Ore Research & Exploration Pty Ltd, a member organisation of Flagstaff GeoConsultants

The use of assay standards is critical to the verification and quality control of sampling programs from the grass roots level through to prospect evaluation and grade control. In their absence, precious exploration expenditure may be wasted chasing phantom anomalies, orebodies may remain undetected or, in extreme cases, mine closures may result from overestimation of grades at the feasibility stage. In today’s competitive climate, mining company managers are more accountable for decision making and must be seen to be employing best practices in all aspects of their operations. An integral part of this approach is to have in place a robust QA/QC program covering the acquisition of assay data. This can only be achieved by the routine use of good quality reference standards. These alert the geologist to potential analytical problems as they arise and provide ongoing evaluation and monitoring of laboratory performance.

Paul Hamlyn, a director of Flagstaff GeoConsultants, has been supplying standards to the mining industry, through his company Ore Research & Exploration Pty Ltd (ORE), for over 12 years. While many major mining companies incorporate assay standards in their sampling programs the number of smaller explorers that still have no checks and balances in place to ensure the integrity of their assay data is alarming. The cost of implementing quality control systems should reflect the risk to the mining company, investors and financiers and the size of the proposed investment. In this regard, assay control can be considered a form of insurance adding value to an exploration or mining project.

There is a long held view amongst geologists and geochemists familiar with the preparation of gold ore standards that “you can’t make a silk purse out of a sow’s ear” i.e. an ore characterised by the presence of coarse gold makes an inferior standard. With the implementation of innovative technology we can mill ores to an extremely fine particle size. Pulp standards having mean particle sizes of 5 to 10 microns are routinely prepared. This has resulted in an outstanding improvement in the homogeniety of matrix matched standards from client-supplied nuggety source materials. It is an effective way to recycle pulp residues and coarse rejects. Similar improvements in homogeneity are observed in standards prepared from concentrate samples. As part of the quality control of these matrix matched standards intensive homogeneity evaluation is carried out on multiple (generally 15-25) 0.1 to 1g analytical subsamples using high precision instrumental neutron activation analysis at Becquerel Laboratories. Irrespective of the pedigree of source materials a very high level of homogeneity, previously unattainable, is routinely achieved.

In using standards to monitor assay data quality users have always been faced with the dilemma of apportioning standard variability in repeat assays between sampling error (i.e. standard inhomogeneity) and measurement error (analytical precision). Our new generation standards are characterized by a level of uniformity that renders errors attributable to sampling negligible in a conventional fire assay determination. They can therefore be used to directly and quantitatively evaluate laboratory analytical precision and accuracy without the usual ambiguity associated with standards of lesser quality.

In addition to preparing and certifying site specific standards for clients such as Newmont, AngloGold, Rio Tinto, WMC Resources, Newcrest, MIM, Pasminco, Abosso Gold and Pacmin Mining, Flagstaff, through ORE, supply their own range of gold and base metal standards to projects within Australia and overseas. Expert advice is available to ensure robust assay quality control procedures are in place prior to the commencement of all sampling programs.