ONTARIO GEOLOGICAL SURVEY TILL SAMPLING: PAST SUCCESS AND FUTURE WORK FOR CRITICAL MINERAL EXPLORATION

Over the past 40 years till sampling programs completed by the Ontario Geological Survey (OGS) have aided in mineral exploration for a variety of different deposits using indicator minerals and till matrix geochemistry. For example, the dispersal halo of the Ring of Fire chromite deposit, located in a remote part of Northern Ontario, was detected by one of these campaigns. Other till sampling programs originally targeted at specific deposit types, like gold, can now be reassessed for their critical mineral potential by reanalyzing the archived samples. The OGS has initiated a multi-year project to analyze the fine fraction (<0.25 mm) non-magnetic heavy mineral concentrate of these legacy indicator mineral samples from across Ontario for modal mineralogy using SEM energy dispersive X-ray spectroscopy. The goal of this project is to create an indicator mineral dataset that details the regional potential for critical mineral deposits across the province. The first samples reanalyzed were from the 1988 Rainy River gold exploration project in northwestern Ontario. This area is close to many prospective granites and pegmatites, posing and ideal location to test the reanalysis methods. To further aid critical mineral exploration in Ontario, a regional (10 km spacing) till sampling program was initiated during the summer of 2023 around the Georgia Lake Pegmatite Field near Lake Nipigon. The region was selected because the advanced lithium prospects are well studied, allowing the known ore minerals and whole rock geochemistry of the pegmatites to be related to the indicator minerals and matrix geochemistry observed in till. The goal of the project is to define the surficial response of lithium bearing pegmatites in till so that till sampling can be used in other regions to find lithium deposits. Altogether, till sampling completed by the OGS has proven successful for critical mineral exploration and ongoing projects continue to define the regional prospectivity of Ontario for critical mineral deposits.