TIMING OF DOLOMITIZING FLUIDS IN ZEBRA DOLOMITE BODIES OF NEVADA’S BASIN AND RANGE PROVINCE

Zebra dolomites, in Nevada, are commonly found in spatial and stratigraphic proximity to economic ore deposits. Zebra dolomite is a product of hydrothermal alteration resulting from the inter-fingering of dark, fine-grained replacive dolomite and light, coarse-grained saddle dolomite bands. Paleomagentic analysis was conducted to determine whether hydrothermal fluids relating to zebra dolomitization occurred co-eval with economic ore deposits (shortly after deposition of host rock) or during one of the many orogenic episodes up to and including the Basin and Range. Dolomites within Cambrian age Hamburg Dolomite and Nopah Formation; Devonian age Simonson Dolomite, McColley Canyon Formation; Mississippian age Monte Cristo Limestone; and Permian age Arcturus Formation were subjected to paleomagnetic analysis to test if timing of diagenetic alteration leading to the formation of a zebra dolomite could be determined.

Thermal demagnetization of zebra dolomite and host carbonates in this study uncovered a characteristic remanent magnetization (ChRM) likely carried in magnetite, with a maximum unblocking temperature of 500° C. After correction for Cenozoic tilting, the calculated virtual geomagnetic pole of 53 ֯N, 161 ֯W (dp=5.6, dm=5.6) corresponds to the late Jurassic- early Cretaceous portion of the apparent polar wander path for North America. This ChRM, interpreted as a chemical remanent magnetization (CRM), may represent a re-magnetization acquired during mobilization of hydrothermal fluid flow during relaxational extension periods of the Sevier Orogeny or Central Nevada Thrusting. The NW direction is similar to an intermediate temperature component, interpreted to reside in pyrrhotite and/or magnetite, reported by other workers from Paleozoic rocks throughout Nevada. A failed conglomerate test of breccia clasts, coupled with field observations of zebra dolomite outcrops in the host Monte Cristo limestone, suggest zebra dolomitization occurred prior to this fluid-flow event; therefore, zebra dolomitization must be older than late Jurassic - early Cretaceous in age. These findings differ from previous studies of zebra dolomite at Hancock Summit and the Delamar Range in southern Nevada which show a Triassic-aged CRM in the Guilmette Formation, which could be the timing of the zebra formation.