New Mexico Mineral Symposium — Abstracts

Well-crystallized zeolites are known from a number of localities in Colorado and New Mexico, mainly in aMyqdules in basaltic to andesitic lavas but also from several other types of geologic environments. Perhaps the best known occurrence, first described in 1882, is in the potassic basalt ("shoshonite") of North and South Table Mountains, Golden, Colo. (Wajdschmidt, 1939, Colo. Sch. Mines Quart., v. 39, 62 p.). Analcime, thomsonite, and chabazite are found here in abundance, plus lesser amounts of natrolite, mesolite, stilbite, scolecite, levyne, cowlesite, gonnardite, garronite, and offretite, and the associated non-zeolite mineral, apophyllite. Nearby, in Lakewood, Colo, an unusual occurrence of crystals of heulandite and stilbite within plant fossils was found in 1982 during housing excavations in the Denver Formation (Late Cretaceous to Paleocene). Here, volcanic-ash-bearing sandstone Contains fossil wood, seeds, twigs, and roots which have been partially or completely replaced by the zeolites (Modreski and others, 1983, Rocks and Minerals, v. 59, p. 18-28). Amygdules in basalt near Wolf Creek Pass in Mineral Co. and Archuleta Co, Colo. (Harmer, 1976, Mineral. Rec., v. 7, p. 272), contain sprays of mordenite, plus heulandite, analcime, natrolite, and the rare barium¬zeolite, wellsite. The vesicles are lined with blue celadonite and dark-brown, crystallized balls of nontronite, an iron-bearing clay mineral.

In New Mexico, the most outstanding locality for well-crystallized zeolite minerals is probably in andesitic lavas along the East and Middle Forks of the Gila River; Catron Co. and Grant Co. (Haynes, 1983, N.M. Geology, v. 5, p. 841,-85). Zeolites found here include mesolite, chabazite, stilbite, 'heulandite, analcime, levyne, thomsonite (white, fibrous spheres), and gonnardite (white, compact spheres). Several zeolite species,. including natrolite, tetranatrolite, and analcime; occur as microcrystals in gas cavities of the phonolite sill at Point of Rocks, Colfax Co., N.M. (DeMark, 1984, Mineral. Rec, v. 15, p. 149-156), associated with other sodium-rich and silica-poor minerals.

Other zeolite occurrences in basaltic lava flows include heulandite and white, fibrous mordenite near La Madera, Rio Arriba Co, N.M., and phillipsite near Hunter Mesa, Colfax Co, N.M. In alkalic intrusives, analcime and thomsonite are reported from Wind Mountain, Otero Co, N.M., and natrolite, thomsonite, stilbite, and analcime from South Park, Park Co, Colo.

Natrolite and analcime are reported as alterations of nepheline in carbonatite at Iron Hill, Gunnison Co, Colo, Hlnd stilbite and analcime from altered nepheline in phonolite at Cripple Creek, Colo. In skarn-type contact metamorphic deposits, stilbite, scolecite, chabazite, and laumontite, plus apophyllite and prehnite, occur at Italian Mountain, Gunnison Co, Colo, and laumontite occurs at the San Pedro mine, Santa Fe Co, N.M. Heulandite crystals and white, fibrous mordenite occur in breccia at the Rex/Smuggler mines, Hillsboro, N.M., and fine-grained leonhardite is abundant in hydrothermaily altered quartz monzonite of the Chalk Cliffs, Chaffee Co, Colo.

In addition to the coarsely crystallized zeolites that are best known to mineral collectors, much more widespread--and economically important--are zeolites that form by post-depositional recrystallization of tuffaceous sediments, particularly from alkaline-lake environments. These zeolites, including clinoptilolite, mordenite, analcime, chabazite, erionite, and phillipsite, are typically very fine-grained but occur in large tonnages that can sometimes be mined for industrial processes that make use of their molecular sieve, ion-exchange, or other properties. Post-depositional recrystalliza¬tion is a common mode of occurrence for zeolites in Tertiary or Quaternary sedimentary basins in the western states, including Colorado and New Mexico. One prominent example is the Green River Formation (Eocene) of northwest Colorado and adjacent Wyoming and Utah, in which tuffs and oil shale contain abundant analcime, plus local clinoptilolite, mordenite, natrolite, and harMotome. Zeolite deposits in tuffs also include those near Buckhorn, Grant Co, N.M. and near Winston, Sierra Co, N.M.