USGS Professional Paper 1697 - Alaska Resources Library

32 Metallogenesis and Tectonics of the Russian Far East, Alaska, and the Canadian Cordillera
that contain Early Silurian SEDEX deposits (Howards Pass
metallogenic belt described below). The major deposits are in the
Anvil district are the Faro, Vangorda Creek, Grum, Firth, DY,
and Swim (table 4) (Nokleberg and others 1997a,b, 1998).
Anvil District SEDEX Zn-Pb-Ag Deposits
The Anvil district contains six pyrite-bearing, stratiform
Zn-Pb-Ag (Au-Cu-Ba) deposits and two stratiform pyritic
Cu-Zn occurrences, which extend southeastward along strike
for 45 km. The deposits and occurrences are hosted in distinctive,
Early Cambrian graphitic phyllite, which forms a
district-wide metallotect. Ore from the three principal deposits
(Faro, Vangorda, and Grum) consists of massive pyrite, pyrrhotite,
sphalerite, galena and marcasite with patchy barite in a
siliceous gangue. Higher ore grades are associated with barite.
Pyrite-bearing massive sulfides are overlain by barite-bearing
massive sulfides, and are underlain by pyrite-bearing quartzite
that grades laterally into ribbon-banded graphitic quartzite and
graphitic phyllite. The Faro deposit (fig. 12) occurs approximately
100 metres stratigraphically below the contact between
phyllite and quartzite of the Early Cambrian Mount Mye
Formation and calcareous rocks of the Cambrian and Ordovician
Vangorda Formation. At the northern end, the deposit is
intruded and contact metamorphosed by the Cretaceous Anvil
SW
S
Overburden
Cretaceous
Diorite, quartz/feldspar porphyry dykes
Cambrian and Ordovician
Calcareous phyllite, metabasalt, calc-silicate
Carbonaceous phyllite
Noncalcareous phillite and schist
batholith and related dikes. For the district, the combined,
premining reserves were 120 million tonnes grading 5.6
percent Zn, 3.7 percent Pb, and 45 to 50 g/t Ag (Jennings and
Jilson, 1986). Faro, the largest deposit, ceased production in
1997. The Vangorda deposit, a smaller mine, containing 7.1
million tonnes of ore, was largely exhausted between 1990 and
1993 (Brown and McClay, 1993), and the Grum mine produced
16.9 million tonnes of ore from 1993 to 1996.
Origin of and Tectonic Controls for Anvil Metallogenic Belt
The Anvil metallogenic belt is hosted by sedimentary
rocks of western Selwyn Basin that are part of the Cambrian
through Devonian passive margin of the North American
Craton (Nokleberg and others, 1994c, 1997c; Monger and
Nokleberg, 1996). The host sedimentary strata represent a transition
from shelf to slope facies. The coincidence of southwestward
thickening in the graphitic phyllite with a linear array
of alkaline basalt volcanism centers suggests that rift-related
synsedimentary faults may have served as conduits for the
SEDEX fluids. However, demonstrable feeder zones have not
been observed. The elongate subbasins hosting the deposits
are interpreted as forming during Middle Cambrian to Early
Ordovician extension and faulting (Jennings and Jilson, 1986).
The resulting structural conduits provided concentrated exha-
Staurolite line
Other Features
NE
White mica alteration envelope
Quartzose disseminated sulphides
Massive sulphides
Contact
1,216 m
1,070 m
0 100 M
Figure 12. Faro sedimentary-exhalative Zn-Pb-Ag deposit, Anvil metallogenic belt, Canadian Cordillera.
Schematic cross section. Adapted from Jennings and Jilson (1986). See figure 3 and table 4 for location.