The Eurelm mining district is in the western San Juan Mountains in southwestern Colorado and includes 'fill area of about 30 square miles from which ores of gold, .silver, copper, lead, and zinc have had a gross value of more than $63 million. Two mines, the 'Sunnyside and Gold King, yielded more than 90 percent of the dollar value of production. This report, although centered on the Eurelm district, also deals with parts of several adjoining mining districts, all of which nrc genetically related to the same center of volcanism and periods of mineralization. Chiefly volcanic rocl\:s are 'exposed in the area; they have accumulated to a thickness of more than a mile on a basement of Precambrian, Paleozoic,-and Mesozoic rocks. This complex of eruptive rocl{S, which is divisible into three major and several minor units, consists of lava flows, breccias, tuff-breccias, tuffs, and welded ash-flow tuffs that in composition average rhyodacite-quartz latite but range from andesite-basalt to rhyolite. Associated with nnd intruded into these eruptive rocks are many smnH dil{es, sills, nnd irregularly .shaped plutons. The San Juan Formation at the base of the volcanic succession is exposed almost entirely outside a large volcanic depression called herein the .San Juan depression. Next higher in the succession and confined mostly witJlin this depression is the Silverton Volcanic Group, which has been subdivided in order of decreasing age into the Picayune Formation, Eureka Tuff, Burns ]j.,ormation, and Henson Formation. At the to,p of the succession is the Potosi Volcanic Group, which is widespread in the western San Juan Mountains and subdivided into several formational units, only one of which is exposed within the area of this re-•port. The dominant structural feature of the Eureka district and environs is the :Silverton cauldron, about 10 miles in diameter, which lies within the western half of the San Juan volcanic depression, about 15 miles wide and 30 miles long. Outward from the •Silverton cauldron, the volcanic and basement rocks ar~ broken by systems of radial and concentric fractures; the rocks within and adjacent to the cauldron site are broken, tilted, and irregularly faulted. The central-east part of the Silverton cauldron coin.ddes npproximately with the Eureka district; the district thus has intersecting major ring faults bounding a subsided cauldron block and radial graben faults that extend northeast outward from the cauldron center. Coincident with or just following later. stages of eruptive activity, the volcanic rocks throughout and around the !San Juan volcanic depression were intensely altered propylitically. .Subsequently, the metalliferous deposits were formed within and along the numerous fissure and fault systems. 'rwo major types of mineral deposits and associated alteration products are •recognized within and around the Silvel'lton cauldron: (1) chimney or pipelike deposits with intensely leached and decomposed walls consisting of clays, alunite, 2 GEOLOGY AND ORE DEPOSITS, SAN JUAN MOUNTAINS, COL...

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Plate 1. Generalized geologic map showing principal structural areas in the San Juan region, Colorado. .In pocket 2. Geologic map showing structure of the Silverton volcanic center and areas nearby,

Late T e rtia r y and Q u a te rn a ry formations. E]T e r t ia r y v o lc a n ic r o c k s .In tru s iv e bodies and dikes c h ie f ly e a r ly T e r tia r y .M e s o z o ic a n d e a r ly T e r t ia r y fo rm a tio n s . P aleozic fo r m a tio n s .P re -C a m b ria n m assifs. G ra n it e , schist and gneiss.

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