Graduate Thesis Or Dissertation
 

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  • Ore deposits of the Butte district formed at ca. 66 to 62 Ma within a host rock of Butte Quartz Monzonite (ca. 76 Ma). Deposition of the Lowland Creek Volcanics Formation (ca. 53 to 50 Ma) on top of the Butte Quartz Monzonite and its ores, followed a period of extensive uplift, erosion, and unroofing. The amount of post-mineral tilting related to Cenozoic normal faults has been controversial. In the northwest part of the district, the Lowland Creek Volcanics dip 10 to 50 degrees northwestward. From which data, Proffett (1973, 1979) inferred that the underlying ore deposit is similarly tilted. Paleomagnetic studies of the Butte Quartz Monzonite in the central and western part of the district suggest lesser tilts of 5 to 17 degrees north to northwest (Geissman et al., 1980z, b). In an effort to resolve the question to the amount of post-mineralization tilting, the structural geology of the Butte district was remapped at 1:12,000 scale. Structural attitudes (n=407) were collected on originally subhorizontal sill-like bodies of aplite that are cogenetic with the Butte Quartz Monzonite. Examination of these data supports a hypothesis that the amount of tilting related to Cenozoic normal fault block rotation varies spatially across the district. Sheeted sill-like bodies of aplite in the northwest part of the district are tilted 34 degrees northwestward. West of Big Butte, sill-like bodies of aplite are gently tilted 16 degrees northwestward. Sill-like bodies of aplite in the northern and eastern parts of the district are tilted 10 to 15 degrees north to northeastward. In the southern part of the district, sill-like bodies of aplite on Timber Butte are tilted 20 to 25 degrees northeastward. In the northwest part of the district, the Lowland Creek Volcanics and underlying Butte Quartz Monzonite are tilted 10 to 50 degrees northwestward by closely spaced (0.3 to 0.6 km) northeast-striking, southeast dipping normal faults that exhibit moderate displacements (less than 0.8 km). These northeast striking normal faults exhibit diminishing amounts of offset and northwest tilting southwestward along the faults and successively decreasing amounts of offset and tilting southeastward, perpendicular to the faults. The northeast striking normal faults include some moderate northwest dipping faults. Large north south striking, widely spaced (~9.5 km) Basin-and Range type normal faults cut northeast striking faults. These faults localized deposits of early Miocene and younger clastic sedimentary rocks in their hanging walls. The largest is the Continental-Klepper-East Ridge fault system which shows increased displacement (from ca. 1.5 to >2 km) and eastward tilting of the hanging wall southward along the fault system. By restoring the orientation of the sill-like bodies of aplite in the northwest part of the district, prior to moderate (10 to 50 degrees) northwest tilting, the sill-like bodies of aplite become gently tilted 10 to 15 degrees northeastward. The restored orientation of the sill-like bodies of aplite is similar to the orientation of the sill-like bodies of aplite southeastward across the district. These observations support the interpretation that the district was gently tilted 10 to 15 degrees northeastward prior to the deposition of the Lowland Creek Volcanics Formation. Analysis of these data identified three episodes of tilting that has tilted different parts of the district in differing amounts supporting both Proffett's (1973, 1979) and Geissman et al.'s (1980a & b) geologic data. The first episode gently tilted the district 10 to 15 degrees northeastward and occurred between 62 Ma (post Main-Stage mineralization) and 59 Ma (pre granite porphyry dike emplacement). The second episode moderately tilted the district 10 to 50 degrees northwest tilting in the northwest part of the district and occurred between 51 Ma (deposition of the Lowland Creek Volcanic Formation) and Basin and Range type normal faulting. Moderate northwest tilting is related to deformation and accommodation along northeast striking normal faults that cut the northwest part of the district. The final episode gently tilted the hanging wall of the Continental fault ~14 degrees eastward and occurred during the middle Miocene related to Basin and Range type normal faulting. By restoring the district prior to the three episodes of tilting, the orientations of quartz porphyry dikes, Pre-Main Stage veinlets, and zonal patterns related to ca. 66 to 62 Ma mineralization-hydrothermal alteration become vertical and symmetrical. This vertical orientation is consistent with most hypotheses of porphyry copper emplacement. Additionally, the two conjugate fault systems occupied by Main Stage veins (ca. 62 Ma) also restore to become normal oblique-slip faults.
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