Summary:
Since the Graphite One Project graphite deposit occurs in a quartz-granite-biotite-sillimanite schist, which is a high grade metamorphic rock, the Graphite Creek mineralization is considered to be of a Flake Graphite Type Mineralization.
The Kigluaik Group consists of amphibolite and granulite facies metamorphic rocks and is therefore divided into two sub-groups, an upper and lower assemblage. Amphibolite grade upper Kigluaik Group schist is exposed on the southern flanks of the Kigluaik mountain range. Pelitic gneiss samples from the upper section of the Kigluaik group have been dated using Rb/Sr to ~735 Ma (Bunker et al., 1979). The basal Kigluaik Group contains granulite grade schist and gneiss, and is exposed on north flank of the mountains. These rocks have no direct counterparts in the adjacent mountain ranges and are believed to represent the deepest crustal rocks exposed in northwestern Alaska (Miller, 1994). The lower Kigluaik Group comprises coarse marble, quartzo-feldspathic gneiss, schist and gneiss of mafic and ultramafic composition, graphite rich schist, and garnet lherzolite.
The Graphite Creek graphite deposit is located on the north side of the Kigluaik Mountains (at about 230 m elevation). More specifically, the graphitic schist occurs on the upslope and footwall surface trace of the reactivated Kigluaik normal fault. The Kigluaik Fault generally strikes at approximately azimuth 250º and dips 75º to the north over a distance of approximately 35 km. Contemporary movement on this fault has uplifted the rugged and youthful Kigluaik Mountains to the south and down thrown the lowlands of the Imuruk Basin to the north (Hudson and Plafker, 1978).
Graphite occurs as high-grade massive to semi-massive segregations and disseminations within amphibolite facies metasedimentary rocks, primarily biotite-quartz schist with zones of sillimanite-garnet-biotite- quartz schist. Based on their apparent association with the Kigluaik Fault, the graphite-bearing schist units strike subparallel to the mountain front and dip north between 40° and 75°.
There are two distinctive graphite-bearing schist intervals at Graphite Creek. The first is sillimanite-garnet-biotite-quartz schist that contains coarse, semi-massive and massive graphite segregations and disseminated graphite. The other interval unit is biotite-quartz schist that typi-quartz schist is the principal host to higher grade graphite and makes up two distinctive layers in the metasedimentary sequence along the north flank of the Kigluaik Mountains. A third potential horizon is defined by ‘pods’ of sillimanite-garnet- biotite-quartz schist. The position of these layers is most likely structurally controlled; that is a folded unit with the third pod-like layer forming in this style as uppermost erosional features (T.Hudson, personal communication, 2012). Hence, shallow-dipping erosional remnants of the southern-most third layer makes up a few discontinuous perched masses at higher elevations. The sillimanite-garnet-biotite-quartz schist layers strike obliquely to the mountain front and dip northwards at 40° to 80°.
The sillimanite-garnet-biotite-quartz schist typically is fine to coarse grained, weathers grey, has a wavy and crenulated schistosity, garnet porphyroblasts (up to 2 cm across), and augen-shaped quartz grains. Discontinuous segregations (lenses and streaks) of coarse high-grade graphite, from centimeters to a few meters thick, are common. These highgrade graphite lenses in the sillimanite-garnet-biotite- quartz-schist have up to 60% coarse, crystalline graphite and were no doubt the sources of hand sorted graphite produced in the early 1900’s. Disseminated flakes of graphite, up to 1 mm or more across, make up several percent of the rock.
The biotite-quartz schist is fine-grained, weathers a rusty ochre colour and has regular layering with individual layers commonly 3 to 10 cm thick. Graphite occurs as disseminated flakes up to about 1 mm across and can make up several percent of the rock. Higher grade graphite-rich layers, varying from 3 to 25 cm in width are present, but are not as common as in the sillimanite-garnet-biotite-quartz schist.