Composition and genesis of silcretes and silcrete skins from the Beda Valley, southern Arcoona plateau, South Australia

Abstract

Detailed petrographic and chemical studies of silcretes collected from Beda Valley near the southern extremity of Lake Torrens, South Australia, have shown that there are at least two distinct types. One type, often found attached to quartzite, has very angular grains of quartz in a matrix very high in titanium. Electron probe studies clearly show that the areas between the quartz grains are mostly titania containing very little silica. These skins, besides containing 2-10% Ti, contain 0.05 to 0.25% Zr with a very high correlation between the ratio of the amount of these elements in the skin to that in the quartzite and it is suggested they have been produced by the loss of silica. The other type of silcrete has subrounded grains of quartz in a matrix of chalcedonic silica. It is also low in titanium (<1%) and low in zirconium (<.04%). It is suggested this type, which in this area is massive with columnar structure, is formed by the addition of silica.