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A Fine Looking Sample – the Importance of Fines in Grade Control Sampling to Mine to Mill Reconciliation


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Author B Reid


Since the commencement of mining operations at the Cowal Gold Operations (CGO) in 2005, reverse circulation (RC) drilling has been the preferred method for grade control (GC) sample collection. This method was selected because it allowed:

  • for multibench drilling (20 m hole depths for two
    benches of mining) to aid mine planning

  • increased data density for comparison against the
    wide spaced ore reserve model

  • angled drill holes to intersect perpendicular to the
    strike and dip of the shallow dipping mineralisation

  • ability to take two metre sample intervals to enhance the understanding of the mineralisation’s short-scale variability and provide the ability to flitch mine.

From June 2008 to December 2013, RC GC was performed using a UDR rotary cone splitter. The field duplicate data over the period displayed strong precision, with a correlation coefficient of 0.94 and an average relative per cent difference (RPD) of 21 per cent for gold. Up to 66.1 per cent of field duplicate data greater than 0.1 g/t fell within ±20 per cent RPD. This is within limits of industry best practice. The Mine to Mill reconciliations over the same period showed the estimated GC grade was 7.7 per cent lower than the calculated head grade (CHG).

In December 2013, the Progradex sampling system (PGX) was introduced at CGO due to its advertised ability to capture 100 per cent of the drill cuttings for the complete sample interval, inclusive of fines. Analysis of field duplicate data indicates that since the introduction of the PGX, the sample precision has drifted. From January 2014 to February 2016, the correlation coefficient was 0.92 and up to 51.2 per cent of field duplicate data, greater than 0.1 g/t fell within the ±20 per cent RPD. Over the same 26 month period, the Mine to Mill reconciliation improved to one per cent lower than the CHG.

Granulometric analysis of the RC samples collected from the PGX indicated the fines material(<212 µm) averaged up to 22 per cent of the samples mass and following assay determination, the fines material represented a weighted average of 31 per cent of the total gold content within the sample.

This paper highlights the:

  • importance of the complete capture of all drill
    cuttings during an RC drill program

  • influence of fines material on the sample quality and

  • impact sampling of the fines material has on the Mine
    to Mill reconciliation performance

  • challenges experienced in commissioning a PGX system.


Reid, B, 2016. A Fine Looking Sample – the Importance of Fines in Grade Control Sampling to Mine to Mill Reconciliation, in Proceedings Ninth AusIMM Open Pit Operators’ Conference 2016, pp 28–38 (The Australasian Institute of Mining and Metallurgy: Melbourne).