BASINAL FLUID EVOLUTION IN THE APPEKUNNY-GRINNELL FORMATION, 2.4 GA BELT SUPERGROUP

At least three post-depositional events, induced by basinal fluids, can be recognized from the conjunction of color, petrographic observations, and geochemical data in Middle Belt siltstones. Red coloration accompanied by K-addition, a green overprint of red siltstones, and dolomitization. Compositional changes involved in these three events are quantitative addressed using Gresens (1967) chemical mass balance. Volume factors (fv) for the three events are established from ratios of elements that are similar (Al, Ti, Lu, Ni), and therefore isochemical: respectively fv=1.03, 1.02 and 1.53. Compositional transformations are expressed as percentage changes; the discussion is confined to changes >10%.

Relative to average upper continental crust (UCC), weathering generated losses (-70 to -90%) of the major element oxides MnO, CaO, Na2O, and P2O5, and the trace element Sr. Losses of Nb, Ta, Zr, and Hf are due to hydraulic sorting. A post depositional potassic event, imparting red coloration involved large gains of K2O, Rb, Cs, MgO, and Li. Given their low solubility in river and seawater, additions of HREE >Th (U) ~ MREE >LREE >Y are also attributed to basinal fluid one. Ba and As are sparingly soluble under surface conditions. Accordingly, depletion of Ba, Mo, Cr, Cu, Pb and Cd, but additions of Sn, W, As, Ag, Sb and Ge likely also stemmed from fluid one.

Overprint of red by green coloration during the basinal fluid two event generated large additions of MnO, MgO, and CaO, Sr, Ba, Y, Zr, Hf, the REEs, Sm to Lu, and the transition metals Mo, Cr, Cu, and Ag. There were depletions of Fe2O3, K2O, Na2O, P2O5, Cs, Hf, Nb, Ta, Th, U, and the transition and semi-metals V, Zn, Sn, W, Pb, Cd, Tl, Ga, As, and Sb. Dolomitization of green siltstone enriched all major element oxides except Ti, Al, and Na. Distinctive chemical transformations are losses of Zr, Hf, Nb, Ta, Tl and U, but gains of Y and La to Ho. Conventionally, fluid movement through the Belt Supergroup has only been studied in the context of mineral deposits. However, fluid advection in the unmineralized Middle Belt is pervasive. Given REE, Zr and Nb mobility, basinal fluids may have been alkaline and oxidizing.

Gresens, R.L., 1967, Composition-volume relationships of metasomatism: Chemical Geology, v. 2, p. 47-55.