Mineralogical and geochemical characteristics of emeralds from the Leydsdorp area, South Africa

Abstract

The world’s oldest emerald deposit occurs in the Gravelotte-Leydsdorp area of the Limpopo Province of South Africa. Emeralds have been mined from the main emerald working, the Cobra pit, since 1927, although emeralds were discovered in the area much earlier. The emerald mineralisation is closely linked with albitite pegmatites, with the emeralds confined almost entirely to metasomatic zones at the margins of strongly deformed pegmatoid bodies in biotite, talc and actinolite schists. While the surrounding Murchison greenstone belt has been extensively studied, very little geochemical information is available for the emeralds in the deposit. This paper documents the geochemical characteristics of 57 low-quality emeralds from the Leydsdorp area, Murchison Greenstone Belt. The emeralds range in size from 3 to 12 mm and were studied using both visual examination and electron microprobe analysis with 960 microprobe analyses on 54 specimens. The emeralds in the present study are subhedral to euhedral with a well-formed prismatic habit, and range in colour from light green to a dark green or bluish green. A number of samples are characterised by colour zoning, typically comprising of a green to dark green transition from core to rim, while small colourless areas within crystals are also seen. Many of the emeralds examined are practically opaque owing to the presence of very abundant mineral (and fluid) inclusions. Numerous small cracks and veins are also present. A wide variety of inclusion species are encountered, comprising dominant biotite, apatite and quartz, as well as a host of other minerals including hematite, pyrite, pyrrhotite, zircon, molybdenite and galena. SiO 2 content varies between 64 and 67 wt%, Al 2 O 3 range from 13 to 17 wt%, MgO concentrations vary between 0.08 and 3.46 wt% and Na 2 O contents range from 0.76 to 2.33 wt%. The Cr content of Leydsdorp emeralds range up to 1.07 wt% Cr 2 O 3 , with an average of 0.23 wt%. Samples are often heterogeneous with a maximum variation of 0.17 to 0.9 wt% Cr 2 O 3 observed in one specimen. Darker green crystals and dark green patches in individual crystals have significantly higher Cr contents compared to light green crystals and crystal sections. An inverse relationship between Al and the amount of Cr, Mg, Na and Cs is seen, while substitution of Al by Fe is also evident. The major and minor element chemistry of the emeralds are broadly similar to other emerald deposits worldwide. The major chromophore appears to be Cr as opposed to V. Na contents are somewhat higher than the world-wide average and this, along with the inclusion suite, could serve to distinguish these emeralds from other emerald deposits.