PROJECT SUMMARY

INTERNATIONAL SMELTER SITE

STOCKTON DISTRICT

BAUER MILL SITE

DRAGON MINE SITE

STOCKTON DISTRICT

This section is intended to be a brief review of the study area. For details of the image processing, ground studies, and spectral databases, please contact Spectral International for additional documentation.

Brief Description of Site

The Stockton (or "Rush Valley") mining district is located in north-central Utah, in the west-central Oquirrh Mountains, about 40 km southwest of Salt Lake City. This Phase I study is most interested in the drainage of mines located east and northeast of Stockton and the historic mills in and near Stockton.

The lead-zinc deposits occur in mineralized veins and mantos-type replacement bodies, hosted in folded and thrusted Paleozoic limestones and shales (Lufkin, 1965; James, 1982). The sediments have been intruded by a number of Early Tertiary igneous bodies of acid-intermediate composition.

The principal country rocks in the Stockton mining district are the interbedded carboniferous quartzites and limestones of the Oquirrh Formation. These are cut by a series of northeast ­ southwest steeply dipping veins, filled with calcite and quartz. Andesite dikes are closely associated with the ore shoots (Brinsmade, 1908).

The mineralized veins consist of quartz, calcite, and varying amounts of sulfides. The sulfides commonly are interbanded with calcite, but at times become massive and fill entire sections of the veins and mantos bodies. The principal hypogene ore minerals are silver-bearing (argentiferous) galena and pyrite together with a little sphalerite. Minor tetrahedrite-tennantite, and chalcopyrite may also be expected. The principal gangue minerals are quartz and calcite. Minor amounts of sericite and chlorite may be present. Bonanza (supergene) ores at surface comprise a similar association of oxides and carbonates such as cerrusite, anglesite, and others.

Economic mineralization in the Stockton area was first discovered in 1864. Aside from nearly 100 small workings on 80 orebodies, there were significant developments at the Honorine, Ben Harrison, Argent, Galena King, and Calumet mines, among others. Anaconda Corp. decommissioned the Honorine Mine and the Bauer Mill and smelter between 1973 and 1976 (Hilton, 1994).

It is believed that about 1,200,000 tons of lead was produced from lead-silver ores between 1865 and 1927. Byproducts included quantities of gold and copper, and occasionally minor zinc (Brinsmade, 1908; Lufkin, 1965). Early exploitation of rich, supergene bonanza ores at the surface soon led to recovery of (argentiferous galena) from underground operation (shafts). These used overhand stoping methods (Brinsmade, 1908). The ore was processed in mills, using gravity separation. The concentrates were 40% Pb and were processed mostly in nearby smelters.

It is reported that the old Honorine Mine, when in production, provided the surrounding arid countryside with sufficient good quality water to support the growth of alfalfa, an essential food for livestock (Brinsmade, 1908). Much of this mine drainage also was performed through the Bauer Tunnel, which exited near the Bauer Mill approximately 2.5 km north of Stockton. The tunnel actually drained many large mines in the Stockton District and provided the mines the ability to greatly lower drainage and mining costs early in the 20th century.

The presence of pyrite in the ore indicates potential for acid generation and consequent heavy-metal leaching. This however is mitigated by the carbonates present, and the actual environmental liability from underground operations, where broken ore, waste, and wall rocks have been exposed to air and water, may be limited.

The historic mill tailings, however, do represent some hazard through potential transportation of fines into the drainages in and near the town; where separated from the carbonate-dominated substrate they may leach and release toxic metals into the environment. Similarly, concentrate spills and smelter fines are rich sources of toxic metals, which can be leached out of the sulfide matrix uninhibited by carbonate gangue.

Why Site Was Chosen

The Oquirrh Mountains study sites of interest to UDEQ and EPA were prioritized as #2 among the five general mining areas for study under the EPA Utah-Watershed AML project. Undoubtedly, the mining districts on the east side of the mountain range were of greater interest than the west-side districts because of their proximity to and being upstream from development in the Salt Lake Valley. However, the west-side districts and mill sites were more intriguing to our team because they represented more varied waste conditions and proffered simpler access considerations.

The Stockton District is the oldest mining district in Utah and, therefore, has the longest mining and milling history and range of age of wastes. Although none of the mines were huge, some produced into the 1960s. Mills were started in the early days of the district and most did not survive more than a couple decades at most. However, the Bauer Mill remained active to some degree into the early 1970s, and represents a classic example of a totally unreclaimed tailings facility, with attendant potential surface and subsurface impacts.

The Stockton District allows evaluation of the hyperspectral technology for mine waste inventorying and characterization where a range of ages and sizes of wastes are exposed. Although the district has broad generalities of geology and does not represent diverse mineralogy among all the waste piles, there also are subtleties of mineralogy related to both the individual orebodies and the overall mineralization system that can be distinguished through the wastes. This will be covered in more detail in the ground studies and spectral database sections.

This is a CASI overview image of the Stockton District. Yellow arrows indicate some of the old mine sites in the district.

SUMMARY OF RESULTS ­ STOCKTON DISTRICT

	The Stockton District represents a typical old mining district with scattered mine dumps of various sizes and nearby mills which mostly did not operate very long in the early decades of the mining effort. The district is of interest as a test of hyperspectral to inventory and characterize dumps for potential chemical hazards, such as acid production and heavy metals. The mills (chemical processing and smelters) are of interest because of the inefficiency of early processing methods, which resulted in the release of heavy metals, especially lead, to the surroundings and downstream watersheds. This district was a source for some of the ore for the Bauer Mill, and therefore, an understanding of it is important to the characterization of the dumps and tailings at the Bauer Mill.
	The most common infrared active minerals in the Stockton Mining District include calcite, illite, Mg-silicate, and gypsum. More accurately, these are the species most readily identified from the aircraft data. Other infrared active minerals identified from the dumps and adits include tourmaline, jarosite, saponite, smectite, and quartz. None of these, except the silica, are present in sufficient amounts to be detected from the air, even with the 4-m pixel size.
	In the samples collected, there was not a great deal of acidic ground water influence. Because these samples have been exposed on the dumps for many years, this implies there either were not many sulfides in the ore and waste initially, or the mining activity was very selective and efficient (i.e., most sulfides removed as ore and sent to mills).
	Gypsum appears selectively at the major mine sites in the district. Because of this, it should be considered as a potential pathfinder for the waste materials. Its presence is hypothesized to be as a weathering product of the calcite (calcium) and the sulfides (sulfur) and, therefore, is found only in association with sulfides or former sulfides.
	Illite is very important for the image processing as it appears to be restricted to the ore, as a product of the hydrothermal alteration in the district. If this association can be unequivocally established, then it can be hypothesized that where illite appears on an image gangue may be found. For this district, even though it is fairly benign relative to toxic materials, this association still is useful for discriminating the dumps.
	The known and suspected mill sites remain under investigation as part of an M.Sc. thesis at the Colorado School of Mines, in an effort to better define whether these features can be discriminated reliably using hyperspectral technology. Many mill sites have been obscured by later buildings and vegetation.
	The Stockton District is important in this study because, although it does not exhibit the characteristics of an acid drainage site, it does contain toxic metals and, therefore, poses a health hazard to the surrounding cultural ground water. The neutral-pH sites are less easy to discriminate with the airborne systems because their signatures are more subtle, which makes the Stockton case study a valuable contribution to the Mine Sites Database.
	With respect to the mine wastes around the Stockton District, hyperspectral data have shown the ability to identify exposures of jarosite and differences among carbonates and clays/silicates that help in understanding the chemistry of both individual dumps and variations across the district. Most of the mine dumps in the district, based on mineralogy, have proven to be environmentally benign. Knowing the mine dumps at Stockton need little remedial action should have a significant monetary impact on UDEQ costs for reclamation of old mining districts.
	The hyperspectral images also have the potential of identifying buildings, dumps, open shafts and adits that pose potential physical dangers and will assist government safety agencies in locating and remediating hazards.

The Stockton District is a typical old mining area, with scattered mines of various sizes and mills that operated early in the history of the area, with most lasting 10 years or less and few surviving past 1900. The Bauer Mill was the only one that continued operation into the mid-20th Century. The mining district proper serves as an example of application of hyperspectral technology to fast inventorying and characterization of the wastes exposed near the mines and for any mill sites that remain wholly or partially exposed (i.e., not built upon by later residential and commercial construction). In this area, the old mills and smelters are associated with elevated to very high lead in their former wastes, which is of concern to government and current residents.

The Stockton District serves as a proving ground for evaluation of an entire mining district. This work has application to government interests for mine and waste inventorying, although airborne hyperspectral sensors probably are best used in a second-level inventorying process rather than in reconnaissance mode (such as with satellite sensors) due to the smaller area covered and the greatly increased data volume. Part of the inventorying and characterization process is to determine, out of a whole mining district or multiple districts, which wastes require further investigation and likely remediation, rather than assuming all wastes are hazardous. This process leads to better utilization of reclamation technology and funds by prioritizing sites and directing limited resources where most needed.

Stakeholders in this process include land management and environmental agencies at all levels and the mining industry. The mining industry has an interest in this technology and sites such as Stockton as a means of identifying environmental risk for continued exploration and mining in historically mined areas. Of course, hyperspectral surveys have direct application to exploration itself, but that is not a primary concern of this EOCAP project.

WATERSHED ASPECTS OF RESULTS

The Stockton District mine dumps appear to present little hazard for acid or metals to the Rush Lake watershed and its intermittent tributaries that flow through the mining area. A possible exception is the Argent Mine due to its abundance of pyritic wastes on the main dump. The historic mills, however, present or presented metal hazards to local residents, based on sampling by UDEQ that showed elevated to very high lead values in the mill wastes.

COMMERCIAL IMPLICATIONS OF RESULTS

With respect to the mine wastes around the district, hyperspectral data have shown the ability to identify exposures of jarosite and differences among carbonates and clays/silicates that help in understanding the chemistry of both individual dumps and variations across the district.

The latter aspect may have application to further mineral exploration in this district, but that cannot be addressed fully within the technical limits of this EOCAP project. Nonetheless, such lines of investigation have obvious commercial potential for mineral exploration companies interested in the district. Most of the mine dumps in the district, based on mineralogy, have proven to be innocuous, especially because of the buffering and neutralization aspects of the carbonate host rocks and wastes.

As with the Dragon case study, knowing that the mine dumps at Stockton need little remedial action has a significant monetary impact on UDEQ costs for reclamation of old mining districts. In other words, they can direct more resources to districts more in need of remediation. The environmental risk for further mineral exploration and production also would be greatly reduced as a result of such hyperspectral surveys.

The old mill sites in the Stockton area are more problematic, mainly because most sites are very old, were not in operation for very long in most cases, and most since have been obscured by buildings and vegetation. Any ability to discriminate such old mill sites and related environmental conditions will have strong commercial implications due to the numerous mill and other industrial sites that have been "reworked" or simply lost over time around the Western United States and many other parts of the world.