New Mexico Mineral Symposium — Abstracts

The past three collecting seasons in the San Juan Mountains have produced interesting mineral finds, mainly microcrystals. The crystals occur in hydrothermal veins, contact metamorphic deposits, bedded replacement deposits, and gossan deposits.

The Silver Link mine, 3.5 mi south of Ouray, Colorado, is situated on sheer cliffs 1,000 ft above the Uncompahgre River and was worked in the 1880s for silver and copper. The quartz-tetrahedrite-bornite-chalcopyrite vein has produced superbly crystallized specimens of anatase, brookite, bornite, chalcopyrite, tetrahedrite, and clear quartz crystals with bornite and tetrahedrite inclusions. Rutile and zircon are also present but are too small to be photographed. Secondary minerals collected so far include brochantite, azurite, and malachite.

Another mine in the Ouray area that has produced a variety of silver-bearing and gangue minerals is the Eldorado mine in Yankee Boy Basin 8 mi southwest of Ouray. The mine was first worked during the silver rush of the 1870s and turned out to be not much more than a prospect with a few short drifts on the vein. In 1986 an exploration drift was driven on the Eldorado vein, and one small silver orebody was mined that produced a number of fine, crystallized microcrystals. The silver-bearing minerals of the hydrothermal vein include native silver, polybasite, pyrargyrite, freibergite, and acanthite. Other ore minerals that occur as microcrystals are galena, sphalerite, and chalcopyrite. The gangue minerals present in the vein are pyrite, marcasite, arsenopyrite, quartz, barite, dolomite, rhodochrosite, kutnohorite, siderite, and calcite. Of special interest are clear barite and quartz crystals containing globular inclusions of pyrargyrite and polybasite.

Just a few miles north of Ouray is the Senorita mine, which is primarily a lead-silver bedded replacement deposit in shales and siltstones. The ore minerals, galena and tetrahedrite, occur in a quartz-barite gangue and are not well crystallized. Secondary alteration of the upper part of the orebody has formed an interesting suite of minerals including malachite, azurite, cerussite, mimetite, and aurichalcite. Other exotic minerals are present but await positive identification.

Near Red Mountain Pass, between Ouray and Silverton, Colorado is a small contact metamor¬phic deposit that was mined for gold and silver in the 1880s. The limestones near the intrusive contact have been highly altered into a small skarn zone. Vugs in this zone contain well-crystallized grossular, tremolite, magnetite, and epidote. The most amazing find though is well-crystallized prehnite on grossular. These may be the finest microcrystals of prehnite found to date in Colorado.

And last but not least is a small lead-zinc-copper gossan deposit near Silverton, Colorado that was worked until the early fall snows hit. The deposit has produced an array of colorful and exotic minerals including anglesite, cerussite, malachite, azurite, posnjakite, chalcoalumite, linarite, brochantite, serpierite, and possibly osarizawaite.