The Canastra Range Diamondiferous Province and the Canastra-1 kimberlite: first primary economic source of diamonds in the country.
The Serra da Canastra Province
integrates one of the four great diamondiferous provinces of Minas
Gerais State, and can be divided into two districts, named as Alto São
Francisco and Médio Rio Grande districts. Kimberlitic and parental rocks
mainly cut mesoproterozoic(?) metasedimentary rocks of the Canastra
Group, in the Alto São Francisco District. In the case of the Canastra-1
diamond-bearing intrusion, the ages are around 120 Ma. In the Médio Rio
Grande district, the main (secondary) source of the diamonds is the
Lower Cretaceous conglomeratic rock occurring in the north side of the
Paraná basin (Bauru Group). The sedimentary source area for these
sediments included the Canastra Range zone. The Canastra-1 body is part
of a cluster with approximately 40 pipes outcropping on the south side
of the Canastra Range, near the sources of the São Francisco River. The
intrusion is made up of two blows aligned along NE-SE, which is the
region's structural trend defined on the metasedimentary rocks of the
Canastra Group. The smaller blow has only negligible diamond content,
while the SE blow is mineralized with an average content of 12 to 18
ct/100 tonnes of rock. There are also significant differences in the
petrographic facies of the two bodies, which is homogeneous in the NW
blow while the SE blow is heterogeneous, with the occurrence of a
mixture of several facies. The mineral chemistry of the pyrope, ilmenite
and diopside showed some resemblance with some diamondiferous African
kimberlites, mainly those of the West African Craton. Data from
experimental mining activities of SAMSUL Mining Company indicated an
excellent quality for this kimberlite's diamonds, with US$180-200/ct
estimated average value; certainly, this is one of the highest values
worldwide.
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