Abstract HU0.0: I80 EXIT 194 GOLCONDA. At the top of the off ramp, turn north (left). Proceed 0.1 mi to the stop sign at the junction with SR789. County mileposts begin here. Turn east (right) on SR789. Steam from active hot springs can sometimes be seen on the north and northeast sides of the town of Golconda. The springs range from 109° to 165°F, are anomalously radioactive, and are actively depositing travertine ( Garside and Schilling, 1979 ). Metals in the spring waters include As (0.02 ppm), Cu (0.05 ppm), Li (0.36 ppm), Mn (0.10 ppm), and Hg (0.0001 ppm). The gravel road to the south at the Interstate off ramp leads to the Adelaide mining district. The district was discovered in 1866 and has intermittently produced small amounts of gold, silver, lead, zinc, copper, tungsten, and manganese. Total gold production has been approximately 40,000 oz. About 25,000 oz of gold were recovered during 1897–1910 from the Adelaide mine (copper-gold skarn), and about 10,000 oz were produced during the 1930–1940s and 1990–1991 at the Adelaide Crown mine (quartz-adularia veins). The ore occurred in three banded cryptocrystalline to chal-cedonic quartz veins, the Crown, the Recovery, and the Margharita. These veins averaged 30 ft in width. The remainder of the gold was recovered from placers mostly in the very early days. The original Adelaide mine was once owned by President Herbert Hoover. Franco-Nevada Mining Corporation is the major property owner in the district (Pete Maciulaites, pers. commun., 2000).

Abstract Paleozoic and Mesozoic rocks in the Osgood Mountains region can be grouped into five terranes based on distinct lithologic, age, and structural characteristics. These terranes are: the Lower Paleozoic Osgood block, the Lower Paleozoic Roberts terrane, the Cambrian and Devonian Dutch Flat terrane, the Mississippian to Permian Golconda terrane, and the Triassic to Jurassic Jungo terrane. Each of these terranes is structurally bounded by moderately to steeply dipping fault zones or melange belts and has a distinct internal structural fabric. Geologic evidence exposed in the Osgood Mountains provides support for a revised model for the Paleozoic tectonic history of the region. Paleozoic tectonic events in Nevada can be reinterpreted in a new framework that portrays the traditional Antler and Sonoma orogenies as complex, transpressive episodes of tectonism along the Paleo-Pacific North American plate margin. Recognizing the accreted Paleozoic terranes of Nevada as tectonic blocks that have experienced significant translational displacement and deformation relative to each other and to the continental margin explains many of the geologic observations that have not been accounted for in other models of the tectonic history of Nevada. Several world-class gold deposits in Nevada, notably the Carlin area deposits, the Getchell region, and Pipeline, among others, are located close to inferred high-angle fault boundaries that can be related to these accreted and dislocated terranes. It is proposed that these terrane boundaries are a first-order control for subsequent gold mineralization in these regions. During younger (Tertiary) mineralizing events, these boundaries served as the preexisting deep crust/upper mantle deep-sourcing conduits, circulating large quantities of auriferous fluids through prospective host rocks.

Abstract Open-pit mining by Pinson Mining Company from 1980 through 1999 yielded a total of 33,750 kg (1,085,105 oz) gold. Oxide ores were mined from sedimentary rock-hosted gold deposits at three locations along the Getchell trend in Humboldt County, Nevada. Most Pinson Mining Company gold production (30,709 kg or 987,348 oz) came from the several deposits of the Pinson mine, located 35 km (21.7 mi) northeast of Golconda. These ores were extracted from carbonates and argillites of the Upper Cambrian to Upper Ordovician Comus formation. The deposits exhibit both strati-graphic and structural control and lie along several structural orientations. Silicification, decalcification, argillization, and fracture-controlled iron oxidation are the principal alteration types associated with these deposits. The Preble mine, located 14.8 km (9.2 mi) northeast of Golconda produced 2,807 kg (90,249 oz) of gold. Ore was mined from interbedded carbonaceous shales, calcareous shales, and silty limestones of the middle member of the Lower Cambrian to Lower Ordovician Preble formation. Mineralization is localized within a broad northeast-striking, southeast-dipping shear zone locally flanked by massive limestone beds. Silicification, phyl-losilicate alteration, and iron oxidation are spatially associated with gold. The Kramer Hill mine, located 3.2 km (2 mi) south of Golconda yielded 234 kg (7,508 oz) of gold. Most of this ore was mined from shattered, interbedded phyllitic shales and impure quartzites of the Twin Canyon member of the Precambrian to Cambrian Osgood Mountain quartzite within the hanging wall of a north-northeast-striking, west-dipping normal fault. Argillization, silicification, and fracture-controlled oxidation are the most evident types of alteration. Gold in the three mines is very fine grained, typically <5 microns in size. Gold mineralization is associated with anomalous Hg, As, Sb, and Tl.

Cumulative Miles 0.0 Log begins at yield sign at Getchell/Twin Creeks mine Road fork: head due north on the mine road. See Day One for log up to the turnoff. Looking west, the south end of the Summer Camp oxide gold open pit is visible behind the Summer Camp waste dump further north. The southern Getchell property boundary is 500 ft south of the open pit. The Summer Camp orebody was discovered in 1984, and mined in 1989–1990. It produced 2.2 M tons grading 0.034 opt. Ore-grade sulfide material (0.2 opt Au) is still exposed on the pit floor and makes up a small satellite resource. Gold ore is hosted in silicified and mineralized hornfelsed Comus formation carbonaceous mudstones and calcareous mudstones along the north-south, east-dipping Getchell fault and intersecting northeast- and southwest-striking faults. Looking northwest, the Riley open pit and underground workings are visible along the range front. Riley is the largest of the tungsten mines that occur along the margins of the Osgood Mountain stock. Scheelite was discovered here in 1917 but was not mined until 1943. The Riley pits expose stratabound and discordant scheelite in garnetite and clinopyroxene (diopside) skarn in limestone protoliths along the intrusive contact of the Cretaceous (92 m.y.) Osgood Mountain stock. From here, the layering you can see in the highwalls of the pits is relic bedding of the stratigraphy that forms dip slopes among the open pits. The open pits are in the footwall of the Getchell fault, well exposed near the mine shack. The Getchell fault dips 45° E, essentially parallel to the bedded exposures in the pit highwalls. The historical Riley shaft head frame is just visible at the north end of the workings near the range front.