Sigma Mineração S.A. (SMSA) is the Brazilian subsidiary of Sigma and is the owner of the mining rights and the holder of mining concessions ordinance which includes the Xuxa, Barrerio, Murial, Lavra do Meio and Nezinho do Chicao deposits.
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Summary:
The deposits within the Project area are considered to be examples of LCT-type pegmatites.
LCT pegmatites represent the most highly differentiated and last to crystallize components of certain granitic melts. Parental granites are typically peraluminous, S-type granites, although some Archean examples are metaluminous, I-type granites. LCT pegmatites are enriched in the incompatible elements lithium, cesium, tin, rubidium, and tantalum, and are distinguished from other rare-element pegmatites by this diagnostic suite of elements. The dikes typically occur in groups, which consist of tens to hundreds of individual pegmatites and cover areas up to a few tens of square kilometres. LCT pegmatites are known to form as far as 10 km from the parental granite and the more distal the pegmatite, frequently the more fractionated. The most highly fractionated rareelement-enriched pegmatites only constitute 1–2% of regional pegmatite populations.
The host rock for the Xuxa pegmatite body is a biotite–quartz schist with a well-developed crenulation cleavage. Pegmatite xenoliths have been observed within the schist, with sizes ranging from a few centimetres to a metre. The pegmatite/schist contact is frequently hornfelsed.
The pegmatite is concordant with the regional foliation, striking northwest–southeast and dipping at 45–55º to the southeast. Drill data indicate the pegmatite has a strike length of 1,700 m, averages 12-13 m in thickness, and can reach as much as 20 m thick. It has been drill tested to 259 m vertical depth. It remains open to the west, east, and at depth.
Pegmatite mineralogy consists of the following minerals, with their approximate vein content: spodumene (20%), microcline and albite (40–45%), quartz (30%) and muscovite (5%). Spodumene occurs as pale green to colourless, elongated, tabular, crystals that can range in size from millimetre to as much as 80 cm in length and be as wide as 10 cm. The spodumene laths are set in a medium- to very coarse-grained groundmass of colourless albite, translucent quartz and pale grey perthitic microcline. Pale yellow– green medium- to coarse-grained muscovite micas may be present. Poikilitic textures of spodumene and quartz are common. Tantalite–columbite and cassiterite can occur in association with albite.
The Xuxa pegmatite dike is found on both sides of the Piaui River but does not crop out in the river valley. Two drill holes were angled to pass below the Piaui River, with one hole drilled from each bank. The drill holes intercepted pegmatite at depth. Core logging showed the spodumene to be weathered and contain replacement textures. The current interpretation is that the Piaui River occupy a fault trace, and that the interpreted fault has thinned the pegmatite body in that location.