Summary:
Four mineralized zones have been outlined as part of the mine plan: Goldfield Main, Gemfield, Jupiter, and McMahon Ridge, from which Centerra is targeting oxide and transition material.
The Gemfield, McMahon Ridge and Goldfield Main deposits are structurally controlled, volcanic-hosted, epithermal gold deposits of the high-sulphidation, quartz-alunite type.
Mineralization
Gemfield Deposit
Gold mineralization in the deposit is restricted to rhyolitic lavas of the Sandstorm Rhyolite. Pyrite is the dominant sulphide mineral. Internally, the rhyolite is composed of strongly flow-banded, often glassy, but generally devitrified, porphyritic rhyolite. The lava flows of the Sandstorm Rhyolite are almost always hydrothermally altered to some degree, and alteration types generally range from propylitic, to argillic, to intense silicification. The widespread distribution of hydrothermal alteration is due to the highly permeable character of portions of the flow-banded stratigraphy. Where encountered in drilling, the formations both above and below the rhyolite are only weakly altered and are rarely found to be more than weakly anomalous in gold.
Gold mineralization in the Gemfield deposit often extends beyond structural boundaries, and in these areas occurs as a low-grade, disseminated gold halo in regularly flow-banded Sandstorm Rhyolite. The low-grade halo almost always forms around high-grade gold mineralization that is generally confined to structures or stratiform, tabular mineralized zones. The origin of the low-grade halo is believed to be related to increased porosity and permeability by virtue of the flow-banded character of the rhyolite. In addition, it appears that gold distribution was further enhanced by a pre-gold stage acid-leach event that opened and connected fluid passageways established along foliation planes related to primary flow-banded volcanic texture.
The deposit strikes roughly 020° and dips 30 to 50° SSW. Using a 0.01 opt Au cutoff, it has a strike length of 2,400 feet, is 1,200 feet wide and has a true average thickness of approximately 100 feet with areas up to 200 feet thick (730 by 350 by 30 to 60 m thick). The depth from surface to the mineralization varies from 10 feet in the NE to 700 feet (3 to 210 m) in the SW.
McMahon Ridge
Gold mineralization is mainly hosted in Milltown Andesite volcanics, which include tuffs, flows and lahars. In addition, some deep mineralization has been encountered in tuffaceous sediments, mainly composed of black shale and tuffaceous sandstone of the Diamondfield (Sandstorm) Formation. Pyrite is again the dominant sulphide. Hydrothermal alteration ranges from weak propylitic, argillic, to intense silicification. With the possible exception of some apparent stratabound mineralization in the Diamondfield Formation, essentially all of the gold mineralization is structurally controlled.
Based on the rather erratic distribution of gold grades with respect to silicification and ledge development, it appears likely that the main gold-mineralizing stage occurred somewhat late during the development of the hydrothermal system. Geologic evidence suggests that the sequence of geologic events at McMahon Ridge probably occurred in the following order: (1) eruption of the latite volcanic sequence and development of the ring-fracture fault system, (2) deposition of the Diamondfield moat sediments, (3) resurgence, uplift, and eruption of the Milltown Andesite, (4) emplacement of the central intrusive complex, (5) onset of the Goldfield hydrothermal system, which initially produced intense silicification, formation of multiple silica ledge zones, and propylitized adjacent Milltown Andesite wall rocks, (6) stage two structural development (continued intrusion, uplift and/or hydrothermal-fracturing and local brecciation of silica ledge zones and adjacent wall rocks), (7) inferred pre-gold-stage acid-leach event, (8) intense argillic alteration of the wall rock/ledge contacts, (9) main-stage gold event, (10) barren, open-space-fill, translucent quartz vein emplacement, and finally, (11) post-mineral faulting. Evidence for this implied sequence of events is based first on the wide range of gold values (nil to >1.0 opt Au) obtained from silica ledge zones, and second, the apparent close association of clay (mainly dickite) with gold in virtually all rock-type assemblages including silica ledges, hanging wall and footwall clay zones, and in some local areas, in propylitized Milltown Andesite or Diamondfield Formation.
The McMahon Ridge deposit has a strike length of approximately 5,000 feet (1,525 m), which includes about 3,600 feet (1,100 m) along the main east-west trend and 1,400 feet (425 m) along the northeast-trending Belmont extension. The mineralized zone, which consists of a number of steeply south-dipping and near-vertical subparallel structures, can be up to 500 feet (150 m) wide with a vertical range of up to 800 feet (250 m). While somewhat curvilinear, the general trend of the main mineralization is roughly east-west. Internally, gold mineralization is very much structurally controlled and the high-grade shoots, while predominantly east-west oriented, are also hosted within northwest and northeast striking cross-structures.
Goldfield Main
Mineralization in the Goldfield Main area is spatially associated with siliceous ledges, with bonanza-grade ore bodies occurring as irregular sheets and pipes within or along the margins of the ledges. Ledges are irregular masses of intensely silicified, brecciated wall rocks or intrusive dikes that occupy pre-existing, structurally controlled, hydrothermal fluid conduits. Where mineralized, the ledges are highly fractured and brecciated with late-stage silica and clay filling the open space. Gold mineralization is associated with this younger silica-clay event. Goldfield Main mineralization, though structurally controlled, is hosted primarily within a porphyritic dacite to rhyodacite flow-dome complex (also referred to as simply dacite or Main district dacite).
Hydrothermal alteration includes phyllic, weak propylitic, argillic, quartz-alunite, to locally very intense silicification. An advanced argillic assemblage of diaspore and pyrophyllite is found locally. The hydrothermal alteration/mineralization sequence in the Goldfield Main area is similar to that interpreted for the McMahon Ridge deposit.
Goldfield Main district ore occurs primarily as native gold associated with bismuth and copper-arsenic-antimony-bearing sulphides and tellurides including bismuthinite, famatinite, and goldfieldite. Native gold is often visible in the rich ores though it can also occur as fine particles within sulphides.
The main mineralized zone strikes roughly NNW and dips steeply east. It is over 5,000 feet (1,525 m) in length and can be followed down-dip to over 1,700 feet (520 m) in true depth, however, the majority of underground workings and past production is within 500 feet (150 m) of the surface, and covers a 1,000 foot (300 m) wide east-west footprint.