The Donlin Creek Au-As-Sb-Hg Deposit, Southwestern Alaska

The Donlin Creek deposit is one of the largest gold occurrences
in Alaska, with a measured indicated and inferred resource of 122.1
million tonnes with an average grade of 2.91 g/t Au (11.4 million
contained ounces). The resource is refractory with most of the gold
bound in the structural lattice of very fine-grained arsenopyrite.
The Donlin Creek district contains a thick sequence of
Cretaceous age graywackes, shales, and siltstones of the
Kuskokwim Group. The Kuskokwim Group has been intruded by 70
Ma, dominantly NE-trending, porphyritic rhyodacite and rhyolite
dikes and sills. The greatest volume of igneous dikes and sills occur
in a NE-trending corridor which overlaps with the bulk of mineralization.
Subareal andesite, basalt, and rhyolite are exposed 10 km to
the west.
Gold-As-Sb-Hg mineralization has been identified over an area
6 km long by 2 km wide and over a vertical extent of at least 400 m.
Mineralization is associated with narrow irregular and discontinuous
quartz and quartz-carbonate veins and veinlets, hairline fractures,
minor breccias, and narrow zones of silica flooding. Veins are structurally
controlled along NNE-trending extensional fracture zones,
and are best developed where these zones intersect competent
lithologies such as felsic dikes or sills, or massive graywacke. The
majority of veins trend about 020¡, and dip steeply to the east and
west.
Veins and veinlets are composed of gray and clear quartz with
white to tan carbonate (ankerite-dolomite-calcite) and contain trace
to 3 percent stibnite-pyrite-arsenopyrite. Many veins lack well
developed textures, however, druse lined vugs, fine-grained comb,
banding, cockade, and bladed textures occur locally. Several narrow
high-grade veins/veinlets have been identified, typically ranging
from a few millimeters to 10 cm in width. Gold values from 20 highgrade
vein samples range from 12.9 to 273 ppm, averaging 57.6
ppm. High-grade veins contain quartz with or without carbonate,
and typically 1 to 5 percent arsenopyrite and pyrite. Veins locally
contain up to 50 percent or more fine-grained pyrite-arsenopyrite.
Dikes and sills through the main Donlin Creek area are altered
to illite or illite + kaolinite with minor carbonate. This illite alteration
is most intense in mineralized zones with associated veining,
but also occurs outside mineralized zones in areas with no obvious
veining. Similar alteration occurs in graywacke and shale but in limited
extent. Available mineralogical, fluid inclusion, and textural
evidence from the Lewis-ACMA-Queen zones indicates veins and
surrounding alteration formed at relatively low temperatures and
shallow depths, likely in an epithermal environment.
The NNE-oriented extensional fractures that host mineralization
at Donlin Creek also occur regionally and are interpreted to be
related to NNE-SSW directed shortening. This regional shortening
event may provide evidence for a 70 Ma compressional event
related to offshore subduction of the Kula Plate.