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Damtshaa |
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Botswana |
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Diamonds
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Damtshaa diamond mine is based on four small kimberlite pipes situated 20 km east of Orapa in north-Central Botswana, in southern Africa (#Location: 21° 18' 31"S, 25° 31' 57"E).
These pipes (B/K1, B/K9, B/K12 and B/K15) were were discovered during exploration undertaken between 1967 and 1972. Damtshaa is expected to yield 5 million carats of diamonds from 39 Mt of ore mined over a 31 year mine life.
The pipes were intruded through both Archaean basement granite-gneiss and the overlying sediments and basaltic lavas of the Mesozoic Karoo Supergroup. The four pipes may be summarised as follows (from descriptions on the Debswana web site):
The B/K1 pipe was discovered by reconnaissance soil sampling in 1967 and was the first kimberlite to be discovered in the Orapa/Letlhakane province. It is 15 km north of Letlhakane village and covers an area of 5.5 hectare as a 600 x 50 to 150 m, north-south elongated body, exposed as a calcrete capped topographic ridge with little or no soil cover. Soft, calcrete altered kimberlite underlies the hard surface calcrete at a depth of 1-2m. The main, northern section of the pipe is a hard, hypabyssal kimberlite with a low diamond content, while the southern section is composed of soft, highly altered tuffisitic kimberlite breccia and covers approximately 1.1 ha.
The B/K9 kimberlite was discovered 3 years later in 1970 from follow-up of a 1969 airborne geophysical survey by air core drilling through 10 m of sand and calcrete cover. The sub-outcrop area of of the pipe is 11.4 hectares, with a 600x300m NW-SE elongation. It has no topographic expression. Near surface, the pipe comprises two lobes of hypabyssal kimberlite at either extremity of the long axis, separated by a breccia composed of basalt wallrock clasts, up to several metres in diameter, enclosed within a weathered/altered kimberlite matrix. This latter lithology may represent a crater facies breccia basin. The basalt clasts comprise between 25 and 75% of the unit, which is between 80 and 110 m thick. Underlying the breccia basin, and invading the hypabyssal lobes are a number of intrusions of both tuffisitic kimberlite breccia and hypabyssal kimberlite.
The B/K12 kimberlite pipe is 800 m north west of the B/K9 pipe and was discovered in mid 1970, from the same 1969 airborne geophysical survey. It was delineated by air drilling below 6 m of Kalahari sand and >6 m of calcrete, again with no surface expression. The pipe is roughly circular with a sub-outcrop area of 3.2 hectares, and a maximum diameter of just over 200 m, decreasing to 2.5 hectares some 100 m below surface. It is composed of a soft, uniformly textured kimberlite composed of sedimentary crater facies infill to a depth of at least 90 m, which differs little from the underlying diatreme facies tuffisitic kimberlite breccia. Wallrock inclusions total around l0%, with up to 50% in localised lenses of basalt/sandstone breccia.
The B/K15 kimberlite was discovered as a heavy mineral soil sampling anomaly in early 1972. It is situated 35 km north of Letlhakane village on an escarpment bounding a prominent salt pan, and is overlain by 2 to 4 m of barren shoreline gravels, with no topographic expression of the pipe. The kimberlite is saturated with saline brines from about 14m below surface. In sub-surface it has an area of 2.5 ha, reducing to 1 ha at a depth of 100 m. The kimberlite is very soft and highly altered near surface, with up to 15% of altered basalt/dolerite/basement wallrock xenoliths. Apart from becoming harder and darker, there is no other obvious change in kimberlite facies with depth, which can generally be described as a diatreme facies tuffisitic kimberlite breccia.
The most recent source geological information used to prepare this decription was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Damtshaa
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