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The largely buried basement of the northern Great Plains includes suture zones and terrane boundaries that represent a significant part of the growth of Laurentia in the Proterozoic. Basement exposures in this region east of the Black Hills are rare. In southeastern South Dakota, southwestern Minnesota, northeastern Nebraska, and northwestern Iowa, small outcrops of the Proterozoic Sioux Quartzite occur. In southeastern South Dakota, Corson diabase sills or dykes have intruded the quartzite. U–Pb ID–TIMS baddeleyite data from a Corson diabase sample yield an upper intercept date of 1149.4 ± 7.3 Ma, suggesting the diabase is related temporally to the Midcontinent Rift (MCR). The similarity in age of this diabase to the Inspiration sill, Pigeon River, Kipling, and Abitibi dykes suggests that early Midcontinent Rift development was not localized within the Nipigon Embayment, but extended along a roughly east–northeast zone from the Abitibi dykes to the Corson diabase. The presence of the Corson intrusions 250 km west of the MCR is hypothesized to represent a failed rift arm within the Superior craton. The greater strength of the Superior craton relative to lithosphere south of the Spirit Lake tectonic zone resulted in a shift of the southwestern rift arm in southern Minnesota along the Belle Plaine fault southeastward to the Iowa border. Alternatively, the apparent northeast trend of known occurrences of the Corson diabase is also consistent with a mantle plume centre explanation for early Midcontinent rifting.

As in the past, gold and, to a lesser extent, silver are attracting considerable attention from the mineral industry. Expenditure on gold exploration dominates exploration budgets worldwide. Gold is most common in the native form, but also occurs as tellurides. Although silver does occur as the native metal, it more often exists as sulfide minerals and sulfosalts, or as minor constituents of common sulfide minerals such as galena, sphalerite, and chalcopyrite. The inertness and high specific gravity of gold account for its concentration by mechanical processes as a heavy mineral in stream and beach environments (placers). Gold and silver are both mobilized by hydrothermal fluids as sulfide or chloride complexes, and precipitated in response to changes in solubility caused by decreasing temperature, or variations in fluid composition, pH, oxygen fugacity. or sulfur fugacity. Gold and silver occur in a diverse range of mineral deposits including recent placers and paleoplacers of gold, epithermal g...

The Glenorchy lode field in the Otago Au-W-Sb metallogenic province of New Zealand is hosted by metasediments of pumpellyite-actinolite facies to lowest greenschist facies. The lodes cut schistosity at a high angle and consist of anastomosing veins of quartz-scheelite-calcite-pyrite-arsenopyrite within zones of crushed and deformed schist up to 6 m wide.Lode quartz delta 18 O values range from 16.0 to 17.4 per mil, background schist contains quartz with delta 18 O values between 13.0 and 14.8 per mil, and the alteration zone quartz delta 18 O values are gradational between the two extremes. Albite and muscovite also exhibit increased delta 18 O values in altered schists relative to background schists. The width of the oxygen isotopic alteration aureole is dependent on schist permeability during mineralization and extends up to 10 m from the lodes. Oxygen isotopic equilibrium conditions were approached in the schists during metamorphism and in the lodes and immediate wall rocks during mineralization. The remainder of the alteration zone is characterized by isotopic disequilibrium. The quartz delta 18 O values in the alteration zone schists probably represent mixtures of hydrothermal quartz (delta 18 O = 17ppm) and metamorphic quartz (delta 18 O = 13-14ppm).The temperature of metamorphism was estimated by oxygen isotope geothermometry to be between 370 degrees and 445 degrees C, although pumpellyite stability experiments by Schiffman and Liou (1977) suggest that 400 degrees C would be the upper limit. Oxygen isotope geothermometry gave a temperature range for mineralization of 280 degrees to 350 degrees C. Relative temperatures can be more reliably stated--mineralization occurred after metamorphism at a temperature at least 80 degrees C less than that of metamorphism.The ore-forming fluid delta 18 O was in the range 7 to 10 per mil, and on this evidence could have been either metamorphic or magmatic in origin, but because there is no spatial association between Au-W-Sb mineralization and igneous activity in the Otago area, a metamorphic origin is favored. A model for the origin and isotopic evolution of the ore-forming fluid is proposed whereby the fluid was derived from metamorphic dehydration reactions in rocks of at least upper greenschist facies. The fluid was trapped in the rocks until uplift of the schist pile led to hydraulic fracturing and release of the fluid to existing fault zones. The fluid migrated rapidly upward, retaining a constant delta 18 O value, and deposited hydrothermal minerals which were 18 O enriched relative to those in the country rock. Ore-forming elements (W, Au) were derived deep in the schist pile, probably also from metamorphic reactions.The oxygen isotope alteration aureole is probably too narrow to be of any practical use in exploration for similar lode deposits, at least in the Glenorchy area.

Schist surrounding the scheelite-bearing Glenorchy Lode was analysed for Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, rare earth elements, W, Pb, Th, and U. The data were used to determine the presence and extent of a chemical halo, to elucidate the origin of the ore-forming elements, and to evaluate the potential of trace element analysis of host rocks in exploration for tungsten mineralisation in the Haast Schist. The trace element halos adjacent to the Glenorchy Lode are less than 1 m wide, and they appear to be closely related to a zone of intensely veined schist which represents a zone of high permeability in the wall rocks during mineralisation. Mass-balance calculations suggest that the tungsten was not derived by lateral secretion from the wall rock immediately adjacent to the mineralised lodes, but originated elsewhere in the Haast Schist, probably at deeper levels in the metamorphic pile. The chemical halos are too narrow to assist tungsten exploration, but study of the chemical varia...

Epigenetic gold-quartz and scheelite-quartz lodes in Otago and other parts of the New Zealand geosyncline crosscut quartzofeldspathic and metavolcanic rocks of the pumpellyite-actinolite and greenschist facies. The lodes, which commonly strike north-west, are spatially associated with a 30 km wide belt of metavolcanic schists, with associated piemontite and sideritic schists, which parallels the axis of the geosyncline. Oxygen isotope and uncorrected fluid inclusion data for Glenorchy and Bendigo material give the range 230–350°C for lode formation, over 100°C lower than the metamorphic temperature of the country rock, and this indicates that substantial uplift and unloading had occurred prior to the hydrothermal emplacement of the lodes. A model is developed to account for the origin of the lode mineralization. A recent oxygen isotope study of the Glenorchy lodes suggested that the ore-bearing fluid was derived from rocks at depth over 150°C hotter than the lode formation temperature, and this is consistent with an origin by dehydration reactions at the greenschist to amphibolite transition. Hydraulic fracturing, induced by rapid uplift and unloading of the pile, allowed fluid migration to higher crustal levels where ore-deposition occurred. The spatial association of the lodes with the metavolcanic suite suggests that metals were derived by trace-leaching from the volcanic suite at the water source, or during migration, whilst tungsten may have been leached from associated manganiferous metasediments which commonly contain high anomolous concentrations of tungsten. Ore deposition occurred in response to lowering of temperature and pressure during fluid migration, and wall-rock interactions, particularly where lower oxygen fugacities were imposed by wall-rocks on the hydrothermal phase.

Mesothermal gold-bearing quartz veins along the Juneau gold belt formed in the early Tertiary from H 2 O-CO 2 (±N 2 ,H 2 S,CH 4 )-rich, low-salinity fluids at temperatures of 200 to 325 °C and pressures in excess of 1 - 1.5 kbar. Isotopically heavy ore-forming fluids, with δ 18 O values between +8‰ and +12‰ and δD values of -20‰ to -30‰ , indicate a deep-seated fluid source. The data are compatible with a model of ore-fluid generation via metamorphic devolatilization within material subducted below the southern Alaska continental margin. Prograde metamorphic fluids infiltrated up zones of mechanically enhanced permeability associated with the Coast Range megalineament and formed the gold-bearing veins within brittle units at higher, retrograding crustal levels. Deposition of gold was accomplished by several mechanisms, including boiling, fluid-wall rock reactions, and decreases in pressure and temperature of the hydrothermal fluids.