O CAMINHO DAS PEDRAS

Abstract

Application of modern geological concepts lead to the identification of different processes involvedin the geodynamic evolution of the southern Serra do Espinhaço, Minas Gerais, Brazil with special referenceto its depositional systems (Tab. 1 and 2), paleoclimatology and synsedimentary crustal extension history.Within this context a detailed analysis of the Espinhaço Rift, especially of the area Sopa - Guinda nearDiamantina (Fig. 1) lead to the identification of the precise magnitude of the source areas of the diamondscontained in the Sopa-Brumadinho Formation and consequently to the location of the primary sourcerocks of the diamonds.The primary source rocks of the diamonds (kimberlites or lamproites ?) are deeply altered either bycontemporaneous Proterozoic weathering or to less extent by recent weathering. The emplacement ofthose rocks along normal faults which controlled the local base level establishes a clear relation of causeand effect (consequence). The intrusions are coeval with the fault movements which defined adjacenttectonic cells where coarse clastics are deposited by the relief as well as the erosion products of the hostrocks ejected by the explosive volcanism (Fig. 4).The granulometric constancy of the conglomerates in the different alluvial fans, the uniform grade ofdiamond within these fans, and the absence of significant fluvial incision demonstrate that the system ofsynsedimentary normal faults was rift prograding, i.e. in the same direction as the sedimentary flow (Fig.4).On the other hand, the quartzitic breccia of the Campo Sampaio Member represents a vent brecciawhich accumulated in maar type craters, sometimes with local sedimentary reworking (Fig. 5).The individual characteristics of the diamonds of each of the diamond-bearing province within thesouthern Serra do Espinhaço (Fig. 1) suggest an intrinsic relationship of origin and space of the depository.Surface peculiarities of the diamonds are contrasting. Stones from the present three hydrographicbasins of the region show remarkable traces (unhas, jaças, many chips) of sedimentary reworking whereaslots of diamonds from conglomerates are found virtually intact, indicating negligible transport from theprimary source rock to the site of deposition within the Proterozoic coarse clastics.