Assessing AMD Risk Using Bulk Rock Chemistry to Estimate Mineralogy

Converting Bulk Rock Chemistry to Mineralogy for Acid and Metalliferous Drainage Risk Management.

Static geochemical data provides the foundation for quantifying the risk posed by sulfidic mine materials, including waste rock, tailings, wall rock, ore and low grade ore stockpiles, heap leach pads and concentrate stockpiles.  A new approach to developing a detailed acid and metalliferous drainage (AMD) risk layer for a mine model was recently developed for the Olympic Dam Fe-oxide Cu-U-Au-Ag deposit.

Major and trace element chemistry data from all resource drillholes are routinely collected at Olympic Dam.  Sixty five analytical parameters, including carbon dioxide, were measured for a dataset of 10,000 samples, each representing a 15 metre drillhole intersection.  A program was developed to allocate the chemical components to 30 ore and gangue minerals known to occur at Olympic Dam, resulting in the production of a highly detailed modal mineralogy for each analysed intersection.  The maximum potential acidity (MPA) and acid neutralising capacity (ANC) values were calculated from the key reactive minerals in each intersection.  Key acidity generating minerals included pyrite, chalcopyrite, bornite and chalcocite.  The dominant acid neutralising minerals included dolomite and ankerite; however, abundant siderite was also present.  Net acid producing potential (NAPP) values were calculated from the MPA and ANC from each sample. AMD classification data identifying non-acid forming (NAF) and potentially acid forming (PAF) materials based on calculated NAPP values was produced for part of the deposit.  Two hundred and fifty samples of drillhole material were collected and submitted to a NATA accredited laboratory for static geochemical analysis to assess the accuracy of the calculated mineralogy and associated NAPP values.  The AMD risk classification for the laboratory data and the calculated mineralogy was shown to be well correlated.

From this work it is concluded that bulk rock chemistry data can be used to accurately calculate mineralogy to develop AMD risk layers for mine block models, potentially dramatically improving AMD management and closure outcomes without the need for additional laboratory-generated geochemical data and at very little relative and absolute cost.

Submitted as an abstract to 9th Australian Workshop on Acid and Metalliferous Drainage.

For more information on the application of this technique to your site contact Earth Systems.