New Mexico Mineral Symposium — Abstracts

The Sunnyside mine is located 7 air miles northeast of Silverton, Colorado, at an elevation of 12,240 ft. The mine is located on what had been the shore of Lake Emma situated in a beautiful alpine valley.

At the height of mining activity in the late 1800's and early 1900's, the Sunnyside mine complex was a self-contained community complete with offices, boarding houses, hospital, and commissary.

During the 1960's, mining was carried out on the Washington vein above the American Tunnel haulage level. During the next ten years, thousands of mineral specimens were collected, the most famous being the fluorite-rhodochrosite crystal groups. In the 1970's, rich gold veins were discovered and mined below Lake Emma. Many fine gold specimens were collected and found their way into private mineral collections.

There is still confusion among mineral collectors as to the correct mine name, and mislabeling of mineral specimens from the Sunnyside mine is common. This problem has come about in the following manner. The American Tunnel is the lower level of the Gold King mine at Gladstone, Colorado. In 1959, the American Tunnel was extended more than a mile to intersect Sunnyside mine orebodies 600 ft below the old mine workings. Thus, the American Tunnel is not a mine but is the lowest transportation and ore-haulage level of the Sunnyside mine; as such, it should not be used as a separate mine name. All the specimens that are labeled American Tunnel, Gladstone, Colorado and other variations should be labeled as follows: Sunnyside mine, Eureka Mining District, San Juan County, Colorado.

At present, the Sunnyside mine is operated by Alta Bay Mines in a joint venture with Echo Bay Mines, Ltd. The Sunnyside mine produces approximately 700 tons of ore per day and is Colorado's largest underground gold mine.

HISTORY
Placer gold was discovered in the San Juan Mountains in 1860 by the Baker Party. In 1871, interest was renewed in the area, but no mining claims were staked because the area still belonged to the Ute Indians. By the summer of 1873, the critical point had been reached between the prospectors and the Indians, and in September of 1873, the "Burnot Treaty" was signed. The San Juan Mountains were now opened for prospecting and settlement.

The Sunnyside vein was discovered by George Howard and R. J. McNutt in 1873, and valid mining claims were staked in 1874. During the next 24 years, the mine was owned by various individuals. In November 1900, Judge John Terry obtained complete control and ownership of the mine. The American Smelting and Refining Company purchased the mine and mill from the Terry family in 1917. Mine improvements and expansion and the construction of a new selective-flotation system made the Sunnyside mine one of Colorado's largest underground mines. In 1927, the Sunnyside mine was the first Colorado mine to produce 1,000 tons of ore per day with a payroll of more than 500 people.

In 1959, the Sunnyside mine was leased by Standard Metals Corporation and operated through the 1960's and 1970's. In June
1978, the mine broke-into Lake Emma and emptied more than four million tons of water and mud into the workings. The cleanup took two
years to complete before the mine was back in operation. In 1985, it was forced into bankruptcy and closed down. Later in 1985, the mine was leased by Echo Bay Mines, Ltd., and put back into production. In 1988, Echo Bay Mines, Ltd., formed a joint venture with Alta Bay Mines, the present operator.

GEOLOGY AND ORE DEPOSITS
The San Juan Mountains consist of thick accumulations of middle to late Tertiary lavas and pyroclastic rocks. The volcanic rocks were unconformably deposited on Paleozoic and Mesozoic sediments and on partially eroded Precambrian volcanics, intrusive rocks, and metamorphosed sediments.

During mid-Tertiary time, approximately 32 million years ago, large-scale volcanic eruptions from a series of central-vent volcanoes deposited rhyodacitic ash and lava flows. About 28 million years ago, renewed volcanic activity erupted voluminous amounts of ash that resulted in the formation of the San Juan and Uncompahgre calderas, which are about 12 mi apart. About one-half million years after the formation of the calderas, pyroclastic eruptions within the San Juan caldera formed the ring-faulted Silverton caldera. Resurgence of the magma later domed the overlying sediments and volcanic rocks between the two calderas. A set of major northeast-trending faults was formed by the doming. Further volcanic eruptions caused the dome to collapse and form a trough--the Eureka graben--between the two calderas.

Mineralization occurs in faults and fractures within the northern walls of the Eureka graben, which is made up of quartz latite lavas, flow breccias, and tuffaceous sediments. These volcanic units are the host rock for the orebodies of the Sunnyside mine. The steeply dipping Ross Basin, Sunnyside, Bonita, and Toltec faults and associated fractures were the main structural control for the localization of the ore shoots and contain the major orebodies of the Eureka mining district. The major ore shoots that make up the Sunnyside mine are along and near the intersection of the Ross Basin and Sunnyside fault zones. The northwest-trending Ross Basin fault zone contains the Mogul, Brenneman, and Belle Creole orebodies, while the northeast-trending Sunnyside fault zone contains the Gold Prince, No Name, Sunnyside, and Spur orebodies. The Washington vein trends roughly east-west and contains the orebodies that are near the intersection of the Ross Basin and Sunnyside fault zone.

The formation of the veins is complex and shows successive fissure reopening and mineral deposition, with each stage having a distinct mineral assemblage. The main ore minerals are sphalerite, galena, chalcopyrite, tetrahedrite, and native gold. The common gangue minerals are quartz, pyroxmangite, pyrite, rhodochrosite, fluorite, and calcite.

MINERALOGY
The common ore and gangue minerals that make up the veins of the Sunnyside mine, in decreasing order of abundance, are:
quartz, sphalerite, galena, pyroxmangite, pyrite, rhodochrosite, chalcopyrite, tetrahedrite, fluorite, and calcite. The remainder of the vein minerals occur in small to trace amounts.

The following is a list of minerals that occur in the veins. It does not include wall-rock alteration minerals.