Geochemical - mineralogical profiles through fresh and weathered till

To use glacial sediments effectively in mineral exploration, it is necessary to understand the causes of vertical compositional variability, whether they are sedimentologic, stratigraphic, or diagenetic. Many useful sedimentologic models are recorded in recent literature, particularly for till and related diamictons, and the importance of understanding contrasting compositions of different stratigraphic units is well recognized, if not always adequately dealt with in mineral exploration literature. Diagenetic alterations of drift, on the other hand, have received scant attention in the drift geochemical literature, except for discussions of the mineralogical and chemical alterations that take place in the true solum. The primary objective of this paper is to document sub-sol um diagenetic alteration of a thick till sheet, which contains abundant concentrations oflabile minerals, some of which have economic significance. The alteration described takes place in the zone of oxidation, above the ground water table. Alteration is particularly easy to document in the many well-exposed till sections of the Appalachian region of southeastern Quebec; colour contrasts and chemical changes in various size and specific gravity fractions of till attest to the effects of post depositional weathering. A secondary objective is to describe mineralogical and chemical partitioning among various grain sizes in till. Mineral phases most susceptible to weathering, carbonates and sulphides, are abundant in Appalachian tills and have long been known to concentrate in specific size fractions because of their varying resistance to glacial abrasion (Dreimanis and Vagners, 1971; Shilts, 1973, 1984). To study mineralogical partitioning of carbonates and sulphides in Quebec till, several samples were divided into six size fractions from 6 mm to particles finer than 2 μm. Detennining the size ranges within which sulphide and carbonate minerals are most highly concentrated allows selection of the best laboratory methods for effective routine analysis of weathered and unweathered samples. A third objective is to compare carbonate contents of the various grain sizes using three different laboratory procedures the Chittick method (Dreimanis, 1962), Leco Carbon Analyzer (Foscolos and Barefoot, 1970), and total dissolution. Glacial deposits are frequently analyzed for carbonate content using one or more of these techniques. Results