Pakaraima ProjectPrevious exploration by the Geological Survey in the 1950’s had identified the presence of ferruginous bauxites in the Pakaraima Mountain area, and concluded that grades were sub-economic. Higher grade bauxite in this area was reportedly formed by weathering of the lower Kopinang dolerite sill, while the higher sill contained a higher silica content, and was not suitable. However work by Bateson (1962) in the Kopinang area showed that weathering of a hornfelsed pelitic unit in the Roraima Formation gave a laterite with both a higher alumina content, and a lower silica content.
In the Pakaraima mountains a number of other forms of mineralisation have been recognised. Diamonds, including rare coloured stones are worked from placer deposits; the source is not known, but this may be derived from fossil placers with Roraima Formation sediments. Potarite, a palladium mineral is often found as a by-product of small scale diamond mining. There are two possible soures for this mineral. Either this is also derived from fossil placer deposit in the Roraima Formation sediments, formed by erosion of even older rocks, or PGE mineralisation occurs either as a result of segregation in the basic sills, or as later vein style mineralisation. Gold also occurs in alluvial deposits in the Pakaraima mountains, and is thought to be derived from possible Witwatersrand style fossil placers in the Roraima Formation.
Fieldwork was done in in the headwaters of the Potaro River in October-November 2000 , with the aim of evaluating the bauxite potential, and exploring for other commodities of commercial value. Topographic relief is high, allowing investigation of various stratigraphic levels of the gabbro sill and the Roraima Formation sediments. In total 124 drainage sites were sampled, in a catchment area of 868 km2, and 22 rock samples were collected of altered, weathered and mineralised material, as well as representative samples of fresh rocks. –30# BLEG samples were analysed for Au, Ag, Pt and Cu, and –80# samples samples analysed for “Au+48” using NAA and ICP by Activation Laboratories in Canada. 22 Fifteen rock samples were assayed, and 13 rock samples were examined petrologically by A. Kemp.
Prior to the start of fieldwork a GIS/digital basemap was made, and air photographs examined using a stereoscope, and a map made of lineaments, lithological boundaries and circular features. This was used in conjunction with field data, JERS Radar and data from previous work to construct an updated 1:50,000 geological map of the area. The available 200,000 scale geological maps show that the geology in the headwaters of the Potaro river consists mainly of large areas of gabbro and dolerite, with a plateau of fairly flat lying Roraima Formation sediments, varying from conglomerates to sandstones with local shale beds. Near the contact of the gabbro sill with sediments hornfelsing has occurred. Some post intrusion faulting has been recognised. Fieldwork showed that generally the available geological maps show the approximate location of different units, but that the doleritic / gabbroic sills have a much more irregular character than shown. Intrusions are only sill like in some areas, and in other areas have an irregular contact with Roraima Formation sediments. Sub-units within the Roraima Formation such as the pelitic units also do not have the layer cake stratigraphy shown on the published map, but have irregular thicknesses, and locally pinch out. Not enough fieldwork has been done in this area to enable the details of sub-units to be mapped out, so in the maps in this report, the Roraima Formation has not been divided. There is good evidence for post intrusion faulting as well as small scale quartz and pyrite veining of the gabbro. Fieldwork has also shown conclusively that in this area lateritisation of the gabbro is incomplete, and abundant small outcrops scattered through-out the fieldwork area indicate that unweathered material occurs relatively near the surface. There was very little evidence of the presence of hornfelsed Roraima Formation mudstones; rocks near the sill contacts were generally quartzites and conglomerates. Some samples of lateritic material and aluminous material have been collected for assay from incised river banks and the base of diamond pits.
Previous work had made no mention of the presence of diamonds in this area. Fieldwork has shown that these have been mined in the past in a number of areas, and in late 2000 were being mined by 4 teams of people, as well as a number of individual porknockers. Diamonds seen were almost all unbroken generally clear crystals often larger than 1m-2mm. The largest diamond seen was just under 1cm in diameter, though full of inclusions. Field investigation suggests that some of the areas being mined are palaeo-channels of the Potaro River. Miners in the area report that diamonds from both the Kopinang and Chenapau areas to the south of the area investigated are similar. Abundant mining activity in the Simang area to the east was also reported.
Gold was only seen in very small amounts in pan-concentrates at 3 sample sites, though there are reports of porknockers earning a living working gold in this area. Geochemical results confirm that low level gold anomalies are present in a few drainages, but are not considered indicative of significant gold mineralisation. Low level gold anomalies may indicate the presence of possible diamond placers.
Alluvial columbite and tantalite have long been known from the Morabisi area, where mineralisation was thought to be associated with pegmatites. Assay results from the Pakaraima Project showed a dramatic surprise; up to 14 ppm Ta in drainage samples wholly derived from the basic sill area. Associated elements include those expected in a basic sill such as Co and Cr, but stream sediments are also high in Mn (max 4400ppm), Mo (max 11ppm), with slightly elevated Th and U. Evidence of the tholeiitc character of the sill includes high Ti (max 7.4% in drainage samples and 14% in rocks). Thin sections of basic rocks from the area show a characteristic late stage fill of opaques with fine quartz and biotite, suggesting the possibility that Ta is associated with a late stage differentiate of the sill. Chemical and petrological data from several other areas of Guyana suggests that similar associations of basic sills with Ta geochemistry and late stage opaques occurs elsewhere.
The whole area is held under a valid large scale “Permission for Geological and Geophysical Surveys”, with a few PPMS (Prospecting Permits Medium Scale) covering specific diamond area.
The following diagrams summarise available information.
Mineral Ocurrence Map (245K)
Geological Summary Map (738K)
Ta anomaly Map (261K)REFERENCE : Heesterman, L.J.L, Kemp, A.W.K, Arjune, B.and Crawford, A. 2003. A Summary of Geochemistry, Geology and Structure in the headwaters of the Potaro River, Pakaraima Mountains. Guyana Geology and Mines Commission, Geoservices Division Technical Report. - Report under preparation.