Geologic Relationships, K-Ar Ages, and Isotopic Data from the Willow Creek Gold Mining District, Southern Alaska
Dawn J. Madden-Mcguire, Miles L. Silberman, Stanley E. Church, 1989. "Geologic Relationships, K-Ar Ages, and Isotopic Data from the Willow Creek Gold Mining District, Southern Alaska", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Willow Creek mining district is located in the Peninsular terrane in Alaska, on the southwestern margin of the Talkeetna Mountains batholith. The district contains exposures of tonalite (74-73 Ma) and adamellite (67-65 Ma) of the batholith and an older unit of schist that has no nearby correlative units. The tonalite (quartz diorite) and schist both host gold-bearing quartz veins in fractures and shears, whereas the adamellite (quartz monzonite) appears to be barren of gold mineralization. Our data suggest that there is a previously unmapped fault along the contact between the mineralized tonalite and schist. The fault may have provided a conduit for mineralizing fluids.
Ceologic relations and K-Ar ages indicate that at least two periods of mineralization occurred at 66 and at 57 to 55 Ma. At 66 Ma, the intruding adamellite provided heat and possibly fluids to the tonalite as suggested by K-Ar ages from a gold-bearing quartz vein and from dikes in the tonalite. At 57 to 55 Ma, the source of heat is uncertain. Both periods of mineralization occurred during right-oblique subduction of the Kula plate beneath the Peninsular terrane and some mineralizing fluids may have originated in zones of metamorphism and partial melting in the descending Kula plate, Fluids of deep origin, magmatic and/or met amorphic, could have moved through a deeply rooted fault system and out into the host rocks along splays and fractures related to a fault separating the schist and tonalite.
Oxygen isotope data suggest that the mineralizing fluids were similar in their oxygen isotope composition to the tonalite, and perhaps the adamellite, but were unlike the schist. This similarity could have resulted if the fluids equilibrated with a plutonic body (tonalite or adamellite) at temperatures high enough that fractionation approached zero. The measured values of δ18O from quartz in gold-bearing veins are 13.2 to 15.8 per mil, with one low value of 9.2 per mil from a pegmatite, and the calculated fluid values are 6.3 to 8.9 per mil. These values occur in veins with ages of 66 Ma, as well as in undated veins. The Pb isotope compositions of sulfides from two veins in the tonalite resemble each other but differ from those in the schist. These compositions suggest that the Pb in veins in the tonalite had a common source. In the schist, the gold-bearing veins may have farmed from the same mineralizing fluid that was modified in Pb isotope composition by circulation through the schist, or from a fluid that was different from the one that produced gold-bearing quartz veins in the tonalite.
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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