Metal Transport from the La Plata Mining District

         Eckel and others (1949) describe the geology, mines, and production from the La Plata mining district near the headwaters of Junction Creek and the La Plata River. The La Plata deposit is a porphyry copper deposit associated with the intrusion of a syenite porphyry. Eckel and others (1949) report production of 2,336 tons of copper ore yielding about 224,000 pounds of copper and about 4,500 ounces of silver from a small open-pit from the Copper Hill Mine in the La Plata River watershed. Minor amounts of gold were produced with byproduct copper and silver from the Bay City Mine. Small amounts of both gold and silver were produced from precious-metal veins primarily from the Cumberland Mine, the Doyle Group, the Durango Girl Mine, the Eagle Pass Mine, the Gold King Mine, the Incas Mine, the Jumbo-Morovoratz Mine, the Lucky Discovery Mine, the Mason Mine and the Neglected Mine. The May Day and the Idaho Mines produced the bulk of the precious metals from the district, reporting total production of about 123,000 ounces of gold and 1,140 ounces of silver between 1903 and 1943. These two mines also reported minor byproduct lead and copper. Both cinnabar and native mercury are reported from the precious-metal veins. Of the mines listed above having recorded production, only the Durango Girl Mine and the Neglected Mine are in the headwaters of Junction Creek.

         In the National Geochemical data base (Hoffman and Buttleman, 1994), six stream-sediment samples were collected from along Junction Creek downstream from the La Plata mining district and two new stream-sediment samples were collected in this study near Durango. None of the stream-sediment samples contained elevated concentrations of copper, lead, or zinc (figs. 6-8) except for sample 95ABD138 which was collected in Durango, inside the city limits. We see no evidence of the transport of metals to the Animas River from the La Plata mining district via Junction Creek.

Metals from the Smelter Sites South of Durango

         The U.S. EPA conducted a preliminary site assessment of the copper smelter site located on the west bank of the Animas River about 1 km south of the confluence with Lightner Creek. Slag from the copper smelter is exposed on the west bank of the Animas River (U.S. EPA, 1996a). Analyses of the slag shows high concentrations of copper, lead, zinc, arsenic, manganese and iron were present. Analyses of bed sediments from the Animas River did not indicate that the slag pile was a source of metals to the Animas River (U.S. EPA, 1996b). No remedial action was recommended by the U.S. EPA (1996a). However, this site cannot be ruled out as the source for the small increase in arsenic concentrations found in the bed sediment data (fig. 28A and B) south of Durango. This small change may not be significant given the larger upstream sources of metals.

         The EPA conducted a preliminary site assessment of buried slag from a lead smelter located on the west bank of the Animas River about 0.2 km south of the confluence with Lightner Creek. Slag from the lead smelter is exposed in the west cut bank of the Animas River (U.S. EPA, 1996c). Analyses of bed-sediment samples from the Animas River show that there are elevated zinc (28,500 ppm) and cadmium (498 ppm) concentrations at the cut bank site that reflect erosion of slag material from the cut bank, but both upstream and downstream, the metal concentrations were not elevated (U.S. EPA, 1996d). No remedial action was recommended (U.S. EPA, 1996c).

Urban Metal Signatures

         Two samples, one from Junction Creek collected in Durango, and one from the Animas River at the park above the 32nd Street Bridge, show some urban metal contamination signatures. These urban metal sources contribute small amounts of metal contamination relative to the metal in the bed-sediments from the area above Silverton.

         In the river-sediment sample from the park above 32nd Street Bridge ( 95ABS137), there are elevated concentrations of zinc and copper (fig. 28H). Elevated dissolved copper concentrations were also noted at this site. The absence of tin in the bed sediments would suggest that this is not likely from particulate brass contamination. The source of this urban metal signature is unknown.

         The increased metal signature in Junction Creek is evident when comparing the data from sample 95ABS138 (collected one block west of U.S. 550 in Durango) with the data from sample 95ABS139 (collected from Junction Creek where it crosses a farm 1.7 km upstream and outside the city limits of Durango sample collected with permission of the property owner). The sample from Junction Creek collected in the city limits of Durango ( 95ABS138) contains elevated concentrations of arsenic, chromium, copper, mercury, nickel, lead, vanadium, and zinc, all metals used in our industrial society. Furthermore, the lead-isotopic compositions in the two samples from Junction Creek differ, probably reflecting lead added to the environment by the combustion of gasoline. These urban metal sources contribute only small amounts of metals to the Animas River.

Selenium and Mercury in Bed Sediments of the Animas River

         Selenium concentrations in bed sediments from Mineral and Cement Creeks, the Animas River, and their respective tributaries are presented in figure 30A. Although the diagram contains many data points, the most important feature of the plot is the small variation in the data set as a whole. Selenium is isochemical with sulfur and substitutes for sulfur in the sulfide minerals. It is not surprising that the sediments derived from the area above Silverton contain 1-2 ppm selenium and that the bed sediments of the Animas Canyon reach contain about 1 ppm selenium. The crustal abundance value for selenium is 0.5 ppm (Fortescue, 1992). With the influx of sediment from tributaries draining the Paleozoic and Mesozoic rocks below the KOA Campground site (72.5 km), selenium concentrations in the bed sediments drop to CAV. In Durango, selenium concentrations in bed sediments again rise to about 1 ppm in response to sediments derived locally from the Mancos Shale which crops out just above Lightner Creek (105 km). Sediments from Lightner Creek are characterized by a large component of Mancos Shale and contain 2.7 ppm selenium. Below the confluence with the Florida River (136 km), the selenium concentration drops again to 0.4 ppm reflecting the large component of sediments supplied by tributaries draining areas underlain by Tertiary rocks (fig. 3).

         Mercury concentrations at several sites above Silverton (fig. 30B) are significantly elevated above the crustal abundance value (0.09 ppm Fortescue, 1992). Five of the sites above Silverton contain elevated mercury concentrations, above 0.3 ppm (about three times CAV). All of these sites, except the Kittimac Mill site, are from the headwaters of the respective drainages where early mining activities took place. We cannot distinguish from these data and published geochemical data on the mineral deposits whether these elevated mercury concentrations are related genetically to the epithermal vein mineralization or to early gold recovery practices by individual miners. However, below Silverton, the only sample that contained elevated concentrations of mercury is the sample from Junction Creek in the city of Durango (102 km) which we interpret as urban contamination. Mercury concentrations in bed sediments of the Animas River below Silverton are generally at or below CAV.

Undiscovered Mineral Resources in the Silverton Area

         The lead-isotopic and chemical data from stream sediments from Mineral and Cement Creeks and their tributaries suggest that the mineralized structures in the Eureka Graben (fig. 4) may extend west of the Gladstone Mine near the confluence of North and South Forks of Cement Creek into the headwaters of Burro Gulch in the Mineral Creek drainage. The lead-isotopic signature of the vein-type ore found in the Sunnyside Mine is present in the sediments of Browns Gulch (fig. 23). The surface expressions of these veins in Prospect Gulch may have been overprinted by the later intrusion and alteration associated with the chimney deposits of the Red Mountain district. Exploration for concealed extensions of the vein-type ore may be warranted in the western portion of upper Cement Creek between Prospect Gulch and Minnesota Gulch west to the headwaters of Browns Gulch where mineralization having the Eureka graben vein-type ore lead-isotopic signature is exposed.