Summary:
Deposit Types
The CK copper-gold deposit does not neatly fit into any specific category or class of conventional deposit models, in part because of the wide array of variability and overlap of assigned deposit model parameters such as geochemical signatures, geologic setting and time frames, and the origin and mechanisms of emplacement of metal-bearing solutions.
Previous authors (Hausel, 1997, 2012; Carson, 1998) have postulated that the CK deposit represents some portion of a copper (Au-Cu) porphyry system, largely based on observations of the nature and occurrence of hydrothermal alteration assemblages exposed in outcrop. According to the U.S. Geological Survey’s Porphyry Copper Deposit Model (John et. Al, 2010) and Preliminary Model of Porphyry Copper Deposits (Berger et. Al, 2008), porphyry deposits consist of disseminated copper minerals and copper minerals in veins and breccias that are relatively evenly distributed in large volumes of rock forming high tonnage, low to moderate grade ores.
While the alteration assemblages encountered within the CK deposit are indeed like those associated with porphyry copper deposits, hydrothermal alteration zones at CK decidedly lack kilometer-scale vertical and lateral dimensions, and potassic and sericitic alteration are clearly not invariably associated with sulfide mineralization, nor are they necessarily temporally, spatially, and thermally zoned with respect to one another. The Proterozoic age of the CK deposit’s host granodiorite and apparent pre- or syn-deformational mineralization further preclude it from classification as a sensu-strictu porphyry deposit.
The CK deposit also exhibits a variety of characteristics that are, individually or in combination, like those of known intrusion-related, iron oxide copper-gold (IOCG), and even orogenic deposits. In each instance, however, the similarities (i.e., age, structural setting, geochemical signature, alterations styles, etc.) are either outweighed by significant differences or are too limited, at present, to support a decisive association with the deposit model.
The regional geologic setting of the CK deposit within the Cheyenne Suture belt is significant, as is the nature of occurrence of sulfide mineralization as disseminations in undeformed granodiorite and in alignment with foliation in foliated to mylonitized granodiorite. Based on the available data and information to date, is suggest that Klein’s (1974) description of the CK deposit as a “structurally controlled base and precious metal deposit hosted in a Precambrian shear zone” is essentially correct. While Klein’s description does not present a conventional deposit model, it does provide a reasonable interpretation on which to base plans for future exploration.
Mineralization
Copper and gold mineralization is largely disseminated, and based on available information to date, occurs solely within the granodioritic plutonic body. Secondary copper minerals, primarily chrysocolla, cuprite, and trace malachite and azurite, as well as secondary iron minerals (hematite, limonite, and jarosite), chalcocite and native copper (flecks and veins) are observed on the surface and define an oxide or supergene zone that extends to depths up to 100 ft below the topographic surface and deeper in fractured or faulted localities. This surficial oxide zone is essentially devoid of magnetite. An intermediate oxide-sulfide or ‘mixed’ zone observed in drill core is characterized by secondary copper and iron minerals as well as primary pyrite and trace chalcopyrite. The mixed zone transitions to a sulfide-dominant zone at depths ranging from 100 to 300 ft, with a significant decrease in oxide mineral content, increase in occurrence of disseminated pyrite and chalcopyrite, and the appearance of magnetite. Within the sulfide zone, sulfide minerals are typically disseminated and very fine-grained, though occasional sizeable pyrite and or chalcopyrite blebs and minor veins and veinlets are observed in drill core.
Sulfide content is modally highest in granodiorite and mylonitic granodiorite and generally ranges, based on visual analysis, from trace amounts to less than 5% of whole-rock content. In addition to pyrite and chalcopyrite, bornite, covellite, molybdenite, and pyrrhotite are also present, as well as trace amounts of very fine-grained native gold, 10 to 250 microns in size (Mountain Lake Resources Inc., 1997). Assay data indicates a significant, if not direct, relationship between metal concentration and sulfide content, particularly chalcopyrite. Coppersulfides are virtually restricted to granodiorite, though trace amounts of chalcopyrite are observed both in mafic dikes within the granodiorite and in the metasedimentary rocks immediately adjacent to the east. Trace to weight-percent amounts of pyrite is also observed in drill core in metasedimentary rocks, aplitic dikes, pegmatites, and mafic dikes, all within the sulfide zone.
Gold mineralization at the CK Gold Project occurs in the west-central portion of the Project area and is distributed in plan-view in an elongate ovoid pattern trending roughly N60°W. The orientation of the mineralized zone is generally coincident with the local trend of shear as interpreted by Klein (1974) and McGraw (1954) based on field measurements of exposed structural fabrics (cataclastic foliation) and fault planes. The primary known mineralized zone is essentially vertical and “keel-like” in shape, as represented by the 1 gpt Au cut-off grade shell with a surface length of 400 ft along strike, width of approximately 200 ft, and depth (thickness) of 600 ft. This higher-grade, central core is surrounded by a halo of lower grade mineralization with an overall length of roughly 760 ft along strike, an average width of approximately 500 ft, and thickness of at least 1,100 ft. Low-grade (<0.5 gpt Au) gold mineralization is open and uniform along strike, both to the northwest and southeast, as well as at depth.
The mineralized zone is crudely bound to the north and to the east by the Northwest fault and the Copper King Fault, respectively. The Northwest Fault strikes west-northwest and dips steeply to the northeast along the northern margin of the mineralized zone. The fault represents an apparent structural control of the CK deposit, as copper-gold mineralization is essentially restricted to south of the fault.
The Copper King Fault trends roughly N30°E along the eastern extent of the CK deposit, truncating known mineralization in that direction. Host granodiorite occurs to the west of the fault, and unmineralized metasedimentary and metavolcanic rocks occur to the east. This contradicts previous interpretations of the fault as normal with a down-to-the-east, nearly vertical, dip slip offset (Hausel, 2012). Based on examination of exposures in prospect pits north and east of the deposit, the Copper King Fault is thought to be Laramide or younger, though it may represent remobilization along a much older, existing fault plane.
A variety of other faults have been interpreted within the Project area based largely on surface expression, indications in drill core, and televiewer data. As noted by Klein (1974), many local structures are generally concordant with the trend of Precambrian shear and may represent more recent (Laramide or younger), shallow depth rejuvenation along previously existing fault planes. Several local structures are discordant with the Precambrian trend of shear, and these are also generally thought to be Laramide or younger based on a lack of cohesion and recrystallization in the faulted material (Klein, 1974). The significance of these structures relative to the CK deposit is likely limited to an associated increase in intensity and/or depth of oxidation and supergene copper mineralization, and potential, small-scale physical displacement of copper-gold mineralization at depth.