Archean Shoshonitic Lamprophyres Associated with Superior Province Gold Deposits: Distribution, Tectonic Setting, Noble Metal Abundances, and Significance for Gold Mineralization
Published:January 01, 1989
D. Wyman, R. Kerrich, 1989. "Archean Shoshonitic Lamprophyres Associated with Superior Province Gold Deposits: Distribution, Tectonic Setting, Noble Metal Abundances, and Significance for Gold Mineralization", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Archean snosnonitic lampropnyres are cotemporal and cospatial with gold mineralization in the Superior province, both being emplaced along major translithospheric structures that demark subprovince boundaries. They occur internal to greenstone belts such as the Abitibi. Lamprophyres constitute a volumetrically minor but widespread component of late kinematic magma-tism, which is most prominently developed in fault graben or dilational jogs where the dikes are associated with alkali gabbros, trachytes, and molasse sediments. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event.
Fresh shoshonitic dikes are characterized by normal background gold contents of 3.9 ±8.1 ppb (lσ) close to the value of 3.0 ppb for the bulk continental crust. Average abundances of As (1.4 ± 0.5 ppm), Sb (0.25 ± 0.25), Bi (0.09 ± 0.02), W (1.9 ± 2.2), Tl (0.11), B (6.2 ± 3.3), Cu (71 ± 38), Pb (7 ± 5), Zn (93 ± 18), and Mo (1.4 ± 1.5) are also close to values of 1.0, 0.2, 0.06,1.0, 0.36, 10, 75, 8, 80, and 1.0 ppm, respectively, in bulk continental crust. Fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly, do not constitute a special source rock. Platinum-group element contents (Ir = 0.84 ± 0.58 ppb; Pt = 5.9 + 26.5, Pd = 5.5 ± 1.8) in conjunction with Cu, Au, and Ni abundances define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements. Sporadically elevated Au abundances in lamprophyres proximal to gold deposits are interpreted as a secondary overprint in dispersion halos. Post-Archean ultramafic lamprophyres from the Superior province have suffered little crustal interaction and do not possess enhanced abundances either of Au (mean = 0.5 ppb) or most Au-associated elements.
Where anomalously rich gold contents occur in alkaline rocks, Au invariably defines a peak relative to Cu and neighboring platinum-group elements on normalized diagrams such that the anomaly is likely the result of a secondary overprint rather than an intrinsic feature of the noble metal budget of alkaline magmas. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites (Brugman et al., 1987) signify that the Archean and Phanerozoic upper mantle had similar noble metal contents such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone. The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas.
Emplacement of the lamprophyres was diachronous from north (2,705 Ma) to south (2,674 Ma) in the Superior province, as was the gold mineralization. Both were related to late transpressional tectonics during successive accretions of individual subprovinces. Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes; the alkaline magmas were derived from depths of 40 to 120 km in the continental mantle lithosphere and asthenosphere, whereas the gold-mineralizing systems were confined to the continental crust.
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The Geology of Gold Deposits: The Perspective in 1988
When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in