Abstract This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.

Abstract Till is a favoured sample medium for locating mineral deposits in glaciated shield terrains of Canada and Fennoscandia because it best reflects the primary composition of the bedrock source area. In the sampling phase, an important and costly component of till surveys, sample density, sample depth and sample method must be chosen according to the needs of the exploration program. Surface till sampling methods in forested areas differ from those used in permafrost terrain. However, in both areas, concentrations of labile ore minerals and their products of decomposition can be detected in the fine fraction (< 2 mm) of weakly oxidized till. In thin drift-covered areas, till samples are collected by hand excavation or trenching at < 5 m depth. In areas of thicker drift, more expensive methods such as reverse circulation rotary drills, rotasonic drills and portable drills are used to collect till samples at depth and to determine lateral and vertical variations in till geochemistry. Laboratory methods are an essential part of till geochemical surveys. The choices of the size fraction and analytical methods are determined by the nature and composition of the expected bedrock target, and by costs.