Graduate Thesis Or Dissertation
 

Study of the gold deposits at the War Eagle mine, Idaho County, Idaho

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  • Renewed interest in the abandoned War Eagle Mine, which is located in the center of the gold districts of central Idaho, provided the opportunity for a detailed study of the deposit. The goal of this study is to determine the genetic history of the deposit with respect to regional mineralizing events. Because the old workings have collapsed, historical descriptions and reports from the period of activity of the mine are combined with current surface and subsurface data to reevaluate the property. The deposits in the vicinity of the mine occur as quartz veins in muscovite-biotite granite, a Late Cretaceous phase of the Idaho batholith. The deposits have been dated at about 71 m.y. by 40Ar/39lAr techniques and are genetically related to the hosting granite. The intersection of major through-going N10-20°E faults and N35°E tension fractures is the consistent structural control. The veins are actually lense-like quartz bodies, which have a maximum length of 15 meters. Quartz, the prominent gangue mineral, reveals three episodes of precipitation separated by episodes of brecciation. The last quartz generation is microsaccharoid quartz that cemented the fragments of pre-existing quartz. The bulk of the mineralization accompanied this episode. Sulfide minerals include pyrite, chalcopyrita, galena, sphalerite, and freibergite. Gold is not present as free grains and probably occurs in solid solution with the base metals in the sulfide minerals. The hydrothermal fluids from which the veins precipitated altered the granitic country rock over 10 meters from the fault conduits. The alteration assemblage associated with the veins is characterized by a pervasive replacement of primary minerals by sericite, quartz, minor calcite, and pyrite. An attempt to determine the source of the hydrothermal fluids through oxygen isotope analysis indicate 6¹⁸0 values between -0.1 and +1.9 permil, from a single mineral quartz analysis averaging 9.3 + 0.2 permil. Further calculations suggest a definite magmatic source for the original fluids. A younger argillic-type of alteration is localized along the fault structures and cross cuts the sericitic assemblage. Hydrothermal alteration is best developed within the 15-meter wide War Eagle shear zone where the ore bodies mined during the 1930's are located. The size and richness of the veins are related to favorable locations along major fault zones that also underwent episodes of shearing. The other vein occurrences on the property are along narrow fault zones with too few tension fractures and no indication of intense shearing. Although the War Eagle deposits occur isolated from the surrounding mining districts, the parallel trends of mineralization, narrow zones of alteration, and similar sulfide mineral assemblages, as compared to the deposits of the Buffalo Hump area (40 air-kilometers to the northwest) indicate an association between the deposits. The War Eagle deposit and the deposits of the Buffalo Hump district are of the same age. Formation followed the emplacement and cooling of the muscovite-biotite granite pluton of the Idaho batholith. The mesothermal veins formed in normal fault zones near the roof of the pluton. These veins were sheared and evolved into quartz lenses. Quartz that recorded multiple structural events host the best deposits.
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