Tertiary igneous intrusive rocks intruded Precambrian limestones and shales of the Newland Formation to form Miller Mountain in the Confederate Gulch and White Gulch area of Broadwater County, Montana. Structure and topography of this area are the result of late Cretaceous and early Tertiary Laramide orogeny that gave rise to folding, faulting, and igneous intrusion. As a result, the dominant structure that affects Precambrian rocks in the area is the York Anticline. The structure has been modified by successive reverse faulting generally associated with the forced emplacement of Tertiary igneous rocks that formed Miller Mountain.

It is believed that the Miller Mountain intrusion may have been emplaced in at least two stages. After the first period of intrusion, a steep reverse fault developed that may be related to the Miller Mountain Reverse Fault (MMRF). A second period of intrusion occurred after the reverse fault formed and may account for the secondary fracture pattern. The secondary fracture-shear pattern is likely related to later magmatic injection into bedding planes and into existing fault planes.

Acid mine drainage (AMD) discharge from the lower adit likely originates from the MMRF, associated fractures, and from the secondary fracture-shear pattern related to the detachment and assimilation of Belt sediments. These faults and associated fractures act as conduits that transport groundwater through overlying Precambrian Belt rocks and its weathered equivalent into the underlying fractured Tertiary quartz diorite and lower mine workings.

• Fractured Bedrock

The mine includes two working levels: upper and lower mine workings. The lower adit drainage is slightly acidic, while the upper mine workings are generally dry. The adit discharge from the Miller Mine flows into Greenhorn Gulch, a tributary of the creek in Confederate Gulch, which eventually empties into Canyon Ferry Reservoir. The surface water in the area immediately surrounding the mine is used as a source of drinking water for cattle and wildlife.

Before remediation, adit discharge ranged from between 5 and 14.5 gallons per minute (gpm) and contained lead at levels that exceeded the National Primary Drinking Water Regulations; aluminum, iron, manganese, pH, and sulfate at levels that exceeded National Secondary Drinking Water Regulations; and nickel that exceeded the aquatic life standard for fresh water.

• Sulfate (0 µg/L)
• Aluminum (0 µg/L)
• Arsenic (0 µg/L)
• Cadmium (0 µg/L)
• Copper (0 µg/L)
• Iron (0 µg/L)
• Lead (0 µg/L)
• Manganese (0 µg/L)
• Nickel (0 µg/L)
• Zinc (0 µg/L)

• Borehole Geophysics
  • Other (VLF terrain conductivity and field gradient magnetometry; downhole camera survey)
• Fluid Loggings
  • Conductivity/Resistivity
• Vertical Chemical Profiling
  • Packer Isolation
  • Multi-sampling port
• Coring
• Tracer (dye) Test
• Other (Lugeon water injection tests)

• Other (Grouting)

The specific goal of this project was to show that the injection of the polyurethane grout reduced the cumulative volumetric flow through the adit by 95 percent. A secondary criterion for success included improving the quality of water exiting the Miller Mine adit by decreasing concentrations of dissolved metals and increasing pH.

Results of the Phase I work indicate that fracture flow was significantly reduced by approximately 77 percent (plus or minus 5 percent), and the metals loading was reduced by at least 80 percent. After the grout had been injected, the maximum recorded total flow from the Miller Mine adit was 3 gpm. Phase II work further reduced metals loading into the underground workings. Average dissolved metal loading reductions of greater than 80 percent were obtained for cadmium, aluminum, zinc, and iron. Reductions of more than 50 percent were obtained for manganese, lead, nickel, and copper.

Although the concentration of dissolved metal at the adit entrance was not reduced, the mass loading of the major metals was reduced by an order of magnitude as a direct result of grout injection into the fracture system. The grouting reduced fracture flow and the metals load as a result of the diversion of fracture flow away from mineralized areas and encapsulation of the sulfide mineralization.

As a result of the technology application, only iron (0.4 parts per billion [ppb]), lead (0.04 ppb), and manganese (3.26 ppb) remain slightly above the National Secondary Drinking Water Regulation levels (iron - 0.3 ppb; lead - 0.015 ppb; manganese  0.05 ppb).

Future grout programs should consider the suggestions listed below:
- Obtain core samples from the site to calculate the relative hydraulic conductivity of the fracture pattern before coring and drilling operations begin. These data may be helpful in selecting an optimum grout for a given fracture system.
- Install an in-line mixer if expansive grouts are used to thoroughly mix grout and water before it is injected to ensure that all injected grout is activated.
- Install an in-line heating system to reduce the viscosity of the grout during injection. This system will provide for greater hydraulic conductivity of the fracture system, increasing the likelihood that the fracture system will be sealed.
- Consider using the Jackleg drill system to drill numerous short grout holes in the vicinity of known seeps and conduits. This system is more mobile and can be used in tight and confined areas.

Reference:
http:www.clu-in.orgproductstins: Mine Waste Technology Program Underground Mine Source Control Demonstration Project. From McCloskey, Lynn, MSE Technology Applications, Inc., Butte MT. Report No: EPS 600-R-05-071, 65 pp, April 2005. Full text available at http:www.epa.govORDNRMRLpubs600r05071600r05071.htm.