Geological predictors for pre-concentration

The mining industry is seeking techniques to optimise ore processing and decrease waste during mining operations. Pre-concentration via screening is a coarse separation technique that can be used to remove uneconomic material prior to cost-intensive grinding, which uses large amounts of energy and water. This technique is based on grade by size deportment, i.e., the tendency for specific mineral phases to naturally fractionate into specific size fractions during breakage of rocks. The magnitude of metal deportment is described through a Response Ranking parameter, i.e., a specific methodology developed by the Cooperative Research Centre for Optimising Resource Extraction, that allows comparison between samples and between deposits.

This project evaluates the influence of geological factors including mineralogy, texture, paragenesis, hardness, geochemistry, magnetic susceptibility, which may influence rock breakage and natural fractionation at a range of scales. Two mineralisation styles in different rock types were studied to understand the relationships between geological features and the propensity of metals to fractionate into certain size fractions.

The first case study is an intrusive related gold deposit hosted in sedimentary rocks. High upgrade potential was shown in coarse grained hard sandstones, with high quartz content and with silica or carbonate cement. Low upgrade potential was shown in softer fine-grained sandstones with siltstone intercalations. These softer rocks had lower quartz content and high clay and white mica content. The mineralisation style also influenced the upgrade potential, samples with high vein density and low content of disseminated mineralisation are more amenable to pre-concentration via screening.

The second deposit studied is an epithermal Zn-Pb-Ag deposit with rhyodacites and volcanic breccia as host rock. In this deposit, hydrothermal alteration played an important role in the upgrade potential. Ores with strong phyllic alteration, characterised by high content of quartz, white mica, and pyrite showed high upgrade potential. Ores with chlorite propylitic alteration and least altered rocks showed moderate to low upgrade potential. Ores with sphalerite and galena veins, and fractures with sphalerite-galena-pyrite filling showed high amenability to pre-concentration via screening. In addition, ores with disseminated sphalerite and galena hosted within hard minerals as quartz and K-feldspar showed high upgrade potential.

Overall, upgrade potential is determined by relationships between the mineralogy and hardness of the host rock plus mineralisation style, e.g., disseminated, in veins, or locked in other minerals. Mineralisation associated with structures such as veins, fractures, bedding, or foliation, often tends to fracture following these pre-existing structures during breakage, exposing the mineralized surfaces during crushing resulting in deportment of the metal into the fine faction. Ores with a disseminated mineralisation can be amenable to pre-concentrate via screening if there is contrast in hardness between the ore minerals and the host rock.

The techniques used in this study could be replicated in core sheds at mine sites, since many of the proxies come from visual logging and portable equipment. The lithological and textural proxies related to upgrade potential have applications for geometallurgical domaining and modelling, possibly impacting resource/reserve definition, mine planning, site scheduling, and material handling on site. Dedicated processing circuits can be used for the material amenable for upgrade. Successful implementation of pre-concentration can reject coarse waste material via screening during an early stage of comminution having the potential of reducing energy consumption at fine grinding stages, leading to less rock being processed, optimising costs, and increasing throughput (metal units). By using pre-concentration via screening potentially it is possible to extract the fraction of material above the cut-off that previously was classified as uneconomic.