: The central Manhattan Mining District has had a long and productive mining history. The area was first prospected for silver and copper in the 1860s with some small production of ore that was shipped to Belmont. The next phase of exploration began in 1905 when the rich lode and placer gold deposits were discovered and mined more or less continuously until 1947. Suma Corporation acquired the property in 1967 but did little exploration until 1972 when they launched a major mapping, sampling, and drilling program to delineate a bulk-mineable heap-leachable gold orebody in the district. By 1973, more than 200,000 tons of ore grading 0.10 opt gold had been delineated and 6500 tons of it was mined and heap-leached later that year, with poor recovery due to the coarseness of the gold. Continued drilling in 1974 and 1975 defined more ore in the East and West pit areas increasing the proven reserves to more than 400,000 tons grading 0.075 opt gold, of which 60,000 were heap-leached in 1974. In 1977, Summa sold the property to Houston Oil & Minerals Company ( later Tenneco/CanAm/ Echo Bay Minerals). More drilling in 1977-1979 increased reserves to 800,000 tons grading 0.09 opt gold, which was considered sufficient to justify construction of a 750 tpd mill from 1981 to 1983 during which time mining was suspended pending mill feasibility studies. In 1984, Tenneco (Echo Bay) resumed mining from the East and the West Pits, which incorporated several smaller open pits from earlier Summa Corp. mining activity. They operated the two open pits for several years using a combination gravity circuit and flotation-cyanidation circuit in the mill. Heap leach operations were also in use from 1988 through 1990. Mining ceased in 1990, shortly after Round Mountain Gold Corporation acquired the site. Milling operations and tailings deposition stopped later that same year. Heap leaching and gold recovery continued until 1993 when heap closure activities started with biological detoxification of the spent ore. Reclamation of the Manhattan mine site from 1994 through 2001 earned Round Mountain Gold Corporation the BLM?s Hardrock Mineral Environmental award in 2004 for its innovative reclamation designs and new technique for treating water used in the mining process. Both Calais Resources and Royal Standard Minerals are now exploring and developing nearby gold mining properties in the Manhattan District (2005).
Comment (Commodity): Ore Materials: native gold and silver, stibnite, microscopic arsenopyrite, realgar, fluorite
Comment (Commodity): Gangue Materials: calcite, drusy quartz, adularia, pyrite, clay, iron oxide minerals; rare fluorite and barite
Comment (Commodity): Commodity Info: Fineness of gold averages about 700.
Comment (Deposit): In the main mined area, gold occurs in drusy quartz-adularia veinlets in brittle, fractured, faulted sandy phyllite of the Gold Hill Formation. Native gold is coarse-grained (.04-10 mm) and occurs on fractures as discrete free crystals and masses. Both the East and West Pit orebodies are characterized by a coarse stockwork of fractures lined by gangue minerals and free gold. Frctures generally range in thickness from 0.16 to 1.3 cm. At the historic Nevada Manhattan/Nevada Consolidated Mine workings, mineralization is localized along the Mud Fault. Ore is controlled by a small north-striking fault in the east part of the mine, and by small fault fissures in the west part of the mine. Gold occurs in solution channels forming rare specks of wire in a muddy matrix of iron and manganese oxides (western part). There is also some replacement along bedding, thinning out away from fissures.
Comment (Geology): The oldest rocks in the Manhattan district are phyllite, schist, quartzite and limestone units of the Cambrian Gold Hill Formation. Of these, a sandy phyllite unit of the Gold Hill Formation has been the most productive gold mineralized unit in the mine area. Locally in the district, the Gold Hill Formation has been overthrust by younger Ordovician sedimentary rocks including the Zanzibar limestone, argillite and quartzite units. The mines in the district are located along the southern margin of the Manhattan caldera, which formed about 25 million years ago. Fluid inclusion data indicates homogenization temperatures of 200-235 degrees C at salinities of 0.3-0.8 wt % NaCl equivalent.
Comment (Identification): This record covers a mineralized area described earlier by MRDS records for the historic Big Four, Reilly, Big Pine,and Little Grey mines as well as other Manhattan District mines which are encompassed by the current mine record, and from which all pertinent material has been incorporated into the current record, but those records should remain in the database for historic reference.
Comment (Location): The mine area is centered about one mile ESE of the townsite of Manhattan, up canyon from the townsite.
Comment (Workings): At the historic Nevada Manhattan/Nevada Consolidated Mine, workings consist of a 500 foot vertical shaft with five levels. The entire district is developed by numerous underground and surface workings, including several open pits. The currently described mine was developed by two open pits: the East Pit (1500 feet by 1000 feet) and the West Pit (1200 feet by 300 feet) developed in the 1980s by Echo Bay Mines Ltd.
Comment (Economic Factors): Lode gold production for the Manhattan District through 1967 was about 193,000 ounces of gold. From 1987 through 1989, the current mine produced more than 62,000 ounces of gold, after which production for 1990-1993 was included with that of Round Mountain Gold Mine to the north. In 1997, proven and probable reserves were reported at 1.7 million tons of ore grading 0.13 opt gold.