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Stratigraphy of the Sandman Low Sulfidation Au Deposits, Winnemucca, Nevada
AdvisorThompson, Tommy B.
Geological Sciences and Engineering
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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. Hosted entirely within mid-Tertiary volcanic, epiclastic, and fluvial rocks, the deposits of North Hill, Silica Ridge, and Southeast Pediment are both structurally and stratigraphically controlled. Abel Knoll is hosted within a mid-Tertiary polylithic breccia body composed of trachy-andesite to andesite, basement phyllites and trace granodiorite, and tuffaceous wall rocks. In the southeast of the project area, the Ten Mile deposit is structurally controlled and hosted dominantly within a late Mesozoic granodiorite stock. 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. Lower in section, a distinctive rhyolite tuff with coarse-grained sanidine-anorthoclase crystal fragments and abundant pumice ± fiamme has been dated at ~26.0 Ma. With a similar age, the Ashdown tuff--exposed to the west of the Sandman project in the Pine Forest and Black Ranges--is petrographically similar with abundant coarse-grained sanidine and pumice fragments; however, the two have differing trace element geochemistry, with the Ashdown Tuff being significantly enriched in Zr and Nb compared to the unnamed tuff found in outcrop at Adularia Hill and encountered in drill core at Silica Ridge and Southeast Pediment. A dacitic to rhyolitic tuff of older--but unconfirmed age--has been encountered in drill core at Abel Knoll, Southeast Pediment, and Silica Ridge. Recognized by a distinctive, moderately welded, basal polylithic volcanic breccia, the Red-Green Breccia tuff package is composed of repetitive lapilli-lithic, fiamme, and vitric airfall tuffs. Excluding rare plagioclase and biotite within the basal polylithic breccia, crystal fragments are largely absent. A lack of datable phenocrysts combined with extensive propylitic, argillic, and quartz-adularia overprints has hindered dating of this tuff unit. The youngest rocks exposed within the Sandman project area include capping mafic flows and related sills and dikes of basaltic trachy-andesite, trachy-andesite, and lesser andesite. These rocks have been dated at ~22.5 Ma. Rocks of a similar age and composition have been documented to the northeast of the Sandman project area within the Bloody Run Hills and Santa Rosa Range. Similar rocks are also exposed to the east in the hills between Winnemucca Mountain and the Bloody Run Hills, and to the north in isolated exposures along the east flank of the Slumbering Hills. With abundant clasts of andesite, the Abel Knoll breccia body is also likely of a similar age. The spatial association of early Miocene dikes, sills, and mafic breccia bodies to middle Miocene low sulfidation Au mineralization may provide a useful tool for future exploration within and outside the project area.