Summary:
Florida Canyon Mine is a large, relatively young epithermal gold deposit adjacent to an active geothermal system. The close spatial association with the geothermal system has led to a general belief that FCM is a hot springstyle, epithermal gold deposit. Hydrothermal alteration assemblages and the mineralogy of both oxidized and unoxidized gold mineralization at FCM have been described and interpreted by Fifarek et al. (2011) as having formed in a low-sulfidation, epithermal environment.
The deposit type is a large fault/fracture-controlled gold system, the overall extent being defined by alteration and oxidation of host meta-sedimentary rocks. Mineralization is preferentially located along major structural trends, in associated adjacent fracturing and rock foliations, and as disseminations in favorable host lithologies.
Rocks of the Rochester Rhyolite, Prida Formation, Natchez Pass Limestone, and Grass Valley Formation are exposed in the FCM area. All these units are of Triassic age. Sills of mafic composition intrude the Prida Formation and sparse, strongly clay-altered felsic dikes locally cut upward into the Grass Valley Formation. The Humboldt City Thrust Fault separated the Natchez Pass and Grass Valley formations from the underlying Prida Formation, and much of the middle and lower units of the Natchez Pass Limestone have been cut out above the thrust fault. The Florida Canyon gold deposits are hosted by the Grass Valley Formation and Natchez Pass Limestone along with sill/limestone contact zones within Prida Formation. The general strike of the stratigraphy at FCM is N30oE with a 30 to 40-degree dip to the west.
The Grass Valley Formation is composed of siltstone with interbedded sandstone lenses, which were metamorphosed to argillite, phyllite, and fine-grained quartzite. At the mine site, the Grass Valley Formation can be separated into layers of silty argillite and quartzite separated by dark gray to black phyllite. The layers of more quartz-rich sediment were more strongly and preferentially fractured and faulted during periods of compressional tectonics relative to the phyllite beds.
As well, extensional tectonics during Basin and Range formation also resulted in more fault and fracture-related permeability in the silty beds relative to the phyllite.
There is a strong N30oE to N50oE structural fabric prevalent in and adjacent to the Florida Canyon deposits, as evidenced by the alignment of quartz veining, shear zones, and well-developed joint sets (Hastings, et al., 1987). Byington (1996) also recognized this important structural control to mineralization, particularly in the Main and Madre (aka Brown Derby) deposits. It has also been noted at other locations, specifically the Northeast Extension (aka Central) deposit that the preferentially mineralized structural trend is west-northwest. The north to north-northeast trending Basin and Range fault system limits the western near surface part of the FCM oxide deposit. The range-front fault system is a series of subparallel normal faults that “stair step” down to the west, with displacement on individual faults ranging from more than 780 feet near the range front to a few feet on parallel structure further to the west (Hastings, et al., 1987). These range-bounding faults are also listric and flatten with depth, which is an important feature in exploring for downdip blocks of mineralization to the west beneath valley-fill alluvium.
Mineralization
The location and geometry of the mineralized bodies at FCM are a result of structure; the presence of favorable silty argillite, quartzite, and limestone host rocks; and the position of the host rocks relative to structural conduits. The higher-grade zones of mineralization tend, in general, to follow the high-angle, northeast- and northwest-trending fault and shear zones. The more moderate- or lower-grade zones are controlled by favorable host rocks more distal to feeder structures.
Rock units that are more favorable hosts to mineralization include silty argillite, hornfels contact zones with mafic sills, karsted limestone, and platy, silty limestone with interbeds of calcareous shale. Local factors that influence the occurrence and geometry of mineralized bodies include variations in folds, foliation, and bedding in favorable units, intersecting structural fabrics, and proximity to low-angle structures (Taylor, 2001). Hypogene mineralization at FCM consists of native gold and electrum associated with quartz, iron oxides, pyrite, marcasite, and arsenopyrite (Hastings et al., 1987). Quartz is the major gangue material. Secondary minerals identified in the FCM deposits to date include gypsum (likely remobilized from the Grass Valley Formation), alunite, barite, native sulfur, calcite, dolomite, fluorite, anhydrite, pyrargyrite, pyrrhotite, and stibnite. There are two types of hydrothermal, epithermal quartz veins at FCM (Hastings, et al., 1987). The most important are vein swarms and stockworks that contain most of the gold mineralization. These veins generally follow a north-northeast trend (Hastings, et al., 1987) and are characterized by colorless, euhedral to subhedral quartz, or banded chalcedonic white to colorless quartz that contains limonite after pyrite (Taylor, 2001).
The second type of hydrothermal quartz veining occurs as large, through-going, banded fissure veins that follow the original north-northeast structural fabric (Hastings, et al., 1987). These veins are interpreted to represent a late hydrothermal event that overprinted the earlier episode of gold-bearing quartz veining and stockworks. These veins are characterized by bands of course, prismatic quartz alternating with bands of cherty chalcedony and only occasionally contain economic gold grades. Milky white bull-quartz veins, considered to be metamorphic in origin, may also be present in the mineralized zones, but they are not gold bearing (Taylor, 2001). Locally, pervasive silicification is generally associated with areas of highdensity quartz veining and/or intense hydrothermal brecciation (Taylor, 2001). Sericite, adularia, clay, and chlorite occur locally in quartz veins, breccia matrix, and on fracture surfaces. There is extensive argillization and bleaching throughout the deposit area, with pervasive hematization that is largely confined to silty units marginal to the bleached areas (Hastings, et al., 1987).
A different style of mineralization has been recognized in the more recently discovered deposits east of the original FCM Main Pit. At Radio Tower, karsted surfaces in Natchez Pass Limestone are replaced by cryptocrystalline silica forming jasperoid and hornfelsed contact zones between mafic sill and limestone are strongly quartz-veined and pyritized. These ore zones represent a likely older event relative to the younger, hot-spring style mineralization in the Main Pit and Madre areas.