Morabisi Project

Previous exploration by the Geological Survey and private companies led to the discovery of alluvial columbite / tantalite deposits in tributaries of the Morabisi River, a left bank tributary of the Mazaruni. The source of these deposits was thought to be pegmatites associated with contacts between intrusive basic bodies and granitic country rock, but this was never confirmed. The Morabisi Mining Company started exploitation of alluvial columbite-tantalite in 1954, with over US$1.0 million invested. By 1957 operations stopped after demand for columbite for the US strategic stockpile ceased, and no alternative market was found. The Columbium
Corporation installed a small suction dredge in Pilgrim Creek in 1957, but by 1960 had also ceased operation.

An exploration programme by the Guyana Geology and Mines Commission (GGMC) was executed in 1999. This project was designed to attempt to establish the source of the columbite / tantalite, better define the extent of the deposits and also explore for other elements of commercial value. Work done consisted of drainage geochemistry, geological mapping, petrology, air-photograph interpretation, and integration of new data with previous information.

Geochemical sampling techniques employed were those commonly used in wet tropical areas, with 107 bulk -30# sediment samples taken using a flocculent with a -80# fraction separated in the laboratory. The fine grained nature of much of the stream sediment meant that obtaining heavy mineral concentrates in the field was difficult, so these were also separated in the laboratory, allowing results from the three media to be directly related. 28 rock samples were also taken. -30# BLEG (Bulk Leach Extractable Gold) samples were analysed for Au, Ag, Pt, Pd and Cu. -80#, concentrates and rocks were analysed for "Au+48" , with 22 rocks also
being examined petrologically. Only Ta was assayed in samples, but historical analyses of concentrates from the Morabisi area showed a Nb:Ta ratio of 4:1.

Results show columbite / tantalite occurrences with associated high rare earth, base metal, Mn, U and W concentrations are related to pegmatites in granites in an oval 10km wide area, broadly ringed by basaltic to doleritic intrusions. The pegmatites in turn appear to be related to a hidden shallow basic body below the granite in the Robello-Kuriaro Creek area, shown by a pronounced magnetic high seen on regional aeromagnetic data. Nb and Ta are frequently related to Sn-W bearing granitic intrusions. In this area this is clearly not the case, with no Sn detectable, although elevated W values are associated with the centre of the magnetic anomaly.
It is postulated that a basic body, with a temperature of over 1000C, partially melted the granite to form pegmatites. Structural concordance of pegmatites and margins of basic intrusions is thought to be due to exploitation of the same lines of weakness.

Precious metals associated with multi-element anomalies in the south of the area are related to quartz veining in metavolcanics. Those in the north-east and west of the area have a different genesis and in some cases appear to be sourced outside the area of study.

Main rock types encountered generally conform to previous descriptions, with geological boundaries from previous work being modified using field data and air-photos. The only unusual rock type sampled was an aplite with garnet and phlogopite. The published geological map had perpetuated a surveying error from the 1940's (Upper Morabisi 10km to the north of the correct position). This has been corrected.

The known area of historical mining is covered by valid Medium Scale Prospecting Permits, but results show that the most anomalous areas are further north. The Guyana Geology and Mines Commission will auction three Prospecting Licences over the main area of interest at a future date.

The following diagrams summarise available information.

Aeromagnetic Summary Map (171K)
Geological Summary Map (205K)
Tantalum Geochemistry Map (140K)
Anomaly Map (128K)
Auction Blocks Map (156K)

REFERENCE : Heesterman, L.J.L., Kemp, A.W. & Nestor, G. 1999. Morabisi Project. A summary of Geochemistry, Geology and Structure in an area with Columbite - Tantalite Mineralisation. Guyana Geology and Mines Commission, Geoservices Division Technical Report GS1/2000.