1.5.1 Primary objective and research questions
Th e overarching question motivating this research is “how can waste rock material characterisation be improved using an integrated geochemical-mineralogical and textural ARD classifi cation protocol?” Th is was not addressed by recent Ph.D theses in this general subject area (e.g.,Weber, 2003; Stewart 2005; Özcelik, 2007; and Servida, 2008). Th erefore, the primary objective was to develop a new approach to ore and waste rock characterisation, which systematically integrates geochemical, mineralogical and textural data in order to improve the accuracy of ARD predictions. A detailed understanding of ARD characterisitics of an ore body obtained at the early stages of mine operations can infl uence decisions on mining, mineral processing and waste disposal so as to reduce the potential for sulphide oxidation. Such an integrated approach must also be adaptable for the classifi cation of waste rock material at abandoned mine sites (Harris, 1997; Feasby and Tremblay, 1995; Strong and Flores, 2008). Th e general research objectives were:
• To establish a protocol for predictive ARD testing of ores and waste rocks that has the potential to be incorporated into resource block modelling.
• To develop a simple scaled textural ARD evaluation scheme which can be used for deposit-scale ARD domaining.
• To deduce a protocol by which samples are better identifi ed for detailed microtextural sulphide characterisation (i.e., element mapping) and kinetic testing.
• To defi ne the application of geometallurgical tools in ARD predictive studies.
Two study sites were sampled in this research, namely, the historic Croydon lode-gold mines and the operational Ernest Henry iron-oxide copper gold (IOCG) deposit, both located in Queensland, Australia (Figure 1.6). Th ese sites were chosen as the style and mineralogy of the ore-deposits diff ered, and both sites represented diff erent stages of mine operations. Th is allowed for the development of a protocol with application both prior to mine operation and post-closure. As ARD is actively being produced at the Croydon mines, research focussed particularly on this site, with results intended to guide future site rehabilitation. Th e research questions specifi c for the case study sites were:
1. Which waste rock groups are acid forming at the Croydon gold mines, and how does texture infl uence acid formation?
2. What are the potentially deleterious elements associated with sulphide minerals identifi ed in the waste rock piles at the Croydon gold mines?
Townsville Cairns Brisbane Croydon Cloncurry Ernest Henry Mount Isa Charters Towers Rockhampton QLD SA 140° 145° 150° 155° 25° 20° 15° miles 0 200 0 kilometres 400
Figure 1.6 Location of the Croydon district and Ernest Henry mines within Australia.
3. Which secondary minerals form in the waste rock piles at the Croydon gold mines, and how do they control the fl ux of potentially deleterious elements to the downstream environment?
4. How can mineralogical and textural analyses be eff ectively integrated into column leach testing, and does this improve the understanding of the leachate chemistry?
5. What are the eff ects of grain size on rate of acid formation and metal leaching for the diff erent acid forming groups identifi ed in waste material from the Croydon gold mines?
6. Which lithologies are acid forming or neutralising at the Ernest Henry IOCG deposit, and how does texture control this behaviour?
7. How can geometallurgical data from the Ernest Henry IOCG operations be best used to defi ne ARD forming or neutralising potential?
Th ese questions were addressed using a combination of well-established, as well as novel and innovative techniques. Specifi cally, this research had the unique ability to explore the application of geometallurgical tools such as MLA-SEM and HyLogger (Quigley, 2008) for ARD characterisation. Further opportunity existed to integrate mineral and element maps using MLA-SEM and LA-ICP- MS to examine the distribution of potentially deleterious elements in sulphides. From this, secondary mineral formation and potential leaching pathways as dictated by the associated mineralogy were better understood.
1.5.2 Th esis outline
Th e following chapters document the Ph.D research. Th e acid rock drainage index (ARDI) and the geochemistry-mineralogy-texture (GMT) approach are proposed in Chapter 2. Th e ARDI has been developed to predict acid formation based on intact rock texture. Th e ARDI evaluates samples by fi ve parameters which infl uence acid formation. Th e ARDI forms part of the GMT approach, which is a revised protocol for classifying solid mine waste (i.e., waste rock). Th e GMT approach consists of three stages and involves parallel use of geochemical, mineralogical and textural analyses. Sample screening is performed at stage-one; routine geochemical tests are performed at stage-two; and advanced geochemical tests and microanalysis are performed at stage-three. Samples from both study sites were used to develop and evaluate the applications and limitations of the ARDI and GMT approach.
A mesotextural classifi cation method for identifying sources of ARD at the historic Croydon-gold mining operations is described in Chapter 3. Th rough undertaking mesotextural classifi cation, waste rock material with a propensity to form acid was systematically characterised using the GMT approach. Th is chapter focussed on evaluating the various techniques and classifi cation methods recommended by the GMT approach in Chapter 2, and identifi ed the best tools for use. Th e acid forming mesotextural groups which posed the greatest environmental risk at Croydon were identifi ed, and subsequently recomended for kinetic testing.
Chapter 4 documents laboratory based column leach kinetic trials performed on four (potentially) acid forming mesotextural groups identifi ed in the waste material obtained from the Croydon gold mining operations. Twelve columns were established, with material from six diff erent samples tested. Two grain sizes (-10mm and -4mm) were used to observe the eff ects of grain size on pH, metal elution and secondary mineral precipitation. Mineralogical and textural changes were also monitored throughout this experiment, and related to leachate chemistry to identify the controls on sulphide oxidation, and deduce the relative rate of weathering. Laboratory-based results were compared with those obtained from a local geochemical study (in which stream sediment and water quality were assessed), and site rehabilitation strategies were proposed.
To explore the application of geometallurgical techniques for predicting acid formation, data from the Ernest Henry iron-oxide copper gold (IOCG) deposit were obtained and used alongside predictive ARD geochemical and mineralogical data as presented in Chapter 5. Samples from two drill
holes were subjected to GMT analyses, with MLA-SEM, Hylogger, assay and mineral hardness data examined in conjunction. Th is provided an indication of how best these tools can be integrated into ARD predictive studies. General conclusions are given in Chapter 6, with the research questions revisited. Recommendations for how the GMT approach can be integrated into the mine life cycle are also given. Finally, this thesis outlines areas for further research.