Systematic evaluation of bulk cyanide leach (BCL) parameters affecting extractability of metals from soils

Abstract

ABSTRACT Partial extraction techniques for analysis of soil and sediment samples have become widely used in geochemical exploration. One such technique is bulk cyanide leach (BCL) which is used in gold exploration and, along with Au, also extracts other elements such as As, Co, Cu, Mn, Mo, Ni and Zn, which are known pathfinders of Au and base metal mineralization. As with all partial extractions, different extraction conditions are likely to affect the recovery of elements obtained by a given BCL protocol. Some of these conditions, such as pH, may be driven by soil properties and thus jeopardize correct interpretation of BCL data; however, such effects have not yet been quantified. We therefore systematically altered key extraction parameters (soil mass, soil: solution ratio, time, degree of agitation, and pH) and measured the effects on BCL extraction of Au, As, Co, Cu, Mn, Mo, Ni and Zn on a single homogeneous soil from mineralized terrain. In addition, the potential effects of soil organic carbon on BCL extraction efficiency was evaluated using a soil amended with different amounts of natural charcoal. Sequential selective extractions were conducted on the soil before and after BCL extraction to better understand the extraction of these elements under BCL conditions. Gold recovery by BCL was found to be insensitive to most differences in extraction conditions, within the solution parameters typically expected in BCL protocols. Charcoal addition to soil, however, significantly reduced gold extractability. Extraction time and pH had the greatest effect on the extraction of all other elements. Increasing pH from 9.8 to 10.4 increased arsenic concentrations by 62%, but decreased Mn and Mo extraction by c . 50%. The extraction of Au, As and Mo was rapid with c . 60% being dissolved within an hour, but Co was only 20% extracted (compared with a 48-hour extraction). The erratic concentrations of Cu, Mn and occasionally Zn suggest their use as pathfinders for mineralization using BCL extracts may be unadvisable. Preliminary metal fractionation results suggest that Au is quantitatively extracted by BCL from a pool not accessible to the other selective extractants used; other elements are removed by BCL from exchangeable/adsorbed or amorphous oxide fractions. This work emphasizes the importance of controlling extraction conditions for bulk cyanide leach assays. The occurrence of charcoal in soils is common even below surface horizons, so that care must be taken in interpretation of BCL gold assays where charcoal is likely to be present.