New Mexico Mineral Symposium — Abstracts

The rugged La Plata Mountains lie on the southwest end of the Colorado mineral belt, southwest of Silverton on the eastern edge of the Colorado Plateau. In 1873, about ten years after the discoveries at Silverton, placer prospectors found uneconomical gold deposits in the La Plata River. Several decades elapsed before the larger lode-gold deposits of the district were discovered, and important discoveries were made intermittently until the late 1930's.

Ore deposits in the district are hosted in Paleozoic and Mesozoic sediments and in diorite-monzonite porphyry sills and dikes of the La Plata laccolith. Gold has been the most important metal produced. The bulk of its production has come from telluride veins and limestone replacements along these veins. Minor production has come from a variety of other deposits: pyrite-base metal veins and replacements, garnet-magnetite skarns, and an interesting, mineralized, svenite stock that contains copper, silver, and minor amounts of platinum-group metals. The veins occupy small fractures related to the uplift of the laccolithic dome. Ore shoots are strikingly controlled by structural changes along strike and dip.

The igneous activity in the area is probably Laramide and, therefore, earlier than volcanic activity in the Silverton area. The ore deposits also are different from those around Silverton and may have formed with their host igneous rocks at greater depths and temperatures. This is supported by the facts that ore shoots are continuous at depth with no mineralogical change and that no wide, boiling-type, alteration haloes surround the deposits.

The authors will discuss the geology and mineralogy of the productive ore deposits and will present fluid-inclusion data on several ores of the district. The region will be discussed in its relation to other mineralized areas of similar structure throughout the West.