Arkose

Let us now imagine a process of hydrothermal alteration of arkose sandstones composed of Mg-chlorite, K-feldspar, K-mica, and quartz. Because precipitating Si02 during alteration is amorphous, we will assume the presence of amorphous silica instead of quartz, and we will consider MgO as the generic oxide / and K2O as the generic oxide j. 

Microlithofacial classification of the sandstones is based on Dott s classification modified by Pettijohn et al. (1972). They are mostly arenites and subarkose and quartz wackes (rare sublithic, sporadically lithic and arkosic). Quartz is the main component of the sandstones (about 60-70 vol. percent). Feldspars (6 vol. percent) are mostly represented by potassium feldspars with plagioclases in lesser amounts. Some micas (muscovite and biotite) and chlorites are observed. Mica content of arenites reaches 3 vol. %, but is higher in the wackes. Heavy minerals present include zircon, sphene, rutile and apatite. Magmatic rocks (volcanic more than Plutonic) are predominant among lithoclasts (about 2 vol. %), but some metamorphic and sedimentary clasts being present too. 

The Horton Group rocks of the Maritimes Basin are hosts to the best documented uranium occurrences (Quarch et a/.1981). The Horton Group can be divided into the upper arkosic Cheverie Formation and the lower Horton Bluff Formation. 

Uranium mineralization in the Windsor area is a typical sandstone roll-front type. The Green Street occurrence that is the focus of this study occurs in thick arkosic sandstones, siltstones, and pebbly conglomerates of the Cheverie Formation (Fig. 2). The occurrence is a remnant tail of a uranium roll front that has subsequently migrated down dip from the outcropping (Ryan O Beirne-Ryan 2007). 

Trenches excavated across the veins are typically flooded by surface water however, three partially flooded trenches have exposed bedrock. From small (< 5m ) outcrops exposed in three of the trenches, it is apparent that the average grain size of the clastic sedimentary rocks and the abundance of coarse-grained detritus, increases from NE to SW away from the buried Caradocian shale, with arkosic sandstone most abundant in the SW near the Christopher vein. An extensive array of 98 drill holes provides excellent downhole, 3-D control on our understanding of the geology. 

Sandstone and conglomerate Wolfville Formation 7 samples Sandstone, arkose, and conglomerates 15 samples Sandstone Cheverie Formation Walton area, Nova Scotia, Canada 26 samples... 

Sandstone Craywacke and arkose Cobalt area, Ontario, Canada Sandstone Japan Niigata Plain Sandstone Typical average... 

A number of Al chlorites in which both octahedral sheets are dioctahedral have recently been described. Dioctahedral Al chlorites have been reported in bauxite deposits (Bardossy, 1959 Caillere, 1962). These chlorites appear to have been formed by the precipitation-fixation of Al hydroxide in the interlayer position of stripped illite or montmorillonite. A similar type of chlorite, along with dioctahedral chlorite-vermiculite, occurs in the arkosic sands and shales of the Pennsylvanian Minturn Formation of Colorado (Raup, 1966). Bailey and Tyler (1960) have described the occurrence of dioctahedral chlorite and mixed-layer chlorite-montmorillonite in the Lake Superior iron ores. Hydrothermal occurrences have been described by Sudo and Sato (1966). 

Kaolinite associated with arkosic sand from Mexia, Texas, U.S.A. analyst F.A. Gonyer. 

A significant percentage of arkoses in the early and middle Proterozoic, and an increase in the importance of mature sandstones (quartz-rich) with decreasing age. Red beds are significant rock types of Proterozoic and younger age deposits. 

There are a few other terms commonly used to describe sandstones. A wacke is a poorly sorted sandstone containing more than 10% argillaceous material (clay). Arenites are well-sorted sandstones having little or no clay in them. An arkose is a sandstone whose particles are derived from plutonic rocks and so consist largely of quartz and feldspar. 

Fig. 2. Comparative schematic sections of the cherty ironformations of the Ukrainian shield. Symbols 1 = schists and gneisses, carbonate rocks of the upper suite 2 = sandy suites (sandstones, conglomerates) 3 = schist suites 4 = cherty iron-schist suites 5 = schist-sandy suites (arkoses, phyllites) 6 = schist-keratophyre suites (keratophyres, tuffs, schists) 7 = schist-ultramafic suites (talc, talc-magnesite, and green schists) 8 = cherty iron-metabasite suites 9 = quartzite-sandstones in metabasites 10 = metabasite suites.

Fig. 18. Scheme of evolution of the composition of sedimentary rocks (after Ronov, 1964) I =jaspilites and their analogs 2 = evaporites 3 = limestones 4 = dolomites 5 = secondary quartzites 6 = clays and their metamorphic equivalents 7 = quartz sands 8 = arkoses 9 = graywackes 10= submarine volcanics, chiefly basic 11 = terrestrial volcanics. 

A Stratified Drift-Arkosic Sandstone Lithology The Bedrock Aquifer of New England... 

Rogers R. J. (1987) Geochemical evolution of groundwater in stratified-drift and arkosic bedrock aquifers in north central Connecticut. Water Resour. Res. 23, 1531-1545. 

Helmold K. P. and van de Kamp P. C. (1984) Diagenetic mineralogy and controls on albitization and laumontite formation in Paleogene arkoses, Santa Ynez Mountains, Cahfomia. In Clastic Diagenesis (eds. D. A. McDonald and R. C. Surdam). American Association of Petroleum Geologists, Tulsa, OK, vol. 37, pp. 239—276. 

Walker T. R. (1984) Diagenetic albitization of potassium feldspar in arkosic sandstones. J. Sediment. Petrol. 54,3-16. 

Figure 8 Observed cumulative mass/age distributions of major sedimentary lithological types. Explanation of abbreviations gr denotes graywackes sh, shales ark, arkoses ss, sandstones dol, dolostones evap, evapor-ites Im, limestones phosp, phosphorites CB, continental basement P, platforms MOB, mature erogenic belts OD, oceanic domain (after Veizer, 1988c).

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