USGS Professional Paper 1697 - Alaska Resources Library

Windermere Creek (Western Gypsum) Chemical-
Sedimentary Gypsum Deposit
The Windermere Creek (Western Gypsum) chemicalsedimentary
gypsum deposit consists of gypsum and anhydrite
that underlie basal carbonate strata of the Devonian Burnais
Formation (British Columbia Department of Mines, and
Petroleum Resources, 1991; MINFILE, 2002). The deposit
and related occurrences form a belt that extends 80 km from
Windermere Creek southeastward to Kootenay River and
Lussier River. Estimated reserves range from 7 to 12 million
tonnes of ore grading 90 percent gypsum. About 6.8 million
tonnes of ore has been produced at four open-pit operations. A
synsedimentary evaporite origin is interpreted for the deposits
and for gypsum in concordant beds in dolostone of the Late
Triassic Whitehorse Formation at Forgetmenot Pass.
Marysville and Mount Brussilof (Baymag) Chemical-
Sedimentary Magnesite Deposits
These chemical-sedimentary magnesite deposits consist
of conformable, interbedded magnesite that is hosted within
quartzites of the upper 100 m of the Early Cambrian Cranbrook
Formation (Grant, 1987; Simandl and Hancock, 1999;
MINFILE, 2002). The thickest beds are as much as approximately
15 m thick and are exposed over a strike length of
5.5 km. The average grade is 40 to 45 percent MgO. Chemical-sedimentary
magnesite forms extensive replacements of
carbonates of the Middle Cambrian Cathedral Formation at the
Mount Brussilof (Baymag) deposit (Grant, 1987; Simandl and
Hancock, 1991; MINFILE, 2002). Estimated reserves are 40.7
million tonnes grading 92.4 percent to 95 percent MgO.
Parson and Brisco Barite Vein and Gypsum Deposits
The Parson barite vein deposit consists of barite and
lesser gypsum in vein and breccia fillings in Early Cambrian
quartzite that is underlain by dolostone and shale (Leitch,
1991; MINFILE, 2002). The mine at Parson produced 75,000
tonnes of barite at unspecified grade from two parallel veins
between 1957 and 1988. The Brisco vein and breccia deposit
occurs in a breccia zone in Ordovician dolostone (Reesor,
1973; MINFILE, 2002). Between 1947 and 1973 the mine at
the deposit produced 140,000 tonnes grading 98 percent barite.
In both cases, early Paleozoic replacement is interpreted.
Origin of and Tectonic Setting for Southern Rocky
Mountains Metallogenic Belt
The Southern Rocky Mountains metallogenic belt contains a
diverse age group of large, stratabound and stratiform deposits of
gypsum-anhydrite, barite, and magnesite. From oldest to youngest,
the ages and modes of formations of the significant deposits are
(1) early Paleozoic replacement for formation of Parson Ba vein
deposit, (2) Cambrian synsedimentary deposition of stratiform
Marysville chemical-sedimentary magnesite deposit, (3) Middle
Cambrian replacement for formation of Mount Brussilof chemical-sedimentary
magnesite deposit, (4) Ordovician replacement for
Mississippian Metallogenic Belts (360 to 320 Ma; figs. 16, 17) 71
formation of Brisco Ba vein deposit, and (5) Devonian synsedimentary
deposition of stratiform Windermere Creek chemicalsedimentary
gypsum deposit. A few deposits are also interpreted
as forming in either the late Paleozoic or Triassic (Nokleberg and
others, 1997a,b). From this short list, at least two major origins
exist, either stratiform evaporate-related deposits, or replacement
vein deposits that formed over a long geologic history. With
further study, the Southern Rocky Mountains metallogenic belt
may be divided into several metallogenic belts that formed during
several tectonic events that affected the passive continental margin
sedimentary rocks of the North American Craton Margin.
Mississippian Metallogenic Belts (360
to 320 Ma; figs. 16, 17)
Overview
The Mississippian metallogenic belts in the Russian Far
East, Alaska, and the Canadian Cordillera are summarized in
table 3 and portrayed on figures 16 and 17. The major belt was
the Northwestern Brooks Range (NBR) belt of SEDEX Zn-Pb
and bedded barite deposits that is hosted in the Arctic Alaska
superterrane. This belt is interpreted as forming during Mississippian-Pennsylvanian
back-arc spreading along North American
Craton Margin. Continuing on from the Middle and Late
Devonian were the Berezovka River (BE), Selennyakh River
(SEL), Sette-Daban (SD), Urultun and Sudar Rivers (URS),
Kedon (KE), Yarkhodon (YR), Northern Cordillera (NCO),
Macmillan Pass (MP), Finlayson Lake (FL), and Gataga (GA)
metallogenic belts. In the below descriptions of metallogenic
belts, a few the noteable or significant lode deposits (table 4)
are described for each belt.
Metallogenic-Tectonic Model for Mississippian
(360 to 320 Ma; figure 29)
During the Mississippian (360 to 320 Ma), the major
metallogenic-tectonic events were (table 3) (1) separation
of North Asian and North American Cratons and Cratons
Margins along a series of oblique-sinistral rifts, (2) ending of
rifting of fragments from cratons and their margins and formation
of associated metallogenic belts, and (3) continuation of
the Sicker arc and associated subduction in the Wrangellia
superterrane. Sedimentation continued along the North Asian
and North American Craton Margins.
Specific Events for Mississippian
(1) From the late Devonian and to the Early Mississippian,
rifting occurred along the eastern margin of the North
Asian Craton Margin (NSV, KN). This event formed the
Kotelnyi (KT), Omulevka (OV), Prikolyma (PR), Nixon Fork-