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The Eureka district hosts mid-Cretaceous (~106 Ma), igneous-related, polymetallic carbonate replacement deposits that have subsequently been overprinted by Eocene(?) Carlin-type gold mineralization, including the Archimedes deposit. This study presents (1) results of new Anaconda-style outcrop geologic map of a structurally complex area measuring 4.5 km2 in the northern part of the Eureka district along the Jackson-Lawton-Bowman normal fault system, (2) structural reconstructions of Tertiary normal faults, as well as (3) reconnaissance trace-element and stable isotopic analyses in an attempt to differentiate between alteration associated with the midCretaceous and Eocene(?) events. The study evaluates the possible relationship of the Jackson fault system to Carlin-type gold. The Jackson fault, which has been proposed as a structural control for both styles of mineralization, extends the length of the Eureka district (21 km). The normal fault system dismembers folds of the Eureka culmination, which involves lower Paleozoic carbonate and clastic rocks. Evidence from structural reconstructions suggests that the north-striking Jackson branch and north-northwest striking Lawton branch have dismembered an anticline-syncline pair that is interpreted to have formed by faultpropagation folding concurrent with movement on the Champion thrust fault in the Early Cretaceous in the Eureka portion of the central Nevada thrust belt. Beginning at the southern edge of the district, the Jackson fault zone is composed of a series of overlapping, steeply (~70°) east-dipping, north-striking fault segments for ~3.8 km. Farther north, the main fault splits into at least three closely spaced, roughly parallel branches, which also dip steeply eastward. The Buckeye fault is likely a fourth branch of the Jackson normal fault system. Contraction and growth of the Eureka culmination occurred concurrent with deposition of the synorogenic Early Cretaceous Newark Canyon Formation at ~116 Ma (Aptian). Mid-Cretaceous intrusions were emplaced and associated carbonatehosted ores formed at ~106 Ma (Albian, i.e., late Early Cretaceous), or ~10 m.y. after contractional deformation. The fact that the northwest-striking, down-to-the-north Ruby Hill normal fault cuts and offsets mid-Cretaceous mineralization and is in turn cut and offset by the Jackson branch effectively precludes the possibility that the Jackson fault acted as a conduit for mid-Cretaceous magmatic-hydrothermal fluids. Carlin-type mineralization at Archimedes is structurally controlled, but by the older, west-northwest striking, down-to-the-north Blanchard and Molly faults. The Eocene(?) Carlin-type overprint on Cretaceous base-metal mineralization in the northern Eureka district may have enriched the tenor of gold mined from carbonate replacement deposits. Soil analyses in the northern Eureka district show subtle Au and As anomalies adjacent to the compound portion of the Jackson fault, and the trace of the fault broadly coincides with a belt of carbonate replacement ore mined in the late 1800s, which is all or in part of mid-Cretaceous age. Mapped alteration patterns (jasperoid, marble, bleaching, and sanding of carbonates), which spatial relationships and isotopic studies suggest are related to the Cretaceous magmatic-hydrothermal system, nonetheless show no relationship to the Jackson fault. The Jackson-Lawton-Bowman normal fault system postdates and offsets not only the carbonate-hosted ores but also Carlin-type gold mineralization and is likely a mid-Cenozoic fault system. It thus has no genetic relationship to either the mid-Cretaceous or Eocene mineralizing systems but may be responsible for preserving both systems on the eastern side of the area. This study underscores the challenge of identifying structural controls on Carlin-type mineralization prior to or beyond the extent of mining-related exposures, especially in structurally complex areas where there is imperfect exposure and where there may be partial spatial overlap between Carlin-type and other mineralizing systems. [ABSTRACT FROM AUTHOR]

Mineral deposits that have been dismembered and tilted by syn- or post-mineral faults can be structurally reconstructed to achieve scientific and practical objectives. Fault surface maps are plan-view projections of the geology of the footwall and hangingwall surfaces of a fault, showing rock units, faults, and other features. Here, we describe how to construct fault surface maps, how to present them efficiently in publications, and how to use them to make three-dimensional, stepwise structural reconstructions, restoring the hanging-wall map against the footwall map. Areas of complex geology containing multiple generations of faults with differing slip directions especially benefit from the use of fault surface maps for three-dimensional reconstructions. Where there are adequate geologic markers and three-dimensional control, the restoration results in numerous slip vectors across the fault surface from which the magnitude of slip in the plane of the fault can be calculated for each vector. Additional geologic constraints are required to assess the amount of tilting that occurred concurrent with slip on each generation of faults. To achieve a three-dimensional structural reconstruction of an area, the restoration process is repeated from youngest to oldest faults, working backward in time. Older faults appear as single lines, commonly known as cutoff lines, on both the footwall and hanging wall surfaces of younger faults. Faults that moved synchronous with the fault surface being mapped (e.g., splays of the same fault) appear as a single branch-line trace in the same position in both footwall and hanging wall maps. Younger, crosscutting faults appear as gaps in the fault surfaces of older faults that are identically positioned on footwall and hanging wall maps, and the gaps widen in the direction of increasing displacement or decreasing dip of the younger fault. Examples from normal faults in the Robinson and Yerington mining districts, Nevada, illustrate the differing appearances of fault surface maps of early generations of faults versus later generations of faults. The types of movement that achieve a restoration differ depending on whether the portion of a normal fault being restored is near or far from either tip where the displacement diminishes to zero. Near-tip regions commonly display an important rotational component pinned near the tip, whereas midfault regions are dominated by a down-dip translational component. Fault surface maps provide an efficient method for portraying the locations and types of three-dimensional geologic controls, thereby allowing readers to see more clearly where geologic interpretations are well constrained versus loosely constrained by the available data. This allows users to evaluate the validity of a proposed reconstruction better, to develop and evaluate alternative hypotheses more easily, and to dispense with unviable alternatives more readily. Structural reconstructions historically have led to discoveries of offset continuations of known orebodies and of new orebodies. Use of fault surface maps for reconstructions facilitates a more threedimensional approach to mineral exploration in complexly faulted regions, such as certain parts of the Basin and Range province. [ABSTRACT FROM AUTHOR]

The Yerington porphyry copper-skarn-iron oxide-copper-gold district is a classic area of continental extension, having been extended more than 150% by multiple generations of Cenozoic east-dipping normal faults that penetrated a minimum of 8 km depth into the crust, initiated at high angles, and rotated to shallower dips until being cut by younger sets of faults. This study examines the Cenozoic normal faults in the vicinity of the Ann-Mason and Blue Hill areas through detailed mapping of two major faults, logging intervals of drill core containing the fault damage zones, and constructing fault surface maps, i.e., geologic maps of the proximal footwall and hanging wall of faults superimposed on structural contour maps of the fault planes. Six normal faults, representing four geometric sets or temporal generations of faults numbered from oldest to youngest are described and analyzed, with particular emphasis on the oldest two generations. Fault surface maps constrain the slip vectors of the faults and their variability along strike. Faults of the latest three generations strike northerly and dip easterly. Faults of the second generation, which are of middle Miocene age, include the Blue Hill (~2.8 km slip with increasing displacement to the north) and Singatse (3.7 km slip) faults. These second-generation faults have damage zones that persist ~15 m on either side of the fault and have hanging-wall splays that merge into the main fault surface. The first generation faults are represented by the 1A fault, which is one of a series of sub-parallel, generally small-offset faults that presently strike southeast and dip steeply to the southwest. Analysis of drill hole data indicates that the 1A fault, which might have the most amount of slip of any fault in the set, has ~230 m of apparent sinistral separation. The incremental untilting of the three later generations of faults restores the 1A fault to a steeply south-dipping fault with normal, dip-slip displacement. The 1A fault appears to connect with a fault that is exposed at the surface west of the area of drill hole constraints and cuts the Weed Heights member of the Mickey Pass Tuff (27 Ma) and is cut by the middle Miocene Singatse fault, bracketing its timing. The structural contour maps reveal new insights into the subsurface geometry of several faults. The Blue Hill fault presently dips ~22° southeast along the northwestern side of a large mullion in the fault plane. A stepwise reconstruction indicates that all faults had initial dips of > 60°, with the Singatse initiating at 73°. The significance of the first generation of faults at Yerington, though poorly understood, may have counterparts in eastern Nevada, where sets of easterly striking normal faults also formed prior to periods of extreme extension. The restorations demonstrate that mineralization in the Ann-Mason and Blue Hill areas originated in the same dike swarm, and thus are genetically related, and that another dike swarm to the south passing near the Casting Copper-Ludwig septum could have porphyry mineralization to the northwest at depth on one or both sides of the Blue Hill fault. [ABSTRACT FROM AUTHOR]

This study documents two magnetite deposits that formed in similar host rocks during the emplacement of the Jurassic Yerington batholith: the Minnesota (Standard Slag) mine and the Pumpkin Hollow deposit. Copper-sulfide skarn deposits in the district, hosted in the same sedimentary units (e.g., the Ludwig and Mason Valley mines), have been well-documented by previous studies. Basin and Range normal faulting in the district has extended and rotated exposed rocks to create Jurassic cross-sectional views through these hydrothermal systems, allowing for documentation of alteration in lateral and vertical extents. Recent work in the Yerington district has documented alteration associations, mineral assemblages, and timing relationships that allow for comparison between various Fe oxide-rich deposits, such as the Cu-poor Minnesota mine and the Cu-rich Pumpkin Hollow deposit, as well as comparisons between these Fe oxide-rich deposits with the Fe oxide-poor copper skarn deposits. Mineralization in the Minnesota mine and Pumpkin Hollow deposits is hosted in Mesozoic carbonate, fine-grained clastic, and Jurassic intrusive rocks and is linked to voluminous Ca(-Na) alteration of associated intrusive units, multiple episodes of brecciation, and replacement dominated by magnetite with subordinate calc-silicates. At the Minnesota mine, Ca-Na alteration of the McLeod Hill quartz monzodiorite is dominated by plagioclase-actinolite-epidote-titanite (sphene) associations. Magnetite mineralization at the Minnesota mine within the Mason Valley Limestone appears to zone from central magnetite replacement with minor pyrite-calcite to magnetite clasts supported by tremolite-calcite-quartz-chlorite ± serpentine matrix at the contact with relict marble and hydrothermal dolomite. The McLeod Hill quartz monzodiorite at Pumpkin Hollow is host to a variety of alteration associations, ranging from plagioclase-actinolite- epidote-titanite to plagioclase-garnet-diopside-calcite. Magnetite mineralization at Pumpkin Hollow is complex, but much of the Mason Valley Limestone replacement is comprised of magnetite with lesser diopside (often replaced by actinolite) ± pyrite-chalcopyrite-pyrrhotite. Crosscutting relationships between igneous rocks and hydrothermal alteration indicate that magnetite-dominated mineralization at both deposits occurred relatively early during emplacement of the batholith, and mainly predates granite porphyry dikes that elsewhere (Ludwig, Mason Valley) are linked to porphyry-related skarn mineralization. Fe oxide-poor systems are dominated by Si-addition (high temperature, anhydrous, calc-silicate mineralization), low volumes of Fe-oxide, and late Cu-mineralization generally linked to granite porphyry dikes. These relationships suggest that Fe oxide-rich systems are distinctly different from Fe oxide-poor systems in the Yerington district. The Fe oxide-rich systems are similar to iron oxide-copper-gold (IOCG) systems documented elsewhere around the world that are linked to processes dominated by external fluids, whereas the Fe oxide- poor systems are magmatic-hydrothermal deposits dominated by magmatic fluid sources. [ABSTRACT FROM AUTHOR]