A mineral quantification method for wall rocks at open pit mines, and application to the Martha Au–Ag mine, Waihi, New Zealand

Effects of mine expansion on geochemical predictions of pit lake water quality: an example from Martha Mine, Waihi, New Zealand

“…It must be noted that Models 1, 2, and 3 are based on the 1997 predictions of the shape of the pit upon mine closure, hydrology, and water balance combined with the wall-rock mineralogy measured by Castendyk et al (2005). They do not represent the updated prediction of pit lake water quality conducted by the mining company for the 2010 expansion.…”

“…Depending upon the geology of the deposit, expansion could also increase the surface area of unweathered, un-oxidised, acid-producing minerals exposed in the pit wall, particularly where a mine in a shallow oxidised ore body is expanded into deeper un-oxidised rocks. Fortunately, drilling and geochemical sampling, combined with mineral quantification (see Castendyk et al 2005), can define the future wall-rock mineralogy in advance of mine expansion so that the effects of expansion can be evaluated.…”

“…Model 3b reflects the removal or covering of fresh-and weathered-argillic rocks above the pit lake surface after steady-state lake levels are achieved. These models are based upon wall-rock mineral data reported by Castendyk et al (2005) and do not represent an updated wallrock evaluation. ANZECC (2000) guidelines for aesthetic drinking water use and recreational use are provided for comparison.…”

“…Hydrothermal fluids altered the parent andesite to rocks that contain quartz, adularia, chlorite, illite, pyrite, calcite, albite, and kaolinite (Brathwaite 1980;Brathwaite & McKay 1989;Jennings et al 1990;Brathwaite & Faure 2002). Castendyk et al (2005) quantified the wall-rock mineralogy of the pit in 2003 using a novel mineral quantification method, and mapped zones of mineral assemblages in the wall-rocks. They identified two areas of hydrothermally-altered andesite, called 'fresh-argillic' and 'weathered-argillic', which had average pyrite concentrations of 8 wt% and 6 wt%, respectively.…”

“…1). The Martha Mine is a particularly good example because it has twice received approval for open pit expansion (in 1997 and 2010), and it is one of the most well-studied open pit mines in the world, with widely disseminated geo-environmental datasets and pit lake modelling results (Castendyk et al 2005;Castendyk & Webster-Brown, 2007a, b). Building from this example, the paper discusses how various components of a pit lake prediction may be affected by mine expansion, and concludes with recommendations for mine managers, regulators, consultants, and modellers that will aid in the upgrade and evaluation of pit lake predictions.…”