Geology topics

Nora K. Foley

Publications and source records attributed to Nora K. Foley.

42 records · Page 3Linked to original sources

USGS1993

Mineralogy, mineral chemistry, and paragenesis of gold, silver, and base-metal ores of the North Amethyst vein system, San Juan Mountains, Mineral County, Colorado

Gold-rich adularia-sericite-type mineralization occurs near the southern margin of the San Luis caldera, at the intersection of the Equity fault and the northern extension of the Amethyst fault system. Mineralized rock is confined primarily to steeply dipping structures in silicified rhyolite and dacite. Intense sericitic alteration occurs at higher levels in the vein system, and wall rock adjacent to some veins is bleached. The ores are multiply brecciated, and vein filling locally shows sedimentary textures. Textural, mineralogical, and chemical criteria indicate that there are at least two partially coextensive associations of mineral assemblages separated by periods of brecciation and sedimentation. An older gold-bearing association consists of two fine-grained ore stages, both of which contain electrum, uytenbogaardtite, tetrahedrite, silver sulfosalts, silver sulfides and base-metal sulfides, and a manganese-rich stage containing the assemblages (1) manganese silicate + manganese carbonate minerals + quartz and (2) magnetite + hematite + pyrite + quartz. A younger crosscutting association contains calcite, adularia, fluorite, and quartz, plus the assemblages (1) coarse-grained basemetal sulfides and (2) hematite + chlorite + quartz. Quartz, manganese-rich calcite, and trace pyrite line late-stage vugs. Mineralogic, lead-isotopic, and fluid-inclusion characteristics of the younger association are similar to those of ores of the southern and central parts of the Creede mining district. In contrast, the gold and manganese-silicate assemblages of the older association are rare to absent in the southern and central parts of the district. The local and early occurrence of the manganese and gold assemblages may indicate that they formed in a small hydrothermal cell that predated the extensive hydrothermal system from which ores of the central and southern parts of the Creede district are proposed to have been deposited (Bethke, 1988). If similar early-stage cells were present in the southern and central parts of the district, they may have been replaced or overprinted by later assemblages, and they may remain to be discovered. In the latter case, mineral assemblages that formed at early stages in the paragenesis hold the most promise for gold exploration.

Colorado↗

USGS1992

Bedrock geology and mineral resources of the Knoxville 1° x 2° quadrangle, Tennessee, North Carolina, and South Carolina

The Knoxville 1° x 2° quadrangle spans the Southern Blue Ridge physiographic province at its widest point from eastern Tennessee across western North Carolina to the northwest corner of South Carolina. The quadrangle also contains small parts of the Valley and Ridge province in Tennessee and the Piedmont province in North and South Carolina. Bedrock in the Valley and Ridge consists of unmetamorphosed, folded and thrust-faulted Paleozoic miogeoclinal sedimentary rocks ranging in age from Cambrian to Mississippian. The Blue Ridge is a complex of stacked thrust sheets divided into three parts: (1) a west flank underlain by rocks of the Late Proterozoic and Early Cambrian Chilhowee Group and slightly metamorphosed Late Proterozoic Ocoee Supergroup west of the Greenbrier fault; (2) a central part containing crystalline basement of Middle Proterozoic age (Grenville), Ocoee Supergroup rocks east of the Greenbrier fault, and rocks of the Murphy belt; and (3) an east flank containing the Helen, Tallulah Falls, and Richard Russell thrust sheets and the amphibolitic basement complex. All of the east flank thrust sheets contain polydeformed and metamorphosed sedimentary and igneous rocks of mostly Proterozoic age. The Blue Ridge is separated by the Brevard fault zone from a large area of rocks of the Inner Piedmont to the east, which contains the Six Mile thrust sheet and the ChaugaWalhalla thrust complex. All of these rocks are also polydeformed and metamorphosed sedimentary and igneous rocks. The Inner Piedmont rocks in this area occupy both the Piedmont and part of the Blue Ridge physiographic provinces. The intensity of deformation and metamorphism increases from west to east in the Blue Ridge. The west flank is mostly chlorite grade or relatively unmetamorphosed, and the central part of the Blue Ridge is mostly staurolite, garnet, or biotite grade, although sillimanite grade rocks occur along the eastern part of the central Blue Ridge in the vicinity of the leading edge of the Hayesville fault. The east flank of the Blue Ridge and much of the Inner Piedmont are at kyanite or sillimanite grade of regional metamorphism except for a zone of retrograde rocks in the Brevard fault zone and a small area of biotite-grade rocks in the extreme southwest part of the Grandfather Mountain window in the northeast corner of the quadrangle. The major mineral resources in the Knoxville 1° x 2° quadrangle are construction materials and a variety of industrial minerals mostly related to either granite and pegmatite or ultramafic rocks. Past production in the quadrangle of metals, which are of secondary importance relative to construction materials and industrial minerals, include copper in massive sulfides of the Besshi type, gold-bearing quartz veins, and residual iron and manganese deposits. Resources are discussed in relation to the Valley and Ridge, Blue Ridge, and Piedmont provinces. The following resources are the most important: A. Construction materials: Dimension stone of the Tennessee marble district in the Valley and Ridge. Limestone and dolomite of the Valley and Ridge. Sand and gravel and crushed stone, widespread throughout the quadrangle. B. Industrial minerals: Feldspar, flake mica, and quartz produced by flotation methods from the Spruce Pine Alaskite (muscovite granodiorite) in the east flank of the Blue Ridge. The district produces about half of the U.S. feldspar and significant amounts of the U.S. flake mica. Olivine produced from alpine-type dunite bodies in the east flank of the Blue Ridge. Talc and marble from the Murphy belt in the central part of the Blue Ridge. Vermiculite produced from a large deposit near Tigerville, S.C., in the Inner Piedmont. Deposit worked out and mine backfilled. Smaller deposits associated with ultramafic rocks in the east flank of the Blue Ridge are now uneconomic and have not been worked in the past 20 years. C. Metals: Copper in three deposits, the Fontana and Hazel Creek mines in the Great Smoky Mountains National Park in the Central Blue Ridge, and the Cullowhee mine in the east flank of the Blue Ridge. D. Organic fuels: The rocks of the quadrangle contain no coal and probably lie outside the maximum range in thermal maturity permitting the survival of oil. The rocks in the Valley and Ridge and for a short distance eastward below the west flank of the Blue Ridge probably lie within a zone of thermal maturity permitting the survival of natural gas. Consequently the western part of the quadrangle is an area of high risk for hydrocarbon exploration. No exploration drilling has been done in this belt.

North Carolina, South Carolina, Tennessee↗

USGS1989

A reinterpretation of the δD H 2 O of inclusion fluids in contemporaneous quartz and sphalerite, Creede mining district, Colorodo: a generic problem for shallow orebodies?

Water extracted from fluid inclusions in quartz from shallow epithermal ore deposits often has a hydrogen isotope composition (δD) different from that of water extracted from inclusions in associated minerals. This difference is usually attributed to the involvement of primary fluids from multiple sources. Isotopic and homogenization and freezing temperature determinations on fluid inclusions from contemporaneous quartz and sphalerite from the epithermal, silver and base metal orebodies of the OH vein, Creede district, Colorado, suggest an alternative explanation. In near-surface deposits, differences between δD H 2 O of inclusion fluids in ore minerals and quartz may result, instead, from contamination during extraction of the fluids contained in primary inclusions by shallow ground water trapped in pseudosecondary inclusions in quartz. Quartz from the OH vein contains two principal petrographically distinct populations of fluid inclusions: primary and pseudosecondary. The primary inclusions have salinities ranging from 5 to 10 equiv wt percent NaCl, and the salinities of pseudosecondary inclusions cluster between 0 and 1 percent. Primary inclusions in quartz from one locality have a measured δD H 2 O value of -69 per mil, while pseudosecondary inclusions at the same locality have a δD H 2 O value of -102 per mil. Both salinity and isotopic values for primary inclusions in quartz are similar to those for primary inclusions in contemporaneous sphalerite. Homogenization temperatures for primary and pseudosecondary inclusions in quartz range from 191° to 280° C and from 199° to 278° C, respectively. The δD H 2 O value measured on fluid inclusions from bulk crystals ranges between -97 and -85 per mil and represents a mixture of fluids from both primary and pseudosecondary inclusions. We interpret the data to indicate that one or more episodes of abrupt incursion of cooler, overlying ground water into the ore zone caused thermal cracking of the quartz crystals during the time interval of mineralization. Subsequent healing of the fractures trapped heated, low-salinity ground water in pseudosecondary inclusions. The abrupt incursions of overlying ground water are speculated to have resulted from either collapse of a transient vapor-dominated region of the ore zone, or catastrophic venting of the system through hydrothermal eruption(s). The unusually high contrast between the salinities of the ore-depositing fluids and the ground water overlying the ore zone allowed recognition of this phenomenon at Creede. It is likely, however, that Creede is not unique. Similar phenomena may be common in shallow ore zones where rapid fluctuation of an interface between a deep, high-temperature thermal plume and an overlying, cooler ground water may be expected to occur. Careful study of the origins of fluid inclusions, particularly in quartz, is essential to characterize the primary ore fluids and to assess the role of ground water in the hydrology of shallow ore deposits.

Colorado↗

USGS1987

Source of lead and mineralizing brines for rossie-type Pb-Zn veins in the Frontenac axis area, New York ( USA).

Veins composed mainly of calcite and some galena, sphalerite, fluorite, and other minerals are widespread in the Frontenac axis area of New York and Ontario. In New York, the veins (Fig. 1) occur mainly west and northwest of Gouverneur (Brown, 1983). The veins, mined in the 1800s for lead, were first described by Emmons (1838). Smyth (1903) called these the "Rossie lead veins," a name now used locally for all veins of this type. Similar veins in southeastern Ontario were described by Uglow (1916) and Sangster (1970). The steeply dipping veins are aligned along extensional fractures in metamorphic rocks of the Grenville Complex and in Paleozoic sedimentary rocks, including the Potsdam Sandstone of Late Cambrian age, and limestones as young as Middle Ordovician. However, the age of emplacement of Rossie veins is unknown. The veins are mineralogically simple, consisting of calcite (~95%) with minor and approximately equal amounts of sphalerite and galena, and sparse fluorite, barite, celestite, and chalcopyrite (Brown, 1983). Paragenetically early minerals are crushed and granulated; later undeformed minerals fill open spaces (Fig. 2). Sandstone dikes that originated from either the overlying Potsdam Sandstone or stratigraphically higher arenaceous rocks cut into the Rossie veins and occupy all available space. Their unstratified nature suggests that sand was intruded under high hydraulic pressure during tectonic events that reopened the veins. Most veins are vertical and narrow (1-3 cm wide) and those that were mined are as much as 1.7 m wide. The veins trend mainly northwest-southeast but have a local east-west orientation. The fractures and veins show evidence of strike-slip faulting with a right-lateral offset due to late reactivation of north-east-trending faults that originally formed during the Proterozoic Grenville orogeny. Relatively rare inclined veins, termed gash veins (Brown, 1983), occur in proximity to some of the regional northeast-trending faults. These veins contain open spaces lined with large, undeformed crystals of calcite (~90%) and green fluorite (~5%). The minerals of the northeast-trending gash veins, which dip to the northwest, are believed to be paragenetically late results of the same mineralization and tectonic event that produced the Rossie veins. For example, the green fluorite in the gash veins is similar in color and occurrence to small fluorite crystals in vugs in the vertical Rossie veins. Associated with the fluorite in both occurrences is coarse crystalline calcite containing tiny tetrahedra of chalcopyrite aligned along crystallographic planes. The present study of fluid inclusions and lead isotopes was done to understand better the nature and possible source of the mineralizing solutions for both vertical and gash veins and the possible origin of the lead and zinc mineralization. Large deposits of essentially syngenetic sphalerite with minor to trace galena occur in the Proterozoic rocks of this region (e.g., Balmat-Edwards, New York). Although none of these deposits are known in the immediate vicinity of Rossie veins, the possibility that Rossie veins contain remobilized metals from these older deposits was considered.

New York↗

USGS1984

Characteristics of some silver-, and base metal-bearing, epithermal deposits of Mexico and Peru

Introduction Lithotectonic, mineralogical, and geochemical data on two silver- and base metal-bearing deposits from Peru and two from Mexico are compiled to facilitate comparisons with other epithermal deposits. Silver and base metal-bearing deposits of Mexico and Peru constitute an important portion of the world silver production derived from shallow, vein-type deposits hosted in volcanic rocks. Although these deposits are generally similar to epithermal deposits of Nevada and Colorado in the western United States, they have some important differences. Because of this, data on the geological attributes of these deposits are very useful for developing models of ore formation that can be used in mineral exploration. The data collected for this compilation are presented in the following pages in summaries of the important characteristics of each deposit. This compilation, which shows the complexities in the geology of epithermal ore deposits in Mexico and Peru, serves as a basis for further comparisons among epithermal deposits throughout the world. The case studies provide data useful to geologists and exploration!'sts interested in developing models of ore formation to be used in exploration for mineral deposits of this type. The deposits described in this report are the Guanajuato district of Guanajuato, Mexico, the Pachuca-Real del Monte district of Hidalgo, Mexico, the Colqui district of Lima, Peru, and the Julcani district of Huancavelica, Peru. Although many characteristics of the geology and geochemistry of this type of deposit were considered, the most important criterion for choosing these deposits was that they have substantial quantities of precious- and base-metal mineralization. Additional criteria for selecting the deposits were that they be hosted primarily by calc-alkaline volcanic rocks of intermediate to silicic composition and that they be younger than Tertiary in age. Many deposits in Mexico and Peru and other parts of Central and South America were excluded because the literature describing the districts is not readily available. Furthermore, many districts have not been examined in detail or the information available is of limited geological scope. The four districts that are compiled in this report were chosen because they are described in abundant literature dating from early mining reports on the general geology and mineralogy to very recent data on detailed geochemical and mineralogical studies. They were chosen as being fairly typical, classic examples of near-surface, low-temperature vein deposits as described by Lindgren (1928) in his treatise on ore deposits (Mineral deposits, McGraw-Hill, 1049 p.). These deposits are similar in aspects of their geology and geochemistry to many of the famous, epithermal silver mining districts in Colorado and Nevada including Creede, Colorado, Tonapah, Nevada, and the Sunnyside Mine of the Eureka district, Colorado, and, in the special case of Julcani, to Summitville, Colorado, and Goldfield, Nevada. The characteristics that distinguish them include overall size, production and alteration assemblage. The information documented in each summary will be used in a forthcoming series of papers on the comparative anatomy of precious and base metal deposits in North and South America.

Guanajuato, Hidalgo. Huancavelica, Lima↗

USGS1983

Comparative anatomy of epithermal precious- and base-metal districts hosted by volcanic rocks: A talk presented at the GAC/MSC/GGU Joint Annual Meeting, May 11-13, 1983, Victoria, British Columbia

In order to distinguish dissimilar from similar features of epithermal districts, lithotectonic, mineralogical and geochemical traits are compiled for 15 such districts. The districts occur in structurally complex settings associated with silicic to intermediate volcanics. Affiliation with subduction environments on a continental scale and caldera settings on a regional scale is common but is not demonstrable for all districts. Most deposits formed near the end of major volcanism, but some formed considerably later. Paleodepth to the top of the ore is 300-600m for most districts, although Au-rich districts appear to be shallower. The lateral extent of the ore zone is highly variable and far exceeds the limited vertical range (300-800m). Most ore was deposited from dominantly meteoric fluids ranging in temperature from 220°-290°C. Salinities ranged from 0-13 wt% NaCl equiv., and typical values were 1-3 wt%. Although noted for eight deposits, boiling is clearly associated with precious-metal deposition in only two deposits. Four districts, typified by Goldfield, Nev., are characterized by a highly sulfidized mineral assemblage, advanced argillic alteration, and ore deposition closely following emplacement of the host rock. The remaining eleven districts highlight a second, discrete type of deposit. They contain adularia, exhibit sericitic ± argillic alteration, and were mineralized significantly after emplacement of the host rock. The latter category includes two subgroups: Ag- and base-metal-rich deposits (e.g., Creede, Colo.), and Au-rich, base-metal-poor deposits (e.g., Round Mtn., Nev.).

Open-File Report↗