Sahasoa blocks. Only the saprolite part of this rock is exposed and fresh rock is absent due to deep weathering.
Vatomina
The Vatomina concession lies within the Anaborian and Manampotsy belts primarily comprising of quartzo-feldspathic migmatiic paragneisses with varying biotite and hornblende.
The major lithological units observed are schists, gneisses and migmatite gneiss with graphite bands along with NNE-SSW trending basic intrusive like dolerite.
The top alluvium soil is around 3-6m thick and comprised of mainly brown to yellowish brown coloured ferruginous minerals. At the bottom of this layer, disseminated flakes of graphite was observed.
Graphite Mineralisation
The graphite deposit in Sahamamy and Vatomina projects are saprolite hosted graphite deposit, epigenetic within the gneissic banded formation (Manampotsy Gneiss), which was deformed and metamorphosed at upper amphibolite to granulite facies.
Sahamamy
At the Sahamamy project site, the exposed thicknesses of the graphitic zones observed at some of the open-pit faces varied between 3-7m (melanosome defining graphitic horizon with interband parting of leucosome (quartzo-feldspathic and altered clay rich bands) defining altogether a saprolitic quartzo-feldspatic gneiss or Manampotsy Gneiss. The strike of graphitic zone which is approximately 850m for the Sahamamy main zone and approximately 600m for the Sahasoa zone was considered the maximum extension direction of the topographic drape of the graphitic horizons shown by Tirupati on their working geological map. There are several bands intersected at different topographic levels and at least two such bands were verified between 320m AMSL and 350m AMSL during SRK’s site visit, thus comprising a total ore horizon thickness of between 20-30m.
Vatomina
The thicknesses of the graphitic zones observed at some of the trench and pit faces in Vatomina project site varied between 3-6m (melanosome defining graphitic horizon with interband parting of leucosome (quartzo-feldspathic and altered clay rich bands) defining altogether a saprolitic quartzofeldspatic gneiss or Manampotsy Gneiss). In Vatomina Project, strike of graphitic zone, which is approximately 900m on an average for the shaded zone was considered the maximum extension direction of the topographic drape of the graphitic horizons as shown by Tirupati on their working Geological map. There are several bands intersected at different topographic levels and at least three such bands were verified between 50m AMSL and 80m AMSL during SRK’s site visit, thus comprising a total ore horizon thickness of 30m.
Present geological mapping depicts a wide single ore zone in Vatomina, well within the saprolite and weathered bedrock reaching down to hard rock formations intersected by drilling at selective drill holes. The banded nature of the ore body is yet to be mapped by Tirupati.
Almost the entire exposed profile at several pit locations in both Sahamamy and Vatomina project areas are highly weathered and altered to saprolite. In general, up to 70m depth is weathered with decomposed rocks all along the eastern coast of Madagascar.
The graphitic carbon content within the mineralised zones varies from 2.5% to around 23% (maximum), with an average of around 4%. The graphite flakes vary from 0.2mm (small flakes) to 5- 6mm (Jumbo flakes) within the graphitic zones.
Structural Geology
Sahamamy
The graphitic zones in Sahamamy imparts a well banded nature of the mineralized formation, with tight isoclinal to recumbent geometry, and is broadly representing first and second order folds (F1- F2), often co-axial and overprinted by third order warps (F3, non-coaxial). Strike of gneissic banding (S0||S1 planes) measured from the graphitic horizon in the Sahamamy mine at different pit faces and at different contour levels is broadly in the range of 270-350 degrees with gentle dips (15-30 degrees) towards South and West.
Strike of the complexly folded graphitic bands observed on several occasion during SRK’s site visit, measured E-W and N-S gneissic bands dipping gently to South and West respectively and contains the graphitic horizons as deeply weathered melanosomes. However, Tirupati has considered the graphitic zone as a composite of a series of asymmetric East verging folds with gentle Westerly to Southerly dipping normal limbs and at places folds with recumbent geometry defining the graphitic zone as a sheet (discounting the inter-ore waste zone, which is minimal within the broad envelop of the mineralized thickness of 20-30m), draped along and as exposed at different contour levels between 320 AMSL and 350 AMSL; this draping has been mapped as a composite graphitic zone which shows an extension of the zone along NE-SW trend in Sahamamy and N-S trend in Sahasoa.
Vatomina
Gneissic bandings are frequently overturned and exhibits asymmetric folded geometry. These broadly represent first and second order folds (F1-F2), often co-axial and overprinted by third order warps (F3, non-coaxial). Strike of gneissic banding (S0||S1 planes) measured from the graphitic horizon in the Vatomina exploration areas at different pit, trench and lavaka faces and at different contour levels are in the range of 320-335° for one set of folded limbs, and 75-95° near hinge zones with highly variable dips towards South and West.
Strike of the complexly folded graphitic bands as observed in Vatomina prospect is similar to Sahamamy. During the site visit, SRK measured E-W and N-S gneissic bands dipping gently to S and W respectively which contains the graphitic zones as deeply weathered melanosomes within a saprolitic horizon. However, Tirupati has considered the graphitic zone as a composite of series of asymmetric folds with highly variable westerly to southerly dipping limbs defining the graphitic zone as a sheet (discounting the inter-ore waste zone, which is minimal within the broad envelop of the mineralized thickness of 20-30m), draped over the topography as exposed at different contour levels between 50m & 80m AMSL; this draping has been mapped as a composite graphitic zone which shows an extension of the zone along ENE-WSW direction with few sleeves along N-S and NW-SE trend.