Our proprietary processing technology is a game changer - we provide quality assurance on the homogeneity of all CRMs produced, regardless of the nature of source materials, including ‘nuggety’ gold ores.
Our procedures do not involve any compositional change to the ore other than grain size reduction. Unlike practices employed by other producers, there is no removal of coarse gold particles by sieving or any addition of fine-grained ‘gold minerals’. The OREAS® range is entirely sourced from naturally occurring ore deposits from various mineralisation styles and when appropriate, blended with geologically matching waste rock to achieve target grades.
The core attribute of a quality reference material is its level of homogeneity, a critical factor that is often overlooked. There should be minimal, ideally negligible, contribution of sampling error from the CRM. This means the CRM itself should not be the cause of analytical bias seen in results due to heterogeneous particle size and disbursement of gold.
All OREAS® gold reference materials undergo homogeneity testing using instrumental neutron activation analysis (INAA) at ANSTO in Lucas Heights, Australia or Actlabs in Ancaster, Canada. INAA is a highly precise, non-destructive technique compared with conventional fire assay. INAA can be used on samples as little as 1g or fraction of a gram. By assaying a reduced subsample, a sampling error is theoretically induced and this enables the absolute homogeneity of the CRM to be quantified. For further information, please see our technical note.
Traditional milling methods (e.g. LM2 bowl and disc pulveriser) are ineffective at reducing the grain size of gold particles. This issue has led to the widespread belief that only synthetic materials or gold ores containing fine grained and homogenously distributed mineralisation can be used to make CRMs.
Figure 1. The gold nugget effect – sampling size and sampling error.
Figure 1 above illustrates a microscopic view of pulverised gold ore. A small sample size could chance upon a nugget or could miss entirely. Larger sample masses are more likely to improve representation and lower sampling errors. The minimum sample size required to obtain a negligible sampling error (e.g. <1% RSD) for a CRM is of critical importance.
As geologists and chemists, we understand the importance of matching CRMs with the field samples. This will ensure commutability of CRMs under the same digestion and fusion methods. For this reason, we have over 130 different OREAS® gold CRMs available covering a broad range of mineralisation styles and grade ranges including orogenic, epithermal (low and high sulphidation), porphyry copper-gold, IOCG, Carlin, Witwatersrand, VMS and magmatic. See our Mineralisation Styles brochure to view graphical plots of Au versus other elements for the different mineralisation styles.