New Mexico Geological Society Annual Spring Meeting — Abstracts

The Jones Hill body, a middle-Proterozoic-aged volcanogenic massive sulfide deposit in the Upper Pecos Watershed, has been targeted for exploration drilling, which raised concerns about acid drainage and metal leaching and their impact on surface and ground waters. To understand the potential effects of rock disturbance at the Jones Hill deposit, this study determined the acid generation potential (AGP) of the ore rock and acid neutralization potential (ANP) of the host rock. Hand specimen study of nearly 1000 feet of cores from six different drill core samples housed at the New Mexico Subsurface Library revealed that the majority of site rocks are plutonic rocks with a range of granitoid compositions (major quartz, plagioclase feldspar, and potassium feldspar). Minor sedimentary rocks from the upper core sections included sandstone, limestone, and shale. The igneous rocks have ubiquitous pyrite which occurs in up to 5% mode. Pyrite is notable because its oxidation produces acidic drainage and heavy metal leaching. Other identified sulfide minerals include chalcopyrite, sphalerite, and galena. Rock magnetic experiments, including magnetic susceptibility vs. temperature analysis and magnetic hysteresis loop measurements, revealed the presence of maghemite, hematite, and other magnetic minerals. The hysteresis curves showed a wide range of magnetic behavior from diamagnetic to ferromagnetic. A variety of magnetic hysteresis parameters supports the presence of a variety of oxide and sulfide minerals. Twenty-five rock samples representing the range of rock types, hydrothermal alteration, and ore mineralization were cut from Jones Hill cores and analyzed for standard acid-base accounting (ABA) and net acid generation (NAG). Four of the metaigneous rocks bearing sulfide minerals had a pH < 4.5 and classified as Potentially Acid Forming. These same samples were at or near the Potentially Acid Forming threshold (NNP < -20). The majority of samples were Non-Acid Forming or Acid Consuming. Upper Pecos River grab samples were collected from sites upstream, along stream, and downstream of the Jones Hill tributaries (Indian Creek and Macho Creek) in Spring 2024 and Fall 2024 to assess baseline spring runoff and fall baseflow conditions. Results showed that the Upper Pecos River belongs to the Calcium-type hydrochemical facies (Ca: 17-40 mg/L; Mg, K, and Na: (0-3.5 mg/L). Sb, As, Be, Cd, Se, Tl, and Zn were not detected in the sampled waters. Cu was detected in two samples (0.5 ppb and 1.9 ppb) and Pb was detected in one sample (0.059 ppb). Both are below the federal government drinking water action levels (Cu 1300 ppb and Pb 15 ppb) (56 FR 26548 § 141.80 (c) (1) and (2)). Results show that the exploitation of the Jones Hill Deposit has the potential to form acid and leach metals. The high-quality Upper Pecos River conditions should be carefully considered before any watershed disturbances.

pp. 26-27