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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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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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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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