Molybdenum (Mo) is a transition metal with an atomic number of 42 and a relative atomic mass of 95.95. Important isotopes are 95Mo, 96Mo, and 98Mo. Molybdenum has a high melting point of 2623
C and one of the lowest coefficients of thermal expansion among commercially used metals. It readily forms hard, stable carbides in alloys and is widely used in steel alloys, including high-strength alloys and superalloys. Molybdenum was discovered in 1781 by Peter Jacob Hjelm and the name is from the Greek “molybdos,” which means “lead,” because it was often confused

with lead. Details on the chemistry and uses of molybdenum can be found at www. rsc.org/periodic-table/element/42/molybdenum.

Barneys Canyon is a sediment-hosted, disseminated gold deposit located 7 km from the large, gold-rich, Bingham porphyry copper deposit. Host rocks for gold mineralization are the Permian Park City dolomite and siltstone and the Kirkman-Diamond Creek sandstone. The gold deposit is approximately 430 m long, 370 m wide, up to 90 m thick and contains 8.5 million metric tons (t) of reserves averaging 1.6 g/t gold. Intrusive igneous rocks are conspicuously absent. The gold deposit is located on the northern flank of the northeast-trending Copperton anticline, an overturned box fold. A small east-striking, south-dipping thrust fault, the Barneys Canyon thrust fault, with 200 m displacement, repeats the Park City Formation, and north-south-striking steep normal faults form a graben in which the gold deposit is located. The Barneys Canyon thrust fault predates mineralization and the Phosphate normal fault postdates mineralization.

Deep Star is a high-grade Carlin-type gold deposit located in the northern part of the Carlin trend. The deposit averages 34.0 g/t Au and by year end 2000 had produced 37.8 t (1,217,000 oz) gold with a remaining reserve of 16.0 t (513,698 oz) gold. The deposit is primarily hosted in brecciated calc-silicate rocks of the Devonian Popovich Formation, with a minor amount of gold in the Jurassic Goldstrike diorite. Intrusion of the syn- and postore Deep Star rhyolite constrains the age of the mineralization. The postore rhyolite is composi-tionally and mineralogically similar to the synore dike and yielded an average 40Ar/39Ar isochron age of 38.3 Ma. Eocene rhyolite dikes intruded active, dilatant north- to northeast-striking faults and/or fractures, providing an important age constraint on the local stress regime at Deep Star during mineralization. Essentially horizontal, west-northwest-directed Eocene extension (291°) is consistent with dextral-normal oblique slip observed on north-south-striking, east-dipping portions of the Gen-Post fault system and dilation and sinistral shear on dike-filled, northeast-striking structures. A right-stepping, releasing bend in the Deep Star fault at its intersection with northwest- and north-northwest-striking subsidiary structures created a deep-tapping dilatant conduit for gold-bearing hydrothermal fluids.

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and <0.25- to >50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.

In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a C02-rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.

Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.

Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, and C02 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolin-ite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.