Over the years many differing approaches have been developed to clearly understand the nature, mode of occurrence, associations and deportment of both free-milling and refractory gold in a wide variety of differing geologic environments and plant products. Many approaches can be taken to characterize the ore which may occur as complex fine grained gold, ultra-fine grained gold, coarse nuggety gold and complex As-rich refractory gold. The approach taken will be dictated by the nature of the ore or plant product, the amount of sample material available, the gold grade and the grain size distribution. In many cases the scope of the study will be limited by sample availability but is none the less possible to generate a significant amount of information. In a typical case a thorough gold characterization study would include the following elements:
* Multiple head assays
To establish the grade, homogeneity, variability, and nugget effect of coarse gold.
* Size fraction assays
Is the gold preferentially concentrated in a particular size class(es)?
* Ore microscopy / petrography of coherent ore slices or fragments
Textural and mineralogical relationships, mode of occurrence and associations of visible gold (>0.5µm in size), microbeam analyses of metallic Au and other Au minerals.
* Preparation of heavy mineral concentrates from sized fractions (screening) of crushed or milled head.
This depends on the sample size where the use of heavy liquids, superpans, tables and other concentrators are utilized to concentrate heavy particles for examination of fractions. Specific refractory gold host minerals will be concentrated by these and other appropriate methods for assay and systematic examinations.
* Systematic scans of the heavy mineral concentrate, middling, and tailing fractions.
This will detect all Au particles >0.5µm in size and combined with SEM-EDS analysis provides semi-quantitative Au & Ag evaluation. The exposed surface area of the grains will be measured and then be converted to equivalent diameter and then weight, along with detailing the mineral association to provide liberation statistics. Finally the particles can be documented by both photomicrographs and imaged using backscatter electron imaging on the SEM to detail texture and associations that effect recovery.
Fine native gold (Au) filling fine fractures in a composite particle of arsenopyrite (aspy) with coarser tetrahedrite (td), galena (gn) and non-opaque minerals (nop).
* Diagnostic Leach Tests
This is designed to statistically determine the association and distribution of gold within the various gravity products to determine the leachable and non-leachable gold occurrences not seen by visual mineralogical methods. This will allow for a complete reconciliation of the accessible gold occurring in the sample. In addition some inference can be made on the refractory amount of gold present based upon recoveries from cyanidation test work and the gold remaining in the residue. This is a less expensive alternative to SIMS or LAMICPMS analysis providing a practical approach to gold assessment that will give an indication of the mineral processing methods to be considered in future test work.
Au recovery per process fraction
Total gold grain association by frequency or weight
The appropriate selection of the approaches detailed above, are used individually or in concert to provide individual gold characterization based upon the available sample size or the specific information required. In the case of initial gold showings and early deposit drilling a reasonably comprehensive study should be undertaken to provide maximum mineralogical information that can assess the future drilling, and impact on the future processing circuit and recovery. An initial detailed characterization can be carried out on selected representative ore types chosen to provide reasonable coverage of the gold grade range present as well as geological and / or mineralogical differences that may be present. This would result in a relatively small number of samples (5-10) and their detailed study should provide a thorough understanding of the nature, mode of occurrence, association and deportment of gold in each. This information can then be used to design an integrated instrumental analysis approach that can be used to analyze large number of samples in an appropriate manner to further understand the deposit as a whole for resource modeling and metallurgical modeling.
Total gold grain distribution by weight