Summary:
The Cliff Creek, Dukes Ridge-Phoenix and AGB zones at the Lawyers Area are all considered to be structurally controlled, low-sulphidation type epithermal gold-silver deposits.
The Ranch Area mineralisation is considered to be high-sulphidation epithermal gold deposits.
Evidence for a number of different mineral deposit types occur across the Lawyers-Ranch Property, including low- and high-sulphidation epithermal gold-silver mineralisation, calc-alkalic porphyry coppergold mineralisation, and iron or copper (± gold and silver) skarn mineralisation.
The Lawyers Area zones consist of a combination of quartz veins, stockwork zones and chalcedony breccia bodies developed along northwest and north-northwest trending fracture systems. Low-sulphidation epithermal gold-silver mineralisation consists of predominantly pyrite (2-5%), with minor chalcopyrite, sphalerite, galena, native gold, native silver, electrum and acanthite in a gangue of quartz, chalcedony, jasperodial chert, amethyst, minor calcite and rare barite. Veins commonly display banded and crusti-form textures typical of low-sulphidation epithermal systems; however, brecciation and alteration related to faulting are more common than classic epithermal textures. The three principle mineralised zones are the Amethyst Gold Breccia (AGB) Zone, the Cliff Creek Zone with its many sub-zones, and the Dukes Ridge-Phoenix Zone. Subsidiary zones (or prospects) are Marmot Lake and Silver Pond Zones.
Low sulphidation (adularia-sericite) epithermal type alteration is characterized by core zones of intense silicification ± adularia and bleaching. At higher elevations within the AGB Zone and within Cliff Creek and Dukes Ridge zones, adularia forms narrow, pink boundaries on vein margins, and outbound of veins replaces plagioclase phenocrysts and groundmass silicate minerals, partly obscuring the porphyritic texture of the wall rocks and described as potassic halos. At AGB, the central potassic alteration grades outward to a propylitic assemblage of epidote-carbonate-chlorite-pyrite. At Cliff Creek and Dukes Ridge zones, adularia on vein margins occurs with sericite flanked by kaolinite. The argillic alteration, accompanied by pyrite and chlorite, forms wide envelopes on the veins and grades outward to a propylitic assemblage similar to the AGB Zone.
Mineralisation on the Property is hosted predominantly within the Metsantan Member of the Lower Eruptive Cycle, which consists of a thick sequence of trachytic ash falls and flows with Kfeldspar megacrysts (Lane et al., 2018). This sequence is overlain by hornblende-bearing andesite crystal tuffs and breccias (Vulimiri et al., 1986). A thick package of flood basalts overlies the trachyandesites and marks the base of the Upper Eruptive Cycle.
Lawyers Area Mineralisation
Low sulphidation style mineralisation present in the Lawyers Area trend is structurally controlled, with the main northwest to north-northwest trending faults considered to be fluid conduits and localities of deposition. Dilation due to extension and displacement along fault structures resulted in the targeted high-grade mineralised vein shoots, such as those observed at the Duke’s Ridge, Cliff Creek and Phoenix Zones. Intersecting fault planes appear to result in a convergence of fluid conduits, concentrating high-grade mineralisation. Deposits are variably truncated and offset, due to a series of post-mineralisation faults. Observations made during mapping and core logging identified sections of the Cliff Creek hanging wall that contain more mineralisation and related alteration, which could indicate a preferred fluid pathway or possible displacement that caused juxtaposition of mineralised zones and relatively unaltered host rock.
High-grade mineralisation in the Lawyers Area deposit is commonly associated with hydrothermal breccia zones composed of translucent to milky/opaque quartz-chalcedony veins and veinlets with varying concentrations of sooty metallic grey fine-grained sulphides (acanthite, sphalerite, pyrite). The hydrothermal breccia zones typically display intense pervasive potassic alteration that commonly resulted in complete replacement of visible phenocrysts. Multi-phase veining and hydrothermal brecciation, and alteration that overprint volcanic textures, indicates that multiple fluid pulses occurred. Mineralisation at Marmot Lake does not appear to be controlled by hydrothermal breccia zones, rather by intervals of strong propylitic and (or) potassic alteration commonly with increased epidote veining and in the groundmass. Mineralisation at Silver Pond has been observed in highly sericitized/QSP altered zones with 10 to 20% disseminated pyrite.
Precious metals mineralisation typically consists of fine-grained native gold and silver, electrum and acanthite. Precious metals are locally associated with minor base metal mineralisation, which includes minor amounts of chalcopyrite, sphalerite and galena. The exception is a base metalrich mineralisation style that occurs in the main Cliff Creek Zone at depths >300 m, as described below.
Ranch Area Mineralisation
High-sulphidation systems are defined by a direct connection between surficial alteration and acidic, high-salinity, high-temperature fluids derived from underlying magmatic intrusions (Hedenquist, 1987).
Mineralisation is generally expressed as widespread argillization and silicification of the Lower Toodoggone Volcanic host rocks. These assemblages are elongated along structures, with major faults acting as fluid conduits and feeder systems to the main alteration bodies. Evidence of stratigraphic control has also been interpreted from shallowly plunging mineralised bodies at the Bonanza and Thesis Zones. Alteration bodies are zoned, with the most intense alteration and leaching occurring proximal to mineralisation and the major controlling structures.
All significant known gold mineralisation is hosted by silica-sulphate and silica-sulphide bodies flanked by argillically altered zones. These bodies are, in turn, controlled by moderate- to steeply-dipping fault zones with north-northwesterly, northwesterly, northeasterly, and north-northeasterly orientations.
Hydrothermal breccias are dominated by barite-silica cement and clasts of silicified volcanics. Gold mineralisation is strongly correlated to the presence of barite and displays weak to moderate correlation to common tracer elements, such as tellurium, bismuth, antimony, arsenic, lead and zinc. The gold-bearing zones have a crudely elliptical shape, which are open-ended along the controlling fault systems for follow-up expansion drilling. In the Bonanza Deposit, some of the gold-bearing zones are considered to have formed by selective replacement/silicification of more permeable tuff units within the volcanic strata. Gold mineralisation at Ranch Area is known to occur over a vertical depth range of ~200 m, extending from ~1,700 m at surface in the Bonanza Zone to ~1,400 m in elevation in the Thesis III Zone and is open at depth in several areas.
Three mineralised zones in the Ranch Area have seen minor past production, including the BV, Thesis III, and Bonanza Zones. Gold mineralisation at the BV Zone is hosted by a quartz-baritepyrite assemblage; the Thesis III Zone by a quartz-barite-pyrite (chalcopyrite, galena, sphalerite) assemblage; and the Bonanza Zone, by quartz-barite and quartz-pyrite chalcopyrite-enargitebornite-(barite) assemblages. Mineral assemblages in the additional zones are similar. A quartzhematite-pyrite assemblage is important at the Ridge and Thesis II Zones.
A total of 35 mineral occurrences are present in the Ranch Area.