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

Abstract

The expansion of open pit mines beyond initial size estimates is a common occurrence in the mining industry, particularly where ore values unexpectedly increase, and where new areas of high-grade ore are identified during mining. The decision to expand a pit should be based not only on expected profits, but also on updated predictions of pit lake water quality based upon the proposed mine geometry. The shape of the pit affects several variables used to generate these predictions, including pit hydrology, wall-rock mineralogy, lake water balance, and physical limnology. A review of pre-expansion geochemical models of the Martha Mine illustrates that changes in the wall-rock composition alone may increase the predicted pH of the pit lake from <5.0 to >6.5. This example underscores the need to update pit lake water quality predictions before open pit mines are expanded in order to best manage environmental risks.