On the geochemistry and origin of the D tree, Wonarah, and Sherrin creek phosphorite deposits of the Georgina Basin, Northern Australia

The early Middle Cambrian phosphorites of the Georgina Basin were deposited in shallow nearshore marine environments varying from lagoonal, estuarine, littoral, to intertidal. Some deposits show penecontemporaneous erosion by stream channels in addition to subaerial weathering during periods of regression. There are three distinct types of phosphorite: mudstone phosphorite, replacement phosphorite, and pelletal phosphorite, the latter two were formed by diagenetic phosphatization of carbonate skeletal sands, bioclastic and micritic limestones, and dolomites. By comparison, the mudstone phosphorite, which predominates in the three deposits studied, shows little textural evidence of such an origin and has been accepted as an orthochemical sediment. However, this paper proposes a diagenetic origin identical to the other types. Some fifteen known deposits occur over a distance of about 1,000 km along the periphery and insular portions of the basin. The geochemistry of two of these deposits, D Tree and Sherrin Creek, which lie 75 km apart on the eastern edge of the basin, has been studied and compared with that of the Wonarah deposit, which lies within the basin 200 km to the west. Fifty-eight analyses reveal four element associations: apatite group (Ca, P, F), clay group (Al, K, Ti, Cr, Zr), heavy minerals group (Fe, Mn, Zn, Ni, Cu), and a leached group. Although the elements Na, S, CO ₂, Y, La, Ce, Sr, U, and Th correlate with apatite in the little-leached Sherrin Creek deposit, they exist as a separate leached group, or correlate with clays, in the more leached D Tree and Wonarah samples. Ba, V, Cu, and Pb do not behave to a consistent trend. The strongest correlations in the leached deposits are at a lower level of significance than at D Tree. Intense Cenozoic weathering extends beyond the maximum known depth of phosphorite. It, together with certain though unquantifiable Cambrian subaerial weathering effects, has been responsible for leaching, increasing in the order Sherrin Creek, D Tree, and Wonarah. Uranium, Th, La, Ce, Y, Ba, Sr, Na ₂O. SO ₃, and CO ₂ are leached from the system to the extent of 45 to 80 weight percent. Comparison of the composition of leached and unleached phosphorites suggests there were discernible differences in the geochemistry of waters in the different embayments at the time the phosphorites were formed.