Kimberlite indicator minerals

Abstract

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This study deals with macrocrystic indicator minerals in the Gibeon Kimberlite Province, southern Namibia. The aim of the work is to study the composition, character and distribution of these heavy minerals from kimberlite pipes and dykes as well as from fluvial sediments of the streams draining the Gibeon area

Samples for this heavy mineral study were taken from the weathered surfaces of 15 kimberlite pipes from nine kimberlite suites (Anis Kubub, Berseba Reserve, Diamantkop, Hinaus 1, Mukorob, Lichtenfels E and Lichtenfels W, Louwrensia and Achterfontein). Additionally, 37 drainage samples were taken, mainly from various trap sites where maximum accumulation of heavy minerals was anticipated. Original samples, approximately 50 kg in weight, were split in half, dry-screened to obtain a 2 mm fraction, and then concentrated. Indicator minerals were hand-picked from 0. 5-1. 0 mm and 1. 0-2. 0 mm fractions of the heavy mineral concentrate. To study the chemical composition of the minerals, a representative number of each indicator mineral type was analysed with an electron microprobe

Ilmenite tends to be the most abundant indicator mineral, comprising generally more than 50 percent of the heavy mineral concentrate, but nevertheless varies considerably in content (from 1 percent at Anis Kubub to 87 percent at Hinaus 1). Garnet shows a similar variation, making up as much as 95 percent at Anis Kubub, but generally representing less than 46 percent. Chrome diopside proportion is generally less than 2 percent, with a maximum of 30 percent at Berseba Reserve. Chromite is relatively sparse, generally representing 1 percent of the mineral separates. Ilmenite, garnet and chrome diopside grains mostly occur as angular fragments and more rarely as whole grains. The 0. 5-1 mm fraction has generally more purple garnets, chromites, and chromium diopside than the 1 -2 mm fraction

Garnet population is dominated by the lherzolite paragenesis but there are also subordinate wehrlite and harzburgite + dunite parageneses. At Mukorob and Hinaus 1, kimberlite pipes contain garnet with 5 wt percent Cr:03, whereas other localities have higher Cr garnets with up to 9 wt percent Cr2O3. However, in all the localities garnet from the drainage sediments are generally enriched in Cr2O3 relative to garnets in the pipes. A significant compositional variation occurs within the GKP ilmenites. Ilmenites from Berseba Reserve are enriched Cr, O, and those from Lichtenfels E are enriched in MgO. Ilmenite (picroilmenite) from the drainage sediments tend to be enriched in Cr2O3, TiQ and MgO compared to those obtained from the pipe samples. Cr-diopsides show a variable compositions but are mainly derived from garnet peridotite sources. The macrocryst chromite population is characterized by Ti-poor/Al-rich and Ti and FeO-rich/Al-poor types. The former is the most redundant type in the studied localities

The appearance of less durable garnet and Cr-diopside, altered ilmenite, kelyphitic garnet, and ilmenite or chromite + garnet and/or gamet + Cr-diopside intergrowths in the downstream sediments usually indicate an influx from an additional source and/or an increase in fluvial energy. In some localities the chemical composition of the indicator minerals in the downstream occurrences is clearly different from that of the sampled source

In the exploration of kimberlite, some of the most important things to consider in the interpretation of the data, are the selection of the sampling site, physical features and composition of the indicator minerals, especially the pyrope garnet. However, the mechanical weathering of the indicator minerals in the stream sediment make it hard to deduce whether the observed differences in chemical composition is due to the influx from an additional source or weathering of the minerals