New Mexico Geological Society Annual Spring Meeting — Abstracts

Mine waste rock piles are anthropogenically created landforms in mining areas that are derived from open pit and underground mining methods. These waste rock piles exhibit a diverse mineralogy and geochemistry. In addition to the mineral and chemical heterogeneities: the waste rock piles contain variable grain sizes. There are 100,000s of waste rock piles in New Mexico of varying size and composition. One ofthe controversial problems facing scientists today is how to sample these waste rock piles quickly and economically in order to adequately assess and prioritize envirorunental hazards.

A sampling strategy was developed to geochemicaUy characterize four mine waste rock piles in the Hillsboro mining district in Sierra County, New Mexico. The piles represented four different mineral deposits and include: a placer gold waste rock pile (Site A), a Laramide polymetallic vein waste rock pile (Site B), a carbonate-hosted Pb-Zn waste rock pile (Site C), and a carbonate-hosted Ag-Mn waste rock pile (Site D). The waste rock piles have highly variable grain sizes. Chemical analyses of six size fractions indicated the less than 0.25 mm mesh typically contained the highest metal concentrations. In addition to each of the grain size fractions, chemical heterogeneity is found within an entire waste rock pile. As an example, 45 samples were analyzed by XRF for each ofthe sample sites within a grid overlying the Laramide polymetallic vein waste rock pile (Site B). Copper values range from 460-2700 ppm, lead from 73-730 ppm, zinc from 120-1400 ppm, and arsenic from 4-76 ppm.

Each of the four waste rock piles were sampled using unique grids which conformed to their respective dimensions. To insure equal treatment of the samples, sampling was performed using a less than 0.25 mm stainless steel sieve. Each of the waste rock piles were sampled with densities of 15, 30, and 45 samples within each of the respective grids. This study indicates that heterogeneous mine waste rock piles can be adequately sampled and chemically characterized by homogenizing samples collected from a grid containing 15-30 sample cells.