AN INTEGRATED PROCEDURE TO PREDICT PIT LAKE GEOCHEMISTRY: PART I. MINERAL QUANTIFICATION, MARTHA MINE, WAIHI, NEW ZEALAND

Water-rock reactions play a critical role in the chemical evolution of pit lakes. We present a mineral quantification method to spatially and quantitatively define the wall rock mineralogy in active open pit mines. The method involves: (1) field observations and extensive sampling, (2) X-ray diffraction analyses (XRD), (3) selection of representative samples, (4) X-ray fluorescence and Leco furnace geochemical analyses, and (5) mineral calculations. We apply this method to the low-sulfidation epithermal Au-Ag deposit at the Martha Mine in Waihi, New Zealand, which will close in 2007 to become a 192-m-deep pit lake by 2012. Alteration minerals observed in the field and defined by XRD analyses on 125 samples show a heterogeneous distribution in the mine, with calcite restricted to one region of the pit wall, widespread pyrite, and ubiquitous silicate minerals. Eight mineral associations display the following mineralogy: (1) weakly-altered: plag-qtz-opx-chl-ill-adu; (2) propylitic: qtz-ill-chl-kao-cal-pyr; (3) fresh-argillic: qtz-kao-chl-ill-pyr-cal; (4) weathered-argillic: qtz-ill-chl-kao-alu-pyr; (5) oxidized: qtz-ill-adu-kao-chl-goe; (6) potassic: qtz-ill-adu-chl-alb-pyr-kao-alu; (7) quartz veins: qtz-ill-adu; and (8) post-mineralization deposits: glass-plag-opx-mag-qtz-bio. Mineral calculations on 46 representative samples assign geochemical data to observed minerals to quantify mineral concentrations (wt%). Calculated concentrations match mineral concentrations (vol%) observed through thin section petrography to within approximately 5%, and validate the method. Under oxidizing conditions, subaqueous pyrite-bearing associations may generate significant acidity whereas subaqueous calcite-bearing associations may contribute alkalinity. Silicate minerals in each association may also contribute to alkalinity over long time periods. Knowledge of the distribution of associations can help mine managers design effective rehabilitation programs. However, a knowledge of dissolved oxygen concentrations in the lake, as modeled in Part II, is essential before integrated models of water-rock reactions in the future Martha lake can be developed, as pyrite bearing-associations may generate little or no acidity under reduced subaqueous conditions.