Environmental geology of gold and coal mines, Reefton, New Zealand


Hewlett, Lucy (Lucy Hewlett)


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At Reefton, mesothermal gold deposits occur in the Ordovician Greenland Group basement, and bituminous coal deposits occur in overlying Eocene Brunner Coal Measures. These different resource types occur in close proximity to one another in the '" same topography, vegetation and climate. Both types of resource/have been mined for X many years and as a result historic mining areas discharge water with chemistry reflecting the mineralogy of the deposits. Coal measures release natural acid rock drainage throughout the region due to oxidation of pyrite in coal and associated sediments. The acid generating capacity of the Brunner Coal Measures exceeds the acid neutralising capacity. Levels of acidity are increased in mining areas, and pH near 3 is common. Water discharging from coal mines contain Pb (up to 0.768 ppm), Ni (up to 0.08 ppm), Zn (up to 1.36 ppm) and Cu (up to 0.286 ppm). Gold mines contain pyrite, arsenopyrite and stibnite. Discharge waters from these sites have pH near 7 because of the high acid neutralizing capacity of the basement rocks. Water discharging from gold mines contains elevated As (up to 58.9 ppm), Ni (up to 0.271 ppm), Zn (up to 0.83 ppm), Sb (0.106 ppm), Cu (up to 0.121 ppm), Fe (up to 486 ppm). Both coal and gold mining sites have iron oxyhydroxide precipitates in drainage waters. These precipitates are more abundant at gold mining sites, and one deposit is up to 5m thick. Hydrated Iron Oxides (HFO) occurs in discharge from coal mines but occurs in . smaller quantities and often downstream from the source. Gold mine iron oxyhydroxide contains Zn (up to 353 ppm), Ni (up to 592 ppm), Cr (up to 17 ppm), Cu (up to 72 ppm), Pb (up to 29 ppm), Sb (up to 1180 ppm) and this material is particularly high in As (up to 20 wt%). Most of the material is amorphous but distinctive zones (mm scale) are As rich (up to 30 wt% As oxide). Pharmacosiderite, K2Fe4(As04)3 (OH)5.6.3H20, is a secondary mineral in one deposit. These iron oxyhydroxide deposits are useful indicators for the metals mobilised into the environment by oxidation in the historic workings. Iron oxyhydroxide at coal mine sites contain Zn (up to 89 ppm), Ni (up to 10 ppm), As (up to 163 ppm), Cr (up to 23 ppm), ,)9Jup;-t'\ 16), and Cu (up to 11 ppm). J arosite group minerals, schwertmannite, an~ g~dt~i!~)lave been identified in the coal mine iron '- ----- oxyhydroxide, although much of the material is amorphous.

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xiv, 204 leaves : ill. (some col.), maps (some col.) ; 30 cm.


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Hewlett, Lucy (Lucy Hewlett), “Environmental geology of gold and coal mines, Reefton, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/411.

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