Tres Equis Prospect, Delta River Mining District, Southeast Fairbanks Census Area, Alaska, USAi

Regional Level Types
Tres Equis Prospect	Prospect
Delta River Mining District	Mining District
Southeast Fairbanks Census Area	Census Area
Alaska	State
USA	Country

The Tres Equis breccia prospect is important because it demonstrates that the Fish Lake complex contains high-grade nickeliferous massive sulfide.
Location: The Tres Equis prospect is located one-quarter mile northwest of peak 3710, east of the headwaters of Eureka Creek below Eureka Glacier. The prospect is at the center of the SE1/4 section 12, T. 19 S., R.7 E., Fairbanks Meridian.
Geology: The Tres Equis prospect is in the Fish Lake ultramafic-mafic intrusive complex, a layered lopolith of Late Triassic age considered to be a comagmatic with lava flows of the Nikolai Greenstone (Nokleberg and others, 1991). The rocks at the Tres Equis prospect consist of layered wehrlite cut by irregular, mainly gabbroic dikes. A 50- to 100-foot-thick layer of taxitic gabbro overlies the wehrlite and is overlain in turn by a thick, massive peridotite of a later magmatic cycle. The taxitic gabbro contains leucocratic and anothorositic phases and commonly has clinopyroxenitic margins. The dikes range from melagabbro to leucrocratic pegmatite. Magmatic layering in the lopolith strikes east to southeast and dips 45 to 50 degrees to the south. The dikes also strike east to southeast but cut the layering at high angles. All units are cut by high-angle, north- to northeast- trending faults having offsets of 20 to 50 feet. The Tres Equis deposit is a magmatic breccia in the lower portion of the gabbroic unit of the second magmatic cycle of the Fish Lake complex. The breccia strikes east to southeast, subparallel to the gabbroic dikes. The breccia is along the margin of an irregular dike and consists of clasts of peridotite and gabbro in an oxidized and leached (post-mineralization) matrix. Some of the clasts, which are as much as 2 feet in diameter, contain disseminated pyrrhotite, chalcopyrite, and pentlandite. The sulfides also form networks and massive clots containing individual crystals more than an inch across. The average grade of six clasts containing massive sulfides was 3.2 percent copper, 5.2 percent nickel, 0.33 percent cobalt, 1,750 parts per billion (ppb) palladium, and 28 ppb platinum. A net-textured clast assayed 0.3 percent copper, 1.0 percent nickel, 0.07 percent cobalt, 712 ppb palladium, and 100 ppb platinum (W.T. Ellis, unpublished data, 1996).
Workings: One hydraulically washed hand trench 20 to 30 feet wide and 100 feet long was completed in 1996. A 627-foot hole was drilled in 1997; two 200-foot holes were drilled in 1998. None of the drill holes penetrated significant mineralization. The occurrence is covered by active Fort Knox Gold Resource claims.
Age: Late Triassic, the age of the host rock.
Alteration: The host rock is oxidized and leached. The peridotite is moderately serpentinized; hairline joint fractures are filled with fibrous serpentine (chrysotile). The gabbro dikes are amphibolitized along their margins but are essentially fresh for a half -inch into the dikes.

Commodities (Major) - Cu, Ni, Pd; (Minor) - Co, Pt
Development Status: None
Deposit Model: Ni-Cu-PGE in differentiated mafic-ultramafic sill.

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Link to USGS - Alaska:	MH104

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References