Page 71 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Geochemistry of Mine Dump Material from the Lake Valley Mining District
By J.R. Herring and V.T. McLemore1
Abstract
This report characterizes rocks and soils from the surfaces of mine dumps in the Lake Valley mining district, Sierra County, south-central New Mexico, as to their concentrations and ranges of several geoenvironmentally significant trace elements (GSTE). The study focuses on implications of possible release of these trace elements into the environment. The mine dumps are remnants of silver and manganese underground mining oper­ ations and consist of heterogeneous mixtures of low-grade ore and waste rock.
The considered set of GSTE (Ag, As, Ba, Bi, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Sn, V, and Zn) is selected for the notable enrichment of its various elements relative to average shale and (or) for geoenvironmental significance of the various elements in terms of known toxicity or environmental mobility. Trace element enrichments above a baseline significance level are combined into a single GSTE score for each dump that ranks the collective presence of elevated element concentrations above baseline concentrations. Although the score itself does not indi­ cate the potential for these trace elements to move into the envi­ ronment, it can be combined with other criteria, such as water leach data, to indicate those mine dumps that have high collec­ tive presence of GSTE in significant amounts that are easily releasable into the environment. Finally, the score can also be considered with other criteria of the dump composition, such as the presence of sulfide or carbonate minerals, that can increase or decrease the release of GSTE into the environment.
The dumps with highest potential impact on the environ­ ment are those with high GSTE scores, low abundance of car­ bonate minerals, and high abundance of sulfide minerals. Dumps with these criteria are enriched in zinc and secondarily in antimony. Lead enrichments upwards of 20,000 ppm also occur in dumps with high GSTE scores, but these dumps also have relatively abundant carbonate and only minor sulfide, which argues for limited acid water generation and, hence, low potential for mobilization of the lead. Elements with the great­ est enrichments above a significant baseline concentration, nota­ bly lead, antimony, and zinc, have low solubility fractions of their bulk solid concentrations, which reduces the amounts that might be released into the environment. Conversely, manga­ nese, molybdenum, and to a lesser extent arsenic have much
1New Mexico Bureau of Geology and Mineral Resources, Socorro, NM 87801. (Formerly New Mexico Bureau of Mines and Mineral Resources.)
greater solubility fractions and will solubilize proportionally more of their bulk concentrations from the mine dump solids. pH concentrations of the 24-hour leachate samples are alkaline and do not indicate acid-generation potential of the minerals and water.
Introduction Background
The Lake Valley mining district lies within the Caballo Resource Area, which is a Bureau of Land Management (BLM)­ administered region including all of Sierra and Otero Counties, south-central New Mexico. At the request of the BLM, the U.S. Geological Survey (USGS), in cooperation with the New Mex­ ico Bureau of Mines and Mineral Resources, undertook a geo­ logic and mineral resource evaluation of six BLM-designated Areas of Critical Environmental Concern. In addition to these studies, the USGS sampled abandoned mine dumps throughout the Resource Area in an effort to characterize the base- and pre­ cious-metal composition of the mined deposits. This report consists of geochemical characterization of mine dump material in one of those areas mined for silver, lead, zinc, and manganese in southwestern New Mexico during the past century. The intent is to provide information to aid decisions for subsequent reclamation of some or all of these mine dumps.
Lake Valley District—Location, Geology, Ore Deposits, and Mining History
The Lake Valley mining district is located about 25 km south of Hillsboro in south-central New Mexico. The locations of Lake Valley and neighboring mining districts in Sierra County are shown in figure 1. No current mining takes place in Lake Valley.
Two styles of mineralization have occurred, resulting in (1) fissure replacement bodies along fracture zones and faults, and (2) tabular, bedded replacement deposits. The fissure replace­ ment bodies along faults and fracture zones are irregular, steeply dipping deposits that cut across bedding planes. Adjacent to the mineralized fault zones, tabular, stratabound deposits have replaced the upper Alamogordo Member near its contact with
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