Stratigraphy of the Sandman low sulfidation Au deposits, Winnemucca, Nevada

Newmont Mining Corporation's Sandman Au project consists of five recognized, mid-Miocene low-sulfidation epithermal deposits hosted primarily in mid-Tertiary volcanic, volcaniclastic, and fluvial rocks, and to a lesser degree within basement Triassic rocks and late Mesozoic intrusions. These deposits include North Hill, Silica Ridge, Southeast Pediment, Abel Knoll and Ten Mile.;Excluding the Ten Mile deposit, which is controlled by northeast-striking faults, deposits occur along north to north-northwest extensional structures. Intersections with northeast structures and the presence of Early Miocene mafic dikes also played a role as fluid conduits. Post-mineral structural offset affects all deposits. Older fault reactivation and movement along younger Basin and Range structures resulted in the development of north-south-striking grabens and half grabens and lesser intervening northeast grabens.;Basement lithologies consist of Triassic metasedimentary rocks dominated by phyllitic mudstones and siltstones with lesser degrees of quartzites and rare limestone units interbedded. Regional foliations strike northeast and dip moderately to the northwest. Basement rocks have been intruded by late Mesozoic granitic stocks to the east and southeast of the project in addition to local diking in phyllites on the flanks of Blue Mountain to the west. Lying unconformably on basement rocks is an approximately 700->1300 foot thick package of Tertiary volcanic, epiclastic, and fluvial rocks. Lithologies consist of northeast striking, gently southeast dipping dacitic to rhyolitic airfall tuffs, pyroclastics and distally sourced ash flow tuffs with interbedded fluvial and lacustrine volcaniclastic siltstones, sandstones, and conglomerates. The presence of thick, distally sourced fluvial rocks in the northern portions of the property represent a major east-west (?) Oligocene to early Miocene paleodrainage. These rocks may be related to the inferred Cretaceous age Pansy Lee Conglomerate to the east, although this remains to be definitively proved. Additionally, further to the south drill intercepts have encountered approximately two hundred feet of organic and sulfur-rich, varved and laminated lacustrine sediments. Early Miocene basaltic trachy-andesite, trachy-andesite, and andesite sills and dikes intrude and cap the Sandman stratigraphic section.;Subtle topography limits outcrop exposures of mid-Tertiary rocks to capping mafic flows within the Basalt and Little Basalt Hills and more resistant quartz-adularia altered ridges occurring in the northwest of the Sandman project area. Additionally, extensive Quaternary deposits of the Crescent Dune Field cover much of the northern part of the property. Three field seasons were spent mapping surface exposures and trenches and logging drill core and reverse circulation chips from the project area leading to the recognition of three distinct tuff units within the Sandman stratigraphic section. Petrography, trace element geochemistry, and radiometric age dating was used to create a detailed stratigraphic column linking variable stratigraphy from deposit to deposit.;Within the northwest of the property, mapping and drilling has indicated a paleodrainage-controlled, marker fiamme unit---dated at ∼25.4 Ma---linking the stratigraphic section across Silica Ridge, Adularia Hill, and North Hill. Outcrop appearance with abundant large fiamme, a sparse phenocryst assemblage of sanidine-plagioclase-anorthoclase-beta quartz-biotite, radiometric age dating, and trace element geochemistry confirm this marker fiamme unit to be the regionally widespread rhyolitic Nine Hill Tuff. Named for the type locality in the Sierra Nevada foothills just north of Carson City, this pyroclastic unit is widespread and has been correlated with the "D" unit of the Bates Mountain Tuff in central Nevada near Austin. The Sandman project area is the furthest north documented location of the Nine Hill Tuff. No source caldera has yet been identified, though through regional distribution it has been suggested to lie beneath sediments within the Carson Sink. (Abstract shortened by UMI.).