Mineralogy: The missing “M” in AMD prediction?

The previous blog of 5th December 2013 Flotation Mineralogy: Valid and Valuable examined the role of mineralogy in flotation.  Indeed, many of these same ideas, challenges and solutions are experienced when dealing with environmental characterisation of ore deposits.  However, the challenge extends further as often environmental managers and superintendents are left with smaller budgets, and fewer staff to undertake such predictive characterisation works.  The key to predicting environmental issues such as formation of acid mine/rock drainage (AMD/ARD) lies in understanding the mineralogy.  If there was just one piece of information or test that one could perform in this field, it would be to obtain mineralogical data, and yet, the current trend is to collect as much geochemical data (i.e., net acid generation and net acid producing potential values) as possible.  Furthermore, when such geochemical data is collected, it is rarely correlated back to the original mineralogy, or indeed to the lithology from which the sample originated. Therefore, how can an ARD block model be produced based on a handful of numbers which do not have any mineralogical or lithological context?

The State of Play

Current ARD mineralogical assessments primarily focus on using optical microscopy and X-ray diffraction (XRD) to determine the modal mineralogy and identify acid-generating sulphide phases.  However, such techniques are limited to samples classified geochemically as ‘uncertain’ and not routinely performed.  Sophisticated techniques such as scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA) are occasionally used to supplement these studies with the objective of gathering information about fine-grained mineral phases, reaction rims and solid-solution minerals.  The use of automated mineralogy for ARD characterisation of tailings is also now undertaken, but this does not directly aid the development of robust predictive ARD block model.  Most recently, there has been a renewed interest in utilising whole-rock geochemical data and modal mineralogy data to quantify mineralogy, and through careful validation (which can now be performed simply using bench-top XRD instruments e.g., the Bruker D2 Phaser at the University of Tasmania), such a method demonstrates promise in allowing for an ore deposit to be fully understood.  Whilst all of these mineralogical techniques are of use, what is absent is a robust simple mineralogical tool or tools that can be used on a large number of samples (i.e., in accordance with best practice sampling guidelines as outlined by the Australian Government).  Indeed, the three hurdles identified by Dr Chris Greet in Flotation Mineralogy: Valid and Valuable also apply here: turn-around time, expense and validity.

‘Simplicity is the ultimate sophistication’

The acid rock drainage index (ARDI; Parbhakar-Fox et al. 2011) is perhaps the best example of a simple mineralogical evaluation scheme that requires the assessment of a given sample area by five key parameters (i.e., sulphide content, morphology, alteration, neutralizer content and mineral associations) that influence ARD formation.  Such a method only requires trained personnel to perform the evaluation (therefore reducing turn-around time and expense) with samples continually taken for validation geochemical testing, as per stage-one of the GMT approach (Parbhakar-Fox et al. 2011) introduced in the blog on 1st November 2013: Reinventing the ARD testing wheel.  To aid the ARDI assessment, other techniques can be performed to aid mineral identification, such as carbonate staining or short-wave infrared analyses to correctly identify alteration minerals (i.e., potential secondary neutralisers).  This first pass assessment of mineralogy allows for any data generated by geochemical tests to be better understood, and also provides an insight of the samples texture and its influence on ARD formation.  Any samples identified as complex or as an end-member (i.e., highly acid forming or neutralising) can be better recognised and nominated for further mineralogical interrogation using more sophisticated tools introduced previously.  To undertake ARDI assessments as routine (on waste rock and marginal ore materials) would be the ultimate goal to improve ARD characterisation.  However, to achieve this requires a significant cultural shift by the mining industry, who resist change as more funds would have to be spent during the early stages of mine operations.  But the benefits of reducing post-closure costs and avoiding reputational damage (particularly for ore deposits prone to ARD formation i.e., massive sulphide, porphyry, SEDEX) are arguably more significant.

The next steps…

The conclusions given in  Flotation Mineralogy: Valid and Valuable are reiterated here but in an environmental context.  First, a simple field-based ARDI assessment must be performed routinely on a given sample interval (as site-specifically determined).  Next, the appropriate mineralogical tools must be used in a focused study to determine the mineralogical and textural controls on ARD formation on a selection of well-chosen representative samples. T his data, alongside others generated by geochemical and geometallurgical tools will allow for better ARD prediction and block modelling.

The challenge is to change the perception that mineralogy is only required for exploration, resource development, and processing studies. Mineralogy is the key required by all and will help the mining industry unlock a more environmentally conscientious future.

For more information on AMD prediction consulting or analysis, please contact MinAssist.

References:

Parbhakar-Fox, A.K. et al., 2011. Development of a textural index for the prediction of acid rock drainage. Minerals Engineering.