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.
Figure C1. Map showing extent of coal measures on the West Coast. BCM and PCM are Brunner and Paparoa Coal Measures, respectively.
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).
Figure C2. Map showing extent of coal measures in Southland.
There are no mine drainage chemistry data or acidbase 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:
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:
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):
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
- Geological Information
- Formation name
- Map unit name
- Informal or local name
- Sedimentary information, composition, texture, structures
- Igneous/metamorphic information, mineralogy, textures
- Alteration textures and minerals
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.
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).