Summary:
Marigold is located in the Battle Mountain mining district of north-central Nevada within the Basin and Range physiographic province bounded by Sierra Nevada to the west and the Colorado Plateau to the east. The Battle Mountain district hosts numerous mineral occurrences, including porphyry copper–gold, porphyry copper–molybdenum, skarn, placer gold, distal disseminated silver-gold, and Carlin-type gold systems. The gold deposits at the Marigold Mine are best classified as Carlin-type gold deposits (CTGD). Gold mineralizing fluids were primarily controlled by fault structure and lithology, with tertiary influence by fold geometry. Gold occurs natively as submicron scale particles in fractures in association with iron oxide. Rocks within the Marigold mine area are oxidized to a maximum depth of approximately 450 meters. The Buffalo Valley gold deposit is a distal disseminated silver-gold deposit and formed along a southeast trending zone of felsic porphyry dikes and faults.
Marigold
The gold deposits at Marigold are considered Carlin-type and cumulatively define a north-trending alignment of gold mineralized rock more than eight kilometres long. Gold mineralizing fluids were primarily controlled by fault structure and lithology, with tertiary influence by fold geometry. Within the Valmy Formation, higher gold grades are observed in the hinge zones of open folds that trend west–north-west and plunge gently. When viewed down plunge, the undulation of these folds is mimicked by gold mineralized horizons. The deposition of gold was restricted to fault zones and quartzite dominant horizons within the Valmy Formation and high permeability units within the Antler sequence.
In unoxidized rocks, gold occurs in arsenic-enriched overgrowths on pre-ore pyrite. Arsenopyrite is also present on pre-ore pyrite grains but is not auriferous. Geochemically, the gold mineralization event is characterized by elevated arsenic, barium, antimony, and mercury, among others. Gangue minerals include quartz, arsenopyrite, stibnite, calcite, clay, and barite. Hypogene sulfide minerals do not occur in ore as these gold-bearing phases are not amenable to heap leaching.
In oxidized rocks, gold occurs natively in fractures associated with iron oxide. Rocks within the Marigold mine area are oxidized to a maximum depth of approximately 450 m. The redox boundary is not consistent throughout the property and is substantially influenced by lithology. Shale, argillite, and siltstone units are frequently unoxidized adjacent to pervasively oxidized quartzite horizons.
A silver and base metal mineralizing event at Marigold includes a mineral association of chalcopyrite, argentiferous tennantite, galena, and sphalerite. The absolute age of this event is unclear, although it may be related to late Cretaceous magmatism in the district.
Buffalo Valley
The Buffalo Valley project is located approximately 14 km southwest of the Mackay complex at Marigold and eight kilometres southwest of Trenton Canyon on the immediate western flank of the Battle Mountains. Early works relating to deposit genesis have variably ascribed the Buffalo Valley gold system to distal disseminated silver-gold, porphyry copper-molybdenum, and gold skarn deposit models. Recent work tends to favor the distal disseminated silver-gold model as most of the gold mineralization is associated with quartz+sericite+pyrite (QSP) veins and veinlets that postdate development of the various hornfels and skarn alteration assemblages. The Buffalo Valley deposit is hosted by Eocene felsic dikes and metasedimentary rocks and basalt of the Mississippian-Permian Havallah sequence that are pervasively altered to skarn and hornfels in the vicinity of the deposit area.
The most prominent fault set in the Buffalo Valley project area are south striking structures that define the range bounding fault system. The structural block that hosts the Buffalo Valley deposit is bound by the Range Front fault to the east and the moderately dipping (34 to 48 degrees) Front fault to the west (Seedorff et al., 1991). Bedrock in this block is exposed at surface and may indicate increased transfer of slip to the Front fault or other subsidiary faults in the vicinity of the deposit. The Front fault is mineralized but also offsets Quaternary alluvium, constraining the minimum age of initiation to the early Oligocene and latest slip to the Quaternary. Dikes and hydrothermal fluids exploited dilational SE-striking relay structures related to N-S oriented master fault structures (Rhys, 2022; internal communication). This fault zone is well characterized by a large aeromagnetic anomaly that can be traced for more than seven kilometres (Doebrich and Theodore, 1996).
The Buffalo Valley gold deposit formed along a southeast-trending zone of felsic porphyry dikes and faults. Gold occurs in arsenian iron sulfide overgrowths on pyrite in sheeted QSP veinlets within the central granodiorite and dacite porphyry dikes, subparallel to dike margins in the country rock, and within faults (e.g., the Front fault). Outboard of the intrusion’s gold mineralization is stratiform in receptive horizons of Havallah sequence metasedimentary rocks. In general, gold concentration decreases with increasing distance from the granodiorite and dacite porphyry dike system. Although most of the gold mineralization at Buffalo Valley occurs in QSP veinlets that overprint skarn alteration assemblages, a lesser amount of gold is documented as native grains within garnet and amphibole crystals associated with prograde skarn development (Reid et al., 2010). Minerals associated with gold mineralization in oxidized zones include scorodite, manganese and iron oxides, calcite, and clay.
Trenton Canyon
The Trenton Canyon property is located approximately 5 km south of the Marigold deposit and comprises an area of approximately 34 km2. Trenton Canyon is separated from Marigold by the southwest-striking Oyarbide fault, a range-bounding fault on the northern flank of the Battle Mountains. Gold deposits at Trenton Canyon are hosted by siliciclastic and carbonate rocks of Cambro-Ordovician and Pennsylvanian-Permian age proximal to potentially genetically related Eocene felsic dikes. The gold deposits are on the margin of a calc-silicate and hornfels alteration aureole attributed to emplacement of the Cretaceous Trenton Canyon stock, exposed on surface approximately one kilometer southwest of the historical South pit.
A Cretaceous base metal mineralization event, thought to be related to emplacement of the Trenton Canyon stock, is characterized by a sulfide association of pyrrhotite, chalcopyrite, and pyrite in unoxidized samples, and a gangue association of tremolite, calcite, muscovite, diopside, and garnet. This event pre-dates Eocene gold mineralization at Trenton Canyon, which is characterized by a sulfide association of auriferous arsenic-bearing iron sulfides and argentiferous tennantite, and a gangue association of quartz, carbonate, phyllosilicates, clays, carbon, and stibnite.
Gold mineralization at the South, West, and East pit areas is primarily hosted in a network of transtensional faults locally intruded by Eocene dikes and sills. Hydrothermal and/or phreatomagmatic breccias within these structures typically contain increased concentrations of gold. Gold mineralization is well confined to structures, although a small (several meter) halo of lower grade, more disseminated mineralization may be present. Quartz veining, illite, iron oxides, and iron hydroxides (goethite) are the primary indicators of gold mineralization where oxidized.