Stratigraphy and correlation of glacial deposits of the Rocky Mountains, the Colorado Plateau and the ranges of the Great Basin

The Rio Grande Rift System in Colorado

Zincian spinel [(Zn,Fe,Mg)Al2O4] is an accessory mineral in a variety of rock types, in particular, metamorphosed massivesulfide deposits, pegmatites, aluminous metasedimentary rocks, skarns, marbles, and sulfide-bearing granulites. Textural and compositional studies of zincian spinel in thirteen Proterozoic sulfide deposits metamorphosed to the amphibolite and granulite facies in Colorado, many of which are spatially associated with Mg-rich alteration zones, shed new light on the use of zincian spinel composition as a guide to sulfide ores. Zincian spinel from the Independence, Bon Ton, and Sedalia deposits and some cases from the Green Mountain, Betty, Caprock, Cinderella, and Cotopaxi deposits show compositions that correspond to zincian spinel associated with metamorphosed massive-sulfide deposits hosted by hydrothermally altered Fe‐Al-rich metasedimentary and metavolcanic rock units elsewhere in the world (Gahnite44‐90Hercynite0‐45Spinel0‐25). However, other compositions of zincian spinel from these last five deposits and from three locations in the Wet Mountains (Marion and Amethyst prospects and an unnamed prospect) are depleted in Zn and enriched in Mg and Fe. These examples of Mg-rich zincian spinel occur in rocks in or adjacent to metamorphosed massive sulfides dominated by Mg-rich minerals such as phlogopite, anthophyllite, gedrite, tremolite, forsterite, cummingtonite, sapphirine, cordierite, enstatite, and clinohumite. The composition of zincian spinel in and surrounding metamorphosed massive-sulfide deposits is influenced by reactions involving Zn and Fe sulfides, as well as by bulk-rock composition, which in the case of many metamorphosed massive-sulfide deposits in Mg-rich alteration zones in Colorado, causes a marked enrichment in the spinel component of zincian spinel, to Gahnite0‐65Hercynite0‐50Spinel25‐90. Knowledge of the geological setting must be considered when using zincian spinel as a guide to metamorphosed massive-sulfide deposits.

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Zincian spinel associated with metamorphosed proterozoic base-metal sulfide occurrences, colorado: a re-evaluation of gahnite composition as a guide in exploration

The central Colorado volcanic field (CCVF) consists of at least ten late Eocene/Oligocene (38–29 Ma) eruptive centers and their volcanic products dispersed over approximately 22,000 km 2 in the Southern Rocky Mountains. The CCVF originally covered most of the Sawatch Range, southern Front Range, Wet Mountains, northern Sangre de Cristo Range, and the areas between. Outflow aprons extended onto the High Plains to the east, merged with the San Juan volcanic field to the southwest, and over­ lapped the Colorado Mineral Belt on the north and west. The CCVF was severely dissected by Neogene faulting and erosion leaving widely scattered remnants, the largest of which is the Thirtynine Mile volcanic area (previously known as the Thirtynine Mile volcanic field), located between South Park and the Arkansas River. Most of the larger erosional remnants, including the Thirtynine Mile remnant, consist of a combination of products of local eruptive centers and regional ignimbrites erupted from caldera sources in or near the Sawatch Range. We have developed a time­stratigraphic framework for the CCVF using 40Ar/39Ar dating and pale­ omagnetic characterization of regional ignimbrites (ash­flow tuffs) as stratigraphic markers to corre­ late remnants of the volcanic field. 40Ar/39Ar ages of sub­caldera plutons and non­caldera rhyolitic and andesitic volcanism help flesh out this framework. Our interpretation of the eruptive history of the CCVF is based on our database of 83 single­crystal laser­fusion analyses of sanidine, 45 resistance­ furnace step­heating analyses of other phases (biotite, hornblende, groundmass), and paleomagnetic study of 40 sites. Caldera volcanism in the CCVF was active between 36.7 and 32.9 Ma. erupting seven regional ign­ imbrites that spread mainly eastward along paleovalleys on the late Eocene erosion surface. Four caldera centers have been recognized in the Sawatch Range (Grizzly Peak, Mount Aetna, Bonanza, and Marshall Creek); three of these calderas erupted four of the regional ignimbrites (Antero Tuff, 33.8 Ma; Badger Creek Tuff, 33.8 Ma; Thorn Ranch Tuff, 33.7 Ma; and Gribbles Park Tuff, 32.9 Ma). Caldera sources have not yet been determined for three other regional ignimbrites (Wall Mountain Tuff, 36.7 Ma; tuff of Stirrup Ranch, 36.5 Ma; and East Gulch Tuff, 33.7 Ma). Two ignimbrites (Fish Canyon Tuff, 27.8 Ma; and Carpenter Ridge Tuff, 27.4 Ma) erupted from the San Juan volcanic field and lapped onto the CCVF. Other eruptive centers within the CCVF include Cripple Creek (32.5–30.9 Ma), Silver Cliff–Rosita Hills (35.4–32.0 Ma), Guffey (36.1 Ma), Waugh Mountain (31.6 –31.3 Ma), and Nathrop (30.4–28.9 Ma). Dated plutons in the Sawatch Range are Mount Princeton batholith (34.3 Ma), Mount Aetna Quartz Monzonite (34.1 Ma), and Mount Antero Granite (29.6 Ma). Three fossiliferous sedimen­ tary deposits were also dated; they are Antero Formation (33.8 Ma), Pitch­Pinnacle Formation (33.7–32.9 Ma), and Florissant Formation (34.1 Ma). Compilation of more than 300 chemical analyses from the literature and from student theses indi­ cate that rocks of the CCVF are calc­alkaline to alkaline in composition and relatively rich in potassi­ um. Ignimbrites are dacites, trachydacites, and rhyolites. Lavas are dominantly trachyandesites and trachydacites. Alkalinity increases eastward toward the Rocky Mountains–High Plains boundary. Cripple Creek and the Silver Cliff­Rosita Hills centers erupted highly alkaline rocks and are precious metal mining districts of the Great Plains Margin type. A­type granites and topaz rhyolites were emplaced along the flanks of the Rio Grande rift between about 30 and 29 Ma.

Geochronology of the central Colorado volcanic field

Pattern of shallow ground water flow at Mount Princeton Hot Springs, Colorado, using geoelectrical methods

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D.C. Cox

Papers

Geology and mineral deposits of the Poncha Springs NE quadrangle, Chaffee County, Colorado, with a section on Fluorspar mines and prospects

Geologic map of the Poncha Springs NE quadrangle, Chaffee County, Colorado