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Temporal and Spatial Constraints on Sedimentary Rock-Hosted Gold Deposits: Pequop Mountains, Nevada
AdvisorRessel, Michael W.
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Discovery of sediment hosted, distal disseminated gold at the Pequop Mountains (Pequops) inthe early part of the twenty-first century spurred new research interest in tectonic setting andprocesses where such deposits will form. At the Pequop gold is disseminated in Paleozoic siltycarbonates and dissolution breccias associated with faults, folds, and dikes. These deposits exist~120 km east of well-known Au trends in Nevada, in metamorphosed terrane formerly consideredless prospective for this style of deposit. Well-studied Carlin-type gold deposits along the Carlintrend (Nevada) formed during the Eocene (42-36 Ma) at shallow depths of 1-3 km in rocksunaffected by regional metamorphism. In contrast, distal disseminated deposits at the Pequopsformed between 33 and 38 Ma and are hosted in variably metamorphosed rocks exhumed duringbrittle crustal extension associated with the Ruby Mountains-East Humboldt metamorphic corecomplex (RMEH). Exhumation of the RMEH occurred over pulsed intervals of cooling in the lateEocene, Oligocene and Miocene. The Pequops show intervals of extensional faulting andexhumation that are temporally similar to exhumation at the RMEH.Gold mineralization at Long Canyon postdates Jurassic mafic dikes and sills, which host ore andprovide a source of iron for sulfidation and mineralization. Age of mineralization at West Pequopis constrained by spherulitic rhyolite dikes dated at ca.40 Ma, are hydrothermally altered and hostgold mineralization. High-silica rhyolite dikes at West Pequop differ from andesite through lowsilicarhyolite lavas of the ca. 39 Ma Nanny Creek volcanic field to the northeast suggesting twoloci for Eocene magmatism. Spherulitic textures in the Eocene rhyolites where they intrude highgrademetamorphic rocks suggest: 1) very shallow level of emplacement at ca. 40 Ma, and 2)significant exhumation occurred pre-40 Ma.Geochronology and thermochronometry on illite, zircon and apatite from igneous andsedimentary rocks bracket the age of sedimentary rock hosted Au mineralization at the Pequops.U-Pb ages of zircon from altered intrusions are Jurassic and Eocene, consistent with regionalmagmatism while apatite fission-track and (U-Th)/He ages indicate cooling progressed from ca. 51Ma on the east flank to 4 Ma on the west flank. Illite from mineralized rocks resulted in 40Ar/39Arages ranging from ca. 55.5 to 33.3 Ma. Illite from Au-mineralized sedimentary breccias provide astrong indication that gold mineralization likely occurred ca. 33.3 Ma, Based on thermochronologyand cross cutting relationships, Au mineralization in the Pequops occurred between 38 to 33 Maand at depths of 1 to 4 km, similar to other sedimentary rock-hosted Au deposits in Nevada.Pequops disseminated gold deposits are unique however, since there is indication for a second,Miocene age of mineralization, ca. 16 to 22 Ma.U/Pb apatite and AFT data are consistent with exhumation beginning in the Eocene throughthe mid-Miocene. Clustering of several apatite U/Pb ages from 41-32 Ma may alternately reflectcooling of an Eocene intrusion at depth. Eastward variation in AFT data from ~22-163 Ma,respectively suggests the area straddled the Oligocene 110°C isotherm required to anneal apatite,and thus was relatively shallow. Eocene and younger mineralization also occurred relativelyshallow, <4 km.New geologic mapping combined geochronology, and thermochronometric data providenumerical constraints on Cenozoic exhumation of the Pequops. Low-angle normal, or detachment,faults are prominent features in northeastern Nevada, particularly east of the RMEH. The flat-lyingPequop fault at the Pequops records the earliest extensional faulting and is temporally and spatiallycorrelative with other low-angle normal faults documented in proximal ranges. Earliest extensionon the Pequop fault ceased by ca. 38 Ma. (U-Th)/He zircon and apatite thermochronometry yieldage patterns across the range that are interpreted to represent progressive east to west exhumationof the range from the late Eocene through Miocene. Thermochronology data also provide evidenceof a thermal ‘re-heating’ event recorded by asynchronoous cooling ages interpreted to reflect theage of distal disseminated gold mineralization (ca. 33-38 Ma). Eastward rotation of the centralPequops has been recorded by 40 to 45° ESE dip of the Eocene Nanny Creek volcanic units and isaccommodated by slip on the Pequop and Independence Valley NW faults. Regionally these largenormal faults are accommodating brittle extension of the crust related to the exhumation of theRMEH.