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Kimberlte Exploration Methods
Tanzanian Royalty Exploration Corp.'s NEW Approach
HISTORICAL BACKGROUND
The history of kimberlite exploration in Tanzania began just before the First World War when diamonds were discovered at Mabuki village, located north of Mwadui in the Shinyanga area. That was soon followed by the 1940 discovery of the largest economical kimberlite pipe found to date, the Mwadui pipe, by Canadian geologist Dr. Williamson. The pipe has a diameter of 1.5km and was discovered using loam sampling techniques. The discovery initiated a countrywide diamond rush resulting in more than 300 kimberlite pipes being discovered. Seven of these were economically exploited. Among them are the Mabuki, Usongo (Nzega), Kizumbi, Loya, Nyanghwale and Mwadui Mines
(refer to map).
In 1979 Geosurvey International conducted an aeromagnetic survey in the Shinyanga region. Approximately 100 anomalies were identified within the Mwadui diamond field. Ground follow-up by Tanex Ltd., a subsidiary of De Beers, led to the discovery of 27 kimberlites, of which 4 were found to be diamondiferous (Gobba, 1989).
In 1992 Tanex, which was under new management, conducted an airborne survey to cover an area that included the Mwadui, Nzega and Kahama diamond fields. More than 500 magnetic targets were selected for ground follow-up. Heavy mineral sampling formed a significant part of this program. Kimberlitic indicator mineral occurrences were followed-up with infill sampling and anomalous areas where subsequently drilled. This led to the discovery of more than 100 additional kimberlitic pipes.
Almost 75% of the known kimberlites discovered in Tanzania to date, are a direct result of magnetic surveys with the remainder of pipes without a magnetic expression having been discovered by using loam sampling methods.
CONSTRAINTS OF EXPLORATION METHODS USED BY PREVIOUS EXPLORERS
Magnetic Surveys:
In both the 1979 and 1992 airborne surveys the world class Mwadui pipe does not show up as a magnetic anomaly. This suggests that:
• The top of the pipe is buried by its thick crater facies sediments overlying the diatreme.
• There is deep weathering.
• A combination of both of the above.
It is estimated that 25 -- 30% of kimberlites in Tanzania do not have a magnetic signature due to the above factors.
Loam Sampling techniques for Indicator Mineral Analysis:
The ineffectiveness of the loam sampling method, especially within thick transported sediments such as mbuga, is due to the following factors:
• Mbuga is clay rich. Weathering and erosion will not result in the required dispersion of coarse indicator minerals that can be identified positively at mineral laboratories.
• Comparisons are made with Kalahari Desert conditions where indicator minerals are found above >100m of transported dune sands due to the activity of termites. This should be done with caution, as mbuga, a reducing clay, is not a favourite habitat for termites as seen by the low density of termite mounds on mbuga in Tanzania.
• Previous explorationists essentially disregarded kimberlitic grains that were scattered within this overburden and identified.
Tanzanian Royalty Exploration Corp. believes that these factors are possibly the main reason for the discovery of only a few economic kimberlites in Tanzania. A few kimberlitic grains that were never followed up within the mbuga may well represent a subtle "fingerprint" of a buried kimberlite pipe. The presence of thick mbuga and recent rift sediments are fundamental factors that have hindered diamond exploration in the past.
THE TANZANIAN ROYALTY EXPLORATION CORP. APPROACH
Despite these difficulties Tanzanian Royalty Exploration Corp.initiated a dynamic diamond exploration program by acquiring large areas of ground, both proximal and away from the Mwadui - Shinyanga diamond field, in addition to areas west and northwest of the Kahama diamond fields (see map).
The company's approach is to explore for kimberlites in under-explored terrain principally beneath mbuga and recently transported sediments and clay rich soil using both magnetic and hand auger-sampling methods. The streams in this part of Tanzania are mainly low energy due to the immature nature of the erosion profile. Field reconnaissance revealed that most of the streams consist of mbuga but an older higher energy stream (palaeochannel) exists immediately below the mbuga. Tanzanian Royalty Exploration Corp. therefore introduced hand-operated augers to collect samples from within palaeochannel streambeds that typically occur below 2 to 10m of mbuga cover.
A bulk sample is collected throughout the entire profile of the overburden but preferentially from a coarse basal river channel immediately below the mbuga where one is present. The channel is typically less than one meter thick, and is the same horizon that artisinal miners are exploiting diamonds from around the Mwadui alluvial fields. Bulk samples are washed and all liberated grains are gravitated by experienced technicians using a "Zambian jig". Collected concentrates are sent to the Mineral Services Laboratory in Cape Town for indicator mineral identification, sorting and electron microprobe analysis whenever kimberlitic indicators are picked.
Tanzanian Royalty Exploration Corp. is currently concentrating its exploration efforts proximal and to the northwest of the Nyanghwale diamond mine located within the Kahama diamond field (Area 1 on the map). A work program based on the newly developed auger sampling technique will be initiated in the region surrounding the Mwadui diamond cluster (Area 2 on the map) early in 2006.
WORK CARRIED OUT TO DATE
AREA 1
Previous work
Due to thick sediments and mbuga cover, the area was not intensively explored in the past. Earlier explorers relied on surface loam samples and no airborne magnetic surveys were flown. Poor sample results discouraged further work.
Work Conducted by Tanzanian Royalty Exploration Corp.
Magnetic Surveys
In 1999 Tanzam 2000 conducted a regional aeromagnetic survey over all the licenses shown in Area 1, predominantly for gold exploration. Lines were flown north-south with a spacing of 500m. Approximately 240 "pipe-like" anomalies were identified from the survey and ranked according to their magnetic signatures. All airborne magnetic data was submitted to three independent geophysical consultants for analysis and assessment. The consultants collectively selected in excess of 150 magnetic targets of which 107 fell within Tanzanian Royalty Exploration Corp. license boundaries and were recommended for further ground magnetic follow-up.
Ground magnetic follow-up was initiated in 2000. For each target magnetic readings were taken at 25m intervals at a line spacing of 50m. All acquired data was sent to the company's consultant geophysicist for modeling. Targets were re-prioritized based on the regional magnetic data, geological background, and ground magnetic signatures picked.
Heavy Mineral Sampling (HMS)
The first kimberlite indicator mineral sampling program took place in 2000 using conventional loam sampling methods over the ground magnetic targets. However, this exercise yielded negative results and not a single indicator mineral was identified. The presence of thick mbuga over these targets proved to be a major challenge. It was concluded that the conventional sampling method proved effective in granitic terrain and less effective in clay rich soil (mbuga) areas.
As part of the company's kimberlite condemnation protocol for Area 1 it was suggested that a stream sediment sampling program be initiated for indicator minerals to effectively rule out any doubt as to whether a pipe without a magnetic expression may be present. Stream sampling exercise was initiated over Area 1 (see map)
using both conventional and new augering heavy mineral sampling techniques. Approximately 300 samples were collected during this initial "trial" program.
Results to Date
The ground magnetic follow-up succeeded in confirming approximately 40 aeromagnetic anomalies that, from a geophysical point of view, are regarded as "high rank". Initial sampling over magnetic targets (using conventional methods) had yielded negative results. Two reasons can explain this:
• The bodies are not kimberlites.
• The loam samples were collected from transported sandy and mbuga soils and do not represent bedrock directly below the target.
Samples taken on other magnetic targets and streams using the hand auger sampling method yielded a good number of kimberlite indicator grains. This has significantly upgraded the nearby targets and clearly indicates that for some bodies the conventional sampling method was not effective.
Picked grains were sent to the Mineral Service Laboratory in South Africa for identification. Kimberlitic grains were subsequently probed by means of an electron microprobe and were confirmed to be kimberlitic and most likely to be associated with diamonds, as indicated on the map. The mineral chemistry of some grains is indicated on the map.
The approximate locations of the grains are indicated as areas of discovered positive indicators on the map. Using the more effective auger sampling technique many of the licenses that previously returned a zero heavy kimberlitic minerals content, are now showing positive results. As a result of this level of success, a more detailed follow-up stream sampling program has been completed to "home in" on the source of the kimberlitic grains. Ground magnetic follow up was carried out upstream of the kimberlitic grains in order to locate any subtle magnetic anomaly. Furthermore, some of the indicator minerals recovered are located close to the higher ranked magnetic targets. These areas are the prime focus of exploration at present and results support drilling most of these targets. Drilling is scheduled for late 2005.
Based on this recovery of kimberlite indicator minerals, a localized NW-SE secondary kimberlite corridor has been identified by Tanzanian Royalty Exploration Corp. as shown on the map
. The corridor encompasses the known diamondiferous pipes of the Kahama, Bukene and Loya diamond fields, higher ranked magnetic targets and newly discovered kimberlitic grains. This is an important corridor and much effort will be directed on this region.
AREA 2
Licenses located in Area 2, are located close to the Mwadui diamond mine and extend further south where surficial cover consists principally of thick rift sediments and mbuga. Earlier explorers intensively investigated the area but the presence of a complex overburden profile, including Proto Lake Victoria sediments, hindered previous sampling efforts. A number of known kimberlite pipes which are located in the granitic terrain are sporadically scattered throughout the area, as seen on the map. Earlier explorer Tanex however, did recover a good number of kimberlite indicator minerals, though their source(s) was never satisfactory determined.
Future Work Planned for Area 2
Area 2 is challenging for 2 reasons:
• Numerous companies, including Tanex, have explored it since the discovery of Mwadui.
• The complex overburden makes exploration that much more difficult and therefore expensive.
Kimberlites are known to occur in clusters and this can be clearly observed in Tanzania. Tanzanian Royalty Exploration Corp. believes that a primary regional north south trending corridor controls the location of these clusters, as shown on the map
. The diamond stability field defines the eastern boundary. As a rule of thumb, east of approximately longitude 34 degrees one moves out of the all important diamond stability field for kimberlites due to the effects of the East African rift system. The western boundary of this corridor is temporarily fixed and could change as more results are received.
Based on the known work that Tanex has carried out in this area, licenses have been chosen where the least exploration for kimberlites has been conducted. These are typically in areas of mbuga cover. Work on these licenses will start soon and will involve the following:
• Stream-sediment sampling using both conventional and hand augering sampling methods. A systematic auger-sampling program has also been planned over mbuga-covered areas.
• Ground follow up work over magnetic targets that are indicated on the map.
• Re-examining the known pipes within the Tanzanian Royalty Exploration Corp. licenses. Heavy mineral concentrates will be collected and submitted for heavy mineral collection and mineral chemistry studies.
SUMMARY
The Tanzanian Craton remains a highly prospective area with regard to kimberlite exploration largely because exploration has not been adequately carried out in areas with complex overburden. The use of hand augers as a sampling tool in addition to conventional loam sampling methods, combined with high-resolution ground magnetic surveys has proven to be most successful in locating kimberlites concealed beneath thick surficial sediments and mbuga cover. The recent acquisition by Tanzanian Royalty Exploration Corp.of their own RAB/RC drill rig will enable quick follow-up drilling of identified kimberlitic indicators and magnetic targets within the identified diamond corridors described in this brief summary.
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