Summary:
Mineralization
Gold mineralization is spatially associated with the Dolphin stock but predominantly occurs in the Fairbanks Schist. Within the area tested by drilling, the Fairbanks Schist is largely hornfelsed, presumably in response to the intrusion of the Dolphin stock.
Gold mineralization is hosted by discrete, high-grade quartz veins, veinlets, and areas of vein stockwork that form vein swarms, within a broad structural corridor comprised of the Dolphin stock and schistose Fairbanks metasedimentary rocks. The Cleary Hill Vein swarm (“CVS”) mineralization dips to the south and plunges southwest towards the Dolphin intrusive, with the mineralization increasing in abundance toward the Dolphin intrusive, especially along the intrusive schist contact margins.
Intrusive-Hosted Sulfide-Quartz Veinlets
The highest gold grades in the Dolphin intrusive are associated with sulfide disseminations and 0.1 to 5 mm sulfide-quartz veinlets. Gold also occurs with disseminated euhedral arsenopyrite (1 to 5 mm) that appears to belong to an earlier, higher-temperature mineralizing event (McCoy and Olson, 1997), and in fault gouge enriched with sulfides, sulfide-rich veins, and locally as narrow sulfide-quartz veins less than 15 cm thick.
Pyrite and arsenopyrite are the most common sulfide minerals although stibnite, lead-antimony sulfosalt minerals, tetrahedrite, scheelite, galena and sphalerite occur locally. McCoy and Olson (1997) identified two distinct varieties of arsenopyrite in the Dolphin gold deposit based on arsenopyrite geothermometry and age relations. Older arsenopyrite from quartz stockworks (90.1 Ma) formed at higher temperatures, whereas younger arsenopyrite from shear zones formed at lower temperatures (88.3 Ma). McCoy also noted that older “hotter” arsenopyrites were finer-grained compared to younger “cooler” arsenopyrites, which are generally coarse and blade finer-grained compared to younger “cooler” arsenopyrites, finergrained compared to younger “cooler” arsenopyrites, which are generally coarse and blades. Furthermore, the high-temperature arsenopyrite contains particulate inclusions of gold, whereas the lowtemperature arsenopyrite contains maldonite (a gold-bismuth mineral). Although stibnite and antimony sulfosalts are not uncommon in the deposit, geochemical studies suggest that high antimony values are generally associated with low gold values. Evidence suggests that the fluids evolved towards increasing base metals and antimony with time.
Auriferous Quartz Veins
High-grade auriferous quartz veins (2 cm to 3 m), hosted in metamorphic rocks, occur at numerous locations, and were the source of all previous gold production from the Property. Their general mineralogy, morphology and structural setting are summarized below.
Auriferous quartz veins occur both parallel to and cross cutting the primary host rock foliation at very high angles. A large number of veins dip south, although some dip north. Vein thickness is variable and ranges from a few centimeters to a few meters over short distances along both strike and dip. Pinch- and-swell features, bifurcations and splays are characteristic. Discrete auriferous quartz veins commonly have sharp wall rock contacts but can grade into shear zones, suggesting a continuum between this type of gold quartz veining and shear-hosted gold described below. In contrast to the high-grade quartz veins, barren, translucent or milky-colored metamorphic quartz most commonly occurs as seams or boudinage subparallel to the primary foliation of the host rocks.
Auriferous quartz veins consist of hydrothermal quartz with minor to trace amounts of sulfides. The quartz is opaque to milky-white and locally grey to mottled grey and white. Bands or laminations parallel to vein walls are not uncommon, and vein centers commonly contain vuggy or comb quartz crystals. Silicified vein breccia is also common and may comprise the entire vein or be restricted to bands within the banding sequence. This suggests that there were most likely multiple, possibly alternating, episodes of silicification and deformation.
Auriferous quartz veins rarely contain more than 5% total sulfides and average 1-3%. The most common sulfide is pyrite, although other sulfides are locally present, including arsenopyrite, stibnite, jamesonite, tetrahedrite, galena and sphalerite. Scheelite is present in a few specific veins and is notably abundant in the Cleary Hill and Wyoming vein. Visible gold typically occurs as crystals, coarse flakes, filigree, or wires suspended in quartz or mingled with sparse, scattered sulfides.
Shear and Breccia-Hosted Veinlet Zones
Shear and breccia-hosted auriferous veinlet zones occur within some of the same shear zones that host major auriferous quartz veins and are likely parts of the same mineralizing event. The key characteristic of these zones is that they may contain sufficient polyphase veinlet density and gold grade to justify bulkmining methods.
Shear and breccia-hosted veinlets consist largely of quartz with variable amounts of sulfides, although locally the veinlets may consist largely of sulfides with lesser amounts of quartz. Sulfide-quartz veins within shear-hosted zones generally are less than a few centimeters in thickness. Locally these veins form vein sets with spacings of a less than one meter, resembling a sheeted vein system (vein swarm). The veins are discontinuous along strike and dip, and commonly grade into broken veins, vein breccia, and zones of sugary, granulated crushed quartz material. Higher quartz vein and veinlet density is generally indicative of higher gold values.
Shear and breccia-hosted veinlet zones are characterized by pervasive sericite and clay alteration as well as by localized silicification and carbonate alteration. In addition, these zones are typically highly oxidized near the surface and contain locally intense iron, arsenic or antimony oxides. Most veinlets are subparallel to the strike and dip of the zone.
Deposit Types
Recent discoveries in the Fairbanks District have identified a series of distinctive mineral occurrences that appear to be genetically related to mid-Cretaceous plutonic activity that affected a large area of northwestern British Columbia, Yukon, Alaska and the Russian Far East (Flanigan and others, 2000). This work, based on extensive geologic and structural mapping and analytical studies (major and trace element analysis, fluid inclusion microthermometry, 40Ar/39Ar geochronology, and isotope analysis) has provided new information regarding gold metallogenesis in the Fairbanks District (Baker and others, 2006; Burns et al., 1991; Lelacheur et al., 1991; Hollister, 1991; McCoy et al., 1994; Newberry et al., 1995; McCoy et al., 1995). A synthesis of this information (Hart et al., 2002, Hart 2007, McCoy et al., 1997, Lang and others, 2001) suggests a deposit model in which gold and high CO2-bearing fluids fractionate from ilmenite series, I-type mid-Cretaceous intrusions during the late phases of differentiation.
The gold is deposited in anastomosing pegmatite and/or feldspar-selvage quartz veins. Brittle fracturing and continued fluid convection lead to concentration of gold-bearing fluids in intrusions and schist-hosted brittle quartzsericite shear zones. Carbonate and/or calcareous metabasite horizons host W-Au skarns and replacement deposits. Structurally prepared calcareous and/or carbonaceous horizons may host bulkminable replacement deposits. These occur most distal to the intrusions within favorable host rock in the Fairbanks Schist and Chatanika Terrane.