Department of Geology

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Dave Craw
Geology Department
University of Otago
Dunedin, New Zealand
dave.craw@otago.ac.nz

Notes for symposium on:
Metal contaminants:
Sources and effects on human health and the environment
Wellington NZ
February 2003
Geological controls on metals in the New Zealand environment


Geological background

Essential features

  • Main rocks of New Zealand relevant to this topic are summarised in Figure 1. This map shows where the principal rocks are exposed at or near the surface. In detail, the basement (bedrock) is exposed on ridges and mountains. Intervening valleys, basins, and coastal plains are filled with sediments of varying ages. These topographically low areas are readily apparent on the maps in Figures 2 and 3.
  • Basement is greywacke (old ocean sediments) and schist (same sediments transformed with heat and pressure) beneath most of both islands.
  • Greywacke and schist are chemically similar over large areas, so the basement is essentially uniform.
  • South Island is dominated by greywacke and schist.
  • Exceptions are granites and related rocks on the west coast of the South Island (not important for environmental metal mobility).
  • Locally thick sediment covers some basement, especially in the North Island. Some of these sediments are coal-bearing, which has some environmental significance.
  • Young volcanic rocks were erupted on to basement and sediments, and form some mountains.
  • Volcanic rocks are most important in the North Island, especially Taupo Volcanic Zone, Coromandel Peninsula, and Northland.


Metals in the environment

  • Principal naturally occurring metals that are environmentally significant in the New Zealand geological setting are listed in Table 1.
  • Metals occur in distinctive associations that are related to the geological setting. These associations are listed in Table 2, in relation to geological settings described in following sections.


Gold deposits

Gold is the main metallic resource that has been mined in New Zealand for over 150 years. Deposits of other metals do occur, but nearly all these are geological curiosities only and have had no economic significance.

  • New Zealand has had a favourable geological setting for gold deposit formation (a geological "accident").
  • Gold is accompanied by other metals, which were discarded by early miners. Hence, much of the science of geological controls on metals in the New Zealand environment is tied to the geology of gold deposits.
  • Two distinctly different gold deposit types occur. Both of these are hydrothermal deposits, i.e., they were formed by hot water moving through cracks in the rocks before the rocks were deeply eroded.
Gold deposits in schist and greywacke ("mesothermal" deposits)

These deposits formed by hot water moving through rocks as the surrounding rocks were uplifted from deep (10 km) in the Earth's crust. The metals in the deposits were extracted from the surrounding rocks by dissolving trace amounts from a large volume of rocks. The metals were then deposited by sudden cooling and/or pressure change as the waters were driven by earthquakes in the Earth's crust. Characteristic features of these deposits are listed below.

  • Gold occurs with quartz "reefs" or veins in cracks in the rock (mainly schist).
  • Deposits are generally small (10 m scale), localised, irregular, and deposit locations are difficult to predict.
  • Deposits are largely restricted to the South Island, and are scattered through Otago, Marlborough, and northern Westland (Fig. 2).
  • The Oceana Gold Ltd Macraes mine in Otago is an example of this type, as is the proposed mine at Reefton. The Sams Creek prospect (NW Nelson, Fig. 2) has some similarities to this deposit type, but also some affinities to the other type described below.
  • The deposits have high concentrations of arsenic, and often antimony, as well as gold. Typical metal contents are indicated in Table 3.
  • Arsenic and antimony are elevated in surface and ground waters. Typical dissolved metal contents are listed in Table 3, and shown in Figure 4.
  • Antimony has been mined historically from some localities of this type of deposit, but never profitably.
  • Rare mercury bearing deposits occur in Otago (Fig. 2).Volcanic rock related ("epithermal") gold deposits
Gold deposits associated with volcanic rocks ("epithermal" deposits)

These deposits were formed when hot molten rock associated with volcanoes provided a heat source to drive near-surface groundwater into kilometre scale convection systems in the Earth's crust. Geothermal systems such as those exploited at Wairakei and Broadlands are the surface expressions of these systems (to be discussed in a separate session). Characteristics of the deposits are listed below.

  • Deposits are commonly associated with quartz veins which are locally continuous for 100's of metres.
  • These deposits are accompanied by large amounts (over 100's of metres or even km) of rock that have been altered to clays by hot water.
  • Deposits occur in the North Island, mainly in the Coromandel Peninsula and Taupo Volcanic Zone (Fig. 3).
  • The Martha Mine at Waihi is an actively mined example of this deposit type, and the nearby Golden Cross mine is in the latter stages of site rehabilitation.
  • Gold is accompanied by arsenic, cadmium, copper, lead, zinc, and some antimony (Table 2, 3).
  • Cadmium, copper, lead and zinc are commonly mobile in surface and ground waters, accompanied by lesser amounts of arsenic and/or antimony (see Table 3, Figure 4, and section on acid rock drainage).
  • Similar deposits, with mercury but little gold, form where hot springs have reached the surface in the past (e.g., Puhipuhi, Northland; Fig. 3). Some of these have been mined historically for mercury.
  • These mercury-rich deposits release arsenic and minor mercury into the environment.
Alluvial gold
  • Gold eroded from veins has been concentrated in sediments in valleys, basins and coastal plains as alluvial gold deposits in all gold bearing areas of New Zealand.
  • These alluvial gold deposits have been, and are, an important resource especially in Otago and Southland.


Acid rock drainage


Acid generation
  • Iron sulphide minerals, pyrite and marcasite (FeS2), readily decompose in oxygenated waters at the surface.
  • One of the products of this decomposition reaction is sulphuric acid.
  • This acid lowers the pH of the immediate environment, and can spread downstream (= "acid rock drainage")
  • Environmental pH <1 is possible in this situation.
Acid neutralisation
  • The acidification can be limited if the surrounding rocks contain reactive minerals, especially calcite (calcium carbonate).
  • There is sufficient calcite in schist to neutralise acid from pyrite in schist-hosted gold deposits, which almost invariably yields environmental pH between 6 and 8.
  • There is insufficient calcite in most volcanic rock related gold deposits to neutralise acid from pyrite oxidation, and acid rock drainage is common.
Metal mobility
  • Low pH encourages environmental mobility of cadmium, copper, lead, and zinc in streams and groundwater.
  • Very low pH (<2) facilitates mobility of aluminium, iron and manganese in streams and groundwater.
  • Arsenic and antimony are more soluble at neutral pH than at low pH.
  • Hence, volcanic-related gold deposits typically yield acid waters with elevated Cd, Cu, Pb and Zn, with some As. (Table 1, 2, 3, Figure 4)
  • Schist-hosted gold deposits can yield waters with very high As and Sb (Table 3, Figure 4).


Other environmental metal-producing geological settings


Coal bearing sediments

  • Coal occurs in river and lake sediments resting on basement rocks in many parts of New Zealand. Most important mining areas are shown in Figure 5.
  • Coal and associated sediments commonly contain sulphur in the form of pyrite (FeS2). Sulphur is highest in coals associated with sediments formed near the sea coast.
  • Sulphurous coal exposed at the surface produces sulphuric acid during oxidation (acid rock drainage).
  • Pyrite in coal generally contains traces of arsenic, which is released with the acid rock drainage.
  • The acid also dissolves other metals from associated sediments. The most common metals dissolved in New Zealand coaly sediments are copper, nickel and zinc.


Groundwater

  • Groundwater is extracted from young sediments in basins, valleys, and coastal plains throughout New Zealand (Figure 6).
  • Most groundwater is of high quality and is readily replaced by rain and river recharge.
  • Some groundwater resources are being extracted at the limits of sustainability, and deeper resources are being tapped.
  • Deeper waters move more slowly, are older, and can interact with more with rocks, and have potential for containing elevated metal contents.
  • Groundwater from coal-bearing sediments also has the potential to become acidified with enhanced metal dissolving properties (e.g., Southland, North Otago; Figure 6).


Arsenic in groundwater: the Bangladesh analogy

  • Groundwater in many areas of Bangladesh has elevated arsenic concentrations, leading to what has been called the largest case of mass poisoning in human history.
  • Key geological features which have contributed to high arsenic in Bangladesh are:
    • Water is in young sediments derived from active mountains.
    • The sediments contain only background arsenic concentrations.
    • Because the sediments were transported and deposited rapidly, they still contain minerals which are susceptible to chemical interaction with groundwater.
    • This groundwater interaction can dissolve arsenic at low levels.
    • Water levels in sediments fluctuate in seasonal wet/dry climate.
    • This water level fluctuation can induce chemical processes which concentrate the arsenic.
  • Many of these geological features apply for New Zealand groundwater (Fig. 6).
  • Thus far, groundwater sources tapped in New Zealand have sufficient water movement to ensure that any dissolved arsenic is diluted to low levels.
  • Some slope gravels in Central Otago have all of the above characteristics, and arsenic levels are slightly elevated locally (marginally above drinking water limits).

Naturally elevated vs "polluted" or "contaminated" metal concentrations

The significance of mining
  • All of the settings and processes described above are natural geological phenomena, and elevated levels of trace metals are normal; i.e., "background" is higher at some sites than others.
  • Metal mobility can be enhanced or accelerated by mining activity, leading to localised metal contamination or pollution of streams
  • The total amount of metals going down streams from mines is not necessarily increased, but mine excavations can focus the discharge into small areas at higher concentrations.
  • Natural erosion can have similar effects to mining on metal mobility.
  • Historic miners discharged mine tailings directly into streams, and these streams have now been largely naturally rehabilitated by floods.
  • Mine tailings have been stored on mine sites since the 1960's, in response to environmental concerns.
  • Elevated levels of trace metals from these tailings facilities should be expected as part of the naturally high background of many mining areas.
  • Monitoring of tailings is essential to ensure that trace metal levels do not rise above natural levels, especially during periods of low rainfall when downstream dilution is lessened.

Specific examples of metal mobility in a variety of geological settings

Mercury in alluvial gold
  • Alluvial gold in some Southland valleys has recently been found to contain a high mercury content (Fig. 2).
  • Dissolved mercury in streams is elevated above background but is lower than drinking water limits.
  • Miners melting recovered gold have inadvertently inhaled and ingested high levels of mercury.
    Arsenic at Barewood mining area, east Otago
  • Historic mining of schist-hosted gold has exposed arsenic-bearing rock and discharged arsenic-bearing tailings to a stream which has now become a wetland (Figure 7).
  • Arsenic in soil and wetland sediment is strongly elevated (to >1000 mg/kg) above regional background of 10 mg/kg (Figure 7A).
  • Dissolved arsenic concentrations reach nearly 1 mg/L in places (Figure 7B).
  • Several cows died from reputed arsenic poisoning in the area in the 1980's.
  • The wetland discharges into the Taieri River, an important recreational and water supply river, but arsenic levels are low when the discharge reaches the river in normal rainfall seasons.

Antimony at Endeavour Inlet, Marlborough
  • Antimony was mined historically from schist hosted veins above Endeavour Inlet (Figure 8).
  • Groundwater emerging from old underground mines has strongly elevated arsenic and antimony.
  • Many streams and the principal river have naturally elevated As and Sb.
  • Tailings from the old processing plant contain some extremely high levels of reactive arsenic and antimony minerals, and are located beside the principal river.
  • The river with elevated As and Sb discharges to farmland and the Queen Charlotte Track, a popular tourist walk (Figure 8).
Mine tailings, Te Aroha
  • A volcanic-related mineral deposit occurs above the town of Te Aroha (Figure 3).
  • The underground Tui Mine extracted copper, lead and zinc sulphides in the 1960's, and the mine tailings were deposited nearby (Figure 9).
  • Water discharging from the mine and tailings area has high trace metal contents, especially zinc (Figure 9).
  • Waters near the town of Te Aroha have trace metal concentrations near to drinking water limits (Figure 9), because of downstream dilution under normal rainfall conditions.
  • The town's water supply was partly obtained from these waters.
  • Metal contents became unacceptably high at times of low rainfall, and a different water supply source was necessary. Road gravel, Puhipuhi, Northland
  • The Puhipuhi area (Figure 3) has several mercury-rich deposits formed by ancient hot spring systems associated with eruption of volcanic rocks in the area (Figure 10).
  • These deposits are rich in quartz and are therefore hard compared to surrounding soft clay-rich rocks.
  • This material was quarried for road gravel on public and private roads in the area (Figure 10).
  • The road gravel contains marcasite (FeS2) with trace amounts of mercury and arsenic, and some other As/Hg bearing minerals.
  • The marcasite oxidises in rainwater, and roads typically have a pH of 3.
  • Complaints occurred of discharges from roads of acid waters with sediments containing elevated As & Hg.
  • Extensive sealing and concrete guttering has been necessary to prevent these discharges into waters used for agricultural purposes.

Bibliography

The preceding notes and diagrams have been compiled from the following sources.
  • Beaumont, H. M., Tunnicliffe, J. C., Stevenson, C. D., 1987. Heavy metal survey of Coromandel streams. Water Soil Misc. Publication, N.Z. Government, Wellington 104, 17-47.
  • Black, A.; Craw, D. 2001. Arsenic, copper and zinc occurrence at the Wangaloa coal mine, southeast Otago, New Zealand. International Journal of Coal Geology 45: 181-193.
  • Brathwaite, R. L., Christie, A. B., Skinner, D. N. B., 1989. Hauraki goldfields: regional setting, mineralisation and recent exploration. In: Mineral deposits of New Zealand (Kear, D., ed). Australasian Institute of Mining and Metallurgy Monograph 13, 45-56.
  • Carter, D. A. 1983. Heavy metal survey in the Coromandel. A case study in design of water quality surveys. Water Soil Misc. Publication 63, N.Z. Government, Wellington.
  • Craw, D., 2000. Water-rock interaction and acid neutralization in a large schist debris dam, Otago, New Zealand. Chemical Geology 171, 17-32.
  • Craw, D., Chappell, D.A., 2000. Metal redistribution in historic mine wastes, Coromandel Peninsula, New Zealand. New Zealand Journal of Geology and Geophysics 43, 187-198.
  • Craw, D.; Pacheco, L. 2002. Mobilisation and bioavailability of arsenic around mesothermal gold deposits in a semiarid environment, Otago, New Zealand. The Scientific World Journal 2: 308-319.
  • Craw, D., Chappell, D., Reay, A., Walls, D., 2000. Mobilisation and attenuation of arsenic around gold mines, east Otago, New Zealand. New Zealand Journal of Geology and Geophysics 43, 373-383.
  • Craw, D., Chappell, D., Reay, A., 2000. Environmental mercury and arsenic sources in fossil hydrothermal systems, Northland, New Zealand. Environmental Geology 39, 875-887.
  • Craw, D.; Chappell, D.; Black, A. 2002. Surface run-off from mineralized road aggregate, Puhipuhi, Northland, New Zealand. New Zealand Journal of Marine and Freshwater Research 36: 105-116.
  • Davey, H.A., van Moort, J.C., 1986. Current mercury deposition at Ngawha Springs, New Zealand. Applied Geochemistry 1, 75-93.
  • Leon, E.A.; Anstiss, R.G. 2002. Selected trace elements in Stockton, New Zealand, waters. New Zealand Journal of Marine and Freshwater Research 36: 81-87.
  • Merchant, R.J., 1986. Mineralisation in the Thames district, Coromandel. In: Guide to the active epithermal (geothermal) systems and precious metal deposits of New Zealand In: Henley, R.W., Hedenquist, J.W., Roberts, P.J., (eds) Monograph Series on Mineral Deposits 26, 147-163.
  • NWQMS (National Water Quality Management Strategy) 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Paper 4, Volumes 1 and 2. Australian and New Zealand Environment and Conservation Council (ANZECC), and Agriculture and Resource Management Council of Australia and New Zealand.
  • Pang, L., 1995. Contamination of groundwater in the Te Aroha area by heavy metals from an abandoned mine. N.Z. Journal of Hydrology 33, 17-34.
  • Williams, G.J., 1974. Economic Geology of New Zealand. Australasian Institute of Mining and Metallurgy Monograph 4.
  • Wilson, N.J.; Hunter, K.A.; Craw, D. 2002. Elevated arsenic and antimony levels in a South Island mesothermal mineralised zone. AusIMM NZ Branch 2002 Annual Conference Proceedings, Australasian Institute of Mining and Metallurgy Publication Series 6/02: 81-86.
  • Youngson, J.H.; Wopereis, P.; Kerr, L.C.; Craw, D. 2002. Au-Ag-Hg and Au-Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer-source relationships. New Zealand Journal of Geology and Geophysics 45: 53-69.

Figure captions

  • Figure 1. Geological map of New Zealand, highly simplified to emphasise the geological features important for defining trace metal associations.
  • Figure 2. Topographic map of the South Island, showing the locations of principal mining areas in schist-hosted hydrothermal deposits, and some other related localities mentioned in the text.
  • Figure 3. Topographic map of the North Island, showing the locations of principal mining areas in volcanic-related hydrothermal deposits, and some other related localities mentioned in the text.
  • Figure 4. Visual comparison of typical trace metal associations and concentrations in schist-hosted and volcanic-related gold deposits. Comparison is also made between observed elevated levels of trace metals and various regulatory guidelines (Table 3). Typical pH ranges for the different deposit types are shown also.
  • Figure 5. Topographic map of the New Zealand, showing the locations of principal coal mining areas in which acid rock drainage and minor dissolved trace metal anomalies can be expected.
  • Figure 6. Topographic map of the New Zealand, showing the locations of principal areas in which groundwater is extracted. Typical aquifer characteristics are described, and some sites of potential trace metal elevation are indicated (as discussed in the text).
  • Figure 7. Sketch maps of a wetland in the Barewood mining area, east Otago, adjacent to schist-hosted gold and arsenic bearing rocks. Map A shows the range of arsenic contents of soils and sediments in the wetland. Map B shows the range of dissolved arsenic in waters in the wetland.
  • Figure 8. Topographic map of the Endeavour Inlet antimony mining area, Marlborough, showing some sources of elevated dissolved arsenic and antimony in streams in the area.
  • Figure 9. Sketch map of the Te Aroha area, south of the Coromandel Peninsula. The location of the Tui Mine is shown in a volcanic-related hydrothermal mineral deposit. Summaries of dissolved metal concentrations in waters below the mining area are indicated (data from Pang, NZ J. Hydrology 1995).
  • Figure 10. Geographic map of the Puhipuhi area, Northland, with locations of principal mercury deposits formed associated with volcanic rocks in the area. Roads are gravelled with rock from two of these sites, containing elevated mercury and arsenic, and these rocks generate low pH runoff.