Dolphin Prospect, Fairbanks Mining District, Fairbanks North Star Borough, Alaska, USAi

Regional Level Types
Dolphin Prospect	Prospect
Fairbanks Mining District	Mining District
Fairbanks North Star Borough	Borough
Alaska	State
USA	Country

Location: The Dolphin intrusive body forms an elongate northeast-trending exposure covering a 2,000 foot by 1,200 foot area on the ridge between Willow Creek and Bedrock Creek; NE1/4 sec. 25, T. 3 N., R. 1 E., of the Fairbanks Meridian.
Geology: In 1995, Freegold discovered significant disseminated gold mineralization in the granodiorite and tonalite of the Dolphin stock (Freeman and others, 1998). Additional soil sampling conducted in 1995 and 1996 delineated a large northeast trending gold anomaly (+100 ppb gold) which closely mimics the south contact of the Dolphin stock. The mineralization is controlled by the district-scale Dolphin shear zone. The gold-in-soil anomaly also is highly anomalous in silver, arsenic, lead, antimony and zinc. The anomaly forms the southeast portion of a broad gold-in-soil anomaly which extends from the west bank of Willow Creek to Bedrock Creek. The Dolphin stock forms an elongate northeast-trending exposure of equigranular biotite granodiorite covering a 2,000 foot by 1,200 foot area at the surface. Drilling along the northern edge of the stock encountered calc-silicate skarn at a vertical depth of 400 feet, suggesting the north edge of the stock dips steeply north . The southern contact of the stock dips shallowly to the south. Drilling on the northeast end of the intrusive suggests that the main stock branches into a dike swarm in that area and that the main stock may be faulted at depth. Mineralization is open to the west into Willow Creek and below 1000 feet, the deepest hole drilled on the deposit. During 1995-96, Freegold conducted 15,559 feet of reverse circulation drilling in the Dolphin prospect in 46 drill holes. In 1998, a single vertical core hole was drilled to a depth of 1,033 feet. This hole encountered only intrusive rock over its entire length and both compositional phases of the Dolphin stock were encountered. Upper portions (0-622 feet) of the intrusive consisted of granodiorite while tonalite was encountered below 622 feet. The tonalite phase is texturally similar to the main granodiorite body but with abundant biotite and possible amphibole. The contacts of both the dikes and the main stock were loci for well developed brittle fracturing as evidenced by abundant crushed quartz and clay. The tonalite has a distinctive geochemical signature of relatively elevated Ni, Li, K, Na, Mg, V, Sc, and Ti. A chloritic and sericitic altered intrusive has also been recognized; given the chlorite content, the rock likely represents altered tonalite. Most of the visible sulfide mineralization is in the form of arsenopyrite, pyrite, stibnite, pyrrhotite, and pentlandite. As would be expected the pyrrhotite and pentlandite has only been recognized in the mafic tonalite and does not seem to be associated with the quartz veining. Significant intervals of rock with more than 0.03 ounces of gold per ton were encountered at depths well below previous drilling, and the hole bottomed in anomalous gold mineralization. Sulfide mineralogy is dominated by 1 to 5% disseminated pyrite and arsenopyrite, and pervasive zones with more than 50% sulfides are characterized by large amounts of galena, tetrahedrite and sphalerite. Up to 2% disseminated scheelite is locally abundant. Electron microprobe analyses and polished section microscopy reveal native gold and bismuthinite as inclusions in arsenopyrite. Visible gold is rare and is hosted in white, quartz stockwork veins in drill cuttings and core. Preliminary metallic sieve analyses also suggest little or no nugget effect (Adams, 1997). Bottle roll analyses from granodiorite in the oxide zone recovered 82% to 92% of the gold, while analysis of the sulfide-rich altered granodiorite recovered less than 10%. Gold grade at the Dolphin prospect is controlled by the thin quartz veinlets, typically less than 1 mm, which cut the host intrusive. Vein orientations vary with no apparent preferred orientation for mineralization. At least three separate hydrothermal events are present in the Dolphin core. The Dolphin deposit consists of multiple flooded zones of alternating silicification and sericitization, with local scheelite-bearing carbonate zones and sulfide rich zones containing elevated base metal values. Scheelite-bearing carbonate alteration zones generally contain lower gold values and tend to form halos around the higher grade gold intercepts. Silica flooding occurs throughout the intrusive and locally reaches up to about 90% SiO2. Silica flooded zones often contain chalcedonic green, brown or black quartz. In some drill holes the flood silicification in granodiorite results in near-complete obliteration of original igneous textures. Chloritic alteration is rare. Preliminary resource calculations estimated a gold resource of approximately 600,000 ounces grading 0.020 ounces of gold per ton (Adams, 1997).
Workings: During 1995-96, Freegold conducted 15,559 feet of reverse circulation drilling in the Dolphin prospect in 46 drill holes. In 1998, a single vertical core hole was drilled to a depth of 1,033 feet (Freeman and others, 1998).
Alteration: The Dolphin deposit consists of multiple flooded zones of alternating silicification and sericitization, with local scheelite-bearing carbonate zones (Freeman and others, 1998). In some drill holes the silicification of the granodiorite results in near-complete obliteration of original igneous textures. Chloritic alteration is rare.
Reserves: Preliminary resource calculations estimated a gold resource of approximately 600,000 ounces in material with 0.020 ounces of gold per ton (Adams, 1997).

Commodities (Major) - Au; (Minor) - Ag, Bi, Te
Development Status: None
Deposit Model: Low-grade igneous-hosted gold deposit; disseminated veins and shears.

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Link to USGS - Alaska:	LG112

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