Mine Rock Piles Research Articles

Isotope hydrology of the intermontane Elk Valley, British Columbia: an assessment of water resources around coal mining operations

ABSTRACT This study aimed to synthesise and interpret stable isotopic data (δ 2H and δ 18O) from various sources to understand the isotope hydrology around coal mine operations in Elk Valley, B.C., Canada. The data, including precipitation, groundwaters, seeps, and mine rock drains, were used to construct a local meteoric water line (LMWL) for the Elk Valley, evaluate the spatiotemporal isotopic composition of its groundwater, and assess mine seepage and mine rock drain discharge. The study revealed a robust LMWL relation (δ 2H = 7.4 ± 0.2 · δ 18O – 4.3 ± 4.1). The groundwater and seep data indicated a winter season bias and a north–south latitudinal gradient, suggesting rapid near-surface groundwater flow without significant post-precipitation evaporation. Porewater isotope samples from unsaturated mine rock piles (MRPs) showed site-specific evaporation patterns, potentially due to convective air flows or exothermic sulphide oxidation. This research revealed the influence of groundwater and meltwater on rock drain discharge. Based on evaporative mass balance calculations, MRPs seasonally contributed ca. 5 %(December base flow) and 22 % (snowmelt) to drain discharge. The findings underscore the value of stable isotope data collections in the Elk Valley to help better define and quantify the hydrology–hydrogeology, including a better understanding of evaporative conditions in MRPs.

The Role of Microorganisms in the Formation, Dissolution, and Transformation of Secondary Minerals in Mine Rock and Drainage: A Review

Mine waste rock and drainage pose lasting environmental, social, and economic threats to the mining industry, regulatory agencies, and society as a whole. Mine drainage can be alkaline, neutral, moderately, or extremely acidic and contains significant levels of sulfate, dissolved iron, and, frequently, a variety of heavy metals and metalloids, such as cadmium, lead, arsenic, and selenium. In acid neutralization by carbonate and silicate minerals, a range of secondary minerals can form and possibly scavenge these potentially harmful elements. Apart from the extensively studied microbial-facilitated sulfide oxidation, the diverse microbial communities present in mine rock and drainage may also participate in the formation, dissolution, and transformation of secondary minerals, influencing the mobilization of these metals and metalloids. This article reviews major microbial-mediated geochemical processes occurring in mine rock piles that affect drainage chemistry, with a focus on the role of microorganisms in the formation, dissolution, and transformation of secondary minerals. Understanding this is crucial for developing biologically-based measures to deal with contaminant release at the source, i.e., source control.

English

Past open pit mining operations at the Chevron Mining Inc. (CMI) Questa Mine in Questa, New Mexico, resulted in several large overburden rock piles. The rock piles are generally steep, angle of repose (2:1 and steeper), acidic (pH 2.5 - 7.0) and composed of a variety of igneous rock types. Pre-mining and currently undisturbed vegetation in the area of the mine site is dominated by mixed conifer forests. Forestry is the approved post-mining land use for this mine. Since the early 1990's studies have been conducted examining the potential for establishing the forest vegetation directly on the overburden rock piles. In the fall of 1996 and spring of 1997 two operational scale plantings at two locations on the mine rock piles were conducted to examine both the logistics associated with planting trees and shrubs directly on angle of repose slopes and the subsequent survival and growth performance of the transplanted vegetation. Short-term (three year) overall survival averaged 81%. There were survival differences between the two planting sites and among the three types of plants being transplanted with crop trees (conifers) averaging 87%, nurse trees (deciduous) averaging 63% and shrubs averaging 92%. Long-term (twelve year) survival averaged 43% and differed between planting sites with the lower elevation (2,500 m) planting site averaging 45% and the higher elevation (3,000 m) site averaging 41% overall survival. The resulting vegetative communities differed between the two planting sites with the higher elevation site having a higher proportion of crop trees compared to the lower site that had comparatively a higher proportion of shrub species. Implications of the results relative to future reclamation plantings are discussed.

Development of a Modified in situ Direct Shear Test Technique to Determine Shear Strength Parameters of Mine Rock Piles

Abstract A modified direct shear test apparatus was designed and used to measure cohesion and friction angle of rock pile materials. Two test apparatuses were constructed, a 30-cm square metal shear box and a 60-cm square metal shear box. In addition to the shear box, the testing apparatus has a metal top plate, a fabricated roller plate, normal and shear dial gages with wooden supports, and two hydraulic jacks and cylinders with a maximum oil pressure of 70 MPa (10,000 psi) and a load capacity of ten tons. The main difference between the in situ shear box and its conventional laboratory equivalent is that the in situ shear box consists of a single box that confines an excavated block of rock pile material. The lower half of the block consists of the rock pile material underneath the shear plane that is a semi-infinite domain. This modification in the shear test apparatus reduces the time needed for block preparation, helps perform several tests at different levels of the same sample block, and allows for accommodating large shear displacement with no reduction in the area of the shear plane.

Isotopic and textural discrimination between hypogene, ancient supergene, and modern sulfates at the Questa mine, New Mexico

Abstract Chemical and mineralogical changes due to pyrite weathering are of interest with respect to understanding long-term physical stability of mine rock piles at the Questa mine, New Mexico. The ability to discriminate between ancient and modern processes is important for establishing the extent of modern weathering within the piles. Initial inventories of sulfur minerals and representative isotope compositions in rocks from orebodies, the hydrothermal alteration zones associated with orebodies, hydrothermal alteration scars, and mine rock piles were determined. Ore body sulfides have δ 34 S CDT of 0 ± 4‰, typical for sulfides formed by magmatic processes in stockwork Mo systems. Pyrite from alteration scars has a wide range of δ 34 S values from 0.0‰ to −13.6‰. Sulfate from the ore body has markedly positive δ 34 S (5–10‰) accompanied by positive δ 18 O SO 4 values (6–15‰) reflecting equilibrium formation from magmatic fluids. Sulfates from alteration scars have δ 34 S values over a broad range, similar to alteration scar pyrites, from −10.6‰ to 0‰ and δ 18 O SO 4 of 0 ± 3‰. Sulfates with fine grained, delicate, and euhedral mineral habits suggesting recent formation within the mine rock piles, have δ 34 S values similar to orebody pyrite and alteration scars but more negative δ 18 O SO 4 values (−3‰ to −10‰). Sulfates from all three sources occur in these piles, and their stable isotope values have proven useful in differentiating them and their environments of formation (i.e., hypogene, ancient supergene, and recent weathering). Correlating the isotopic compositions with textures allows petrographic assessment for the origins of sulfate minerals in the rock piles, but this must be applied with caution because some sulfate mineral recycling has occurred.

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.

SECOND YEAR TRANSPLANT SURVIVAL ON CONSTRUCTED TEST PLOTS, QUESTA MINE, QUESTA, NEW MEXICO

Chevron Mining Inc. established a revegetation test plot program to evaluate the potential cover depth, soil amendment, and planting treatments for reclamation of the Questa Mine rock piles. The Questa Mine is located near the Village of Questa, New Mexico at an elevation of approximately 2438 m. The test plot program consisted of constructing multiple test plots on 3 slope gradients: 3 horizontal units to 1 vertical unit (3h:1v slope gradient), 2h:1v slope gradient, and flat gradient (platform) test plots. Each test plot was constructed with 3 cover soil depth treatments over the existing rock pile material. These cover soil depth treatments included: no cover, 30 cm, and 90 cm of cover soil material. Soil amendment treatments varied between the sloped and flat gradient test plots. The sloped gradient test plots received treatments of forest soil mycorrhizal inoculant or no inoculant. The flat gradient test plots received treatments of phosphate fertilizer, forest soil mycorrhizal inoculant, or no amendment. After construction, each test plot was seeded, hydromulched, and planted with transplant tree and shrub seedlings. The transplanted tree seedlings consisted of 2 categories: nurse species and crop species. The nurse species consisted of fast establishing, short-lived species, which will shade and protect the crop species. The crop species consisted of multiple conifer species, which represent the post-mining land use plant community. The crop and shrub species were planted at a constant density, while the nurse species were planted at 2 different densities. Overall survival of all seedlings (2008) for platforms, 3h:1v, and 2h:1v slopes was 47%, 44%, and 46% respectively. Of all the treatments applied to the test plots, the one that consistently made a significant difference in survival was the depth of cover soil. The overall survival of all seedlings in 2008 for 0, 30, and 90 cm covers was 22%, 59%, and 58% respectively.

English

Biological monitoring was initiated on the Red River in 1997 to evaluate the effects on aquatic biota of open pit mine operations and mine rock piles over nearly a 40-year period. The general public perception was that the mine has had severe effects on the aquatic biota due to the absence of robust biological populations adjacent to the mine. However, initial biological monitoring data provided evidence that the observed negative impacts to fish and benthic invertebrates in the Red River near the mine were actually caused by naturally occurring thermal scars upstream of, and within, the mine area in the Red River drainage. These data indicated the open pit mine and mine rock piles did not measurably impact the suitability of the Red River to support aquatic organisms. Continued biological monitoring has reinforced these earlier conclusions, and has shown that the aquatic biota is most likely limited by episodic summer rain storms which simultaneously add large amounts of sediment and degrade water quality in the Red River downstream of the hydrothermal scars, although groundwater appears to be a contributing factor in low baseflow periods. Fish and macroinvertebrate parameters decreased immediately downstream of the town of Red River, demonstrating that the negative impacts to the aquatic biota begin well upstream of the Molycorp Mine property. Long term biological monitoring has demonstrated the complex nature of factors structuring the aquatic biota in the Red River, and demonstrated the value of well-designed monitoring programs. Additional

English

Mining companies and their consultants generate large amounts of data at mine sites today. One of the most effective tools to enter, store, report, and utilize these data are by relational databases. The New Mexico Bureau of Geology and Mineral Resources has developed a database, the Mine Rock Pile database, to incorporate the characterization of mine rock piles for potential acid-mine drainage and rock pile stability. This database is being applied to an investigation at Molycorp's Questa mine in New Mexico. There are six main tables in the database: Samples, Drillholes, Test pits, Analytical data, Photographs and Bibliography, with more than 100 supporting tables. The data to be incorporated in this database includes locational data, climatic data, mineralogy, chemical analyses, geologic data on drill holes and test pits, historical and recent photographs, and other data. Existing data as well as newly collected data will be incorporated into the database. The purposes of this database are 1) to provide easy access to computerized data that will aid in the physical, mineralogical, chemical, and geotechnical characterization of the mine rock piles and 2) to create links to other databases at the mine and at NMBGMR. Forms allow for standard collection and input of data in the field and laboratory. Standard Operating Procedures (SOPs) are incorporated throughout the database to insure acceptable quality of sample collection and laboratory procedures. Once the data are entered into the database with locations, the data can easily be converted to GIS format for displaying on maps. The data can be imported into spreadsheets and used in most commercial software for data manipulation and interpretation.

Physical and Geochemical Characterization of Mine Rock Piles at the Questa Mine, New Mexico

AbstractFor Abstract see ChemInform Abstract in Full Text.

INFILTRATION TEST PLOT STUDY FOR MINE ROCK PILES AT QUESTA MINE, NEW MEXIC0

ABSTRACT An infiltration test plot study has been initiated at the Questa mine, located in the Sangre de Cristo Mountains in northern New Mexico. The objective of this study is to determine the key processes controlling infiltration into the mine rock piles. This study is part of a comprehensive site investigation carried out in support of the development of a closeout plan for the Molycorp Questa Mine. From 1965 to 1983 large-scale open pit mining at the Questa mine produced over 300 Mio tons of mine rock, which was end-dumped into various steep valleys adjacent to the open pit. As a result, the mine rock piles are typically at angle of repose and have long slope lengths (up to 2000 ft), and comparatively shallow depths (~100-200 ft). Initial analyses suggested that net infiltration may vary significantly across the site due to differences in the local micro-climate (caused by differences in elevation and aspect). In order to account for these differences, test plots were located on sites representative of low, mid and high elevation conditions. The test plots consist of 92” deep HDPE tanks (94” dia), which were backfilled with local mine rock material. The lysimeters are instrumented with suction and temperature sensors as well as access tubes for measuring volumetric moisture content at various depths. The lysimeters are free-draining and leachate from the test plot is monitored using a tipping bucket. An automated weather station was erected at each site to monitor all relevant climate parameters. This paper describes the design and installation of these test plots and summarizes monitoring results obtained to date. The results of this infiltration study will be used to assess the effectiveness of alternative closure measures.

An Empirical Technique for Predicting the Chemistry of Water Seeping from Mine-Rock Piles

An Empirical Technique for Predicting the Chemistry of Water Seeping from Mine-Rock Piles