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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Natural Contributions of Acidity and Metals to Surface Waters of the Upper Animas River Watershed, Colorado

By Dana J. Bove,¹ Winfield G. Wright,² M. Alisa Mast,³ and Douglas B. Yager⁴

Copper-molybdenum (Cu-Mo) porphyry and acid-sulfate hydrothermal systems, which are noted for their association with naturally degraded waters, are areally extensive within the upper Animas River watershed. Integrated geologic and aqueous geochemical studies of three such systems in the Mount Moly, Topeka Gulch, and Prospect Gulch areas provide critical insight into pre-mining stream conditions and natural processes of metal mobilization and attenuation within the watershed.

The Mount Moly area is underlain by over 4.5 square miles (mi²) of intensely altered and pyritized rock related to a 25.5 million year old (Ma), low-grade, Cu-Mo porphyry system. The main zone of Cu-Mo mineralization is coincident with intense quartz-sericite-pyrite (QSP) altered rock, high fracture densities, and up to 5 volume percent (vol. %) pyrite. QSP-altered rock grades outward into weak-sericitic/intense propylitic (Wk-S) (<0.5 vol. % pyrite) and finally into propylitized (chlorite ±± epidote ±±± calcite) rock. Stream waters draining the QSP alteration zone had pH values <3.4 and Ficklin metal (FM) sums (Cu+Zn+Pb+Ni+Co+Cd in micrograms per liter) averaging 360. Streams influenced chiefly by Wk-S altered rocks were slightly less degraded with pH values from 4.9 to 5.8 and FM sums of 12 to 80. Natural waters draining propylitic-altered rocks (PROP) were less affected still, with a median pH value of 6.0 and FM sum of 14. By comparison, mining impacted waters in the same area had a median pH value of 4.4 and FM sum of 1,613. Mass-balance calculations indicate up to 35 percent of FM in waters draining the Mount Moly area are derived from natural sources.

The headwaters of Topeka Gulch are underlain by intensely altered rock (>0.3 mi²) at the margins of a 23 Ma acid-sulfate hydrothermal system centered near Ohio Peak. Downstream of Ohio Peak, rocks are dominantly QSP-altered (>3-5 vol. % pyrite) with pyrite mostly oxidized above stream level. PROP-altered rocks are present in the lower one-third of the basin. Weathering of pyrite in the QSP assemblage produces highly acidic waters with a median pH of 3.5 and FM sum of 355. An abandoned mine in Topeka Gulch is present near the PROP/QSP transition zone. Water discharging from the mine contains high concentrations of dissolved sulfate and alkalinity, which control the stream-water chemistry downstream to the subbasin outlet. The high-pH/sulfate-rich mine water (pH of 6.9, FM sum of 72) is probably buffered by propylitized rocks containing abundant calcite present in microfractures and as primary mineral replacements. Mass-balance studies indicate that over 90 percent of the FM sum at the mouth of Topeka Gulch is naturally derived.

The northern part of Prospect Gulch encompasses the margins of an extensive 23 Ma acid-sulfate hydrothermal system comprised of QSP and quartz-alunite ± pyrophyllite-altered rocks (0.6 mi²) with high metal content and 8-10 vol. % pyrite. In contrast, rock in the southern part of the basin is predominantly PROP-altered, representing the margins of the acid-sulfate system. Large, mineralized fault structures (>150 feet wide) cross-cut the basin, and mineralized breccia masses are present south of the stream. A large zone of quartz-alunite-altered rock is associated with the most naturally degraded waters (pH of 3.3 and FM sum of 285). In contrast, waters draining the argillic/PROP margins of the acid-sulfate system are less degraded with pH values from 3.4 to 3.8 and FM sums <10. Waters influenced by propylitized rocks are the most pristine, with pH values >6.0 and FM sums <70. Similar to Topeka Gulch, calcite-bearing, propylitized rock buffer sulfate-rich waters that interacted with QSP-altered vein structures. Mine-impacted waters in Prospect Gulch had higher FM sums than their natural counterparts, with a median pH value of 3.2 and FM sum of 4,300.

¹U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dbove@usgs.gov)

²U.S. Geological Survey, P.O. Box 3367, Durango, CO 81302 (wgwright@usgs.gov)

³U.S. Geological Survey, MS 415 P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (mamast@usgs.gov)

⁴U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dyager@usgs.gov)

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