| Abstract/Notes | Granite, graywackes, and metadiabase, presumably Precambrian in age, are found only in small isolated areas within the mapped region.An aggregate thickness of 2,400 ft of sediments, representing a part of each Paleozoic period, overlies the Precambrian rocks. Within the mapped area they are not exposed in a simple continuous sequence. The ages and chief lithologic characters of the Paleozoic rocks are: (1) Cambrian (?) Bliss Sandstone--hematitic, glauconitic, or quartzose; (2) Ordovician El Paso Limestone--bluish-gray slabby to massive, with abundant yellow-brown silty partings; (3) Ordovician Montoya Limestone basal sandstone, massive to tin, black or gray, limestone or dolomite, with abundant chert; (4) Silurian Fusselman Dolomite--massive gray or brownish-gray; (5) Devonian Percha Shale black fissile shale, green limy shale; (6) Mississippian Lake Valley Limestone--gray to black massive limestones with chert and thin-bedded shaly limestone; (7) Pennsylvanian Magdalena Limestone--massive limestone, shale; and (8) Permian Abo Formation--massive red shale, red sandstone. Rock units 1-4 contain little shale, whereas 5 through 8 have no dolomite. Igneous sills are much more abundant at the shale horizons in the upper Paleozoic rocks than elsewhere in the section, presumably because of the lower mechanical resistance of the shales.Tertiary igneous rocks may be divided into three groups, from oldest to youngest: 91) andesitic and latitic rocks, which include the intrusive quartz monzonite porphyry; (2) rhyolitic extrusives and intrusives; and (3) andesitic volcanic rocks. The total estimated thickness of the Tertiary volcanics is approximately 3,000 ft. Terrestrial conglomerates, sandstones, and shales occurring in a nearby area between the rocks of items 2 and 3 contains a fossil flora dated between the limits early Miocene and early Pliocene. The andesitic and latitic rocks are intensely altered on a regional scale, whereas the younger igneous rocks are not.Feldspars in the shallow-depth quartz monzonite porphyry dikes and sills exhibit optical anomalies suggesting that both high- and low-temperature forms are present.The tuffaceous rhyolite, with both pyroclastic and flow features, probably flowed for the most part either as a loose, granular, essentially solid mass lubricated by liquid, or by means of actual liquid flow. Oxidized biotite of the tuffaceous rhyolite may indicate a temperature during extrusion greater than 400-500 deg. C, whereas the green hornblende may represent one or more of the following: (1) conversion from the brown to the green varieties during cooling; (2) an extrusion temperature below 850 deg. C; or (3) a low oxygen concentration during the extrusion. Large concentrations of xenoliths in the tuffaceous rhyolite may either represent a vent agglomerate or indicate proximity to a fissure zone. Such xenolith zones are geologically contemporaneous and consanguineous with the enclosing tuffaceous rhyolite.Abundant intrusive rhyolitic bodies, consanguineous with the extrusive tuffaceous rhyolite on the basis of compositional and textural similarity, suggest proximity to a major eruptive center. Both intrusive and extrusive rhyolitic rocks may have been emplaced as crystalline, and glassy, granular masses. The following features, commonly regarded as being restricted to plutonic rocks, occur in this volcanic igneous environment: (1) quartz-sanidine pegmatite bodies in rhyolite porphyry; (2) evengrained xenomorphic-granular lenses in rhyolite porphyry; (3) primary granophyric crystallization and by sanidine crystallization on corroded quartz phenocrysts, in glassy rhyolitic dikes containing 10% phenocrysts; (4) perthitic phenocrysts in a rhyolitic sill; (5) geochemical culminations in silicon and potassium produced by a lack of equilibrium during the formation of crystalline reaction rims about accidental xenoliths enclosed in rhyolite vitrophyre.The above phenomena do not substantiate the hypothesis that volcanic processes cannot produce plutonic features.Late andesite flows and agglomerates display a repeated cyclical sequence ranging from massive flow rock through scoriaceous rock in turn overlain by a pumiceous agglomerate, and are partly interbedded with the early gravel deposits.Quaternary events include deposition of later gravels, formation of several stream terraces, and erosional dissection of the Black Range.Moderate folding of the sediments resulted probably from their adjustment to the undulatory surface produced by a highly-faulted basement complex, with possible assistance from igneous intrusive forces. Thus, high-angle normal faulting, predominantly of a north-south trend, appears to be the major structural determinant. Faults are more abundant in the pre-Devonian rocks, because the Percha shale was capable of plastic deformation and thereby compensated for, and insulated the upper Paleozoic rocks from, minor ruptures in the earth's crust. The major periods of faulting were concluded before the eruption of the rhyolitic rocks.The best explanation of the extremely irregular distribution of the quartz monzonite porphyry is that it was emplaced along high-angle faults or fractured zones, and spread laterally into shale horizons of low mechanical resistance.Lead, zinc, silver, manganese, copper, and gold ores with an estimated total value of more than 7 million dollars have been produced from the Kingston mining district, the Gray Eagle, and Grandview mines. Discontinuous pockets of high-grade ore have been found in fissures and fault zones, or as replacement bodies in the Fusselman, Montoya, and El Paso limestones and dolomites, which would, barring erosion, underlie the Percha shale. The ore deposits were formed after the quartz monzonite porphyry was emplaced and before all younger rocks were formed. Future exploration for ore deposits not now exposed at the surface might well be directed toward areas of arched Paleozoic rocks or those areas where Paleozoic rocks are covered by sills of the quartz monzonite porphyry. |
|---|
