3.2 Analysis of rocks from the proposed mine site
Field observations and laboratory analysis of rocks to be disturbed by mining are used to assess the likely quality of mine drainage. Field observations are useful for the selection of rocks for further analyses and interpretation of data once analytical results are obtained. Laboratory analysis of rock composition and reactivity can enable quantitative assessment of the mine drainage chemistry that can be expected from a prospective mine area. The interpretive value of quantitative analyses is improved by having a good collection of geological data, a robust geological model, and thorough observations of field characteristics or rock samples. Analysis procedures vary from rapid and standardised laboratory analyses to specialised testing procedures that can be designed to investigate site-specific geochemical issues. This section provides a guide to the appropriate analyses that should be undertaken, and their interpretation.
3.2.1 Important minerals and field observations
The type, abundance and reactivity of minerals within rocks determine the chemistry of mine drainage, associated potential environmental impact, and management or treatment strategies required. The most important groups of minerals that influence mine drainage chemistry are sulphides and carbonates (Plumlee and Logsdon 1999; Appendix C.3). Oxidation of sulphide minerals causes acid production and strongly influences the trace-element geochemistry of mine drainage. In general, carbonate minerals neutralise acid produced by sulphide minerals and also contribute trace elements. There are several other groups of minerals, including secondary minerals that form after sulphide oxidation, that influence acid-producing or neutralising characteristics of rocks.
The distribution of sulphide, carbonate and secondary minerals varies within and between rock types within a coal or gold deposit. Therefore a general geological description of any samples collected is essential for interpretation of analytical data. Detail of what should be included in a general geological description is provided in Appendix C.4.
Field or hand-specimen observations of minerals in rocks to be disturbed by mining are a qualitative tool for assessment of mine drainage potential. These observations are useful for the selection of rocks for further analyses (see section 3.2.2) and interpretation of data once analytical results are obtained. Important observations include:
- General geological description of a rock type or sample (Appendix C.4);
- The presence of primary sulphide minerals, particularly pyrite and arsenopyrite;
- The presence of carbonates, particularly calcite and siderite; and
- The presence of secondary minerals that indicate the reactivity of rocks when exposed to surface such as Fe3+ (ferric) oxides, hydroxides and hydroxy-sulphates, Al hydroxides, sulphate minerals.
Further details, including photos of different rock types, are provided in Appendix C.5. Field observations of minerals in hand specimens or outcrops are qualitative and should not replace laboratory analysis of rocks. Rather, these observations assist with the interpretation of laboratory data.
Field observations should also be completed on a regular basis throughout all mining and resource development phases so that previously unidentified rocks that have implications for mine drainage chemistry are identified, analysed and appropriately managed.
3.2.2 Sampling strategies for geochemical characterisation of rocks
Quantitative laboratory analysis of rock geochemistry is required to make definitive statements about likely mine drainage chemistry from a proposed new mine site. Quantitative tests for acid production potential are collectively called acidbase accounting, while trace element concentrations in rocks can be determined by X-ray fluorescence (XRF). Quantitative testing can be undertaken for several purposes and sample collection strategies will differ accordingly. However, fresh samples are always required as weathered rocks can give false-positive results in many acidbase accounting tests. For the purposes of characterisation of rock at a new mine site or mine development, some general rules have been developed during the research programme that can be applied to ensure analysis density is sufficient.
Sampling density should suit the complexity and variability of rock types in the mine development area. For example, coal measure formations require more samples than homogeneous rock formations because coal measure formations contain a wide variety of rock types. Samples collected during site characterisation for acidbase accounting or trace element analysis should be collected with the following aims:
- Characterisation of representative rock types within the sequence of rocks to be disturbed by mining including ore rocks or coal;
- Identification and sampling of specific geological features that have anomalous acid-producing or neutralising characteristics. This includes:
- For coal deposits geological structures such as faults or beds that contain pyrite or calcite, and the roof and floor of coal seams, could have anomalous acid-producing or neutralising characteristics;
- For hard-rock gold deposits anomalous acid-producing or neutralising minerals or trace elements could be localised in mineral alteration zones and faults; and
- For alluvial gold deposits the presence of oxide minerals or sulphide minerals should be noted and gravel coarser than 2 cm should not be included in the sample.
- Sufficient data should be collected so that statistically meaningful acidbase accounting values can be identified or calculated for important rock types.
In general, sampling of rocks for environmental purposes can be completed at the same time as the collection of exploration data. The mine drainage implications of all rock types can be treated in a similar manner to exploration data so that three-dimensional models of rocks with different environmental implications can be compiled and these rock types can be selectively mined and managed appropriately (see also section 4.4.1). Additional sampling of potentially acid-forming rocks will be required throughout resource development and mining so that rocks continue to be appropriately managed (see also Chapter 8).
How many drill holes should be sampled?
The density of data required for assessment of mine drainage chemistry is usually less than that required to identify and define a coal or mineral deposit. Therefore, data collection for mine drainage assessment can accompany exploration or resource definition drilling and not all drill holes need to be sampled for environmental purposes.
Where geology is complex (>5 different rock types present in any 20 m of core) at least one drill hole should be sampled for every block of 250 × 250 m. Where geology is simple (<5 different rock types present in any 20 m of core) at least one drill hole should be sampled for every block of 1000 × 1000 m. The decision to use a drill-hole density greater than 250 × 250 m for acidbase accounting analyses should be made by an experienced geologist.
The drill-hole densities recommended here provide minimum data for an initial assessment of acidbase accounting data or trace element data. Once operational it is likely sample density will need to increase for management.
How many samples per drill hole?
Density of sampling within drill holes should ensure that the complete sequence of rocks to be disturbed by mining is sampled. At least 510 samples of each rock type encountered in the sequence that is to be disturbed by mining should be submitted for acidbase accounting analysis. At most exploration targets, it is likely there are 4-8 different rock types; therefore a minimum of 2040 rock samples should be analysed for acidbase accounting for each block of 250 × 250 m.
If drill cores are not collected and/or drilling processes produce rock chips, then rock chips may also be collected for analysis. Prior to submission for analysis the rock chips should be sieved and rinsed because they may be contaminated with additives used during drilling. These additives can alter results of the analyses.
Where alluvial gravels are to be sampled, samples should be taken from areas that are beneath the oxidised zone. The oxidised zone can be identified by the presence of Fe oxide staining (Appendix C.5).
In summary, the following samples should be collected from each drill hole sampled for environmental analysis:
- A minimum of five samples from each rock type to be disturbed by proposed mining;
- Roof and floor rocks from coal seams in coal deposits;
- Rocks from the alteration zone at hard-rock gold deposits; and
- Rocks from below the oxidised zone in alluvial sediments.
What kind of samples should be collected if there is no drilling?
Where drilling is not part of the exploration programme, or it is not practicable to undertake drilling, alternative sample types should be collected. This might include samples from:
- Rock outcrops and/or road cuttings where rocks have been exposed;
- Trenches or test pits; and
- Old mine workings.
The strategy and density of the sampling should be similar to that for drill cores. Five to ten samples of all rock types that will be disturbed by mining should be sampled for acidbase accounting analysis for each 250 × 250 m area, and selective sampling of rocks with acid-forming or neutralising implications should be completed. Samples collected from such locations are typically of poorer quality than drill cores because of weathering processes. Weathering generally affects the top 10 m of outcropping rocks and this can influence the acidbase accounting properties of the rock. Effort should be made to collect the least weathered rock where possible and to provide detailed geological descriptions (Appendix C.4).
3.2.3 Acid-base accounting (ABA)
Acidbase accounting (ABA) tests identify the rocks that have the potential to change pH or increase the acidity or the alkalinity of mine drainage chemistry. There are several reasons to undertake ABA analysis, including:
- To determine the presence or absence of potentially acid-forming (PAF) rocks;
- To determine the presence or absence of acid-neutralising rocks;
- To predict mine drainage chemistry;
- To establish relationships between specific rock types and acid production or neutralisation;
- To optimise management of waste rock or inter-burden with respect to mine drainage chemistry; and
- To select rock types for more detailed geochemical analyses.
Acidbase accounting analyses provide information on the geochemical characteristics of the rocks, but ABA does not provide information on the rate (kinetics) at which different rocks react or trace element concentrations. However, relationships between ABA properties, rock reactivity and trace element concentrations could be determined if sufficient additional data such as kinetic test information or data from historical mine drainages were integrated.
Acid-base accounting analyses are the most common tests carried out to determine whether mines will produce acidic drainage. In general, ABA analyses identify the maximum amount of acid produced, and the maximum amount of acid that can be neutralised (base) by a rock during weathering. Acidbase accounting results can be combined with geological data relating to the distribution of different rock types to identify particular rock types or areas of concern, which can impact mine planning.
A brief description of the different tests that are commonly used, their limitations, and a brief guide on the interpretation of the results are listed below with further details provided in Appendix C.6.
Maximum potential acidity (MPA)
- Total sulphur (S) is determined and the maximum possible acid generation is calculated assuming all S is sourced solely from the mineral pyrite (FeS2) with the results expressed as units of kilograms of H2SO4 per tonne of rock (kgH2SO4/t).
- Usually MPA values are between 0 and 200 kgH2SO4/t.
- MPA analyses are commonly combined with ANC analyses for interpretation.
- There are some important limitations to MPA testing that should be understood when interpreting the results of MPA analyses (Appendix C.6).
Acid-neutralising capacity (ANC)
- ANC is the amount of acid that can be neutralised by a rock sample and mostly relates to the amount of carbonate minerals within that sample.
- ANC is measured by the amount of acid consumed when a crushed rock sample is added to a known quantity of acid.
- ANC is best measured in units of kilograms of H2SO4 per tonne of rock (kgH2SO4/t) so that it can be directly compared with MPA.
- ANC values for rocks are commonly between 0 and 200 kgH2SO4/t, although limestones have higher ANC values (>200).
- ANC analyses are commonly combined with MPA analyses for interpretation.
- Rocks with less than 1 kgH2SO4/t should not be relied on for ANC.
Net acid production potential (NAPP)
- Net acid production potential (NAPP) is the calculated difference between MPA and ANC, i.e. NAPP = MPA − ANC, and is reported in units of kg H2SO4 per tonne of rock (kgH2SO4/t).
- Positive NAPP values are acid producing, while negative NAPP values are acid neutralising (see also Table 1).
Net acid generation (NAG)
- NAG testing assesses net acid production during weathering. Specifically, a crushed rock sample is oxidised to release acid that reacts with the neutralising minerals in the rock.
- Titrations (for acidity and pH measurements of the NAG solution are used to quantify the acid-producing potential).
- Net acid generation potential is measured in kgH2SO4/t and the end pH of the NAG solution is also measured.
- Samples with more than 10 kgH2SO4/t or a pH of less than 4.5 are significantly acid forming.
- Samples with 110 kgH2SO4/t and pH less than 4.5 are moderately acid forming.
- Samples with less than 1 kgH2SO4/t and pH less than 4.5 are weakly acid forming.
- Samples with pH greater than 4.5 are non-acid-forming.
There are some important limitations in the applicability of NAG analysis, and potential for false-positive interpretations. These are discussed in further detail in Appendix C.6).
- Paste pH is a field-based analysis that determines the readily soluble acidity in crushed rock and is commonly used as a qualitative tool to identify and manage acid areas.
- This analysis is carried out by mixing a crushed rock sample in a 1:5 volume ratio with deionised water and measuring the pH immediately.
- A sample with a pH less than 4.5 indicates the rock has high stored acidity and will contribute acid rapidly during weathering.
- Paste pH is not an indicator of long-term acid contributions from rock because it does not analyse those components that require long-term exposure to air and water to release acid.
- The carbonate bomb field test combines a powdered rock sample with concentrated acid to release carbon dioxide (CO2) gas. The pressure of gas in the bomb is used to estimate the carbonate mineral content of the sample (Appendix C.6).
- The carbonate bomb can be used as a field method to assess the distribution of rocks that contain carbonate minerals and ANC of rocks.
Interpretation of acidbase accounting analyses
MPA, ANC and NAG analyses are the primary analyses that are useful in determining the acid-forming status of rock samples. Table 1 provides a generalised guide for the interpretation of acidbase accounting results in relation to the acidity of the collected rock samples.
Table 1 Summary to interpreting acidbase accounting results
> 10 kgH2SO4/t
Highly acid producing
Highly acid producing
Moderately acid producing
Moderately acid producing
Low no acid producing
Low no acid producing
Low no acid neutralising
Moderately acid neutralising
Highly acid neutralising
Commonly, acidbase accounting data are plotted with NAG values on a y-axis and NAPP or the ratio of MPA/ANC on an x-axis (Figure 8 and 9). Graphs of this type divide samples between the four quadrants of the diagram and into fields that are acid-producing, non-acid-producing, and uncertain. Samples that plot as uncertain usually do so due to interference in the analytical method or a breakdown of the assumptions underlying the test method (see Appendix C.6). The geological description of the sample (Appendix C.4) is often the most important piece of information used to interpret samples that plot in the uncertain quadrants.
The results of acidbase accounting are provided on a per sample basis and need to be considered alongside geological data, particularly the volume of each different rock type, to determine the potential mine drainage chemistry. These data should be integrated with geological models, mine scheduling information and kinetic test data (section 3.2.5) to provide the most complete interpretation. In general, if all samples are NAF or have negative NAPP and have high NAG pH and low NAG acidity values then mining will not produce substantial AMD and the whole deposit would be considered to be non-acid-forming (see section 5: Coal non-acid-forming). If any samples are PAF or have a positive NAPP value then interpretation of the geological data is required to determine the extent of the occurrence of the PAF rock and the overall likely impact as few as 5% PAF samples or samples with positive NAPP could result in the production of substantial AMD. If PAF rocks are distributed sporadically and are equalled in quantity by rocks with neutralising capacity (negative NAPP values) then it is unlikely that the mine will produce AMD, although AMD issues may still arise as a result of preferential flow of AMD through the rocks and/or AMD seeps. If PAF rocks represent a small but predictably acidic suite of samples and are not balanced by rocks with negative NAPP values then mining of these areas is likely to produce AMD.
Ongoing monitoring of the rocks disturbed during mining (Chapter 8) will be required on a regular basis to ensure that rocks with implications for mine drainage chemistry (i.e. PAF rocks, or rocks with neutralising capacity) are identified and appropriately handled.
If ABA analyses indicate PAF rocks, read section 4.2 for further description of predicting the chemistry of mine drainage
If ABA analyses indicate NAF rocks, read section 5.2 for further description of predicting the chemistry of mine drainage
3.2.4 Geochemical testing to assess the trace element content of rocks
The trace element concentration of rock samples is commonly determined by X-ray fluorescence (XRF). This method provides information about the total trace element concentration of the rock but it does not provide any information about the form the trace elements are in (speciation). Trace element speciation determines the mobility or reactivity of trace elements during weathering processes. Trace elements present in reactive minerals such as sulphides and carbonates are considered highly reactive and can easily be released into dissolved form. In contrast, trace elements in silicates or some oxides are less reactive and unlikely to be released in dissolved form.
3.2.5 Assessing the reactivity of rocks kinetic tests
Kinetic testing, or rock reactivity testing, provides greater detail about rock drainage chemistry, in particular trace element chemistry of mine drainage, and rate of acid or alkalinity generation. Specifically, kinetic tests are designed to determine changes in mine drainage chemistry with time. These changes occur because the rates at which reactive minerals such as sulphides and carbonates weather is variable. In general, kinetic tests expose a rock sample to laboratory-simulated weathering or field-based weathering, and leachate chemistry is analysed frequently. Kinetic tests can be designed to provide information on:
- Sulphide oxidation rates and mine drainage evolution trends;
- Carbonate reactivity and alkalinity generation;
- Lag periods prior to acid-producing or neutralising reactions;
- Trace element concentrations likely to be present in mine drainage;
- Effectiveness of mine drainage management methods; and
- Optimisation of management procedures.
Kinetic tests are unlikely to be required during initial geochemical assessment of a site but provide critical information to enable optimal mine site management, in particular waste-rock management and effective mine drainage treatment. However, these tests are likely to be required if acidbase accounting analyses or XRF analyses indicate that the mine drainage requires treatment.