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Early Mesoproterozoic intrusive breccias in Yukon, Canada: the role of hydrothermal systems in reconstructions of North America and Australia

Thorkelson, DJ, Mortensen, JK, Davidson, GJ, Creaser, RA, Perez, WA and Abbott, JG 2001 , 'Early Mesoproterozoic intrusive breccias in Yukon, Canada: the role of hydrothermal systems in reconstructions of North America and Australia' , Precambrian Research, vol. 111, no. 1-4 , pp. 31-55 , doi:

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In northern Yukon, Canada, numerous breccia zones of early Mesoproterozoic age (ca. 1.6 Ga) are targets for
mineral exploration. Collectively termed Wernecke Breccia, they are characterized by disseminated specular hematite
and local enrichment of Cu, Co, U and Au. The breccias are hosted mainly by the Paleoproterozoic Wernecke
Supergroup, a 13-km thick basinal to platformal succession of carbonate and fine-grained clastic rocks. Brecciation
occurred after the Wernecke Supergroup was fully lithified, deformed, and locally metamorphosed.
The breccia zones were generated by forceful explosions of volatile-rich fluids within the crust. The source of the
fluids is uncertain, but may be related to igneous intrusions at depth. Rapid expansion of the fluids shattered large volumes of country rock, mainly sedimentary rocks of the Wernecke Supergroup, and dioritic to syenitic rocks of the Bonnet Plume River intrusions. In the central parts of the breccia zones, fragments underwent considerable motion, and in some cases became rounded from abrasion. Venting of brecciated rock and fluid is considered likely, but surface deposits are nowhere preserved. At one locality, large blocks of country rock foundered into open space near the top of a breccia zone, forming a fallback megabreccia. Faulting may have been active concurrently with brecciation.
Breccia fragments are cemented together by hematite, quartz, carbonate, chlorite, feldspar, mica, and other
minerals. In most cases, clasts and wallrocks were hydrothermally altered, leading to metasomatic growth of
secondary minerals including flecks of hematite or rhombs of dolomite. Widely disseminated earthy hematite and
local potassic alteration in the breccia clasts resulted in color changes from original drab hues of gray and brown to
striking pink and red. Clasts with embayments rimmed with secondary minerals such as specular hematite are
evidence for the dissolution of clasts or their diagenetic cements by hydrothermal fluids. The main phase of
brecciation and metasomatism occurred at ca. 1.6 Ga, as indicated by a 15955 Ma U-Pb date on titanite.
Subsequent minor hydrothermal events related to emplacement of the Hart River intrusions and Bear River dykes
occurred at 1382.87.4 Ma (U-Pb rutile) and ca. 1270 Ma (U-Pb baddeleyite), respectively.
Mineralized breccias at and near the Olympic Dam deposit in South Australia mineralogically and texturally
resemble, and have nearly the same age as, the Wernecke Breccias. These similarities suggest that both breccia
provinces developed from related systems of hydrothermal activity, and provide additional evidence for models
linking the cratons of North America and Australia in Proterozoic time.

Item Type: Article
Authors/Creators:Thorkelson, DJ and Mortensen, JK and Davidson, GJ and Creaser, RA and Perez, WA and Abbott, JG
Keywords: Proterozoic; Yukon; Rodinia; Breccia; Geochronology; Continental reconstructions
Journal or Publication Title: Precambrian Research
ISSN: 0301-9268
DOI / ID Number:
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