Summary:
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 led to concentration a 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 favourable host rock in the Fairbanks Schist and Chatanika Terrane.
Mineralization
Gold mineralization is hosted in a Dolphin multi-phase stock and in Cleary Hill schists at approximately 1:5 proportions. Mineralization is genetically related to the Dolphin stock and conforms to an intrusionrelated model (IRGM).
In general, gold mineralization comprises tabular-shape bodies that dip SE at 40-50 degrees and more gently 10-15 degrees in a SW direction. Limits of mineralization are primarily tectonic and defined by two sinistral faults (DSF and TSF) and the CRCS shear zone. Mineralization extends for approximately 1,800 meters from the SW to NE, is open to the SW, and is from 800 to 1,000 m wide, i.e., the approximate distance between sinistral faults. Mineralization has been traced from surface to a maximum vertical depth of 700 m.
Gold Mineralization is represented by a combination of: tabular shaped low-grade mineralization (>0.5 g/t Au), in several levels – Upper, Main and possibly Lower, characterized by sharp NW and SE boundaries defined by two sinistral faults – the Newsboy and Tamarack, and 2) higher-grade gold corridors (>1.0 g/t Au).
Gold mineralization in the Upper level (0-150 m) correlates with As, Ag, Pb, Sb and Zn and reflects a combination of stockwork and vein styles of mineralization. Gold mineralization in the Main level is accompanied primarily by arsenic; base metals are rare.
Similar combinations of low- and higher-grade gold mineralization are recognized at various elevations. There are two or three relatively continuous high-grade corridors oriented approximately ENE and two or three high-grade corridors oriented NWN. The first group includes the Tolovana, Colorado, Wyoming and Wackwitz veins.
Two modes of gold mineralization are present in the Dolphin deposit:
Intrusion- and schist-hosted quartz (quartz-sericite, quartz-feldspar and quartz-sulfide) sheeted veinlets/veins, quartz stockwork veinlets/veins and shear and breccia-hosted gold-bearing veinlets. This represents an early stage of “invisible”, micron-sized gold mineralization, characterized by a relatively simple gold-arsenic geochemical association, where gold grades are above 0.5 g/t Au and increase with alteration intensity and veinlets/veins density.
Auriferous sulfide-quartz veins and disseminations such as those exploited at historic underground workings (Cleary Hill, Saddle Zone and Hi Yu), and later stage mineralization, that occupies the upper part of the deposit. This represents late-stage gold mineralization, whose geochemical assemblage includes silver, lead, antimony and zinc. The silver to gold ratio for the entire deposit (>0.5 g/t Au) is 0.74.
In addition to Au-bearing grey transparent quartz veins, there are also randomly distributed, narrow (30 – 50 cm) Au-bearing veins that are present at any depth and are possibly part of a separate gold event, predating Cleary Hill veins.
Mineralogy and geochemistry of the two modes of gold mineralization differ.
Pyrite and arsenopyrite are the most common sulfide minerals present; stibnite, lead-antimony sulfosalt minerals, tetrahedrite, scheelite, galena and sphalerite are also present locally.
Arsenopyrite is associated with both modes but varies in morphology and trace element content. Lowtemperature arsenopyrite tends to be coarse-grained, while the high-temperature variety is much finergrained. Arsenic almost replicates gold distribution and correlates with high gold grades.
Silver, lead and zinc-bearing minerals (tetrahedrite, silver/lead sulfosalt minerals, galena and sphalerite) and their respective geochemical anomalies indicate distal disposition and are primarily confined to the Upper level.
Antimony minerals (stibnite and antimony sulfosalts) and their associated anomalies are distal and spread more broadly and deeper, primarily above gold mineralization in the Main level.
Distal elements in the Upper level coincide with the Dolphin stock carapace and the Cleary Hill antiform.
Sulfur follows both distal and proximal geochemical assemblages, but is coincident with the latest mineralogical event orientation and dips NE.