In New Mexico, there are at least 100,000 abandoned mine waste rock piles with widely ranging mineralogical and geochemical compositions. To better understand the environmental implications of metal mobility in regions of minimal precipitation (< 250 mm/year) a mineralogical and geochemical study was implemented for five mine waste rock piles, some of which contained sulfides, as well as the drainage systems from these areas in the Hillsboro mining district. Furthermore, this study will give insight into the mineralogical mechanisms governing metal mobility in arid environments.
The average metal content of a Laramide vein waste rock pile is 1,200 ppm copper, 230 ppm lead, 550 ppm zinc, and 26 ppm arsenic, but a stream sediment sample directly below the area contains 190 ppm copper, 52 ppm lead, 150 ppm zinc, and 8 ppm arsenic. The mineralogical assemblage of the same Laramide vein waste rock pile consists of (in order of abundance) quartz, albite, microcline, chlorite, illite, hornblende, muscovite, pyrite, chalcopyrite, hematite, minor bornite, chalcanthite, and actinolite. The stream sediment contains igneous rock fragments, quartz, albite, chlorite, hematite, minor pyrite, calcite, gypsum, and cuprite.
Mineralogical characteristics of mineral grains and their weathering rind products were examined with the electron microprobe to examine the chemical breakdown of minerals that release metals to the environment. Several minerals have weathering rinds of different mineralogical and chemical compositions than their cores. Pyrite and chalcopyrite appear to be the most reactive to surficial weathering and tend to show the thickest weathering rinds. Chemical composition of a pyrite core/rind transect showed iron concentration increased from 46% to 66% while sulfur decreased from 54% to 2%. Chemical composition of a chalcopyrite core/rind transect showed copper concentration decreased from 33% to 3%, iron increased from 30% to 58%, and sulfur decreased from 34% to 0.3%. Sulfur lost from the system will be removed by the formation of sulfuric acid after oxidation occurs. Clays, iron oxide/hydroxide, iron sulfate, and primary texture rinds exist in the waste rock piles analyzed.
Metal ratios of mine waste rock pile/stream sediment sample are copper 6.3, lead 4.4, zinc 3.6, and arsenic 3.2. Metal mobility, therefore, can be described in the following manner: As > Zn > Pb > Cu. Supergene oxidation of galena created a cerrusite rind, which effectively shields the galena grain from further weathering and release of lead and sulfur. This may decrease the level of lead mobility depending on the overall acid producing capability of the waste rock pile. Metals present in sulfide mineral weathering rinds are in higher concentrations than those in rinds surrounding oxides or silicates. Oxides and silicates need more time and water to break down and showed at most a minimal clay rind. This indicates metal mobility is higher in the sulfide minerals, which therefore enhances metal availability to the environment.