The Pasminco Century Mine (PCML) is located 250 kilometres north-north-west of Mt Isa in Queensland. As one of the world's largest zinc mines, PCML comprises mining and milling operations at Lawn Hill, concentrate filtering and shipping facilities at Karumba in the Gulf of Carpentaria, and a 304 km slurry pipeline connecting the two operations._x000D_
Mineralisation at Century occurs in the upper part of the Lawn Hill formation and is comprised of siltstones, shale and sandstone. Pyrite occurs immediately above the mineralised sequence, and throughout the footwall carbonaceous shale forming the lower proportion of the deposit. Zinc predominantly occurs as sphalerite (ZnS) and lead as galena (PbS). These sulfides are very fine-grained and may oxidise rapidly upon exposure. The pyrite, sphalerite and galena are the main potential sources of acid rock drainage (ARD) or heavy metal leachate at the mine site._x000D_
To minimise the generation of ARD in the waste rock dumps, they are constructed in a specific manner and waste rock is classified into the following three classes based on competence and acid forming/consuming capabilities: Class 1: Competent rock, non-acid forming or acid-consuming material._x000D_
Class 2: Non-competent, non-acid forming or acid-consuming material._x000D_
Class 3: Acid-forming material._x000D_
The generalised Waste Rock Dump Design for current operations comprises an outer zone of Class 1 material and inner zone with Class 2 and 3 material. Limestone is the Class 1 material used for both its structural support and acid neutralising capacity. Reserves of limestone at the mine site are limited. Net acid generation (NAG) tests confirm that a mix of limestone and shales can be net acid consuming._x000D_
Field based and laboratory based trials have been established at PCML to quantify blending levels of limestone and shale that will produce leachates both neutral in pH and low in metal/salt concentrations. This paper will discuss the first six months results (of a two-year sampling program) from the trials._x000D_
FORMAL CITATION:McIlwaine, L M and Ridd, M, 2004. Assessment of leachate from blended waste rock material at the Pasminco Century Mine - initial results from a two-year study, in Proceedings New Leaders' 2005, pp 15-22 (The Australasian Institute of Mining and Metallurgy: Melbourne).