The central part of the Bayhorse Mining District is located in the Salmon River Mountains in north-central Idaho between the towns of Challis and Clayton. The area is underlain by metasedimentary rocks of Early to Middle Paleozoic age that were profoundly affected by the emplacement of a plutonic complex in Middle Cretaceous time, both of which were later intruded and covered by volcanic rocks of Early Tertiary age. Both intrusive events were accompanied by
significant mineralization. These basement rocks and the associated mineral deposits have been partly exposed by post-Miocene uplift and subsequent glacial and deep stream erosion. The stratigraphic succession within the Bayhorse area consists of a series of alternating pelitic, carbonate and quartzite units that range from Latest Cambrian to Middle Ordovician in age. Five sedimentary rock units have been described within the area of study and consist, from oldest to youngest, of the Garden Creek Phyllite, Bayhorse Dolomite, Ramshorn Slate, mixed lithology sequence and Clayton Mine Quartzite. The lithologic and textural varieties of the Lower Paleozoic
rocks, combined with regional considerations, collectively indicate that the Bayhorse area was transitional between marine shelf areas to the west, north and east and deeper miogeosynclinal areas to the south for most of Paleozoic time. Central Idaho was affected by magmatic activity continually from Late Jurassic to Middle Cretaceous time. Synchronous with the onset of batholithic scale magmatism was folding and thrust faulting of the Paleozoic sedimentary rocks. The magmatic activity culminated in the formation of the Idaho Batholith and related outlying plutons, one of which is locally represented by the Juliette Creek intrusive complex. Geologic evidence indicates that the Juliette Creek intrusive complex represents the upper parts of a much larger and somewhat deeper plutonic mass that was forcefully emplaced into the surrounding sedimentary rocks at depths ranging from 4 to 5 miles along anticlinal axes that paralleled the north-south structural grain of the region. In approximate order of emplacement the exposed part of the intrusive complex consists of quartz diorite, granodiorite grading to granite, and quartz-feldspar porphyry. The effects of thermal metamorphism were variably imposed upon the adjacent sedimentary rocks and the resulting changes in the lithologic characteristics of the country
rocks aided in the modification of the pre-existing local structure by the forceful emplacement of the intrusive complex. Hydrothermal alteration and sulfide metallization are predominantly structurally controlled and spatially, temporally and probably genetically related to the Juliette Creek intrusive complex. Fluorite mineralization is related to the later igneous activity of Early Tertiary age. The emplacement of the intrusive complex was of major importance in preparing the ground for the two later episodes of mineralization by significantly altering the pre-existing local structure and lithologic characteristics of the sedimentary rocks The predominant structural feature of the district consists of two parallel elongate folds that formed in the Paleozoic sedimentary rocks by eastwardly directed compressional movement. Subsequent ethplacement of the Juliette Creek intrusive complex has locally modified the pre-existing structure and caused the sedimentary rocks to break along predictable zones of weakness. The sulfide metallization is related to the upper parts of a large hydrothermal system that may be associated with stock-work molybdenum or porphyry-copper type mineralization at depth.
After this major period of magmatic, tectonic and hydrothermal activity the rocks of the district were again affected by a later, but similar sequence of events that culminated in the eruption of the rhyodacitic, andesitic and basaltic flows and pyroclastic deposits of the Challis Volcanics and the deposition of significant fluorspar in Early Tertiary time.
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