Archaean tholeiite

Arndt et al. (1997) compared the compositions of Archaean tholeiites with those of modern basalts from ocean-ridge, ocean-island, and arc environments (Fig. 3.22). They showed that Archaean tholeiites have higher SiOa and FeO, and lower incompatible trace element concentrations (although enriched in Rb and Ba relative to modern MORB), compared to their modern equivalents. Many of these geochemical features of Archaean basalts match those of modern arc basalts, but there are also important differences. This led Arndt et al. (1997) to the important conclusion that "the major- and trace-element characteristics of Archaean basalts are matched by no common type of modern basalt." The uniqueness of Archaean tholeiitic basalts requires that they are either from a source different from that of modern tholeiites, or are the product of a different melting process. Arndt et al. (1997) interpreted the low incompatible trace element concentrations and high Si, Fe, Ni,... 

FIGURE 3.22 The trace element composition of Archaean tholeiites compared to modern enriched-MORB and normal-MORB, normalized to the primitive mantle (after Arndt et al.( 1997). 

Figure 4.20 Rare eardt element abundances nonnaUzed to chondridc meteorite values plotted against atomic number for an Archaean tholeiitic basalt (sample 11 — RoUinson, 1983). The same sample has been normalized using five different sets of values the numbers refer to the columns of normalizing values listed in Table 4.5.

Aftabi and Azzaria (1983) studied the distribution of Hg in ore and host rocks from the Sigma Au Mine, Val D Or, Quebec. The deposit occurs in deformed Archaean metavolcanic rocks of ultrabasic and tholeiitic composition which are intmded by diorite porphyries and undeformed porphyry dykes. Gold mineralisation occurs in sub-vertical and sub-horizontal veins associated with chloritisation, silicification, albitisation, sericitisation and minor carbonatisation. Background Hg contents in the metavolcanic host rocks averaged about 5 ppb compared with 80 ppb in ore veins and rocks associated with ore. However, the distribution of Hg was found not to be wider than that of Au, Ag, As, Zn or Cu in the various host rocks (Fig. 12-6). 

---