Abstract NW¼SW¼SE¼Sec.12, T.33N., R.5E., Silver Mine Dam on St.Francis River, Madison County, Missouri; Rhodes Mountain 7½-minute quadrangle.From the junction of Missouri 72 and Missouri D at Oak Grove, follow Missouri D south and southwest for 3.3 mi (5.2 km) to the parking lot adjacent to the picnic area on the east side of the road, about 200 ft (60 m) south of the bridge across the St.Francis River (Fig. 1).Cross the road and walk uphill west then north onthe prominently marked foot trail leading to the historic Einstein Silver Mine and the Silver Mine Dam.The trail roughly parallels the river for about 2,000 ft (600 m) to the mine.About 1, 500 ft (450 m) Missouri D , the trail joins an old wagon road carved into bedrock; bear right and follow the old wagon road downhill to the mine and dam.To return to the parking lotretrace the trail to the mine or, if conditions permit, cross the St.Francis River at the dam and follow the trail along the eastern bluffs of the river.The site is on public land.

Abstract Mac Canyon is located on the northwestern flank of the Pilot Mountains, approximately 3.6 mi (6 km) east of Mina, in west-central Nevada. Mina is situated on U.S. Highway 95 approximately 33 mi (50 km) southeast of Hawthorne and about 76 mi (120 km) northwest of Tonopah. Mac Canyon is accessible by a good gravel road that leaves the southeastern quadrant of the Mina townsite (Fig. 1). The road ends at an old mine dump (suitable for parking) approximately 0.15 mi (0.25 km) south of the mouth of the canyon. A fair trail leads north along the range-front fault of the Pilot Mountains and is an easy walk to the canyon mouth. Some problem may be encountered in locating the appropriate road from Mina to the northwestern flank of the Pilot Mountains, because it is not marked. If local directions are sought, ask for the road to Water or Spearmint Canyon (site of the town water supply). Location of the town airstrip and use of the accompanying copy of parts of the Mina and Sodaville quadrangles should be sufficient direction. It is important to note that some of the roads indicated on the map have moved since publication of the quadrangle maps. Roads are good and passenger cars can be used. It is doubtful that buses could negotiate the road to the parking site, but they probably could approach to within about 2.4 mi (4 km) of the trail head.

The eastern Lake Mead region, to the north of the belt of metamorphic core complexes that define the Colorado River extensional corridor, underwent large-magnitude extension in the middle to late Miocene. We present two speculative new models for extension in this area that resolve several puzzling and paradoxical relations. These models are based on new field mapping and structural, geochronologic, and thermochronologic data from the northern White Hills, Lost Basin Range, and south Wheeler Ridge. The Meadview fault, a previously underappreciated structure, is an east-side-down normal fault that separates the northern Lost Basin Range to the west from south Wheeler Ridge to the east. Proterozoic crystalline rocks of the northern Lost Basin Range yielded an apatite fission-track (AFT) age of 15 Ma, whereas 2 km to the east, across the Meadview fault, crystalline rocks of south Wheeler Ridge yielded a 127 Ma AFT age. Similarly, at the south end of the Lost Basin Range, crystalline rocks with ca. 15 Ma AFT ages lie within 5 km of crystalline rocks of Garnet Mountain that yielded a 68 Ma AFT age across the Grand Wash fault. Neither of these relations can be explained by existing tilted crustal section or tilt-block models. In our “classic” metamorphic core complex model, the Grand Wash fault (breakaway), the Meadview fault, and the South Virgin–White Hills detachment represent different structural levels of a single, regional detachment that was active between ca. 16 and 11 Ma. The hanging wall of the detachment consists of rocks at south Wheeler Ridge, the Paleozoic ridges, and possibly part of the crystalline basement of the Gold Butte block, sedimentary and volcanic rocks in the hanging walls of the Salt Spring and Cyclopic Mine faults, and possibly stranded tilt blocks beneath the Grand Wash Trough supradetachment basin. The footwall, exhumed by subvertical simple shear and characterized by middle Miocene AFT ages, includes the central and western Gold Butte block, Hiller Mountains, and crystalline rocks of the White Hills and the Lost Basin Range. The east-dipping Meadview fault bounds the crystalline core on the east; the west-dipping South Virgin–White Hills detachment bounds the core on the west. Therefore, the Grand Wash fault represents the structurally highest part of the detachment, and the South Virgin–White Hills detachment represents the structurally deepest exposed part of the detachment. In the modified core complex model, the Grand Wash, Meadview, and South Virgin–White Hills detachment faults are separate structures, and the Grand Wash Trough is a “trailing-edge” basin bound on the east by the Grand Wash fault and on the west by the Meadview fault. The South Virgin–White Hills detachment is the main detachment along which extension was accommodated, and the Meadview fault is a major antithetic normal fault that facilitated exhumation of the core at the trailing edge of the detachment system.

Abstract The Blackhawk landslide is located in southeastern Lucerne Valley at thesouthern edge of the Mojave Desert 85 mi (135 km) east of the Los Angeles, California, civic center. It lies across the eastern half of the line separating the Cougar Buttes and Big Bear City, California, U.S. Geological Survey7½-minute Topographic Quadrangles. Its distal (lower) end is readily apparent on both the map and the ground near 34°25'N, 116°47'W. The easiest way to reach the landslide is to take I-15 to Victorville, then California 18 east through Apple Valley, approximately 25 mi (40 km) tothe center of the town of Lucerne Valley, and finally County Highway 247 (CH247, Fig. 1) east 8.7 mi (13.9km) to its intersection with Santa Fe Road (SFR, Fig. 1). The steep, gray, 50-ft (15-m) scarp approximately 2,000 ft (600 m) to the southeast is the distal edge of the landslide. It can most conveniently be visited on foot either from the prospectors' road (moderate clearance or four-wheel drive advisable) that enters the highway at Santa Fe Road or from the highway itself about 0.6 mi (1.0 km) farther east. Features along the west side of the landslide can be reached along Blackhawk Canyon Road (BCR, Fig. 1; moderate clearance or four-wheel drive advisable), which crosses the highway 1.05 mi (1.7 km) west of the Santa Fe Road intersection. Features within the landslide lobe can be reached by taking Blackhawk Canyon Road south from the highway 1.05 mi (1.7 km) to the remains of a jeep trail that goes eastward, following it (four-wheel drive strongly advisable) to the dry wash nearest the landslide, and driving up the wash. Alternatively, these features can be visited on foot by hiking from the roads to the west and south. Features of the proximal (upper) part of the landslide can best be reached by taking County Highway 247 east from the Santa Fe Road intersection 3.25 mi (5.2 km) to a well-graded mine road marked by a large white marble boulder, then taking the mine road (low hill on right 4.4 mi (7. 1 km) from highway is eastern lateral ridge of Silver Reef landslide) southwest 5.3 mi (8.5 km) to a prospectors' road (intersection about 1,400 ft (425 m) northeastof Round Mountain), next taking the prospectors' road (moderate clearance or four-wheel drive advisable) west approximately 0.25 mi (0.4 km) to where it crosses another similar road, and finally taking the other road northwest 1.6 mi (2.6 km) to the mouth of Blackhawk Canyon (BCM, Fig. 2). Features in the vicinity can be visited either on foot or, in most cases if road conditions permit, by vehicle (moderate to high clearance and low gear or four-wheel drive advisable). Good views of the landslide lobe on the alluvial slope and of its source area on Blackhawk Mountain can be obtained from the mine road (high clearance and low gear or four-wheel drive strongly advisable) that climbs the ridge just east of Blackhawk Canyon (VP, Fig. 2).

Abstract Production from the Mercur mine was completed in March of 1997. From September 1982 through March 1997, over 2 million ounces of gold were produced from 22 million tons of ore. Gold production prior to 1982 is estimated to be approximately 1.5 million ounces. The gold deposits of the Mercur mine are situated in the southwest Oquirrh Mountains, approximately 30 miles south of Tooele, Utah. The gold deposits of Mercur lie along the eastern limb of the NNW-trending Ophir anticline. The anticline is one of a series of NNW-trending folds that have an asymmetrical shape due to folding of upper plate rocks during Sevier-age thrusting. The Ophir anticline plunges both to the north and south, forming a dome. The Mercur, Sacramento, Marion, and Golden Gate pits were the main deposits developed at Mercur. Mineralization developed along east-to northeast-striking faults and the north- to northwest-striking Mercur fault. Dilation and extension are seen along the east- to northeast-striking faults. It is postulated that the east-to northeast-striking faults represent extensional features that began with the doming of the Ophir anticline. The north- to northeast-striking Mercur fault is inferred to be related to the regional folding. Gold mineralization at Mercur occurs as sediment-hosted, disseminated (Carlin-type) deposits. Mineralization is hosted in a 200- to 240-ft sequence of thin-bedded, fossiliferous limestones and siltstones that form the top of the otherwise micritic Topliff Member of the Mississippian Great Blue Formation. This sequence of rocks, known locally as the Mercur Mine Series, is conformably overlain by the Long Trail Shale Member, which acted as an impermeable cap to the deposits. Intense oxidation, decalcification, argillization, and silicification are evident. Many parts of the deposits remain unoxidized. Carbon flooding is also present. Fracturing and decalcification were critical in preparation of the host rocks, increasing permeability and allowing access for mineralizing fluids. Calcite and quartz veinlets are common in the deposits, along with sulfides which include pyrite, realgar, and orpiment. Jasperoid is present in many areas as a conformable mass along the base of, and extending up into, the lower half of the Mercur Mine Series. The Porphyry Peak granodiorite (36.7 Ma) crops out north of Mercur as a series of NNW-striking dikes. The Eagle Hill rhyolite (aplite; 31.6 Ma) forms a combination sill and dike complex that bounds the Sacramento deposit on the south and west sides. Several hydrothermal breccias have been encountered at Mercur, the largest of which, the Thickener Breccia, accounted for over 90,000 ounces of gold production. The breccia is a heterolithic assemblage of rocks belonging to the Mercur Mine Series as well as to units above and below the Mercur Mine Series. Also present in the breccia are clasts of the two igneous rock types present in the district. Gold mineralization at Mercur is interpreted to have taken place after the emplacement of the Eagle Hill rhyolite. The presence of igneous clasts in the mineralized Thickener Breccia and the presence of rhyolite in a mineralized collapsed flower structure in the Sacramento pit suggest possible post-intrusion age mineralization.

Marysvale, Utah, is an area of special geological interest. The district is a source of consistent uranium production where ores occur in veins related to igneous activity. Here is an unusual illustration of the nature of hydrothermal alteration in connection with vein formation. The region furnishes data on the relationship between centers of igneous invasion and the sedimentary accumulations of the Colorado Plateau. The area of uranium production lies in the vicinity of the Prospector, Freedom No. 2, Bullion Monarch, and near-by workings and constitutes a zone about a mile long by half a mile wide. Numerous prospects lie in a surrounding area about 7 miles across. Ores occur throughout a vertical distance of 1500 feet as indicated by drill holes and mine workings. Hypogene uranium occurs as pitchblende associated with black fluorite and fine pyrite in breccia and distributed veinlets along veins. Nearly vertical uranium veins range from a few inches to several feet in thickness. Veins pinch and swell, and ore shoots are either vertical or exhibit a steep west rake. Supergene uranium minerals occur near the surface. Umohoite is a new hydrous uranium molybdate first discovered in the Freedom No. 2 mine. The mineral occurs in the ore-bearing vein along with pitchblende. This occurrence at depth and associated with pitchblende rather than the oxidized uranium-molybdenum minerals ilsemanite and jordisite suggests hypogene origin. Earlier hydrothermal alteration produced numerous masses of alunite which occur in the rocks of the Bullion Canyon volcanic series; these are mostly pyroxene andesite and tuff. Radioactivity is low in these masses. Later hydrothermal alteration is associated with the uranium-bearing veins. Clay minerals occur along vein zones which cut quartz monzonite or granite and contain mixed aggregates of kaolinite, illite, halloysite, and montmorillonite. These are cut by broken bands of silicification with pitchblende either disseminated or in streaks. Chlorite occurs along vein borders, and adularia is a vein associate. Minor alunite is a late alteration product. Clay appears to diminish at depth. Near the surface some veins abut unconformably overlying rhyolite. Here clay masses may be found which contain oxidized ore that extends upwards into rhyolite fractures. The Potts fraction, Bullion Monarch open cut, and Lucky Strike ore bodies represent such accumulations. Clay zones extend beyond the ore-bearing portions of veins which constitute a small portion of the total length of clay veins in the area. On the other hand, ore is associated with alteration. Mount Belknap glassy rhyolite forms dikes, fills vents, and appears to be associated with diatremes. In places glassy dikes have been converted to masses of clay minerals by hydrothermal alteration. In the Bullion Monarch mine pitchblende is associated with glass, and ores occur in some of the argillitized dikes. The occurrence of uranium in late Tertiary alteration zones confirms Pb:U age determinations which place the age of Marysvale uranium at about 10,000,000 years. Marysvale lies along the western margin of the Colorado Plateau near the center of an igneous area of about 4000 square miles. Quartz monzonite and granite have invaded plateau strata and Bullion Canyon post-Wasatch Tertiary volcanics. All have been eroded. The eroded surface has been covered in places with Mount Belknap volcanic rocks. Rafted blocks of metamorphosed plateau sediments suspended in quartz monzonite intrusive plugs suggest that these strata underlie the exposed volcanic sequence. Several square miles of Paleozoic and Mesozoic sediments along the Deer Trail Mountain escarpment west of Marysvale indicate the character of the sediments which lie deeply buried beneath the volcanic rocks near the intrusives. The Antelope Range in which the uranium deposits are located lies in a graben between the Sevier fault on the east and the Tushar fault zone on the west. The graben has been disrupted north of the Tushar fault by the invasion of the Mount Belknap volcanic series.

Abstract The South Cañon Number 1 Coal Mine fire, in South Canyon west of Glenwood Springs, Colorado, is a subsurface fire of unknown origin, burning since 1910. Subsidence features, gas vents, ash, condensates, and red oxidized shales are surface manifestations of the fire. The likely success of conventional fire-containment methodologies in South Canyon is questionable, although drilling data may eventually suggest a useful control procedure. Drill casings in voids in the D coal seam on the western slope trail are useful for collecting gas samples, monitoring the temperature of subsurface burning, and measuring the concentration of gases such as carbon monoxide and carbon dioxide in the field. Coal fire gas and mineral condensates may contribute to the destruction of floral and faunal habitats and be responsible for a variety of human diseases; hence, the study of coal gas and its condensation products may prove useful in understanding environmental pollution created by coal mine fires. The 2002 Coal Seam Fire, which burned over 12,000 acres and destroyed numerous buildings in and around Glenwood Springs, exemplifies the potential danger an underground coal fire poses for igniting a surface fire.