CHEMICAL-COMPOSITION OF BANDED IRON-FORMATIONS OF THE GRIQUALAND WESTSEQUENCE, 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 undergro und mine and an open pit mine were carried out. Out of these 19 open p it mine samples were excluded from the interpretation of the analytica l 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 shaw that the chemical composition of the iron-formation s is virtually independent of their stratigraphic and geographic local ities over hundreds of kilometres across Griqualand West, if averaged element distribution patterns are compared. The major element composit ion of mesobanded iron-formation samples (magnetite chert and magnetit e-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.% P2O(5). The trace elem ent 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 S r, <4 to 5 ppm U (detected in only one sample), <2 to 240 ppm Rb, <4 t o 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 C r, <4 to 16 ppm V and <10 to 177 ppm Ba. The intercalated stilpnomelan e lutites have a very similar gross composition but regularly display higher concentrations of Al2O3, TiO2, K2O, and Zr. They have a differe nt origin but certainly bear the imprint of the BIF environment and mu st be considered a elastic 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 ar e best to reconcile with a model which assumes hydrothermal fluids as major sources of BIF. It can be concluded, from the general geochemica l uniformity of BIF throughout the depository and from the absence of clear-cut relations of elemental distributions and ratios to crustal c omponents (documented by low K/Rb, high Rbl Sr ratios, no relevant cor relation between TiO2 and Al2O3), that a basinward hydrothermal system acted as a source for the Fe and Si in the BIF. The geochemical simil arity between the microbanded Kuruman Iron Formation and the interbedd ed granular and microbanded Griquatown Iron Formation suggests deposit ion of the iron-formations in the same chemical environment over the e ntire basin from the inception of these conditions until their termina tion by elastic input. The somewhat lower trace element concentrations in the Griquatown Iron Formation as compared to the Kuruman Iron Form ation probably do not mean an environmental change, but may reflect th e diagenetic and tectonic evolution of the iron-formations. Regular bu t short-lived interruptions by distal volcanic ash had no influence on the bulk BIF composition. On the contrary, the volcanics reveal a str ong 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 e specially with those of the Hamersley Basin, leads us to the conclusio n that the BIF of the Griqualand West Sequence are representatives of typical, large-scale iron-formations of the Precambrian, which all for med in chemically very similar environments.