Chemical composition of banded iron-formations of the Griqualand West Sequence, Northern Cape Province, South Africa, in comparison with other Precambrian iron formations
Mesobanded lithotypes (band rhythmites) of banded iron-formation (BIF) from the Griquatown and Kuruman Iron Formations of the Asbestos Hills Subgroup (Transvaal Supergroup, South Africa) have been sampled. 142 major and trace element analyses from diamond drill cores, an underground mine and an open pit mine were carried out. Out of these 19 open pit mine samples were excluded from the interpretation of the analytical results because of a different behaviour of the alkali elements. It is shown that other published data seem to suffer from the same effect. The results show that the chemical composition of the iron-formations is virtually independent of their stratigraphic and geographic localities over hundreds of kilometres across Griqualand West, if averaged element distribution patterns are compared. The major element composition of mesobanded iron-formation samples (magnetite chert and magnetite-carbonate chert, riebeckite-carbonate chert and ironsilicate chert) varies between 30 and 51 wt.% SiO2, 23 and 66 wt.% Fe2O3T, < 0.02 and 0.14 wt.% TiO2, < 0.04 and 1.9 wt.% Al2O3, < 0.02 and 1.05 wt.% MnO, 1.48 and 7.5 wt.% MgO, 0.14 and 12.09 wt.% CaO, 0.11 and 4.26 wt.% Na2O, < 0.007 and 2.39 wt.% K2O and < 0.01 and 1.57 wt.% P2O5. The trace element contents are generally low and range from < 1 to 2 ppm Nb (detected in only one sample), < 2 to 23 ppm Zr, < 3 to 31 ppm Y, < 2 to 152 ppm Sr, < 4 to 5 ppm U (detected in only one sample), < 2 to 240 ppm Rb, < 4 to 4 ppm Th, < 4 to 19 ppm Pb, < 3 to 4 ppm Ga, < 4 to 33 ppm Zn, < 6 to 69 ppm Cu, < 7 to 17 ppm Ni (detected in only one sample), < 5 to 59 ppm Cr, < 4 to 16 ppm V and < 10 to 177 ppm Ba. The intercalated stilpnomelane lutites have a very similar gross composition but regularly display higher concentrations of Al2O3, TiO2, K2O, and Zr. They have a different origin but certainly bear the imprint of the BIF environment and must be considered a clastic contaminant of the otherwise chemically or biochemically precipitated iron-formations. Clastic contamination and subsequent hydrothermal alteration are the most plausible agents which effected the element distribution pattern of the BIF, because they are best to reconcile with a model which assumes hydrothermal fluids as major sources of BIF. It can be concluded, from the general geochemical uniformity of BIF throughout the depository and from the absence of clear-cut relations of elemental distributions and ratios to crustal components (documented by low K/Rb, high Rb/Sr ratios, no relevant correlation between TiO2 and Al2O3), that a basinward hydrothermal system acted as a source for the Fe and Si in the BIF. The geochemical similarity between the microbanded Kuruman Iron Formation and the interbedded granular and microbanded Griquatown Iron Formation suggests deposition of the iron-formations in the same chemical environment over the entire basin from the inception of these conditions until their termination by clastic input. The somewhat lower trace element concentrations in the Griquatown Iron Formation as compared to the Kuruman Iron Formation probably do not mean an environmental change, but may reflect the diagenetic and tectonic evolution of the iron-formations. Regular but short-lived interruptions by distal volcanic ash had no influence on the bulk BIF composition. On the contrary, the volcanics reveal a strong overprint by the regional major element BIF chemistry. Therefore, the relationship between volcanogenic rocks and banded iron-formation seems to be coincidental and related to basin development. The general geochemical similarity of the Griquatown and Kuruman Iron Formations with other Proterozoic and Archaean iron-formations in the world and especially with those of the Hamersley Basin, leads us to the conclusion that the BIF of the Griqualand West Sequence are representatives of typical, large-scale iron-formations of the Precambrian, which all formed in chemically very similar environments. © 1995.