Geology topics

Jerry C. Olson

Publications and source records attributed to Jerry C. Olson.

3 recordsLinked to original sources

USGS1977

Age and tectonic setting of lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in South-central Colorado

Alkalic igneous rocks were emplaced into heterogeneous terrane of Precambrian X and Precambrian Y rocks about 570 m.y. ago (Cambrian or upper Precambrian) in the Powderhorn area in Gunnison County and about 520 m.y. ago (Cambrian) in the McClure Mountain, Gem Park, and Democrat Creek areas in the northern Wet Mountains, Fremont and Custer Counties. The radiometric ages are based upon studies by K-Ar, Rb-Sr, and fission-track methods. Associated with these alkalic rock complexes are numerous thorium-bearing veins and red syenite dikes; some, if not all, in the northern Wet Mountains were formed about 495 m.y. ago. Diabase, gabbro, and other mafic dikes appear to be slightly younger than the thorium veins in the Powderhorn area and both older and younger in the Wet Mountains region. In the Powderhorn district, an older group of syenites also intruded Precambrian rocks as plugs or small stocks about 1350-1400 m.y. ago. The various dike rocks and the thorium veins were formed in extensive, deep fractures, indicating a condition of tension or shear in this part of the crust during Cambrian or very late Precambrian to Ordovician time. The localized alkalic magmatisin may reflect melting spots in the mantle. In the two principal areas, 135 km apart, the episodes of alkalic magmatism differ in age by about 50 m.y., the younger toward the east. The age relations might be explained by migration of the sites of localized melting or volatile enrichment in the mantle or westward movement of the continent above a single site.

Colorado↗

USGS1960

Beryl-bearing pegmatites in the Ruby Mountains and other areas in Nevada and northwestern Arizona

Pegmatite occurs widely in Nevada and northwestern Arizona, but little mining has been done for such pegmatite minerals as mica, feldspar, beryl, and lepidolite. Reconnaissance for beryl-bearing pegmatite in Nevada and in part of Mohave County, Ariz., and detailed studies in the Dawley Canyon area, Elko County, Nev., have shown that beryl occurs in at least 11 districts in the region. Muscovite has been prospected or mined in the Ruby and Virgin Mountains, Nev., and in Mohave County, Ariz. Feldspar has been mined in the southern part of the region near Kingman, Ariz., and in Clark County, Nev. The pegmatites in the region range in age from Precambrian to late Mesozoic or Tertiary. Among the pegmatite minerals found or reported in the districts studied are beryl, chrysoberyl, scheelite, wolframite, garnet, tourmaline, fluorite, apatite, sphene, allanite, samarskite, euxenite, gadolinite, monazite, autunite, columbite-tantalite, lepidolite, molybdenite, and pyrite and other sulflde minerals. The principal beryl-bearing pegmatites examined are in the Oreana and Lakeview (Humboldt Canyon) areas, Pershing County; the Dawley Canyon area in the Ruby Mountains, Elko County, Nev.; and on the Hummingbird claims in the Virgin Mountains, Mohave County, Ariz. Beryl has also been reported in the Marietta district, Mineral County; the Sylvania district, Esmeralda County; near Crescent Peak and near Searchlight, Clark County, Nev.; and in the Painted Desert near Hoover Dam, Mohave County, Ariz. Pegmatites are abundant in the Ruby Mountains, chiefly north of the granite stock at Harrison Pass. In the Dawley Canyon area of 2.6 square miles at least 350 pegmatite dikes more than 1 foot thick were mapped, and beryl was found in small quantities in at least 100 of these dikes. Four of these dikes exceed 20 feet in thickness, and 1 is 55 feet thick. A few pegmatites were also examined in the Corral Creek, Gilbert Canyon, and Hankins Canyon areas in the Ruby Mountains. The pegmatite dikes in the Dawley Canyon area intrude granite and metamorphic rocks which consist chiefly of quartzite and schist of probable Early Cambrian age. The granite is of two types: a biotite-muscovite granite that forms the main mass of the stock and albite granite that occurs in the metamorphic rocks near the borders of the stock. The pegmatites were emplaced chiefly along fractures in the granite and along schistosity or bedding planes in the metamorphic rocks. Many of the Dawley Canyon pegmatite dikes are zoned, having several rock units of contrasting mineralogy or grain size formed successively from the walls inward. Aplitic units occur either as zones or in irregular positions in the pegmatite dikes and are a distinctive feature of the Dawley Canyon pegmatites. Some of the aplitic and fine-grained pegmatite units are characterized by thin layers of garnet crystals, forming many parallel bands on outcrop surfaces. The occurrence of aplitic and pegmatitic textures in the same dike presumably indicates abrupt changes in physical-chemical conditions during crystallization, such as changes in viscosity and in content of volatile constituents. Concentrations of 0.1 percent or more beryl, locally more than 1 percent, occur in certain zones in the Dawley Canyon pegmatites. Spectrographic analyses of 23 samples indicate that the BeO content ranges from 0.0017 to 0.003 percent in the albite granite, from ,0.0013 to 0.039 percent in aplitic units in pegmatite, from 0.0005 to 0.10 percent in coarse-grained pegmatite, and from less than 0.0001 to 0.0004 percent in massive quartz veins. The scheelite-beryl deposits at Oreana and in Humboldt Canyon, Pershing County, are rich in beryllium. Twelve samples from the Lakeview (Humboldt Canyon) deposit range from 0.018 to 0.11 percent BeO, but underground crosscuts have failed to intersect similar rock at depth. Beryl locally constitutes as much as 10 percent of the pegmatitic ore at Oreana. The beryl was not recovered during tungsten mining at Oreana and is now in the tailings of the mill at Toulon, Nev. The percentage of beryl is lower than the Oreana ore because of dilution by tailings from other ores milled at Toulon. Beryl has been found in many pegmatite dikes in the Virgin Mountains. Both beryl and chrysoberyl occur in dikes on the Hummingbird claims, north of Virgin Peak, in Mohave County, Ariz. Spectrographic analyses of 5 representative samples of the principal dike on the Hummingbird claims range from 0.055 to 0.11 percent BeO.

Arizona, Nevada↗

USGS1954

Thorium and rare earth minerals in the Powderhorn district, Gunnison County, Colorado

Thorium has been found since 1949 in at least 33 deposits in an area 6 miles wide and 20 miles long in the Powderhorn district, Gunnison County, Colo. The district is composed largely of pre-Jurassic metamorphic and igneous rocks, which are chiefly if not entirely pre-Cambrian in age. The metamorphic and igneous rocks are overlain by sandstone of the Morrison formation of Jurassic age, and by volcanic rocks of the Alboroto group and Hinsdale formation of Miocene and Pliocene (?) age, respectively. The thorium deposits occur in or near alkalic igneous rocks in which such elements as titanium, rare earths, barium, strontium, and niobium occur in greater-than-average amounts. The greatest mass of the alkalic igneous rocks the Iron Hill composite stoc,- occupies an area of 12 square miles in the southeastern part of the district. The age of the thorium deposits, like that of the alkalic igneous rocks, is not known other than pre-Jurassic. The thorium veins and mineralized shear zones range from a few inches to 18 feet in thickness and from a few feet to 3,500 feet in length. The veins are composed of calcite,.dolomite, siderite, ankerite, quartz, barite, pyrite, sphalerite, galena, goethite,. apatite, alkali feldspar, and many other minerals. The thorium occurs at least partly in thorite or hydrothorite. Sparse xenotime has been tentatively identified in one deposit. Several minerals containing rare earths of the cerium group as major constituents are found in carbonate veins near Iron Hill. Bastnaesite has been identified by X-ray methods, and cerite and synchisite are probably present also.The fluorapatite in some veins and in parts of the carbonate rock mass that occupies 2 square miles in the central part of the Iron Hill complex contains rare earths of the cerium group, generally in amounts of a fraction of a percent of the rock. The radioactivity of the deposits appears to be due almost entirely to thorium and its daughter products The ThO 2 content of selected highgrade samples from the Little Johnnie vein is as much as 4 percent. The ThO 2 content of the veins is generally less than 1 percent, however, and is only 0.05 to 0.1 percent in many of the veins studied. The little Johnnie vein, which was mapped in detail, can be traced discontinuously for a distance of more than 3,500 feet. The thoriumbearing material occurs as irregular veinlets and thin films introduced into the fault zone. The mineralized shear zone ranges from less than 6 inches to 5 feet in thickness. Near its west end the vein is broken by many faults in a zone that marks the edge of a roughly circular fault block, 1 1/2. miles in diameter, that has dropped 1,000 feet or more since the deposition of Miocene volcanic-rocks that now floor the Milkranch basin.

Colorado↗