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Appendix C    Geochemistry

C.1              Background geological information – West Coast

C.1.1     Greenland Group Rocks

Greenland Group Rocks are Cambrian to Ordovician in age (c. 400-500 million years old) and form the basement rocks for large parts of Westland. The Greenland Group includes alternating mudstones and sandstones that have been metamorphosed to a hard-rock sequence. These rocks host hard-rock gold deposits in quartz veins and shear zones, particularly in the Reefton area, where a new mine was opened in 2007. Greenland Group rocks have been intruded by granites and contain scattered carbonate minerals, particularly calcite; alteration zones around gold deposits contain calcite and iron-bearing dolomite. Sulfide minerals, especially pyrite arsenopyrite, and stibnite, occur in and adjacent to gold deposits (Christie and Braithwaite 2003).

At a regional scale the Greenland Group rocks are not acid forming because of the ubiquitous but low carbonate content. At the mineral-deposit scale acid-forming rocks are present but uncommon.

C.1.2     Paparoa Coal Measures

Paparoa Coal Measures (Figure C1) are Cretaceous to Paleocene in age (c. 100-50 million years old) and were deposited in river and lake settings in a fault-controlled basin that was inland from the coast at the time. Thick accumulations of clean peat developed in these narrow fault depressions (Newman and Newman 1992; Moore et al. 2006). Paparoa Coal Measures were deposited on thick gravels of the Cretaceous Pororari Group or older basement rocks (Greenland Group greywackes, and granites). Sediments accompanying the coal contain abundant rock fragments from the underlying basement rocks. Greenland Group rocks contain scattered carbonate minerals (Christie and Braithwaite 2003), which can contribute to acid neutralisation capacity (ANC). Groundwater alteration of the Paparoa Coal Measures include clay minerals (kaolinite and illite), carbonates (siderite > magnesite > calcite) and silica overgrowths (Boyd 1993; Boyd and Lewis 1995).

Data collected to date indicate low acid-producing potential (MPA) in the Paparoa Coal Measures at the regional scale. This is interpreted to relate to low sulphur in the depositional environment because the sediments formed in an environment removed from the sea (Pope et al. in press).

C.1.3     Brunner Coal Measures

Brunner Coal Measures (Figure C1) are Eocene in age (c. 40 million years old) (Pocknall 1992) and were deposited in a river or estuarine environment close to the sea, at a time when sea levels were rising and covering the coals after they formed (Nathan 1986; Leask 1993; Flores and Sykes 1996). Brunner Coal Measures were deposited on top of Greenland Group basement rocks, granite, and locally, the older Paparoa Coal Measures (see above) (Nathan et al. 2002). The Brunner Coal Measures are overlain by the Kaiata Formation, which was deposited in the sea. The Kaiata Formation is mainly mudstone (generally called Kaiata Mudstone) with some sandstone layers (called Island Sandstone locally). The Kaiata Formation sediments are dominated by quartz (Newman 1988) with some feldspar (Titheridge 1992) that was derived from Greenland Group and granite basement rocks.

Calcite fossils occur throughout the Kaiata Formation, increasing in numbers with distance above the Brunner Coal Measures. The abundant rock fragments of the Paparoa Coal Measures are not present in the Brunner Coal Measures and Kaiata Formation. Groundwater alteration of Brunner Coal Measures includes abundant kaolinite formation. Pyrite is common as scattered grains (sometimes very fine grained; Weisener and Weber in press) or as a cement. Rare calcite and dolomite nodules occur in the Garvey Creek Coalfield (Newman 1988). The relative proportions of pyrite and calcite in the Kaiata Formation immediately above the Brunner Coal Measures and their effects on acid-base accounting (ABA) are described by Hughes et al. (2007).

At a regional scale the Brunner Coal Measures are moderately to highly acid forming. They were deposited in a sulphur-rich tidal environment and are overlain by marine sediments that also contain sulphur, and that supplied additional sulphur to the coal measures during burial. The overlying Kaiata Formation has acid-forming and acid-neutralising characteristics and at Stockton these characteristics have been stratigraphically and geochemically defined (Hughes et al. 2004, 2007; Weber et al. 2006). However, it is unclear if the stratigraphic controls on acid-forming compared with acid-neutralising rocks that are used for management at Stockton will have regional significance throughout the West Coast. Additional analyses of the Kaiata Formation are required.

C.1.4    Rotokohu Coal Measures

Rotokohu Coal Measures are early Miocene rocks (c. 20 million years old) that form part of the Bluebottom Group. They are underlain by marine mudstones and overlain by non-marine sediments. Insufficient data have been collected to generalise about the acid-forming potential of the Rotokohu Coal Measures. However, acid mine drainage (AMD) has not been reported from this formation to date.

C.2     Background geological information – Southland

C.2.1    Morley Coal Measures

Morley Coal Measures (Figure C2) are part of the Ohai Group and occur mostly within the Ohai Coalfield (Turnbull and Allibone 2003). The Morley Coal Measures vary between 0 and 210 m thick, are the main coal-bearing unit in the Ohai Coalfield and have an extensive mining history by both underground and opencast methods. They are Late Cretaceous in age (c. 80 million years old) (Warnes 1988, 1990; Raine 1989). The coal deposits and associated sediments were deposited in a river valley setting removed from the sea, similar to the Paparoa Coal Measures (above) (Sykes 1985, 1988; Bowman et al. 1987; Shearer 1995). Drainages from mines in the Morley Coal Measures typically have low acidity, circum-neutral pH and elevated suspended solids (Craw et al. 2008).

C.2.2     Beaumont Coal Measures

Beaumont Coal Measures (Figure C2) are part of the Nightcaps Group and occur within the Ohai Coalfield and surrounding basins. Beaumont Coal Measures are between 20 and 25 m thick within the Ohai Coalfield but are thicker outside the coalfield. Beaumont Coal Measures have been mined at a small scale in the Ohai Coalfield and are Eocene in age (c. 40 million years old) (Couper 1960; Pocknall 1990; Raine 1989). Beaumont Coal Measures were deposited in river environments near to the sea, and are overlain by lake and marginal marine sediments of the Orauea Mudstone (Bowen 1964).

There are no mine drainage chemistry data or acid–base accounting from the Beaumont Coal Measures. However, the marginal marine environment of formation of Beaumont Coal Measures and overlying Orauea Mudstone is similar to that of the Brunner Coal Measures and overlying Kaiata Formation (described above), and similar acid-base accounting issues are to be expected.

C.2.3     Gore Lignite Measures

Gore Lignite Measures (Figure C2) are Late Oligocene to Miocene in age (c. 25-15 million years old) (Pocknall 1990) and were deposited in a river delta and flood plain environment near to the sea (Isaac and Lindqvist 1990). The Gore Lignite Measures locally rest on marine sediments, including limestones, and the lower parts of the lignite measures are interlayered with marine sediments (Chatton Formation) (Isaac and Lindqvist 1990). Sediment for the Gore Lignite Measures was derived from the Otago Schist, local basement greywackes, and some erosion of pre-exisiting coal measures (Isaac and Lindqvist 1990; Youngson et al. 2006). Groundwater alteration of Gore Lignite Measures includes alteration of some components of the sediment to clays (Craw et al. 2008), occasional silicification of plant remains (Isaac and Lindqvist 1990), rare carbonate cementation and irregularly distributed sulphide (pyrite or marcasite) cementation of sandstone and conglomerates (Youngson 1995; Falconer et al. 2006).

Mine drainage from the Gore Lignite Measures has been investigated at mine pits (Dune 2001; Mulliner 2006; Mulliner et al. 2006). In general, mine drainage chemistry is circum-neutral. Rarely pH below 5 occurs and rarely this can contain elevated trace element concentrations. Datasets of acid-base accounting information for the Gore Lignite Measures are rapidly improving and indicate that most rocks are not acid forming but occasional acid-forming rocks are present (Pope et al. in press).

C.3    Minerals with implications for mine drainage

C.3.1    Sulphide minerals

Sulphide minerals control the acid production potential and strongly influence the trace element geochemistry of mine drainage. Pyrite and marcasite are the most abundant acid-producing sulphide minerals in rocks likely to be disturbed by mining on the West Coast and in Southland. Pyrite oxidises to produce acid:

(1) FeS₂ + 7/2 H₂O + 13/4O₂ → Fe(OH)₃ + 2 SO₄^2- + 4H⁺

Pyrite oxidation is a complex and stepwise process that is often catalysed by microbial processes (Evangelou 1998). Breakdown of other sulphide minerals can release trace elements with or without acid:

(2) PbS + 2O₂ → PbSO₄ 
(3) ZnS + 2O₂ → Zn²⁺ + SO₄^2-

Sulphide minerals are commonly impure and can contain several % of other transition metals that substitute for Fe in the mineral structure. Therefore breakdown of pyrite can also substantially increase trace element compositions.

C.3.2     Carbonate minerals

Acid-producing potential decreases with increasing abundance of common carbonate minerals such as calcite, dolomite, ankerite and magnesite. Calcite and dolomite are present in some rocks disturbed by coal mining in the West Coast and Southland. Dolomite, ankerite and siderite carbonate minerals are common in rocks disturbed by hard-rock gold mining on the West Coast and in Southland (Christie and Brathwaite 2003):

(4) CaCO₃(s) + 2H⁺ (aq) = Ca²⁺ + H₂O + CO₂(g)

Carbonate minerals can also be the source of trace elements, because trace element impurities are common in carbonate minerals and trace-element-specific species can form such as ZnCO₃, MnCO₃, CdCO₃ and many others.

Most carbonate minerals have acid-neutralising capacity except siderite:

(5) FeCO₃(s) + 2H⁺ = Fe²⁺ + CO₂(g)+ H₂O
(6) Fe²⁺ + 1/4O₂ + H⁺ = Fe³⁺ 1/2H₂O
(7) Fe³⁺ + 3H₂O = FeOH₃(s) + 3H⁺

C.4    Geological description

The following information should be recorded when describing rock samples:

Location of sample Outcrop description or drill-hole name and coordinates Sample depth

Date collected

Date analysed

Geological Information Formation name Map unit name Informal or local name

Sample description Colour Sedimentary information, composition, texture, structures Igneous/metamorphic information, mineralogy, textures Alteration textures and minerals Weathering

Rationale for sample collection Representative of a particular rock type Selected because it contains minerals that have implications for acid production or neutralisation: Sulphide minerals (pyrite, pyrrhotite, arsenopyrite, etc.) Secondary minerals after sulphide oxidation (gypsum, Fe oxyhydroxides) Carbonate minerals (calcite, siderite, magnesite, etc.)

C.5    Field and core samples

The following section provides a brief description and pictures of key minerals that have mine drainage implications (acid forming, acid neutralising, arsenic forming). Their presence during hand or core sampling indicates that there are definite implications for the quality of the mine drainage likely to be produced, and this should be taken into consideration during further geological sampling (e.g. sample selection for acid-base accounting analyses). However, these minerals are not always easy to see in hand specimens and cores, and formal chemical analyses are needed to confirm their presence or absence.

C.5.1     Pyrite

Pyrite (FeS₂), the primary acid-forming mineral in most AMD areas, can be identified in rock samples. It commonly has a gold colour (bright or dull) (Figures C3 and C4), although it can be greenish grey when fine grained or crushed and ground by drilling (Figure C5).

Figure C3. Pyrite in coal, Taratu Formation, South Otago.	Figure C4. Pyrite (elongate grey patches) in core from Brunner Coal Measures.	Figure C5. Pyrite cement (greenish grey) filling in between quartz white grains in quartz gravel, Taratu Formation. Acid runoff has killed grass around this specimen and stained the gravel brown with Fe oxyhydroxide.

C.5.2    Arsenopyrite

Arsenopyrite (FeAsS) is the most common source of As in mine drainage and is almost always present in hard-rock gold deposits. It has a silver-grey colour when coarse grained (Figures C6 and C7) and a black colour when fine grained or crushed by drilling. Grey stibnite (Sb₂S₃) superficially resembles arsenopyrite when it is fine grained in quartz veins (Figure C8).

Figure C6. Silvery grey arsenopyrite (FeAsS) crystals in a gold-bearing vein. The arsenopyrite crystals have a characteristic diamond shape and distinctive striations on crystal faces.	Figure C7. Grey streaks and patches in an auriferous (gold-rich) quartz vein are rich in fine-grained arsenopyrite.
Figure C8. Grey stibnite (Sb2S3) superficially resembles arsenopyrite when it is fine grained in quartz veins (as seen in core, left), but is much softer and has long thin crystals (right).

C.5.3     Calcite

Calcite (CaCO₃) is the most common mineral that contributes acid-neutralising capacity to rocks. It can be formed during deposition of sedimentary rocks as cement, or as fossil shells. Calcite also forms during hydrothermal alteration of rocks after burial, and occurs in most hard-rock gold deposits as veins and scattered grains in rocks adjacent to veins (Figure C9). Calcite is commonly white and can be distinguished from other white minerals because it dissolves effervescently with addition of 10% HCl.

Figure C9. Small white calcite veins in Greenland Group rocks that host gold deposits.

C.5.4     Gypsum

Gypsum (CaSO₄) is a sulphate mineral that can be present in hydrothermally altered or weathered rocks and, when present, is generally associated with pyrite. Gypsum can also occur as a salty encrustation on rocks that have been recently drilled or as a coating on rock surfaces (Figure C10). For this reason, sampling of fresh unweathered core is preferred for acid-base accounting measurements (Figure C11). Occurrence of gypsum in a core indicates that the core will contain elevated sulphur (S). However, S associated with gypsum is non-acid-forming and pyrite-specific S analyses are required to determine the acid-forming potential of rocks that contain gypsum.

Figure C10. Gypsum encrustation on core from Gore Lignite Measures.	Figure C11. Gypsum on gravel.

C.5.5     Secondary iron oxyhydroxides

Secondary Fe oxyhydroxides are a suite of minerals (including ferrihydrite, schwertmannite, jarosite and others) that form when Fe oxidises or when water that contains dissolved Fe is neutralised. Iron ARDs are commonly rusty coloured and occur as a surface coating on rocks (or drill core) (Figure C12). Jarosite (Fe oxyhydroxysulphate) can have a distinctive butter-yellow colour (Figures C13–C15). The Fe in these secondary precipitates can come from pyrite (Figures C16 and C17) and therefore can be an indirect indicator of acid-forming minerals in a rock. However, Fe oxyhydroxides are an unreliable indicator of acid-forming conditions because Fe can also come from other minerals (including carbonates) or fluids associated with rocks (Figures C18 and C19).

Figure C12. Iron oxide staining (deep brown precipitate) on rocks in a creek draining an acid-producing coal mine. When the creek is low, the acid drainage dominates, and Fe is readily dissolved in the creek water. During rain events, the creek pH rises because of rain dilution, and Fe oxyhydroxide is deposited on the pebbles.	Figure C13. Mixed jarosite and iron oxide on coal.
Figure C14. Jarosite: butter-yellow staining on surface of rock.	Figure C15. Jarosite: butter-yellow staining on surface of quartz gravels.
Figure C16. Fe oxyhydroxides associated with pyrite in Brunner Coal Measures.	Figure C17. Iron oxide staining (top half of picture) indicating an oxidised zone in alluvial gravels; the lower half is unoxidised and contains pyrite.
Figure C18. Fe oxyhydroxides (brown patches) that are not related to pyrite in conglomerate from the Gore Lignite Measures. Fe oxyhydroxides here are localised onto specific fragments in the rock that contain other Fe minerals such as chlorite (an iron silicate).	Figure C19. Iron-magnesium-calcium carbonates. These are less common than calcite, but can contribute to acid neutralisation. Even small amounts of Fe in these minerals cause a distinctive orange weathering appearance, as in the veins from a gold mine shown above.

C.5.6     Scorodite

Scorodite is a green arsenic oxide mineral that forms on arsenic sulphide minerals such as arsenopyrite (Figures C20 and C21). Fine-grained grey arsenopyrite can be difficult to distinguish in rocks and cores, but the green scorodite coating is a useful indicator of As-bearing rock in a mine setting.

Figure C20. Grey arsenopyrite has a coating of green scorodite (FeAsO4.2H2O) that forms from oxidation of the arsenopyrite.	Figure C21. Green scorodite coats a sample of dark sheared schist containing abundant arsenopyrite. Specimen is 20 cm across.

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