Summary:
Geology
The project lays within the Walker Lane mineral belt and the Southwestern Nevada Volcanic Field (SWNVF). The regional stratigraphy includes a basement of Late Proterozoic to Late Paleozoic metamorphic and sedimentary rocks. Basement rocks are overlain by a thick pile of Miocene volcanic and lesser sedimentary rocks of the SWNVF, ranging in age from approximately 7.5 to 15Ma. The pre-Tertiary rocks exhibit large-scale folding and thrust faulting, having been subjected to compressional deformation associated with multiple preTertiary orogenic events. The stratigraphy of the SWNVF is dominated by ash flow tuff sheets erupted from a cluster of nested calderas known as the Timber Mountain Caldera Complex. The southwestern edge of the caldera complex lies approximately 10km east of the North Bullfrog project. The stratigraphy of the SWNVF includes voluminous ash flow tuff sheets, smaller volume lava flows, shallow intrusive bodies, and lesser sedimentary rocks.
Deposit type
Miocene rhyolitic volcanic tuff and flows host the lowsulphidation epithermal deposits at North Bullfrog. Steeply dipping structures contain high-grade gold and silver epithermal vein mineralisation and occur within pervasively quartz-adularia altered volcanic rocks which also host broad disseminated lowgrade mineralisation. Mineralisation at North Bullfrog is typical of other low-sulphidation type gold systems in and around the Walker Lane trend in Nevada.
Mineralisation style
Gold mineralisation at North Bullfrog is primarily hosted in the middle Miocene Sierra Blanca tuff. Two styles of precious metal epithermal mineralisation are present at the project: high-grade, structurally controlled fissure veins and associated stockwork zones, and low-grade disseminated or replacement deposits within altered volcanic rocks. Two district-scale north striking normal faults are the dominant structural features in the project area, but several smaller-scale faults between them are important controls for distribution of hydrothermal alteration and gold mineralisation.
Mineralisation characteristics
High-grade mineralisation at North Bullfrog is hosted in steeply dipping quartz-pyrite-electrum veins and associated vein stockwork zones. Disseminated low-grade adulariaquartz-pyrite mineralisation is the second-most important mineralisation style. The structurally controlled mineralisation can be distinguished from the older earlier alteration style event, and is generally associated with a white to light brown illite or illite-adularia alteration overprint. Observed textures are typical of low-sulphidation epithermal veins and include bladed quartz pseudomorphs after calcite, crustiform banding, and milky chalcedonic quartz with distinct but fuzzy banding. The bestmineralised veins at YellowJacket are grey translucent quartz stock work veins with little distinctive internal structure. Grains of native gold can often be observed in this quartz along with electrum, acanthite (Ag2S) and pyrite. In general, the silver to gold ratio associated with vein mineralisation is greater than 6:1 and locally can be 100:1 or more. The disseminated mineralisation has not been age-dated but based on cross-cutting relationships appears to be older than the high-grade vein mineralisation which has been dated at ca. 11.6Ma.
The North Bullfrog project is a combination of four mineralised deposits comprised of YellowJacket, Sierra Blanca, Jolly Jane, and Mayflower. The YellowJacket deposit is a very continuous high-grade vein within the moderate-grade stockwork mineralisation. The other three deposits are low to medium-grade.
YellowJacket
The YellowJacket Vein Zone consists of a massive quartz vein surrounded by hanging wall and footwall quartz stockwork zones. Such quartz vein and stockwork mineralization is found at YellowJacket and along the crest of North Sierra Blanca ridge. Observed textures that are typical of low sulphidation epithermal veins include bladed quartz pseudomorphs after calcite, crustiform banding and milky chalcedonic quartz with distinct but fuzzy banding. Veins with these textures may be relatively barren or have high-grade gold. The most common and best mineralized veins at YellowJacket are grey translucent quartz stockwork veins with little distinctive internal structure. Grains of native gold can often be observed in this quartz. There is generally little wall rock alteration associated with grey translucent quartz veins. However, white illite overprint of earlier quartz-adularia is often observed in the general vicinity of grey translucent stockworks. The illite overprint can locally be rather intense creating selvages around jasper veins and destroying all the feldspar in the rock.
The primary minerals associated with the vein-style mineralization are gold, electrum, acanthite (Ag2S) and pyrite. Petrographic studies have also documented pyrargyrite (Ag3SbS3), stromeyerite (AgCuS), proustite (Ag3AsS3), chalcopyrite (CuFeS2) and covellite (CuS). Sphalerite has been observed as a late cavity infill. In general, the silver to gold ratio associated with vein mineralization is >6:1 and locally can be 100:1 or more.
Sierra Blanca
The greater Sierra Blanca area includes the Savage Valley, Sierra Blanca, North Sierra Blanca, YellowJacket, Air Track Hill and Air Track West areas.
Mineralization at Sierra Blanca can be classified into the following styles:
- Early, large-volume, pervasive low-grade disseminated gold mineralization associated with quartz-adulariapyrite alteration.
- Middle, structurally controlled disseminated gold mineralization associated with overprinting illite-adulariapyrite alteration and gold enrichment (NW10, NE30, NE50, NE60, Liberator Faults).
- Late, high-grade gold associated with quartz veins and quartz stockwork veining (YellowJacket Vein Zone).
Jolly Jane
The Mineral Resources at Jolly Jane consist of the older alteration-style quartz-adularia-pyrite disseminated mineralization. The primary host rock is the Sierra Blanca Tuff, and secondary host rocks include the Savage Valley Dacite, Pioneer Formation, Savage Formation and rhyolite of the North Bullfrog Suite. Mineralization is controlled by a combination of small-displacement, high-angle feeder structures and the highly brecciated Sierra Blanca Tuff. The deposit is largely pseudo-stratabound within the Sierra Blanca Tuff. Minor quartz stockwork veining occurs throughout the Jolly Jane area, but no YellowJacket-style gold-silver enrichment event has been identified to date. All of the current Mineral Resources at Jolly Jane are oxide.
Mayflower
The historic Mayflower mine was developed on en-echelon quartz-calcite veins and stockwork zones along the NWstriking, steeply SW-dipping Mayflower Fault Zone (MFZ).
Vein and stockwork mineralization are proximal to individual fault splays. Late stage black manganiferous calcite occurs as veins and breccia fillings along the mineralized structures. This calcite is mineralized and is thought to be a late vein stage of the waning hydrothermal system.