Composition of glacial sediments in Canadian Shield terrane, southeastern Ontario and southwestern Quebec: applications to acid rain research and mineral exploration

Abstract

Glacial sediments that overlie Precambrian rocks belonging to the Central Metasedimentary and the Central Gneiss belts in southeastern Ontario and southwestern Quebec receive some of the most acid precipitation in North America. Sensitivity of drift to changes induced by acid loading depends on at least three important aspects of composition: 1) carbonate minerals, 2) clay-sized detritus, and 3) trace elements. The first two provide buffering capacity, and the last may be displaced into vegetation, groundwater, and surface water as a result of exchange or hydrolysis reactions with hydrogen ions. In the eastern part of the study area, significant buffering capacity is provided by carbonate-enriched trains of fine-grained drift that extend southwestward in Ontario across Precambrian bedrock composed of both calcareous and noncalcareous lithologies. Part of the carbonate and clay-sized detritus was derived from Paleozoic limestone or dolomite outcrops underlying the Ottawa and St. Lawrence valleys. In contrast, relatively modest quantities of carbonate are found in drift overlying Precambrian marble in the study area farther west in Ontario and in Quebec. The small influence of Precambrian marble on glacial sediment composition over a wide area can be attributed to physical characteristics of these strata. Hard, massive, coarse-grained Precambrian marbles were less susceptible to processes of glacial erosion and comminution than the thinly bedded, fine-grained Paleozoic lithologies. In most cases, trace element concentrations in drift are closely related to the composition of the underlying bedrock and may vary widely over short distances because of the complex geology of the Canadian Shield. High concentrations of arsenic, mercury, copper, and silver, for example, occur in anomalous concentrations in drift overlying outcrops of metasedimentary and metavolcanic rocks in Ontario known to host arsenical gold deposits.