USGS Professional Paper 1697 - Alaska Resources Library

and barite in siliceous black turbiditic shale of Famennian
(Late Devonian) age (Paradis and others, 1998). The deposit
has estimated reserves of 18.1 million tonnes grading 2.38 percent
Zn+Pb (Insley, 1991; Paradis and others, 1995). The sulfide
bodies occur in three stratigraphic levels in the Gunsteel
Formation of the Earn Assemblage in different thrust-bounded
panels. The host sedimentary rocks are interpreted as part of a
Devonian-Mississippian clastic wedge.
Origin of and Tectonic Setting for Gataga Metallogenic Belt
The SEDEX Zn-Pb-Ag-Ba deposits of the Gataga metallogenic
belt are interpreted as forming during deposition of
basinal clastic rocks of the Earn Assemblage during the early
to late Famennian over a time span of not more than 7 million
years. This relatively brief episode occurred during or immediately
after a period of continental-margin arc formation and
subsequent rifting of the North American Craton Margin in the
early Mississippian (Paradis and others, 1998). The rifting is
interpreted to have influenced sedimentation and volcanism in
the Yukon-Tanana and Kootenay terranes (Nokleberg and others,
1994c, 1997c; Monger and Nokleberg, 1996; Nokleberg
and others, 2000). The SEDEX deposits of the Gataga metallogenic
belt are interpreted as forming during a relatively brief
period immediately after rifting (Paradis and others, 1998; this
study). However, other metallogenic belts containing Devonian-Mississippian
SEDEX deposits are interpreted as forming
over a longer age range, from Frasnian (early Late Devonian)
at Macmillan Pass to Tournasian (Early Mississippian) in the
Cassiar terrane.
Robb Lake Metallogenic Belt of Southeast
Missouri Zn-Pb Deposits (Belt RL), Northern British
Columbia
The Robb Lake metallogenic belt of Southeast Missouri
Zn-Pb deposits occurs in northern British Columbia
(fig. 17; tables 3. 4) (Nokleberg and others, 1997b, 1998;
Nelson and others, 2002) and is hosted in the Proterozoic to
Devonian passive continental units of the Rocky Mountains,
which constitute part of North American Craton Margin. The
Southeast Missouri Pb-Zn deposits in the Robb Lake metallogenic
belt occur in secondary breccias of either solution or
tectonic origin in folded dolostones mainly of the Silurian and
Devonian Muncho-McConnel Formation (Taylor and others,
1975; Nelson and othes, 2002). Occurrences also are located
in the underlying Silurian Wokkpash and overlying Devonian
Stone,Dunedin, Pine Point, and Slave Point Formations (Macqueen
and Thompson, 1978; Nelson and others, 1999; Paradis
and others, 1999). The significant deposit is at Robb Lake.
Robb Lake Southeast Missouri Zn-Pb Deposits
The Robb Lake deposit consists of sphalerite, galena and
pyrite that occur primarily in tabular and lenticular zones parallel
to bedding in dolomite collapse breccias of the Silurian and
Middle and Late Devonian Metallogenic Belts (387 to 360 Ma; figures 16, 17) 69
Devonian Muncho-McConnel Formation (EMR Canada, 1989;
Dawson and others, 1991; Mining Review, 1992; Nelson and
others, 2002). The deposit occurs on the west limb and crest of
a large south plunging anticline. The deposit occurs in an 8 km 2
area, and consists of a series of interconnected, bedding-parallel
and crosscutting breccia bodies with a matrix of dolomite,
sphalerite, galena, pyrite, quartz, calcite, and pyrobitumen, and
peripheral veins and stockwork. (Nelson and others, 2002). Estimated
reserves are 7.1 million tonnes grading 4.7 percent Zn
and 1.5 percent Pb (Mining Review, summer, 2000). A significantly
larger, but less defined resource for the district and belt is
an estimated 20.1 million tonnes grading 5.1 percent combined
Pb and Zn (Dawson and others, 1991).
Origin of and Tectonic Controls for Robb Lake
Metallogenic Belt
The southern part of the Robb Lake metallogenic belt
occurs along a Devonian carbonate facies front in shales of the
Besa River Formation to the west for 250 km, extending from
Mount Burden on the south to the CTV and DODO deposits
near Tuchodi Lakes on the north. The stratigraphic setting for
these occurrences, which consists of a carbonate front adjacent
to the major Great Slave Lake Basin, is interpreted as analogous
to that at Pine Point (Nelson, 1991).
Some occurrences in the northern part of the Robb Lake
metallogenic belt contain fluorite, barite, and pyrobitumen and
are poor in sphalerite, galena, and pyrite. This feature suggests
continuity with the southern end of the Liard metallogenic belt
of Southeast Missouri Ba-F deposits, which occur in similar
Devonian host rocks (Nelson and others, 2002). The timing of
mineralization is poorly constrained but is interpreted as preearly
Tertiary (pre-Laramide) on the basis of rotated geopetal
structures (Manns, 1981) or as Devonian or Missippian on the
basis of isotopic studies (Nelson and others, 2002).
The Robb Lake metallogenic belt exhibits characteristics
similar to the Liard metallogenic belt of Southeast Missouri(?)
Ba-F deposits (described above) and to the Gataga Lake metallogenic
belt of SEDEX deposit (described above). Sulphur
isotope analyses exhibit heavy values for barite in all three
metallogenic belts (K.M. Dawson, unpub. data, 1995). These
data and a similar geologic setting suggest a genetic relation
between the two deposit types and three metallogenic belts. A
similar rifting origin is interpreted for all three metallogenic
belts. A major period of Late Devonian and Early Mississippian
rifting along the North American Craton Margin is
interpreted with rifting away of the Yukon-Tanana and Kootenay
continental-margin terranes and formation of the Robb
Lake and coeval metallogenic belts (Nokleberg and others,
1994c, 1997c; Monger and Nokleberg, 1996; Nokleberg and
others, 2000). Nelson and others (2002) propose a somewhat
contrasting interpretation of back-arc and intra-arc spreading,
and exhalative activity for the genesis of the Robb Lake and
related, coeval metallogenic belts. This back-arc and intra-arc
spreading would be coeval with the slightly older Kootenary
arc as interpreted by Nokleberg and others (2000).