Material involved in the present study was supplied by Sopemi Ltd (Brazil) via the Diamond Trading Company (London). Samples are referred to throughout by numbers prefixed by BZ (Brazil) and JH (Jeff Harris) where suffices are adopted according to the methodology described in appendix 3.1.

The São Luiz alluvial diamond deposit lies centrally within the continent of S. America in Mato Grosso State, Juina Province, Brazil, figure 1. The principal deposit is located within channels, palaeo-channels, flats and terraces of the Rio Cinta Larga, a tributory of the Rio Aripuanã. A number of secondary deposits lie in the Rio Cinta Larga catchment area, namely the Rio Vinte e Um, Rio Mutum, Igarapé Porcão, Rio Juininha and Rio Juina-Mirim (Watkins, J. (SOPEMI), 1997, personal communication).

The location of the magmatic source of the deposit is not known, although there are a number of Mesozoic kimberlites recognised in the Juina area (Tompkins, 1992), some of which lie upstream on the Aripuanã river. None of the kimberlites are being mined, but, given that the alluvial diamond production of Juina Province is significant, ~ 10 Mcarats per year (Teixeira, N. (RTZ Mineracao), 1997 personal communication), it is likely that the kimberlites do have some diamond potential. Little has been published on the detailed geology of the area although it is suspected that localised, extensive surveying has been carried out. At present, research is being undertaken on the geochemistry, field relations and diamond prospectivity of Juina kimberlites by Teixeira (RTZ, Mineracao).

From a general geological point of view, Brazil has three major cratonic regions (figure 2) two of which fit with African cratonic regions on reconstructed plate distribution (figure 3). The three Brazilian cratons are:

• The Amazonico Craton which contains the Guiana Shield to the north and the Gaupore Shield to the south. It is within the broad outline of the Amazonico Craton that the São Luiz deposit is located.
• The São Francisco Craton, to the east of the country which is considered to be the western portion of the Congo Craton.
• The São Luis Craton, which lies around the coastal town of São Luis, and is considered to be the southern portion of the West African Craton. This name has no relationship with the São Luiz of the present study.

Additionally, a small craton (Rio de la Plata Craton) has been identified in the south of the country.

Brazilian kimberlites (Svisero, 1995) have been identified as lying along a series of lineaments, figure 2, a common observation for kimberlitic distribution (eg. Hoal et al., 1995). All these lineaments were re-activated during the triggering of Atlantic spreading and saw a degree of volcanic activity (Tompkins, 1992). Juina kimberlites lie on a lineament trending at ~ 125° across Brazil towards Rio de Janeiro. This lineament contains another nine kimberlite clusters in addition to that of Juina (figure 2). The type of volcanism along the lineation appears to vary according to basement type with kimberlitic volcanism within the bounds of the Amazonico craton and volcanism dominated by dunites and lamprophyres within the Parana Basin. Although the Juina kimberlites lie within the general bounds of the Amazonico Craton they, as indeed all other Lineament 125° AZ kimberlites, are not truly cratonic. In fact the Juina kimberlites and the three other kimberlitic fields nearby, all lie within a mobile belt, the Rio Negro-Juruena Mobile Belt, bounding the stable core of the craton, what is termed the Central Amazonico Province, figure 4. This mobile belt is believed to have an island arc affinity (Carrington, 1990 personal communication to Wilding (1990)) caused by docking of the Central Amazonico Province with basement rocks to the south west and has a metamorphic age of approximately 1450-1750 Ma. It has been remobilised during the Proterozoic presumably as it is the most obvious point of weakness within the cratonic area.