Kimberlite Exploration Research Articles

Application of a modified GEOTEM® system to reconnaissance exploration for kimberlites in the Point Lake area, NWT, Canada

Airborne geophysical surveying with electromagnetic (EM) and magnetic methods is an effective reconnaissance exploration tool for kimberlite pipes because the target can have an associated EM and magnetic anomaly. The EM response of kimberlite pipes is most often attributed to weathering alteration in a near‐surface layer, whereas the magnetic response is attributed to magnetite and ilmenite within the deeper unweathered kimberlite pipe. The discrete shape of kimberlite diatremes results in an easily identifiable anomaly pattern. Diamondiferous kimberlites have recently been found in the Northwest Territories (NWT) of Canada, an area glaciated in the Pleistocene and therefore devoid of a strongly weathered zone. By configuring the GEOTEM® airborne EM system to operate at high frequencies (270 Hz) and to take measurements while the transmitter is switched on, weakly conductive bodies may be detected because there is an adequate contrast with the surrounding highly resistive country rock. System modifications also allow the magnetic field to be sampled at an altitude of only 73 m instead of 120 m and ten times per second instead of once a second. This allows much better definition of weak, small magnetic anomalies. Data sets from two test areas (Point Lake and Willy Nilly, near Lac de Gras, NWT) demonstrate the effectiveness of the airborne system for reconnaissance surveying.

Kimberlite xenocryst wear in high-energy fluvial systems: experimental studies

Xenocryst minerals from kimberlite have been mixed with gravels of varied grain size and artificially worn in a rotating drum. Results indicate that the minerals are transported with less wear when the finer-grained proportion of the gravel is increased. This is an important consideration when evaluating the natural dispersion of these minerals from their kimberlite host, and can aid in prioritizing stream-sediment anomalies in kimberlite exploration.

Kimberlite exploration in Tanzania

In 1979 an aeromagnetic survey covering approximately 9 500 sq kms was conducted in the Shinyang Region of Tanzania. Ground follow-up work in the Mwadui Diamond field (1 700 sq km) showed about 100 magnetic anomalies (interpreted by Geosurvey International Ltd) that were possible targets for kimberlites. Twelve of these anomalies coincided with previously discovered kimberlites and fifteen of them were proved to be new kimberlite pipes. Bulk tests showed that four of the discovered kimberlites were diamondiferous but none proved to be economic. The remaining plug-like anomalies were either dolerites, diorites or gabbros. Orientation surveys were done to test ground geophysical techniques namely Magnetometry, Pulse EM, Induced Polarisation and Resistivity over known pipes. Helicopter EM33/2 survey was also tested in an area of 100 sq. kms occuring some 25 km SE of Mwadui Mine. It was found that a combination of Heavy Mineral (HM) surveys and Electromagnetic (EM) surveys were important in locating non-magnetic kimberlite pipes that are filled with epiclastic material. The magnetic method however, has a tendency to give many plug-like anomalies that may not necessarily be kimberlites. A geochemical orientation survey showed that Nb and Ni are most promising indicators of kimberlites in the metasedimentary and granitic environment of the Shinyanga Region.

Exploration for kimberlites in the southwest Goiás region, Brazil: Mineral chemistry of stream sediment samples

Ilmenite and garnet analyses are presented for stream sediment heavy mineral concentrate samples collected during aeromagnetic anomaly follow-up for kimberlites in the southwest Goiás region, Brazil. Based on microprobe analyses of over 200 mineral grains, the samples can be divided into five general categories: (1) those associated with an aeromagnetic anomaly overlying an ultramafic breccia which is considered a possible kimberlite; (2) anomalous ilmenite and garnet analyses associated with a second aeromagnetic anomaly where no specific rock type was identified; (3) high-Mg/low-Cr ilmenite analyses from a highly weathered ultramafic breccia; (4) pyropic garnets (±Cr) from the south of the study area; and (5) background high-Mg/low-Cr ilmenites and various garnet compositions from within the study area. The ubiquitous occurrence of high-Mg/low-Cr ilmenites is considered to be due to numerous unmapped alkaline-ultramafic breccias/intrusions in the southwest Goiás region. Chemical analyses of anomalous high-Mg/high-Cr ilmenites and Cr-pyropic garnets from one weathered breccia suggest a probable kimberlite diatreme with an associated later-stage, shallower-level, olivine melilitite intrusion. A review of the mineral chemical signature of associated Cr-pyrope garnets implies that the kimberlite is unlikely to be an economically viable diamond source.