Selenium Distribution in the Gossan of a Porphyry Copper Deposit, Red Chris Mine, British Columbia, Canada

Abstract

Abstract In 2015, the Red Chris mine in northwestern British Columbia began processing near-surface ores from the Red Chris copper-gold deposit and encountered higher leachable selenium concentrations in process water than had been anticipated from testing of deeper ores. Samples of gossan, near-surface rock, and deeper hypogene rock were submitted for analytical, mineralogical, and kinetic geochemical testing to investigate the origin of the leachable selenium, its mineralogical form, and the release mechanisms. This paper presents testing results and implications for understanding the selenium distribution in weathered rocks at sulfide mineral deposits. Shallow weathered rock, including the gossan, contained higher selenium concentrations and higher ratios of selenium to sulfur relative to the hypogene bedrock. Laboratory weathering and leaching tests indicated that selenium is released from source sulfides by oxidation and retained in a variety of secondary mineral forms. Mean selenium content in the solids was measured as follows: gossan and shallow bedrock >> deeper near-surface bedrock > hypogene bedrock. Water-leachable selenium varied but generally followed a similar trend, with samples of gossan containing the highest water-leachable selenium. Selenium was determined to be preferentially leached from the gossan samples at high pH values, indicating an adsorbed form. We propose that the weathering profile of the Red Chris deposit was previously capped with a well-developed thick gossan, which formed during the Paleogene and was eroded during the last period of glaciation. The current weathered rock reflects the residual base of the former oxidized profile.