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

Geologic Framework of Volcano-Plutonic Igneous Complexes as It Relates to the Upper Animas River and Boulder River Abandoned Mine Lands Studies

By K. Lund,¹ M.J. O'Neill,² D.B. Yager,³ R.G. Luedke,⁴ and D.J. Bove⁵

The upper Animas River and Boulder River study areas are in different parts of the geologically similar San Juan, Colorado, and Elkhorn Mountains, Montana, volcanic fields, which are each estimated to have covered about 10,000 square miles. Igneous activity in the mid-Tertiary San Juan volcanic field formed a well-preserved, nested caldera complex that developed as batholith-scale magmas intruded into shallow levels of the crust. The Late Cretaceous Elkhorn Mountains volcanic field is largely eroded, and the underlying plutonic parts (Boulder batholith) of a large magmatic system are revealed beneath remnants of caldera systems.

Geologic maps compiled for the upper Animas River watershed incorporate previous USGS studies focused on volcano-tectonics and formation of mineral deposits. San Juan volcanism began with eruption of intermediate-composition lava flows followed by large-volume catastrophic eruptions that formed the nested San Juan-Uncompahgre and Silverton calderas. Caldera-related structures and later regional structures provided principal pathways for post-caldera-collapse igneous intrusions and for mineral-laden fluids that followed or filled the structures. Most mineral deposits in the upper Animas River watershed consist of silver, gold, and copper-lead-zinc base-metal sulfide minerals in epithermal vein and hydrothermal breccia-pipe deposits and are closely associated with shallow-level igneous plugs and stocks. Regional-scale pervasive alteration of the volcanic rocks, preserved in various degrees of severity, is associated with late-stage igneous intrusion and hydrothermal mineralization.

Geologic maps used in the Boulder River watershed project originated during intensive study of the Boulder batholith undertaken by the USGS in the 1950's with funding from the Atomic Energy Commission. Emphasis was on characterization of the batholith but, because of interest in radioactive elements, mining history and important metals at most deposits were described. New field work and process-oriented reinterpretation of earlier maps indicate that the roof of the Boulder batholith is exposed at its interface with remnants of the associated overlying Elkhorn Mountains volcanic field. Previous mapping mainly identified textural cooling phases of a single magma. These phases are now interpreted to be chill-margin facies, several more slowly cooled textural phases, volatile-rich roof phases, and cross-cutting bodies of late-cooling magma. The volatile-rich, incompatible-element-rich magma formed the source for lode quartz-vein precious-metal deposits. Conduits and deposition sites were cooling-fracture systems located at the roof of the crystallizing magma and controlled by regional stress patterns. These deposits have only narrow alteration haloes. During mineralization, overlying volcanic units formed a barrier across which hydrothermal systems did not operate extensively. These overlying rocks may have caused local buffering during mineralization as they do at present because of their carbonate content (G.A. Desborough, personal commun., 1997).

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

²U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (jmoneill@usgs.gov)

³U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dyager@usgs.gov)

⁴U.S. Geological Survey, MS 955, 12201 Sunrise Valley Drive, Reston, VA 20192 (bluedke@usgs.gov)

⁵U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dbove@usgs.gov)

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