Geochemistry of critical minerals in mine wastes in New Mexico

New Mexico is home to tens of thousands of inactive mine features in 274 mining districts, including coal, uranium, metals and industrial minerals districts. However, many of these mines have not been inventoried or prioritized for reclamation. Many have existing mine wastes generated during mineral production, which could have potential for critical minerals, especially as the actual mineral production was generally of precious and base metals and not critical minerals (Fig. 1). The purpose of the project described in this article is to characterize and estimate the critical mineral endowment of mine wastes and to “beta-test” U.S. Geological Survey (USGS) sampling procedures. This project is important to the state of New Mexico because critical mineral resources must be identified before land exchanges, withdrawals or other land-use decisions are made by government officials. Future mining of mine wastes that potentially contain critical minerals will directly benefit the economy of New Mexico. Possible remining of mine wastes could clean up these sites and pay for reclamation. Furthermore, this project will include training of younger professional geologists and students in economic and reclamation geology by senior staff. Figure 1: Map of mining districts in New Mexico that have critical minerals. Coalfields are not shown. Other areas in the state, such as high-magnesium dolomites, lithium-bearing playas and coalfields, are not found in specific mining districts and also have critical mineral (CM) potential. Introduction Since its creation in 1927, the New Mexico Bureau of Geology and Mineral Resources (NMBGMR) has collected published and unpublished data on the mining districts, mines, deposits, occurrences and mills in New Mexico and is slowly converting historical data into a relational database, called the New Mexico Mines Database (McLemore et al., 2005a,b; McLemore, 2017; geoinfo.nmt.edu/staff/mclemore/MinesinNewMexico.html). More than 9,000 mines are recorded in this database, of which more than 7,000 are inactive or abandoned. The New Mexico Abandoned Mine Lands Bureau (AMLB) of the New Mexico Energy, Minerals, and Natural Resources Department estimates there are more than 15,000 abandoned mine features in the state (www.emnrd.state.nm.us/MMD/AML/amlmain.html). The U.S. Bureau of Land Management (BLM) estimates that more than 10,000 mine features are on BLM lands in New Mexico, and only 705 sites have been reclaimed (www.blm.gov/wo/st/en/prog/more/Abandoned_Mine_Lands/abandoned_mine_site.html). Most of these mines have existing mine wastes, generated during mineral production and exploration, which could have potential for critical minerals. Critical minerals are mineral commodities that are essential to the economic and national security of the United States, and are from a supply chain that is vulnerable to global and national disruption. In the mining industry, minerals refer to any rock, mineral or other naturally occurring material of economic value, including metals, industrial minerals, energy minerals, gemstones, aggregates and synthetic materials sold as commodities. Thus, the term includes all inorganic substances as well as hydrocarbons, such as oil and natural gas, and carboniferous deposits, such as coal. Many critical minerals are produced in other countries and 100 percent imported into the United States. In many cases, mineral deposits are available in the world for specific critical minerals, but the real challenge for the nation’s economy and security is potential supply disruptions. Disruptions in supply chains can arise for any number of reasons, including natural disasters, labor strife, trade disputes, resource nationalism and conflict. Rare earth elements (REEs) and other critical minerals are essential in most of our electronic devices, such as cell phones, laptops, computer chips, batteries, magnets, wind turbines and hybrid/electric cars (Committee on Critical Mineral Impacts of the U.S. Economy, 2008; Long et al., 2010; McLemore, 2011; McLemore and Gysi, 2023). Other technologies are being developed like solar panels, water purification, desalination, magnetic refrigeration and more efficient light bulbs that require REEs and other critical elements in their manufacture. A variety of minerals, such as magnesium (Mg), REEs, cobalt (Co) and lithium (Li), are required to manufacture batteries, and some of these critical minerals are found in and even produced from New Mexico (Fig. 2), including the mine wastes from previous mines. Figure 2: Periodic table showing critical minerals in New Mexico. Copper (Cu) was added as a critical mineral in 2023 by the Department of Energy (DOE). Figure 3: Locations of areas sampled in the New Mexico mine waste study. Mine wastes have the potential to contain critical minerals, especially as the actual mineral production was generally for precious and base metals and not critical minerals. Therefore, any critical minerals found in the mineral deposit also would be found in the mine wastes, such as mine waste dumps, tailings and slags. Although some of these critical minerals are located in New Mexico, they have not been important exploration targets in the past because demand has been met economically elsewhere, mostly from China. However, with the projected increase in demand for critical minerals and the potential lack of available production from China and other countries, the New Mexico deposits including mine wastes are being reexamined for their critical mineral potential, and several areas of mill tailings are undergoing current exploration. Not only are these data required to delineate favorable mine wastes for the USGS project, but the identification and examination of critical minerals is a high priority of the NMBGMR. This project is important to the state of New Mexico because critical mineral resources must be identified before land exchanges, withdrawals or other land-use decisions are made by government officials. Future mining of potential critical minerals deposits will directly benefit the economy of New Mexico. Possible remining of mine wastes could clean up these sites or pay for reclamation. This project will also allow training of the next generation of younger professional geologists and students in economic and reclamation geology by senior staff. We plan to evaluate typical environmental characteristics of the mine wastes — such as acid base accounting, pH and leaching tests — of the mine features, because these evaluations will be required to obtain permits and provide a safe work environment during mining, reclamation and/or waste reprocessing. The purposes of this project are to (1) test sampling protocols of mine wastes in a variety of mine features, including waste rock piles, stock piles, tailings and pit lakes, (2) determine the potential for critical minerals in mine wastes in New Mexico and (3) compare results between mining districts and types of deposits. Previous sampling of mine wastes in mining districts in New Mexico are compared to new sampling for the USGS mine waste project. Description of areas sampled The NMBGMR has been investigating mine wastes since the 1950s. Samples have been collected for chemical analyses and other characterization data from several mining districts throughout New Mexico as part of prior projects: (1) an inventory and characterization of mine features and evaluation of mine wastes as part of the New Mexico AML project (geoinfo.nmt.edu/hazards/mines/aml/home.html), (2) a study on uranium transport, uranium source characteristics and uranium legacy issues in New Mexico (EPSCoR, geoinfo.nmt.edu/geoscience/research/home.cfml?id=81& project=Uranium+Transport+and+Sources+in+New+Mexico+A+fiveyear+EPSCoR+program), (3) an assessment on the effects of the Gold King Mine spill on the Animas and San Juan Rivers in northern New Mexico and (4) specific mining district studies. More recently, three districts in New Mexico were selected for sampling and testing procedures for the USGS Earth Mapping Resources Initiative (Earth MRI) mine waste project. Samples discussed in this article were collected from several districts in New Mexico, as summarized in Table 1, and locations are shown in Fig. 3. The samples represent different types of mine features, including low-grade stockpiles, mine waste rock piles, tailings and slags, as well as different deposit types. Figure 3: Sieving mine waste rock pile samples. Methods Inventory and mapping of mine features. An important step in this study is to compile all published and unpublished data from existing mines and prospects within the mining districts of New Mexico in order to summarize the mining history and begin the inventory of mine features in the district. Mineral databases were examined, including the Mineral Resource Data System (MRDS) of the USGS, the Minerals Industry Location System (MILS) of the U.S. Bureau of Mines (USBM), the U.S. Forest Service Abandoned and Inactive Mines database, and the Abandoned Mine Land Inventory System (AMLIS). Published and unpublished reports and files at the NMBGMR were also examined. Using these data, mineral occurrences, deposits, mines, prospects, resources, tailings and mills were identified, plotted and entered in the NMBGMR New Mexico Mines Database (McLemore et al., 2005a,b; McLemore, 2017). Waste rock piles and tailings were mapped using a handheld GPS, measuring tape and Brunton compass and/or Lidar. Sketches of selected mines and associated mine features were compiled. The primary purpose of the field inventory was to accurately locate and describe the mine features, identify those features that required more detailed mapping and identify those features suitable for sampling. Project databases. Databases were developed for this project (McLemore et al., 2022). Initially, data were collected and stored in MS Access databases. During the course of the project, to increase reliability, efficiency and security, the data were migrated to MS SQL Server (SQLS), and the Access “front end” forms were reworked to connect to the external database server. Locational and other information of photographs taken in the field, thin sections and hand samples are recorded in the SQLS database. Location and descriptions of samples referred to in this report are in the SQLS database. Sample collection Mine waste features (defined as low-grade stock piles, mine waste rock piles, gob piles, tailings, slag and so on) vary in size, shape, composition, nature of source material, waste management approaches, influence by climate, and site accessibility. These variables will affect safety and access considerations. The number of samples, the type of samples, the sample locations, intervals and other specific procedures depend on the safety, study’s objectives and equipment availability. Two separate splits of each sample are generally collected: one split for chemical analyses that is submitted to the chemical laboratory, and a second split that is archived at NMBGMR. Water sampling of mine waters occurred during this phase of the project but is not discussed in this article. Future reports will describe the sampling and critical minerals potential of mine waters. The first step is to examine the geology and topography of the mine feature. If a feature is large and includes topographical or other distinguishing features (such as benches, separate piles and tailings color) that point to a change in composition or site management, then the feature should be split into separate sample units. A scintillation counter can be used to identify more radioactive zones in the mine feature as separate sample units. Several types of samples can be collected: composite, select or profile samples. Location, type of sample and other descriptive data are entered into the project database. Samples are archived at NMBGMR for future examination. Figure 6: An oxidized profile in a mine waste rock pile at Copper Flat, NM. Composite samples. A composite sample of the mine feature is collected once sample units have been identified in a particular mine feature. Composite samples of waste rock piles are collected using procedures developed by Munroe (1999) and the USGS (Smith et al., 2000; Smith, 2007; McLemore et al., 2014; USGS 2023 memo). This type of sampling was developed by the USGS and poses no stability or erosion risks of the waste rock piles or tailings. Each sampling unit is subdivided into at least 30 cells of roughly equal area. To do this, flags/markers are placed equidistant from each other forming a rough grid, where the flag/maker represents the center of each cell (Fig. 4). If a particular subsample location is inaccessible, an alternative may be selected as close to the original location as possible. GPS locations are also recorded simultaneously while placing flags/markers or while removing flags/markers after sampling. The overburden or overlying surface material (such as leaves, grass, roots, and cover material of tailings) is first removed. Care is taken when removing the cover material from tailings to avoid dilution. Each subsample is collected using identical procedures. The desired volume is collected into the required sieve size (less than 2 mm), and sieved into a single decontaminated bucket or bag, making sure to homogenize while sampling (Fig. 5). For tailings samples that are wet upon collection, the samples are air-dried prior to sieving, and any particle agglomerates, which can form especially upon drying a wet sample, are disaggregated before sieving. Chip samples can be collected as well (especially for waste rock piles where a large percentage of the material is coarse) from the over-sieved material into a separate bucket or bag and broken into chips to remove weathering surfaces. Finally, all the composite samples (fines and coarse) are thoroughly homogenized again and then transferred into appropriately labeled sample containers. Select samples. Select samples include hand specimens of mineralized rocks (including ore samples), grab samples of the mine wastes, and samples from individual cells from anywhere in the mine feature and are collected for specific purposes (such as to identify minerals or separate chemical analyses). These samples are collected separate from the composite sample. The volume or number of samples collected is highly dependent on the type of laboratory test to be conducted and the availability of the sample. Weathering surfaces are removed from the rock samples. If the purpose of sampling is for geochemical analysis, the samples are broken into smaller chips. Grab samples are collected directly from the mine waste feature or the base of the slope to avoid sampling just float material. The sample is stored in a well-labeled bag. Profile samples. Profile samples are generally collected along a vertical exposed section of the mine feature (Fig. 6). The soil profile may be exposed, or a small pit may be dug to reveal a profile. The depth of the pit is dependent on depth of the mine waste feature. A small trowel is used to clean the surface of the profile gently to prevent the boundary lines from caving in. Changes in different lithologies (such as color, thickness, depth texture and grain size) are identified and recorded in the field notebook. Samples are carefully collected by scraping directly into a well-labeled sample bag (the sampling depth is included on the sample bag) or decontaminated container using a small trowel or scoop to avoid cross-contamination from different lithologies. Sampling is done from the midpoint of the profile as much as possible, as sampling the boundary can result in mixing lithologies. One GPS location and waypoint are assigned to the pit, and different sample IDs to each lithology sampled in the profile. Figure 7: Chondrite-normalized (Taylor and McLennan, 1985) rare earth elements (REEs) for samples from mining districts and different deposit types in New Mexico. (See Table 1 for summary of districts sampled and Fig. 3 for locations.) Precautions. Some general precautions in sampling include the following. All equipment should be cleaned to prevent cross-contamination of the sample. Sampling tools (buckets, sampling bags, shovels, trowels and sieves) should be constructed of materials suitable for environmental sampling (typically stainless steel, plastic or aluminum). Devices plated with chrome or other materials should not be used, as they can introduce contaminants to the samples. All equipment used for sampling should be rinsed with deionized water and air-dried prior to use. Disposable gloves should be worn while sieving to avoid contamination. The samples are transported from the field to NMBGMR, where each sample is prepared for analyses. Selected samples are cut, and chips sent for preparation of polished thin sections. The prepared samples are then sent to a laboratory for chemical analyses. NMBGMR standards are submitted blind to the commercial laboratory with each sample batch to assure analytical quality. Figure 8: Nickel (Ni) versus cobalt (Co) for samples from mining districts in New Mexico. (See Table 1 for summary of districts sampled and different deposit types, Fig. 3 for locations and Fig. 7 for legend.) Geochemical analyses Geochemical data are a critical part of geologic mapping and for evaluation for critical mineral resources central to the mission of Earth MRI. Geochemical analyses of samples collected for this study were determined by the USGS laboratory and by ALS Laboratory (a description of methods can be found at ALS Geochemistry Fee Schedule USD (2).pdf and in future reports). Samples were submitted to the laboratories where sample preparation occurred. Duplicate samples and standards were analyzed, and uncertainty of analyses is generally less than 5 percent. Specific analytical methods for each element and additional quality assurance and quality control (QA/QC) are available on request. Chemical plots were created using ioGAS-64 from Reflex. Chemical analyses will be published in future reports. Petrography and mineralogy Hand-sample descriptions of both sawed samples and thin sections were entered into the project’s SQLS database. Polished thin sections of selected samples of the igneous, altered and mineralized rocks were made by Quality Thin Sections. Thin sections were scanned in both plane and polarized light, and selected photomicrographs were taken. The mineralogy of selected samples was determined by visual and petrographic, X-ray diffraction (XRD) and electron microprobe methods.XRD analysis was performed on either whole rock or mineral separates using a PANalytical X’Pert Pro diffractometer at the NMBGMR X-ray Diffraction Laboratory. Analyses were conducted using 45-kV X-ray beam tension and 40-mA X-ray beam current. XRD scans were identified using X’Pert HighScore Plus software, which identifies intensity peaks and matches patterns to a Powder Diffraction File database. XRD data will be available in the final report. Petrographic descriptions, including mineralogy and texture, of thin sections using plane, polarized and reflective light are entered into the SQLS database. Figure 9: Bismuth (Bi) versus tellurium (Te) for samples from mining districts in New Mexico. (See Table 1 for summary of districts and different deposit types sampled, Fig. 3 for locations and Fig. 7 for legend.) Results Potential for critical minerals. Figures 7, 8 and 9 shows some of the chemical plots of mine waste samples from mining districts in New Mexico. Potential for acid rock drainage. Acid rock drainage (ARD) is formed when sulfide minerals are oxidized by meteoric water or atmospheric exposure (weathering). Field characteristics of potential ARD in mine waste rock piles include identification of pyrite and/or jarosite and low pH (Fig. 10). The rate of sulfide oxidation depends on the reactive surface area of sulfide minerals, oxygen concentration and solution pH. ARD potential can be determined by acid base accounting (ABA) and net acid generation (NAG) tests. Figure 10: Acid rock drainage (ARD) plot of waste rock pile at mines examined during the NMBGMR mine wastes project. The ABA procedure consists of two separate tests: the acid potential (AP) test and the neutralization potential (NP) test. ABA was calculated and plotted on the ARD classification plot for waste rock pile samples from the various mines (Sobeck et al., 1978). The assumption is that all carbon (C) in the samples are as CaCO3 (no organic carbon) and that the NAG pH equals the measured paste pH of the sample. The formulas used include: AP (kg CaCO3/t) = 31.25 × S (percent)NP (total C) = 83.3 × C (percent)NNP = NP – APNPR = NP/AP This type of geochemical analysis can be used to develop site-specific criteria for the identification of potential acid-generating rock types. Paste pH is a simple, rapid and inexpensive screening tool that indicates the presence of readily available NP (generally from carbonate) or stored acidity. The outcome of the test is governed by the surficial properties of the solid material being tested, and more particularly, the extent of soluble minerals, which may provide useful information regarding anticipated mining-influenced water quality. For example, acidic paste pH values in combination with elevated sulfate sulfur generally suggest the presence of acidic sulfate salts that potentially could cause short-term or long-term water quality issues. Preliminary conclusions A sampling protocol has been developed that can estimate the chemical composition of mine wastes, including low-grade stockpiles, mine waste rock piles, tailings and slags. Larger mine features are divided into areas for sampling. Composite samples are collected from cells within the sample areas and homogenized into a single sample. Elevated critical minerals are found in some of the mine features in New Mexico, and these features should be evaluated for potential remining. The critical minerals are dependent upon deposit types. Possible remining of mine wastes could clean up these sites and pay for reclamation. n Acknowledgments This report is part of ongoing studies of mineral resources in New Mexico, supported by the New Mexico Bureau of Geology and Mineral Resources (NMBGMR), Dr. Mike Timmons, director and state geologist. This current research is funded by USGS National Geological and Geophysical Data Preservation Program (NGGDPP), Earth Mapping Resources Initiative (Earth MRI) program (G22AC00510), and Department of Energy CORE-CM project DE-FE0032051 (2021–2024). Past funding incudes Energy Minerals and Natural Resources Department (Abandoned Mine Lands Program) and the U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (OSMRE). Additional funding of student research (including laboratory analyses) was by the Mineral Engineering Department of NMIMT, Society for Mining, Metallurgy & Exploration (SME), New Mexico Geological Society (NMGS), NMBGMR Brightstar scholarship, and New Mexico EPSCoR (funded by the National Science Foundation, NSF, award #IIA-1301346). Thanks to Santa Fe Gold Inc. and New Mexico Copper Corp. for access to mining properties. Thanks to Mark Leo-Russell for database support, students of the NMBGMR Economic Geology Group for sample collecting (especially Abena Serwah Acheampong-Mensah, Zohreh Motlagh Kazemi and Devlon Shaver), and Richard Kelley for GIS support. References Bureau of Land Management, 2014, Abandoned Mine Land Inventory Study for BLM-Managed Lands in California, Nevada, and Utah: Site and Feature Analysis. Department of the Interior, Bureau of Land Management, National Operations Center, Denver, CO., 24 pp. www.blm.gov/pgdata/etc/medialib/blm/wo/blm_library/BLM_pubs.Par.79469.File.dat/BLM-AML-Inventory-CA-NV-UT_Nov2014.pdf. Committee on Critical Mineral Impacts of the U.S. Economy, 2008, Minerals, Critical Minerals, and the U.S. Economy: Committee on Earth Resources, National Research Council, ISBN: 0-309-11283-4, 264 pp. www.nap.edu/catalog/12034.html. Long, K.R., van Gosen, B.S., Foley, N.K. and Cordier, D., 2010, The principle rare earth elements deposits of the United States—A summary of domestic deposits and a global perspective: U.S. Geological Survey, Scientific Investigations Report 2010-5220, 104 pp. pubs.usgs.gov/sir/2010/5220/ (accessed 5/1/12).McLemore, V.T. 2011, Rare earth elements foremerging technologies: New Mexico Earth Matters, summer, 4 pp. geo-info.nmt.edu/publications/periodicals/earthmatters/ 11/EM11n2.pdf. McLemore, V.T., 2017, Mining districts and prospect areas of New Mexico: New Mexico Bureau of Geology and Mineral Resources, Resource Map 24, 65 pp., scale 1:1,000,000. McLemore, V.T., 2020, Critical minerals in New Mexico; work needed to realize resources: Mining Engineering, v. 72, no. 2, pp. 31–31, me.smenet.org/abstract.cfm? preview=1&articleID=9501&page=31.McLemore, V.T., Hoffman, G., Smith, M., Mansell, M., and Wilks, M., 2005a, Mining districts of New Mexico: New Mexico Bureau of Mines and Mineral Resources, OF-494, CD-ROM. McLemore, V.T., Krueger, C.B., Johnson, P., Raugust, J.S., Jones, G.E., Hoffman, G.K. and Wilks, M., 2005b, New Mexico Mines Database: Mining Engineering, February, pp. 42–47. McLemore, V.T. and Gysi, A., 2023, Critical minerals in New Mexico: Earth Matters, winter 2023, geoinfo.nmt.edu/publications/periodicals/earthmatters/23/ n1/em_v23_n1.pdf. Munroe, E.A., 1999, Geology and geochemistry of waste rock piles in the Hillsboro mining district, Sierra County, New Mexico: M.S. thesis, New Mexico Institute of Mining and Technology, Socorro, 144 pp. Smith, K.S. 2007. Strategies to predict metal mobility in surficial mining environments, J.V. DeGraff, edi- tor, Understanding and Responding to Hazardous Substances at Mine Sites in the Western United States: Ch. 3. Reviews in Engineering Geology, Vol. 17, Boulder, CO, Geological Society of America. pp. 25–45. Smith, K.S., Ramsey, C.A., and Hageman, P.L., 2000, Statistically-based sampling strategy for sampling the surface material of mine-waste dumps for use in screening and prioritizing historical dumps on aban- doned mines lands; in ICARD 2000; Proceedings from the 5th international conference on acid rock drain- age: Society for Mining, Metallurgy, and Exploration, Inc., Littleton, Colo., pp. 1453–1461. crustal.usgs.gov/minewaste/minewaste.pubs.html. Sobek, A. A., Schuller, W. A., Freeman, J. R. and Smith, R.M., 1978, Field and laboratory methods applicable to overburdens and minesoils: U.S. EPA 600/2-78- 054, Washington, D.C., 203 pp. Taylor, S.R. and McLennan, S.M., 1985, The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publication, Carlton, 312 pp.

Geochemistry of critical minerals in mine wastes in New Mexico

Authored by: Virginia T. McLemore and Evan J. Owen

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