Submitted by Andrew du Toit on Tue, 04/11/2014 - 09:52 | Last updated: Tue, 04/11/2014 - 10:04
Grid Ref (WGS84 Lat/Long in decimal degrees)
Turn Off Mutare to Birchenough Bridge Road near Hotsprings
Basal Conglomerate of Umkondo Group
De Beers discovered the Marange fossil placer diamond deposit in 2003 during the tenure of their exploration licence (EPO 1523) covering the Marange area. The exploration was targeted at discovering kimberlitic diamonds, but the recovery of rounded diamonds in some heavy mineral samples collected from local streams led to suspicions of a secondary source for the diamonds. A search for the possible source led to the discovery of a mineralized conglomerate.
The discovery attracted informal miners from all over the country and abroad. The chaos that ensued led De Beers to voluntarily abandon the area. The government immediately reserved the ground and introduced measures to bring order. Special licences (Special Grants) were issued to the Zimbabwe Mining and Development Corporation (ZMDC) to systematically exploit the resource.
The influx of illegal miners and the leakage of diamonds to the illegal markets resulted in Zimbabwe being put on the Kimberley Process Certification Scheme (KPCS) agenda in 2006. Several KPCS review missions were made to Zimbabwe to assist the country to comply with the minimum requirements of the KPCS. As a result, the ZMDC was advised to seek strategic partners with requisite capital to exploit the diamonds without compromising conditions of the KPCS.
The Marange diamond area is now secured against illegal mining activities, and four companies are commercially exploiting the resource. Families that have been disturbed by the mining activities have been re-settled.
The discovery of diamonds at Chiadzwa in Marange triggered an unprecedented diamond rush, which culminated in more discoveries. Other discoveries were made on the Chikwakwa plateau under Chief Muusha, and on Charleswood Farm east of Chimanimani. The Charleswood deposit is being evaluated by the DTZ-Ozgeo company.
The diamonds are concentrated in a basal conglomerate horizon that sporadically outcrops on the edge of the Umkondo basin in the south-eastern part of Zimbabwe. The conglomerate lying on Archaean granite basement forms base to the 1 100 million year old Umkondo Group rocks comprising over 1 200m thickness of various sediments intruded by basalt and dolerite sills.
The most easily accessible exposure of the basal conglomerate is at Birchenough Bridge where the conglomerate lies on pink granite basement. The conglomerate is in turn overlain by limestone, the so-called lower limestone horizon. This assemblage depicted as ml1 on the 1: 100 000 scale geological map of the area (Watson 1960) is an important exploration guide for the diamondiferous conglomerate.
The diamonds are imbedded in the conglomerate, and also occur in residual soils on the edges of the conglomerate, and in local streams and creeks that drain the conglomerate
The conglomerate, which locally grades into a grit, comprises well-rounded quartz clasts supported by an immature arkosic matrix that resembles the local basement granite in colour and texture. This tends to suggest that the provenance of the conglomerate materials was both local and distal.
The conglomerate that shallowly dips to the east is typically 2-3m thick at best, thinning out laterally to a few centimetres.
The primary sources for Marange diamonds have not been identified. Several kimberlites are known to occur within the environs of the Marange area and beyond. These are however much younger than the age of the Umkondo sediments, and cannot be the sources of the diamonds in the ancient conglomerate. It is however possible that some diamonds in alluvium of current rivers and creeks (diamonds in location 4 in Figure 1) could have come from kimberlites found in the Marange area.
A large percentage of the diamonds are sub-rounded (Plate 1) suggesting that they could have come from a very distal source. Sedimentological studies of the Umkondo sediments show that the basin was fed by rivers coming from the west (Button, 1978), from the Zimbabwe craton. Thus it is envisaged that Archaean or early Proterozoic age kimberlites intruding the craton supplied the diamonds to the Umkondo basin. The diamonds appear to have been deposited in suitable environments on the beaches of an ancient sea of the Umkondo Basin, in a similar way diamonds are currently being deposited on the west coast of Southern Africa.
A unique feature of the Marange diamonds is the brownish colouration on the outer skin of some stones. This is believed to be the result of surface geological processes in the sedimentary environment and the subjection of the stones to radiation emanating from the basement granite, and high heat and pressure as a result of dolerite intrusions ubiquitous in the area, and the Pan-African orogenesis that affected the Umkondo basin. A large percentage of the diamonds are of poor quality as a result. It is estimated that about 10% of the gems are of high quality. Thus the sheer concentration of diamonds locally makes the Marange deposit viable to exploit.
The Marange diamond deposit has not been well studied yet. For instance, the extent and controls of diamond mineralization are not known. Other technical data such as grade mixture, quality and resources of the deposits remain confidential or are not yet known.
Permission required from mining companies operating in the area
Button, A. 1978. A depositional model of the Umkondo Basin in southeastern Rhodesia. 19th Annual Report (1977), Economic Geology Research Unit, University of the Witwatersrand, Johannesburg, South Africa, 13-14. Watson R.L.A. 1969. The Geology of the Cashel, Melsetter and Chipinga Areas. Rhodesia Geological Survey Bulletin No. 60. Petuxov S.N., Simonova O.Y. and Mupaya F.B. 2012. Geology and petrology of the Umkondo diamond deposits with emphasis on the Chimanimani Deposit. Geological Society of Zimbabwe Summer Symposium 2012 Abstracts, p14.
A generalised cross section across the Chiadzwa placer diamond deposit area. 1. Granite basement; 2. Diamond-bearing Umkondo basal conglomerate and arkose; 3. Diamond-rich eluvium shed from the conglomerate; 4. Diamond-rich alluvium; 5. Lower limestone horizon; 6. Lower quartzite horizon; 7. Upper limestone horizon; 8. Upper quartzite horizon.