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Date of Award
Master of Science (MS)
Pevear, David R.
Babcock, R. Scott (Randall Scott)
Tourmaline-bearing breccia pipes are associated with late stage Tertiary porphyritic intrusive rocks in the Silver Star Plutonic Complex.
Whole-rock geochemical analysis of rocks from the complex show a silica vs. alumina/(K2O + Na2O + CaO) trend (Feiss, 1978) toward geochemical conditions favorable for porphyry copper-type mineralization. Copper present in diorite, quartz diorite, and quartz diorite porphyry rocks probably entered silicates, whereas copper formed porphyry copper-type mineralization in the more felsic granodiorite and granodiorite porphyry intrusions.
Optical, x-ray, and chemical analyses indicate two distinct populations of tourmaline present in the breccia pipes; an early "Hemlock Ridge-type" which preceded pyrite, and a later "Black Jack-type" which followed pyrite and preceded chalcopyrite. The Hemlock Ridge-type have refractive indices of ɛ = 1.625 to 1.636 and ω = 1.652 to 1.663, whereas the Black Jack-type have refractive indices of ɛ = 1.643 to 1.652 and ω = 1.671 to 1.675. The ao unit cell dimension of the Hemlock Ridge-type are all less than 15.90 Å, and the Black Jack-type are all greater than 15.90 Å.
Chemically, the earlier Hemlock Ridge-type tourmaline contains 28.5 to 32.3% AI2O3, 4.43 to 8.57% Fe2O3, and 10 to 20 ppm copper, whereas Black Jack tourmalines contain 22.7 to 26.4% A12O3, 11.7 to 16.4% Fe2O3, and 16-76 ppm copper. Although the Black Jack tourmalines are related in time and space to extensive chalcopyrite mineralization, crystal-field effects (Curtis, 1963) restrict copper's entry into the tourmaline structure more than iron. Therefore, copper is reflected indirectly by the high iron content of tourmaline.
Tourmaline analysis is a viable exploration tool for copper in the Silver Star area, and the concepts and methods applied herein are valid, and probably can be applied regionally.
Tourmaline, Breccia pipes, Silver Start Plutonic Complex, Petrology
Western Washington University
Skamania County (Wash.)
Copying of this thesis in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Birk, Robert H., "The Petrology, Petrography, and Geochemistry of the Black Jack Breccia Pipe, Silver Star Plutonic Complex, Skamania County, Washington" (1980). WWU Graduate School Collection. 777.