Dewatering assessment at Hilton Mine, Queensland: An application of gravity methods

Abstract

It was anticipated that the altered rock mass associated with Pb-Zn mineralization within Proterozoic dolomitic siltstones at Hilton, NW Queensland, would create problems during mine development. The material is variably oxidised, very porous, saturated and extensive. Semi-regional gravity data have been used to estimate the volume and extent of such material and assist mine planning and dewatering studies. Interactive, segmented, three dimensional procedures controlled by exploration drilling were used. The interpretation considers the effect of all materials and is not restricted to the high contrast alteration. The altered rock mass was found to be irregular, dip steeply west and to be etched from the ore-bearing horizons. It is up to 400 m deep. The alteration has been controlled by the steeply dipping form of the host rock footwall as well as by local faults and jointing . The shape of the cone of depression, after dewatering for more than five years, has confirmed the interpretation within the first development block and lends confidence to the more regional implications along a strike length of at least 6 km. Analyis has also indicated that the volume and density contrast of the ore zone also varies along strike. The average bulk densities deduced for the ore and alteration zones are 3.1 gm/cm3 and 2.1 gm/cm3 or a contrast of about +0.4 gm/cm3 and ?0.6 gm/cm3 with respect to the Mount Isa Group host sequence.