DIAGENESIS OF THE BURRO CANYON FORMATION, LISBON VALLEY REGION, UTAH-COLORADO
Efem Altinok (M.S. 1998)
ABSTRACT
The diagenesis of the early Cretaceous Burro Canyon Formation, which hosts the main copper ore in the Lisbon Valley area, is the focus of this thesis. The study was accomplished by measurement of eleven stratigraphic sections both near and distant from mineralization and by petrographic analysis. The stratigraphic sections indicate that the Burro Canyon Formation is thinnest in the East Canyon syncline and thickest over the Lisbon Valley salt anticline. This thickness pattern is different from was has been observed along strike in the Moab Valley and Gypsum Valley areas. The diagenesis of the Burro Canyon Formation was studied by means of transmitted and reflected light microscopy, X-ray diffraction, SEM, and coulometric techniques. These analytical methods proved that the paragenetic sequence follows a systematic and chemically consistent order: 1) micritic calcite and hematite, 2) quartz overgrowths, 3) sparry calcite, 4) pyrite, 5) siderite, 6) illite-smectite, 7) kaolinite, 8) quartz overgrowth, 9) sparry calcite, 10) copper mineralization, 11) illite-smectite, and 12) weathering. The paragenetic relationship indicates that the diagenetic history of the Burro Canyon Formation can be divided into two major stages: 1) events which took place between the burial of the Burro Canyon Formation and the initiation of either Laramide orogeny or Miocene uplift of the Colorado Plateau, and 2) events which took place during the unroofing of the Burro Canyon Formation. The earliest diagenetic phases indicate that oxidizing conditions predominated during the deposition and early diagenesis of the Burro Canyon Formation. Computer modeling suggests that this is geochemically reasonable. During progressive burial, the diagenetic conditions became reduced most probably due to the influx of fluids from the overlying marine Dakota Sandsonte and Mancos Shale. Copper mineralization in the Lisbon Valley area is fault related and took place during or immediately after the unroofing of the Burro Canyon Formation. Based on the diagenetic mineralogy in the host Burro Canyon Formation demonstrated by this study, two different models of copper sulfide precipitation are proposed: 1) the replacement of diagenetic pyrite by copper sulfides, 2) replacement of calcite by copper-bearing relatively acidid solutions.
(Co-advisors: Wendy Harrison & M. Hitzman)
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