The Evolution and Relationship of the Western North American Paleozoic Carbonate Platform and Basin Depositional Environments to Carlin-type Gold Deposits in the Context of Carbonate Sequence Stratigraphy

摘要

The Western North American continent developed by passive-margin rifting from a Late Proterozoic supercontinent. This breakup formed a~5,000-km-long passive continental margin from the Late Proterozoic (~600 Ma) through the Devonian (-360 Ma), upon which a -15,000-20,000 foot-thick (-4,500-6,000 m) carbonate platform and basin formed. Carbonate sequence stratigraphy is a powerful tool to understand how this carbonate platform and basin evolved. During Cambrian through Devonian time, eleven 3rd-order eustatic sea level lowering and rising events occurred, as demonstrated by the easterly retrograding and westerly prograding of the carbonate platform. During these sea level lowstands, wave action eroded and undercut the platform reef margins. This resulted in instability at platform margins and slides, slumps, debris flows, and turbidites were transported into deep-water slope, base-of-slope, and basinal environments while karsts formed in shallow water platform interior shelf lagoons and supratidal flats. Sea level low-stand boundaries between shallow-water carbonate platform reef margins and adjacent deep-water slopes are herein defined as "Isochrons". Determining the geographic position of each Isochron is important for understanding the architecture of the Paleozoic carbonate platform through time and space and for predicting the location of "Fairways"-areas hosting Carlin-type gold deposits in deep-water and shallow-water sedimentary rocks. Nevada ranks within the top five gold-producing regions of the world. During the Eocene between -42 and 36 Ma, millions of ounces of gold were hydrothermally transported upward through reactivated and inverted normal and transform faults. Gold was deposited within deep-water carbonate debris flows, turbidites, slides, and slumps, and within coeval shallow-water carbonate karst facies and calcareous siltstone and siliciclastic shale that immediately overlies the karst. Cambrian-Early Mississippian deep-water Carlin-type gold trends in Nevada include: Getchell, Cortez, Carlin, Tonkin Springs and Roberts Mountains; shallow water Carlin-type gold trends include Rain, Emigrant Springs, Eureka, Bald Mountain, Long Canyon and Kinsley Mountain; both shallow-water and overlying deep-water sedimentary rocks host Carlin-type gold deposits along the Cortez, Roberts Mountains, Carlin and Independence trends. Post-Devonian gold has been found in foreland basin Mississippian siliciclastic strata and in Mississippian-Permian carbonate platform sedimentary rocks. Early Cambrian through Lower Mississippian 3rd, 4th and 5th order eustatic sea level lowstands were the main factors responsible for the origin of favorable deep-water and shallow water carbonate gold-hosts.