P e tr o lo g y o f E c lo g itic R o c k s f r o m th e F a ir b a n k s D is t r ic t , A la s k a ABSTRACTEclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystal line schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies.No basic igneous analogs were found for the eclogitic rocks, and their bulk com positions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and thecalcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks.The eclogitic rocks and associated amphibolites are characterized by the follow ing essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants.The garnets in these eclogites are compositionally similar to those from "group C" eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane.The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White's (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by data on garnet-clinopyroxene pairs.Based on experimental data from various authors, and thermodynamic consid erations, these eclogitic mineral assemblages were probably crystallized at tempera tures of 540° to 590°C at 5.5 to 7.5 kb.K40/Ar40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.Eclogite occurrences were associated with two belts of calc-magnesian rocks in a terrane otherwise dominated by pelitic schists. The belts are defined by discon tinuous masses of calc-magnesian schists that are elongate parallel to the northeast strike of compositional layering and crystallization foliation in the surrounding pelitic schists. The calc-magnesian masses contain various metamorphic rock types including amphibolite, impure marble, and eclogitic rocks.Prindle (1913) distinguished three varieties of ec...

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The report is designed to provide current, accurate, and technically reliable information about Alaska's mineral industry. The report depends on the cooperation of individuals, private industry, and government agencies to provide information on their mining projects and activities.In 1990 the sum of expenditures for exploration and development, and the estimated value of production totaled $61 0.6 million. Exploration expenditures rose to $63.3 million, with most of the effort concentrated on hardrock gold projects, some of them in advanced stages. Development expenditures were $14.3 million, considerably less than the $134.3 million in 1989, mainly due to completion of development at the Red Dog1 and Greens Creek mines.Value of mineral production in 1990 was $533 million, doubling the 1989 production value of $277 million.In 1990, with 55 percent of the nation's supply, Alaska led the United States in zinc production. Consequently, U.S. net import reliance for this crucial commodity decreased from 61 to 48 percent.Alaskan silver production accounted for about 18 percent of the domestic mine production. Only Idaho and Nevada produced more.Alaska continues to have the nation's largest placer gold mining industry, which provides jobs to many rural Alaskans.Significant progress in land reclamation and water quality at placer mines indicate that Alaska's mining industry is capable of providing economic diversity and environmentally sound operations in the 1990s.'Names of significant mines and deposits appear in boldface type throughout this report.. . I ICONTENTS m m n n w n t~t t~~m t t n m t n t ! t~~n t t t n n m n n~m t n n m m m m m m~m n m~m m m m

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