A method for quantitative pyrite abundance in mine rock piles by powder X-ray diffraction and Rietveld refinement

Abstract

The abundance of pyrite and other sulfide minerals in mine rock piles is a potentially significant if not a determinative factor in terms of the geochemical and geomechanical evolution of the dumps as oxidation produces acid solutions that drive hydrolysis reactions. A technique is presented here that supports the quantitative determination of pyrite abundance in mine rock dumps by heavy liquid mineral separation to concentrate pyrite for powder X-ray diffraction and then Rietveld method refinement of the diffraction data on a large number of samples using commonly available laboratory equipment. In order to improve and constrain the accuracy of XRD results, binary (pyrite-quartz) and 6-part mineral mixtures (pyrite and rock-forming andesite minerals) spanning a wide range of pyrite concentrations were prepared gravimetrically and run as standards. These standards were then used to minimize errors in pyrite abundance data by constraining key input parameters in the Rietveld refinement. A new polynomial relationship was derived between diffracting crystallite size and the Brindley microabsorption correction input size. This method provides a means to determine uncertainties in pyrite abundance, whereas conventional Rietveld refinement techniques done without the use of standards yield only statistical measures of the least-squares fit, rather than absolute uncertainties in mineral constituent weight percentages. The technique was applied to a number of mine rock pile samples and the uncertainty in the results determined by applying the relationship derived from the 6-part gravimetric standards to the results of the Brindley corrected Rietveld refinements. Uncertainties determined by this method are found to be on the order of ±10% for samples with pyrite content greater than ∼10 wt% and ±30% for samples with pyrite content less than 10 wt%. In order to evaluate the technique’s improvement upon traditional visual mineral abundance estimation the quantitative results are compared to manual volumetric estimates.

Introduction

The mining industry is beginning to appreciate that sulfide-bearing mine rock dumps are among the most chemically reactive rocks known on earth and that their long-term environmental management poses unprecedented challenges and significant costs. This growing awareness of the environmental sensitivity of mine rock piles is complicated by the fact that these rock piles have often been emplaced over a long time period with variable source rocks, while few records were kept, and before concern about their stability developed. Hence, the data relevant to mine rock pile behavior, especially in terms of geomechanical stability and acid production, is scant at best in some mines facing closure, or in those requiring a closure plan in order to keep operating. This paucity of available data on sulfide mineral abundance motivates the development of a new quantitative strategy for mine rock pile characterization required for modeling their long-term geochemical evolution and geotechnical stability. Specifically, pyrite abundance, distribution, and grain size (surface area) in combination with O₂ availability influence to first order both the rate and duration of acid generation by bacterially catalyzed surface oxidation and resultant hydrolysis reactions that affect other minerals in the rock piles, especially silicates and carbonates. The resultant chemical weathering reactions in certain rock types, especially aluminous silicates like feldspars, can yield clays and other minerals, that can cause mechanical and hydrological changes in the physical behavior of the mine rock piles and thus be relevant to geotechnical stability over long (hundred–year) time scales.

As in natural supergene leaching and enrichment systems, it is the exposure of pyrite to bacterially catalyzed oxidation in the air-filled voids in the unsaturated zone of the rock pile that drives the creation of H₂SO₄ and the weathering of rocks under near surface conditions. Acid interacts with rock minerals by hydrolysis (reaction with H⁺ ion) to produce acid rock drainage solutes and secondary minerals such as clays and gypsum formed in situ or along the flow path. Therefore, in supporting geochemical modeling, it is essential to accurately characterize the pyrite content of the mine rock piles and its spatial distribution, specifically in relation to minerals that absorb and buffer acidity.

Mine rock piles in general pose several problems when evaluating and quantifying their mineralogical composition. The manner in which the dumps were emplaced is a complicating factor, in that successive dumping events may have been separated by a considerable length of time, allowing oxidation and weathering to occur at the dump surface. This weathered surface is then covered by subsequent dumping events and a horizon that may be considerably different from the surrounding material is included in the growing pile. The rock fragments comprising the pile are of varying size as a result of blasting and can range from boulders to rock dust, greatly influencing the rate at which weathering and alteration reactions take place by means of increasing the effective surface area available for reaction.

The model was developed and applied to mine rock piles at the porphyry Mo mine at Questa, New Mexico, and has wide application to many other existing mine rock piles worldwide including porphyry Cu and massive sulfide deposits. The Questa Mo mine, owned and operated by Molycorp, Inc., is located in the Sangre de Cristo Mountains in Taos County, northern New Mexico, USA, at an elevation of 2400–2900 m. From 1965–1982, large-scale open pit mining produced over 330 million tons of mine rock, which was end-dumped into various steep-sided valleys adjacent to the open pit. Accordingly, these mine rock piles typically exist at angle of repose and have long slope lengths (up to 600 m) with depths from 30 to 110 m (Lefebvre et al., 2002, Shaw et al., 2002). In general, the rocks hosting this Climax-type porphyry Mo deposit and composing the mine rock piles consist primarily of mid-Tertiary, hydrothermally altered (mostly propylitized) Latir andesite porphyry flows and associated volcanoclastic sandstone with a significant volume of rhyolite dikes and hydrothermally altered (quartz-sericite-pyrite) Amalia welded tuff. The economic mineralization from the open pit consists of high grade (>0.2% by weight) hydrothermally deposited molybdenite (MoS₂). Associated with the hydrothermal mineralization processes are pyrite veinlets and disseminations with an average pyrite abundance of 3.5% by volume (Shaw et al., 2002, Wels et al., 2003).

The goal of this study is to develop a method that supports accurate and rapid quantification of mineralogy, especially pyrite, in mine rock piles. The level of accuracy and the degree to which these results are reliable (i.e. uncertainties), are extremely important in regards to providing meaningful inputs for geochemical modeling, and so the technique must both minimize error and provide for a reliable assessment of these uncertainties. In order to be practical, the analysis strategy must be both accurate and fast as an enormous number of samples are required to adequately characterize the heterogeneous nature of these large mine rock piles.