The Jackson-Lawton-Bowman Normal Fault System and Its Relationship to Carlin-type Gold Mineralization, Eureka District, Nevada
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 mid-Cretaceous 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 fault propagation 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 iso-topic 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.