Precambrian Rhyolite Research Articles

Typomorphic features of placer gold of the Sololi River basin (northeast of the Siberian platform)

Mineralogical features of placer gold and gold-sulfide ore occurrences of the Sololi river basin from the Sololi uplift of the Olenek arch have been studied. According to the typomorphic indicators, two types of gold are distinguished. The first type is gold of a tabular and lumpy shape, often with mineral indentation imprints. The inner structure is represented by granulation, and recrystallization, high-grade intergranular veins, and high-grade shells typical of the gold of intermediate reservoirs. This type of gold is characterized by a high fineness. The sources of this type of gold were gold-bearing intermediate reservoirs of the Permian and Riphean widely developed in the studied area. The second type of gold includes subrounded and poorly rounded gold, ore gold of hooked, tabular, crystalline and angular-lumpy shapes with a pitted-bumpy or flat surface, as well as large growths of gold with quartz. The fineness of this type of gold varies from low to very high. Its primary sources were probably quartz veins genetically related to the metamorphic rocks of the Eekit series and Precambrian rhyolites. Ore occurrence formed by the Late Permian clastic rocks during the processes of the Mesozoic tectonomagmatic activation is considered as an additional source of feeding. Establishment of indicators of low–temperature hydrothermal processes as mineral parageneses, such as mercury gold, cinnabar and barite, suggests shallow and low-temperature formation conditions of this ore occurrence.

Late Precambrian Rhyolite–Granite Volcanic–Plutonic Associations of the Southern Ulutau (Central Kazakhstan)

Late Precambrian Rhyolite–Granite Volcanic–Plutonic Associations of the Southern Ulutau (Central Kazakhstan)

886189 State of stress and natural fractures in a jointed Precambrian rhyolite in south-central Wisconsin : Haimson, B C; Lee, M Y Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P231–240. Publ Rotterdam: A A Balkema, 1987

886189 State of stress and natural fractures in a jointed Precambrian rhyolite in south-central Wisconsin : Haimson, B C; Lee, M Y Proc 28th US Symposium on Rock Mechanics, Tucson, 29 June–1 July 1987P231–240. Publ Rotterdam: A A Balkema, 1987

Metamorphism and timing of thrusting in the Tømmerås Window, central Scandinavian Caledonides

Abstract The core of the Tømmerås Window is made up of Precambrian rhyolites with dykes and sheets of basic rocks. The Precambrian basement rocks are covered by a thin horizon of metasediments, the Bjørndalen Formation, which corresponds to the lower part of the Jämtland Supergroup in the eastern thrust front. The Bjørndalen Formation is in turn covered by a nappe composed of the same lithologies as the core of the window. The rocks of the Bjørndalen Formation were unmetamorphoscd prior to the Caledonian orogeny. Together with available isotopic mineral cooling ages and ages from fossiliferous strata, metamorphic data are used to determine the physical conditions and time constraints for the Scandian overthrusting in the Tømmerås area. It is concluded that the Scandian thrusting occured later than 435 Ma in the Tømmerås area. The thickness of the overthrusted nappe stack was about 30 km. Erosion down to a thickness of about 8 km at 415 Ma gives an erosion rate of 0.5–2.5 mm yr-1. Such rapid erosion is inferred to have been accomplished by tectonic activity.

Digital image processing applied to analysis of geophysical and geochemical data for southern Missouri

Digital image-processing techniques have been used to analyze a variety of geophysical and geochemical map data covering southern Missouri, a region with important basement and strata-bound mineral deposits. Gravity and magnetic anomaly patterns, which have been reformatted to image displays, indicate a deep crustal structure cutting northwest-southeast through southern Missouri. In addition, geologic map data, topography, and Landsat multispectral scanner images have been used as base maps for the digital overlay of aerial gamma-ray and stream sediment chemical data for the 1 x 2-deg Rolla quadrangle. Results indicate enrichment of a variety of elements within the clay-rich alluvium covering many of the interfluvial plains, as well as a complicated pattern of enrichment for the sedimentary units close to the Precambrian rhyolites and granites of the St. Francois Mountains.

Precambrian rhyelites and granites in south-central Wisconsin: Field relations and geochemistry

Research Article| June 01, 1978 Precambrian rhyelites and granites in south-central Wisconsin: Field relations and geochemistry EUGENE I. SMITH EUGENE I. SMITH 1Division of Science, University of Wisconsin - Parkside, Kenosha, Wisconsin S3141 Search for other works by this author on: GSW Google Scholar Author and Article Information EUGENE I. SMITH 1Division of Science, University of Wisconsin - Parkside, Kenosha, Wisconsin S3141 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1978) 89 (6): 875–890. https://doi.org/10.1130/0016-7606(1978)89<875:PRAGIS>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation EUGENE I. SMITH; Precambrian rhyelites and granites in south-central Wisconsin: Field relations and geochemistry. GSA Bulletin 1978;; 89 (6): 875–890. doi: https://doi.org/10.1130/0016-7606(1978)89<875:PRAGIS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Isolated exposures (inliers) of Precambrian rhyolites and granites (1,765 m.y. old) crop out in the Fox River valley and in the Baraboo area of south-central Wisconsin. The geochemical characterization of rock units, in addition to field and petrographic studies, was used to unravel the complex geology of two of these inliers, the Marquette and Marcellon rhyolites, and to determine in a preliminary fashion the geology of the Precambrian igneous terrain between exposures, which is covered by a section of Paleozoic sedimentary rocks and Pleistocene glacial deposits of varying thickness.The rhyolite at Marquette consists of a series of ash-flow tuffs, interbedded with mud-flow breccia, which are broadly folded into a series of normal and overturned asymmetric folds. The rhyolite is cut by a 100-m-thick andesite dike that intruded along a northeast-trending normal fault. The top of the exposed section is a porphyritic rhyolite containing quartz, alkali feldspar, and plagioclase phenocrysts. This unit is underlain by ash-flow tuffs ranging from porphyritic quartz, plagioclase, alkali feldspar rhyolite to rhyolite with only plagioclase phenocrysts. Plagioclase units are phenocryst poor and have Rb/Sr ratios greater than 1; the three mineral units are phenocryst rich and have Rb/Sr ratios less than 1.The rhyolite at Marcellon consists of four ash-flow tuffs folded into a northeast-trending asymmetric antiform. Lithologically, the Marcellon units are spherulitic, flow-banded, brecciated, and massive. Mineralogically, they vary from porphyritic quartz, plagioclase, alkali feldspar rhyolite to plagioclase-bearing rhyolite. Geochemical correlation was used to relate units from one part of the inlier to the other and thus establish the existence of the antiform. This exposure is cut by andesite and basalt dikes.Forty new major- and trace-element analyses for the Fox River valley and Baraboo rhyolites and granites were used to subdivide these rocks into four chemical groups: (1) fine-grained granite at Baxter Hollow and coarse-grained rhyolite dikes at Observatory Hill are characterized by high CaO and low Rb/Sr; these rocks intrude rhyolite flows and may represent an intrusive event that occurred after the major extrusive episode; (2) fine-grained and porphyritic rhyolite at the Marquette exposure and the fine-grained plagioclase-bearing rhyolite at Marcellon have intermediate CaO and Rb/Sr; (3) porphyritic rhyolite and granophyric granite are characterized by low CaO and high Rb/Sr; similarities in chemistry between these two rock types suggest that the granophyric granites are the subvolcanic equivalents of the rhyolites; and (4) rhyolites at the Marcellon and Baraboo exposures are intermediate in chemistry between groups 2 and 3. The four groups are chemically related and show calc-alkalic affinities.Chemical correlation and geologic mapping show that the chemical groups occur geographically as northeast-trending bands across south-central Wisconsin. Granophyric granite lies to the northwest of group 3 porphyritic rhyolites; the porphyritic rhyolites lie in general to the northwest of the texturally variable rhyolites of groups 2 and 4. Structural trends within exposures parallel contacts between chemical groups, thus strengthening the thesis that the chemical trends reflect the geology of the large area of buried Precambrian rock in south-central Wisconsin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Remnant Mountains in Early Ordovician Seas of the El Paso Region, Texas and New Mexico

The Thunderbird mass of Precambrian rhyolite porphyry in the Franklin Mountains of westernmost Texas is a low remnant mountain that protruded above the floor of the Early Ordovician seas. This extrusive rhyolite, dated at 950 m.y., is surrounded and overlapped by limestone of the Early Ordovician El Paso Group. Sparite-cemented masses of rhyolite talus border the mountain, grading into dolomitized biointrasparrudite containing many large angular clasts of rhyolite. This rhyolite mountain and other Precambrian-rock islands in the region provided detritus to the Early Ordovician seas of southern New Mexico, west Texas, and northern Mexico. Preservation of such highlands for much of Early Ordovician time appears to be extraordinary. The El Paso limestone of the Franklin Mountains, however, reflects in entirety an environment ranging from supratidal to shallow-water marine, as evidenced by the occurrence of such features as polygonally cracked micrites, algal stromatolites, digitate algae, and archaeoscyphid mounds. Protection of the remnant mountains from high-energy waves can be attributed to the regional, virtually intertidal, mudflatlike environment; the occurrence of archaeoscyphid biohermal barriers; periods of submergence below wave base; or a combination of these factors. The sedimentary features imply that a vast region of remarkably uniform shallow-water to supratidal environmentals existed during Early Ordovician time.

Zircon Age of a Precambrian Rhyolite, Northeastern Wisconsin

U-Pb isotopic analyses of three cogenetic zircon fractions from a Precambrian rhyolite of the Quinnesec Formation in northeastern Wisconsin yielded a concordia intercept age of 1905 (+30 to −10) m.y. Thus, at least part of the Quinnesec Formation is late or post-Animikie in age and is not correlative with the pre-Animikie complexes of Michigan, as has been previously believed.

REMANENT MAGNETISM AND THE ANOMALY AT COTTONER MOUNTAIN, MADISON COUNTY, MISSOURI

A 2000‐gamma anomaly at Cottoner Mountain, Madison County, Missouri is caused by thermoremanently magnetized black Precambrian rhyolite contained in a volcanic neck. The black color of the rhyolite is imparted by tiny needles of magnetite which may approximate single‐domain grains. The direction of remanent polarization in the rhyolite is S30°W, inclined 35 degrees, and the remanent component is approximately 40 times greater than the natural susceptibility magnetization. A total intensity curve calculated on the assumption that the black rhyolite body is an inclined dike of limited depth extent is a good fit to the observed anomaly. Irregularities in the surface magnetic properties were probably caused by lightning and were acquired after the thermoremanent magnetization.

Southeast Missouri Geology: Details from Annapolis Area and Ketcherside Gap, Missouri: GEOLOGICAL NOTE

New details on the Precambrian rhyolites and Upper Cambrian sedimentary rocks in the Annapolis area and Ketcherside Gap, Iron County, Missouri, indicate that the tectonic style of the St. Francois Mountain area most probably can be compared with plateau (cratonic) areas where germano-type tectonic patterns are present, and not with orogenic areas where thrust faults might be expected as recently proposed by Wheeler (1965). Thick geosynclinal sections, volcanic rocks, lava, metamorphism, schistosity, lineation, folding, and thrust faults are absent. The great areal extent of Precambrian porphyritic rocks is evidence for the widespread volcanism. Later movements were minor as shown by the local occurrence of normal faults. All indications suggest that, in general, the porph ritic rocks are in much the same position as when they formed. Also the distribution of the igneous rocks and rejuvenated movements around the St. Francois Mountain area indicate that large-scale lateral crustal movements have not occurred. The tectonic style of orogenic belts can hardly be transferred to plateau (cratonic) areas, where vertical (germano-type) movements are the rule. As described previously (Zimmermann and Amstutz, 1965), polygonal patterns, diatremes, and volcanic plugs typically accompany these vertical movements and commonly occur at intersections of major faults. The depositional and lithologic features of the sediments across the pronounced Precambrian topographic relief suggest that the sediments were deposited around and on the knobs of Precambrian porphyry without later horizontal movements. The features which suggest this include composition and grain sizes of the dolomites, porphyry fragments and pebbles in the sediments on knob margins, and the galena deposits. Joint patterns in the sediments indicate that the joints formed during compaction.

Origin of the Precambrian Wisconsin Rhyolites

Precambrian rhyolite is recorded in numerous localities in northern Wisconsin and has been described as glassy lava flows or as intrusive material. This study concludes that these rocks are ignimbrites resulting from nuée ardente-type eruption. The evidence is the positive identification of shards, the lack of scoriaceous upper surfaces, and other flow structures.

Pre-Cambrian Rhyolite Discovered in Well in Northwestern Arkansas: GEOLOGICAL NOTES

Pre-Cambrian Rhyolite Discovered in Well in Northwestern Arkansas: GEOLOGICAL NOTES