MIDDLE TERTIARY VOLCANISM AND ASSOCIATED MINERALIZATION IN THE SAN JUAN MOUNTAINS: AN INTERNATIONALLY CLASSIC STUDY AREA

The San Juan region of southwestern Colorado, constituting the largest erosional remnant of the composite volcanic field that blanketed the Southern Rocky Mountains (SRMVF) in the middle Tertiary, has been a focus of continuous geologic studies for more than 100 years. Stimulated by discovery of important base-precious-metal vein deposits, at the beginning of the 20^th century two founders of the Colorado Scientific Society (CSS) undertook some of the earliest detailed stratigraphic and structural mapping of volcanic rocks anywhere in the United States: Whitman Cross and associates in the Silverton quadrangle during 1899-1901 and S.F Emmons and Esper Larsen at Creede in 1911-12. This early work culminated in study of the entire San Juan region, with a geologic map at scale 1:250,000 published in 1935 and a USGS Professional Paper (posthumously) in 1956. More detailed study of volcanic-related mining districts was initiated by Wilbur Burbank at Bonanza 1925, then moving to the Silverton area for the next 40 years; much of Burbank’s work was published by the CSS. A new generation of volcanic studies was undertaken in the 1950s by Thomas Steven (CSS President in 1959) and James Ratté, concurrently with increasing recognition of the presence of enormous ignimbrite sheets and the role of their source calderas as structural controls of ore deposition. Soon, Steven was joined, first by Philip Bethke and Paul Barton in detailed metallogenic studies at Creede, then by me in 1965 for regional study of the Durango 2^o quadrangle. In ensuing years, as innovative petrologic, geochemical, geochronologic, and geophysical techniques became available to provide novel insights into volcanic evolution and magma-generation processes in the region, many additional researchers became involved--too numerous to list fully, but including Olivier Bachmann, Michael Dungan, William McIntosh, Jay Stormer, Jim Whitney, among many others. At present, the San Juan region and the SRMVF are recognized internationally as a benchmark study area for largest-scale silicic volcanism because of the many well documented eruptive events, excellent three-dimensional exposures provided by mountainous terrane, the voluminous framework of geochemical and chronologic data, and intriguing similarities to Cordilleran-margin volcanism elsewhere.