Mineralogical Magazine, February 2006, Vol. 70(1), pp. 51±64
Detrital chrome-spinel grains in heavy-mineral sand deposits from southeast Africa M. POWNCEBY*
AND
P. BOURNE
CSIRO Minerals, Box 312, Clayton South, VIC 3169, Australia
ABS TR AC T
Detrital chrome-spinels are contaminant grains within ilmenite concentrates produced from heavymineral deposits along the coast of southeast Africa. The presence of even minor levels of chromia in the predominantly ilmenite-rich concentrates, downgrades their market value as potential feedstocks for the production of titania pigment. An understanding of their composition can assist in their removal from the ilmenite concentrates. Compositions from a database of close to 900 chrome-spinel analyses shows the major element components and their ranges (in wt.%) are: Cr: 0.4ÿ45.3, Al: 0.0ÿ31.0, Fe: 8.5ÿ69.6 and Mg: 0.0ÿ12.2. Minor components include Ti: 0.1ÿ11.4 and Zn: 0.0ÿ13.7. The chrome-spinel data fall into two compositionally distinct groups. The first group of spinels is dominated by a strong trend reflecting the mutual substitution between Al3+ and Cr3+ in the spinel structure. The second group of spinels is characterized by compositions containing abundant Fe 3O4 magnetite component. The clear division between chrome-spinel compositional types indicates the grains are derived from at least two chemically dissimilar provenances. The compositional differences between the chrome-spinel groups has a positive impact on subsequent ilmenite upgrading treatments as the spinels which contain the highest magnetite component are easily removed via low-intensity magnetic separation procedures.
K EY WORDS :
chrome-spinel, southeast Africa, ilmenite concentrates.
Introduction
THE southeast coast of Africa has been a commercially important source of Ti minerals since widespread aeolian dune placer deposits ®rst came to prominence in the early- to mid-1970s (Fockema, 1986; Force, 1991; Wipplinger et al., 1999). Since then, a large number of economic to sub-economic deposits has been located along the coastline stretching from the vicinity of East London (South Africa) in the south to Mombasa (Kenya) in the north (Fig. 1). At present, all Timineral production in southeastern Africa comes from the large Quaternary beach sand deposits currently being mined along the Kwazulu-Natal coast of South Africa at Richards Bay and Hillendale. Recently however, exploration focus * E-mail:
[email protected] DOI: 10.1180/0026461067010312
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2006 The Mineralogical Society
has moved further north along the coast into Mozambique, Tanzania and Kenya where additional large reserves of ilmenite and associated heavy minerals, rutile and zircon, have been identi®ed. Many of these deposits such as the Corridor Sands project in southern Mozambique and the Kwale project in Kenya are in the process of detailed planning and feasibility studies with the aim of coming on-line as commercial producers within the next 2ÿ5 y. The heavy-mineral sand deposits of southeast Africa are characterized by assemblages dominated by ilmenite (70ÿ90%) but with minor, variable amounts of leucoxene, rutile and zircon. Typical gangue minerals associated with the deposits include magnetite (an Fe-rich spinel), silicates (garnet, pyroxene, andalusite) and chrome-bearing spinels (Hammerbeck, 1992). Of the range of gangue minerals, the presence of chrome-bearing spinels (typically at levels of
M. POWNCEBY AND P. BOURNE
FIG. 1. Location map showing the extent of mineral sands occurrences along the southeast coast of Africa (shaded area), together with sample localities for the ilmenite concentrates examined in this study.
20 wt.%) and high-Cr, low-Al (Cr >35ÿ43 wt.%; Al
