Mineral chemistry, hyperspectral analysis and applications for regional exploration at the Hemlo Gold deposit, Ontario, Canada

Abstract

The Barrick Gold owned Hemlo Au deposit is a world class Archean Au deposit situated in Northern Ontario, Canada with historic production of >21 Moz of Au over 32 years of continuous operation. The deposit has a strike length of ~3 km with a well-documented alteration footprint surrounding mineralization. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of epidote, chlorite and pyrite from within and surrounding the Hemlo gold deposit have identified major and trace element variations in mineral chemistry that allow for the discrimination of deposit-proximal and deposit-distal signatures. Short wave infrared (SWIR) analysis of epidote and chlorite from around the Hemlo deposit can discriminate between end-member compositions of epidote and chlorite and is a useful tool for interpreting mineral paragenesis. Epidote varies in chemistry surrounding the deposit with the highest concentrations of As and Sb proximal to the mineralized zones and can be detected at anomalous values up to 1.5 km further than the documented deposit footprint. Using LA-ICP-MS, As and Sb can be detected at significantly higher concentrations (more than an order of magnitude) within epidote than in traditional whole rock geochemistry, allowing for better targeting of the deposit footprint. Chlorite also displayed variation in trace elements with deposit-proximal chlorite displaying exponentially higher Ti/Sr and V/Co values than deposit-distal and intrusion-related chlorite. The Ti/Sr ratio for chlorite expanded the geochemical footprint of the Hemlo deposit by up to 1 km. Pyrite displayed anomalous enrichments in a number of elements, including Au, Ag, As, Sb, Bi, Te, Mo and W, that could be detected at elevated concentrations proximal to the deposit. Gold, Te and As proved to be the most effective pathfinder elements in pyrite as they were detected at anomalous concentrations up to 2.5 km from the deposit and were detectable in pyrite at much greater concentrations relative to whole rock geochemistry. Several syn-to post-tectonic intrusions that surround the deposit were evaluated using epidote and chlorite chemistry for their potential to create false positive anomalies for mineralization. The distal intrusions of interest displayed no distinctive variation in epidote or chlorite chemistry and did not display false positive anomalies. Intrusion related epidote contain consistently low As and Sb levels relative to deposit epidote was also displays elevated Fe/Al values. Intrusion related chlorite displayed low Ti/Sr and V/Co values relative to the deposit chlorite and was also found to be more enriched in Fe relative to deposit-proximal chlorite.