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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
Economic deposits of precious and base metals are found in a variety of geologic terranes (series of related rock formations). A region’s geology strongly controls the emplacement and deposition of ores and the mechanism of transporting and depositing the orebearing fluids. Mining methods used to extract the ore reflect the nature of the deposit. Underground base- or precious-metal deposits are often found in veins formed when metals-rich hydrothermal fluids are injected along fractures or joints in the preexisting rock. Extracting the ore (rock mined at a profit) requires a shaft or adit for access and workings or tunnels to excavate the rock. During the active period of mining, groundwater is often encountered; the groundwater must be pumped or drained throughout the life of the mine. In the case of a mine that used an adit or shaft for access, drainage may occur long after mining has ended.
Mineralized and unmineralized discrete fractures, joint sets, and faults may control groundwater flow in and near the underground workings. The fractures’ hydraulic conductivity (ability to transmit water) depends on factors such as the degree of alteration or weathering of the rock, the presence or absence of fault gouge, and the connectivity of the fractures. The type of host rock (rock surrounding the ore)—igneous intrusive, metamorphic, volcanic, or sedimentary—controls the density, orientation, and connectivity of fractures. The Boulder Batholith in Southwest Montana, for example, is an igneous intrusive rock hosting a variety of ore bodies. Vertical and horizontal joint sets and faults are common in both mineralized and unmineralized intrusive rock. Although nearly all of the batholith has been slightly altered by hydrothermal activity, intense alteration is found near fractures, faults, and veins. More than 900 underground hard-rock mines have been active in the batholith during the past 140 years. Generally, the mines within the batholith range from small mines limited to small groups of veins with workings that extend only a few hundred feet to larger mines whose workings extend several thousand feet and produced several thousand tons of material. However, the Summit Valley mining district at Butte, MT, has hundreds of mines and thousands of miles of workings. Although groups of individual mines may exploit the larger vein systems, the mines are rarely connected by workings. Adit discharges from most of these mines are recharged from unmined host rock—the recharge area for the smaller mines is likely to be small. The discharge rate is often less than 10 gallons per minute.
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