New Zealand Journal of Geology & Geophysics abstracts
Stratigraphic controls on water quality at coal mines in southern
New Zealand
D. Craw
T. Mulliner
L. Haffert
H-K. Paulsen
Geology Department
University of Otago
PO Box 56
Dunedin 9054, New Zealand
B. Peake
Chemistry Department
University of Otago
PO Box 56
Dunedin 9054, New Zealand
J. Pope
CRL Energy Ltd
PO Box 29415 Fendalton
Christchurch 8540, New Zealand
Abstract Water quality at four coal mines
in southern New
Zealand can be related directly or indirectly to the geology and
mineralogy of the stratigraphic sequence in which the coal mines occur.
The Late Cretaceous Taratu Formation of the Kaitangata coalfield formed
in a marginal marine setting during regional marine transgression, and
marine incursions punctuated coal formation. Abundant authigenic and
remobilised pyrite at the Wangaloa mine (typically 4 wt%S in coal) in
the Kaitangata coalfield has oxidised and caused acidification of mine
waste rocks (to pH 1) and mine waters (to pH 3). Similar or even
greater amounts of pyrite occur locally at the nearby Kai Point mine,
although the mined seam typically has <2 wt%S. However, dissolution
of carbonate concretions in marine sediments immediately overlying the
mined seam at Kai Point mine ensures that acid generated by pyrite
oxidation is neutralised, and the site generally has substrates and
waters with near-neutral or alkaline pH. The close stratigraphic
association between coal and marine sediments at Kai Point mine has
resulted in high B contents in coal (up to 500 mg/kg) about 10 times
higher than at the nearby Wangaloa mine. Both mines have elevated
dissolved B contents in mine waters (up to 6 mg/L at Wangaloa). The
Late Cretaceous coal-bearing sequence at Ohai is entirely nonmarine,
pyrite is rare or absent, and no environmental acidification occurs.
Likewise, the Miocene coal-bearing strata at Newvale coal mine have
negligible pyrite and no acidification occurs from these rocks.
However, iron sulfide cements have formed in Pliocene gravels from
groundwater derived from rain with marine aerosols. Oxidation of this
sulfide material has caused localised acidification at Newvale mine.
Acidification has caused localised and generally short-term elevation
of dissolved trace elements at Wangaloa and Newvale mines, with
dissolved Al up to c. 2.5 mg/L at Newvale. Suspensions of clay
minerals cause turbidity at all mines. Well-washed, coarse (>1
µm), detrital kaolinite at Kai Point, Wangaloa, and Newvale mines
settles readily (within days). Chlorite from disaggregated labile
clasts in overlying Pliocene gravels dominates longer term turbidity at
Newvale, but the turbidity decreases rapidly on a time-scale of months.
Fine-grained (including sub-micrometre) poorly sorted authigenic
kaolinite from labile clasts in the coal-bearing strata at Ohai causes
long-term turbidity, with negligible setting over 9 months. The
stratigraphically controlled processes quantified in this study can be
used to predict the nature and scale of potential water quality changes
in new coal mines in southern New Zealand.
Keywords AMD; pyrite; boron; turbidity;
chlorite; kaolinite;
stratigraphy; mining
G07020; Online publication date 29 February 2008; Received 10 August
2007; accepted 22 January 2008
New Zealand Journal of Geology & Geophysics, 2008, Vol. 51:
59–72
0028–8306/08/5101–0059 © The Royal Society of New Zealand 2008
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