Geology topics
Publications and source records attributed to Terry W. Offield.
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No abstract available.
The Al Wajh gold district contains small deposits of gold-bearing quartz veins located in sheared and altered Neoproterozoic mafic lavas and volcaniclastic sandstone and siltstone. The veins formed during multiple episodes of deformation and have structual and mineralogic features characteristic of mesothermal, low sulfide, gold-bearing quartz veins. Three early deformation episodes (D 1 , D 2 , D 3 ) are interpreted as progressive phases of a major deformation event that culminated about 660 Ma; episode D 4 makes up part of a later event about 620 Ma. The bulk of the gold-bearing veins in the district are located in D 2 and D 3 structures. D 1 veins consist of thin quartz veins in phyllite, D 2 veins are located in throughgoing, steeply dipping sinistral shear zones and in the crests of folds along the shears. D 3 veins, typically associated with carbonate alteration, are concentrated in small thrust faults that in places modify earlier D 2 shears. -from Authors
No abstract available.
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Computer-enhanced Landsat images of the southern Powder River Basin have been used to define facies and linear structural features within the Wasatch Formation. The facies distribution is detectable primarily because of a relation of vegetation density and type to the local substrate. The surface indications of facies are confirmed by sandstone/ mudstone ratios determined from logs of abundant exploration drill holes. These newly defined geologic features are spatially related to known uranium mineral occurrences and are believed to be related to mineralization in the following ways. (1) Major uranium occurrences are virtually restricted to an intermediate grain-size facies of the Wasatch, probably marking the axial zone of the depositional basin. (2) The axial zone is also marked by a change from one structural lineament domain to another, and the structures may have influenced details of fluvial-system patterns and sedimentation and (subsequently) the flow of uranium-bearing ground water. (3) A recently active linear structure may mark the current basin axis; it appears to have some relationship both to groundwater chemistry and the distribution of uranium occurrences, suggesting structural influence on relatively modern ground-water transport and uranium deposition. © 1978 Society of Economic Geologists, Inc.
After expansion of the USGS (U.S. Geological Survey) uranium-thorium program in FY (fiscal year) 1975, new directions of research could be pursued, particularly in sedimentology or genetic stratigraphy, geochemistry-mineralogy, and geophysics. In a relatively short time, many of the new research projects have developed critical information on uranium habitats and improved methods of exploration. In FY 1979 a budget increase of approximately 20 percent is programmed, for a funding base of $6,600,000. The increase is scheduled for research grants and contracts outside the USGS in order to extend the basic program without additional personnel. This outside research will be designed not only to increase basic information on uranium occurrence but also to aid the USGS in its evaluation of uranium favorability in key resource areas for the NURE (National Uranium Resource Evaluation) program of DOE (U.S. Department of Energy).
Computer enhancement, particularly contrast-stretching, reveals a previously unnoticed east-west structural zone across a Landsat image of the southern Brazilian Precambrian shield. In this zone occur the only known economic or near-economic deposits of gold, tin, and copper. Such deposits are typically localized by small east-west structural elements. Non-economic copper occurrences elsewhere in the region appear to be related to major northeast- and northwest-trending lineaments mapped on Landsat images. Mineral exploration should be primarily directed at the main east-west lineament, but two other possible east-west zones might be worthwhile targets also. The major east-west lineament projects through a break in the continental shelf, across the Atlantic along a large transoceanic fracture zone and into the African continent along a mapped tectonic trend that goes through an area that produces copper, gold, and tin. Global geophysical data suggest that the mapped east-west trend in South America is a surface reflection of structures that developed in the tectosphere in Precambrian time and that have persisted until the present. © 1977 Society of Exploration Geophysicists.
High-altitude thermal-infrared images, taken at various times of the day over the Front Range and adjacent plains in Colorado, display many linear and circular topographic features. These features are enhanced by thermal shading effects and commonly are better displayed than in photographs. Comparison with geologic maps of a control area in the Front Range shows that most lineaments drawn from the thermal images represent geologic features, principally faults. A previously unobserved circular feature at the Central City-Idaho Springs mineral district bounds most of the area of gold mineralization; it is a topographic delineation of the most intensively altered ground in the district and may reflect extremely subtle structure related to the presence of an unexposed intrusive body. Lineaments in the plains probably mark subtle structural trends related to the Denver basin and to folds not otherwise expressed at the surface. © 1975 Geological Society of America.
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Comparison of orientation of faults, foliations, and joints, observed during geologic mapping in Colorado, with thermal, photographic, and topographic linears shows that topographic linears are statistically useful indicators of mappable faults and fractures, photographic linears are less useful, and thermal linears, believed to represent zones of moisture concentration, are parallel to faults, foliations, and, a statistically minor, but more open joint set.