Geology, alteration, and mineralization of the True North gold deposit, Fairbanks District, Alaska

Kinross Gold Corporation’s True North Mine located in the central Alaskan
Fairbanks District, lies approximately 18 kilometers northwest of the Fort Knox Mine
and about 28 kilometers northeast of Fairbanks, Alaska. The original resource estimate
for the True North Mine was approximately 631,000 ounces of gold (13.7Mt @
0.046 opt). True North went into production in 2001 and was temporarily shut down
in the spring of 2004. The True North deposit is hosted by the Chatinika Terrane
which is part of the Yukon-Tanana Composite Terrane. The Chatinika Terrane at the
True North deposit consists of an eclogite facies suite, composed of garnet-pyroxenecalcite-
quartz rocks, garnet-pyroxene-amphibole-calcite-quartz rocks, and marble,
and an amphibolite facies suite composed of biotite schist (±garnet), biotite-quartz
schist (± graphite and garnet), amphibolite (±garnet), and quartzite. Locally an
argillite to slate unit is apparently tectonically interleaved with the higher grade units.
The amphibolite facies appears to be a retrograde product of eclogite facies. The diorite
to granodiorite Pedro Dome stock which is interpreted to be part of the same
intrusive complex that hosts the Fort Knox deposit, is the closest known intrusive
body to the True North deposit (≈1.5 miles or 3 kilometers) The age of the mineralization
appears to correspond to the age of intrusion ~88–92 Ma. The Chatinika Terrane
has undergone several regional deformational events prior to emplacement. After the
metamorphism, the high grade Chatinika package was thrust over the lower grade
(lower- to middle-amphibole facies) Fairbanks Schist, which makes up of bulk of the
Yukon-Tanana Terrane in the Fairbanks district. Mineralization associated with the
intrusion of the Pedro Dome stock occurred along high- and low-angle faults. The low
angle faults appear to be sub-parallel to and related to the thrust fault that emplaced
the Chatinika sub-Terrane over the Fairbanks Schist, and mineralized high angle
faults include district scale northeast-trending oblique-slip faults and several generally
north-east trending cross-cutting fault sets. The mineralized faults are cross-cut
by several generations of north-south, northwest and east-west trending faults that
locally offset mineralized faults. All of the mineralized faults show evidence of multiple
stages of movement. Mineralized fault zones range in width from centimeters to several
meters and most have graphitic gouge with a well developed planar, anastomosing
fabric concentrated along slip planes. The graphite is apparently remobilized from
the argillite/slate and/or the graphitic biotite-quartz schist.
Mineralization, which consists of multiple generations of quartz veins, quartz
veins with stibnite, arsenopyrite, ± pyrite, and several generations of carbonate veins,
is found within the fault zones. Carbonate veining locally extends out into relatively
unaltered host rocks for several meters. Veins are crushed and dismembered within the fault zones and intact veins are only rarely preserved. Alteration consists of a halo
of fuchsite-ankerite ± arsenopyrite and pyrite around veins. Where veins are closely
spaced large volumes of rock are altered.