USGS Professional Paper 1697 - Alaska Resources Library

lative metalliferous brines in a reducing environment in which
the deposits formed. Related extensional faulting and emplacement
of Middle Ordovician alkalic basalt dikes, which occur
along the eastern margin of Selwyn Basin and southward to
Gataga Trough, may have served both as a tectonic control on
the development of the sedimentary basins in the Anvil and
Howards Pass areas and as a source of both heat and metalliferous
brines.
The mineral assemblages, host rock age, and geologic
setting for the Anvil metallogenic belt are similar to
those for the Howards Pass and Kootenay metallogenic
belts of the Canadian Cordillera (described below). All
three metallogenic belts are interpreted as forming from
Pb- and Zn-rich fluids resulting during rifting, volcanism,
basinal subsidence, local marine transgression, and related
hydrothermal activity along the passive continental margin
of the North American Craton. Rifting is interpreted to
have formed the Misty Creek Embayment in the Early to
Middle Cambrian (Cecile, 1982), the Selwyn Basin in the
Late Proterozoic to Ordovician (Gabrielse, 1963), and the
Meilleur River Embayment in the Early to Middle Ordovician
(Morrow, 1984), with the latter event marked by alkalic
basaltic volcanism (Fritz and others, 1991). SEDEX occurrences
also formed during these events mainly in the Anvil
and Howards Pass metallogenic belts, and to a minor extent
in the Misty Creek and Meilleur River embayments.
Howards Pass Metallogenic Belt of Sedimentary
Exhalative Zn-Pb Deposits (Belt HP), Eastern
Yukon Territory
The Howards Pass metallogenic belt of sedimentary
exhalative (SEDEX) Zn-Pb deposits (fig. 3; tables 3, 4) occurs
in the eastern Yukon Territory. The belt is hosted in the Selwyn
Basin that constitutes part of a Cambrian to Devonian passive
margin of the North American Craton Margin. The major
deposits are at Howards Pass and Anniv (table 4) (Nokleberg
and others 1997a,b, 1998).
Howards Pass (XY) Zn- Pb SEDEX Deposit
The Howards Pass (XY) sedimentary exhalative
(SEDEX) Zn- Pb deposit (fig. 13) consists of fine-grained,
well-bedded sphalerite and galena with pyrite as stratiform
and stratabound massive bodies, as much as 50 meters thick
and 3 to 4 km long, which are interlaminated with carbonaceous,
cyclical, limy mudstone and chert of the rift-related
Early Silurian active zone of the Ordovician to Devonian
Road River Group (Yukon Minfile, 1984; Abbott and others,
1986a,b; Abbott and others, 1994; MacIntyre, 1991). The
deposit is one of three related Zn-Pb SEDEX deposits, which
occur in an elongate, 20 km-wide subbasin in the eastern Selwyn
Basin. The deposits are interpreted as forming at the base
of the continental slope, about 10 km to 20 km seaward of a
carbonate platform margin. Total reserves and resources are
Cambrian through Silurian Metallogenic Belts (570 to 408 Ma) 33
estimated at about 500 million tonnes grading 5 percent Zn,
and 2 percent Pb (Placer Developments Ltd. Annual Report,
June 1982).
Anniv (OP) SEDEX Zn- Pb Deposit
The Anniv (OP) SEDEX Zn- Pb deposit consists of
sphalerite and galena, which occur in saucer-shaped stratiform
and stratabound bodies in Early Silurian cyclic, rift-related
carbonaceous mudstone and chert of the Ordovician to Silurian
Road River Group (Morganti, 1981; Yukon Minfile, 1984;
EMR Canada, 1989; MacIntyre, 1991). The host rocks interpreted
as part of a Cambrian and Devonian passive margin of
the North American Craton Margin. The deposit averages 13
m thick (maximum 45 m) over a 1.5 km strike length.
Origin of and Tectonic Setting for Howards Pass
Metallogenic Belt
The Zn-Pb SEDEX deposits of the Howards Pass
metallogenic belt are hosted Selwyn Basin, which contains a
sequence of late Precambrian to Middle Devonian, turbiditic
sandstone, deep-water limestone, shale, and chert. The basin
is the offshore equivalent of the shallow-water carbonate and
sandstone of the Mackenzie Platform (Gordey and Anderson,
1993). A major SEDEX event was localized in eastern
Selwyn Basin at Howards Pass, where large stratiform bodies
Limestone
Chert
Chert and sulphides
up to 50% Zn + Pb
Laminated cherty mudstone,
usually contains Zn + Pb,
locally up to 15% Zn + Pb
Calcareous mudstone
and mudstone,
locally up to 10% Zn + Pb
Laminated limestone
Figure 13. Howards Pass sedimentary exhalative Zn- Pb deposit,
Howards Pass metallogenic belt, Canadian Cordilliera. Schematic
stratigraphic column illustrating major stratiform sulfide zones.
Adapted from Morganti (1981). See figure 3 and table 4 for location.