Mahdia ProjectThis was initiated as a joint project between Geological Services, Mineral Processing and the Environmental Division with the aims of :
- Evaluation of gold recovery of land dredges
- Checking black sand composition for other elements of commercial value
- Investigating the environmental effect of mining in the Mahdia area, especially the effects of mercury and acid drainage.
Geological Services took three samples from each dredge visited in October 1999:- feed, post amalgamation black sand concentrate and tailings.
Previous work in the area had established the presence of an alluvial deposit with high gold content, the Proto-Mahdia Channel, which was thought to be mainly derived from the Eagle Mountain area. Alluvial gold is present in many areas outside the Proto-Mahdia Channel, so other sources of gold are obviously present. In some of the dredge sites visited, i.e. the Red and White Hole areas, highly clay-pyrite altered and quartz veined saprolite was being mined, suggesting that at least in the southern part of the Proto-Mahdia Channel, primary minerailisation directly underlies alluvium.
Samples of feed and tailings were analysed for Au, Ag, Cu, Pt and Pd using a cyanide leach method (BLEG). Black sand concentrates were analysed for "Au+48" using INAA and ICP-MS, and were also analysed by conventional fire assay for gold, platinum and palladium. Results show that black sand discarded after amalgamation still contains significant amounts of gold. Correlation between gold assays using INAA and Fire Assay are excellent, confirming that discarded black sand from 5 dredges contains between 5dwt and 1 oz gold / metric ton, while black sand from three dredges contained between 1oz and 1.6% gold. Hopefully only small amounts of black sand with such grades are being discarded!
Dredges appear to be reasonably efficient in extracting gold from the feed material, with most tailings assaying only a fraction of feed grades. Gold grades in both feed and tailings appears to be low, but material was sieved to -30 mesh, suggesting the possibility that much of the gold is coarser. Black sand samples were unsieved. Highest gold grades in dredge feed occur in the Red Hole and St Elizabeth area at the southern end of the Proto-Mahdia Channel, in the area with possible primary mineralisation. The other area with high feed grades occurs at Minehaha
Molybdenum, and other associated elements such as tungsten are elevated only in the southern part of the Proto-Mahdia Channel, in the area where dredges appear to be working saprolite. Conspicuously, Mo is low at Minehaha, the other area with higher feed gold grades.
Mercury content of many discarded black sand concentrates is very high, indicating that this material is an environmental hazard. At least some of the gold being discarded probably occurs as mercury amalgam, but some samples with high gold only contain low amounts of mercury, suggesting that not all gold has been amalgamated.
Historically the Potaro area is famous for the mineral Potarite, a mercury-palladium mineral. Assays of black sand confirm that palladium, with lower amounts of platinum, are present, but concentrations are not sufficiently high to be of commercial significance in this area. Generally samples with the highest gold also have high platinum group elements, possibly reflecting a higher concentration factor from the primary material. Platinum and palladium grades of feed material is very low, though cyanide leach assay for these elements is noted for detecting only a small percentage of the total concentration. A sample of laminated sulphide rich material apparently replacing silty alluvium from the St Elizabeth area assayed 0.8 g/t Au, with high As (120ppm), Mo (316ppm) and 15ppm tungsten.
REFERENCE : Heesterman, L.J.L., Kemp, A.W. & Sampson, E. 2000. A Summary of Geochemistry of Land Dredges in the Mahdia Area, Potaro District. Guyana Geology and Mines Commission, Geoservices Division Technical Report.