Summary:
Previous operators have suggested a range of potential deposit models for the Johnson Tract Deposit, from feeder zone beneath a sea-floor Volcanogenic Massive Sulfide deposit (VMS), to Epithermal within coeval volcanic stratigraphy, to the possibility of mineralization being significantly younger than the host volcanic rocks and instead related to regional intrusive activity and/or structures (Proffett, 1993).
VMS-like aspects include submarine volcanic host rocks, widespread and crudely stratabound anhydrite alteration similar to some Kuroko-type VMS, and strong base metal grades coincident with gold mineralization, whereas deposit morphology at the Deposit, consisting of a quartz-sulfide stockwork and breccia body, and vein textures are more consistent with those found in epithermal-type deposits.
Unlike typical Kuroko-type VMS, the Deposit mineralization appears to be sub-seafloor with no development of stratiform massive sulfide lenses. A note from Proffett (1993) mentions fossilized wood has been mapped above the ore horizon and suggests the volcanics just above the stockwork zone erupted on land, further supporting a link to more of an epithermal type deposit. Further review and comparison of the epithermal type model and the key characteristics of the Deposit suggests a likeness to the intermediate sulfidation model as described by Wang et al., 2019.
One important aspect of the appropriate genetic model to follow is the metallurgy of the Deposit. Unmetamorphosed VMS deposits typically have very fine grained and inter-grown sulfide mineral species requiring very fine grinding to separate the different metal bearing minerals. That is not at all the case at Johnson Tract where relatively course sulfides characterize the sulfide mineralogy indicating a deeper or more porphyry-related model may be at play. The fact that the Deposit is a polymetallic breccia also suggests a porphyry related origin as does the existence of other known porphyry occurrences in the area such as the Easy Creek target. Recent studies in active sub-marine hydrothermal systems in the Kermadec Arc suggest a strong linkage between hot porphyry related fluids and cooler VMS systems as depicted.
Mineralization
Mineralization at the Johnson Tract Deposit forms a steeply southeast dipping, tabular silicified body that contains a stockwork of quartz-sulfide veinlets and brecciation, cutting through and surrounded by a widespread zone of anhydrite alteration (Proffett, 1993). Drilling has defined silicification and mineralization from surface to a vertical depth of approximately 350 m (1,150 ft.), over a total strike length in excess of 600 m (1,970 ft.), and to a maximum true width of 55 m (180 ft.). The main body of mineralization is bound on the east by the southeast dipping Dacite fault.
The Deposit consists of a complex stockwork system of high-angle, 1-10 cm wide veins and breccia zones containing quartz, sphalerite, chalcopyrite, galena, anyhydrite, barite, Fe chlorite and native gold (Steefel, 1987). In addition to veins and diffuse breccias, mineralization is also characterized by massive structureless intergrowths of quartz and sulfides, commonly with very coarse-grained sulfide mineralogy. Veins show characteristics associated with epithermal styles of mineralization. Open-space fill texture is common and breccias consist of subrounded fragments hosted within a sulfide-silica matrix.
Early and relatively minor base metal mineralization (sphalerite) formed with the pervasive anhydritechlorite-sericite alteration. Later base (sphalerite-galena-chalcopyrite) and precious metal mineralization formed over several mineralizing events within the silicified stockwork vein zone. The genetic and temporal relationship between base metal deposition and precious metal deposition is not well understood (Rockingham, 1993). Re-Os dating of a bulk-sulfide separate, containing both chalcopyrite and pyrite from the footwall zone produced an age of 186 ± 6Ma for mineralization. This suggests that mineralization was contemporaneous with Talkeetna Arc volcanism and the Deposition of Talkeetna Formation host rocks (earliest Jurassic, Detterman et al. 1996), and is consistent with the shallow subseafloor setting for mineralization proposed by Steefel (1987).
Alteration
Proffett (2019) summarized the concentric alteration and mineralized zones recorded at Johnson Tract.
Outer Sericite Zone
A broad irregular zone that contains up to a few percent anhydrite and pyrite, with sericite, chlorite, and clay alteration of wallrock. Although most mineralization is recorded in the plagioclase-phyric dacite volcaniclastic rock, the Outer Sericite Zone alteration is seen in rock units stratigraphically above and below.
Anhydrite Zone
Most notable surrounding the Deposit, zones of anhydrite-chlorite-pyrite alteration, commonly exceeding 20 percent anhydrite, are recorded. Anhydrite forms nodules with interstitial chlorite-pyrite which is locally replaced by sericite or clays. Small irregular veins of anhydrite are common throughout. Minor sphalerite is present higher up in some anhydrite-altered zones, either disseminated or as sparse anhydrite-sphalerite veins. Weakly anomalous gold is also known to occur within anhydrite-altered zones, proximal to the inner silicified zone.
Silicified Zone
Within the Anhydrite Zone, a northeast plunging, tabular body of strongly silicified tuffs hosts most of the mineralization. This zone is defined by abundant quartz-sulfide veining, and the replacement of wall rocks with fine-grained quartz. Relict nodular texture is observed locally, replaced by silica, suggesting that silicification may have overprinted earlier anhydrite alteration. Strong silicification and sericitealteration is also closely associated with the more copper-rich ‘footwall’ zone, suggesting that this may represent a feeder to the overlying gold and zinc rich mineralization. Silicified rocks commonly contain >80 wt.% SiO2, compared to ~65 wt.% SiO2 in unaltered dacite tuffs. The silicified zone also contains abundant disseminated pyrite (1-5%), anomalous to high-grade gold throughout, and elevated base metals, commonly outboard of the main Au-rich mineralization.
Veins & Breccia Veins
Several vein and breccia vein types crosscut the Silicified Zone:
• Quartz-pyrite-sphalerite +/- chalcopyrite veins with no obvious open-space textures Breccia veins with open-space textures (coliform):
• o high-grade gold is common;
• o appear to dip steeply to west – northwest.
White quartz, dark chlorite, coarse-grained chalcopyrite, pyrite +/- sphalerite:
• o appear to cross-cut open-spaced breccia veins;
• o high-grade gold is found in the walls, rarely recorded in the veins.
Summary:
The processing of mineralized material will be occurring off-site.
Flowsheet development has focused primarily on the production of separate flotation concentrates for copper, zinc, and lead, and pyrite with the potential cyanidation of flotation concentrates and flotation tailings, to achieve additional gold recovery.
The operating costs for milling were benchmarked off processing plants with a similar throughout and flowsheet, with a toll milling surcharge added.
Recoveries & Grades:
| Commodity | Parameter | Avg. LOM |
|
Gold
|
Head Grade, g/t
| 5.82 * |
|
Silver
|
Head Grade, g/t
| 5.44 * |
|
Copper
|
Recovery Rate, %
| 84.5 * |
|
Copper
|
Head Grade, %
| 0.54 * |
|
Lead
|
Recovery Rate, %
| 72.4 * |
|
Lead
|
Head Grade, %
| 0.71 * |
|
Zinc
|
Recovery Rate, %
| 92.3 * |
|
Zinc
|
Head Grade, %
| 4.72 * |
* According to 2025 study.