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

In northern Yukon, Canada. numerous breccia zones of early Mesoproterozoic age (ca. 1.6 Ga) are targets for mineral exploration. Collectively termed W ernecke Breccia, they are characterized by disseminated specular hematite a nd local enrichment of Cu, Co, U and An. The breccias are hosted mainly by the Paleoproterozoic Wernecke Supergroup, a 13-km thick basinal to platform al succession of carbonate and fine-grained clastic rocks. Brecciation occu rred after the Wernecke Supergroup was fully lithified, deformed, and local ly metamorphosed. The breccia zones were generated by forceful explosions of volatile-rich fl uids within the crust. The source of the fluids is uncertain, but may be re lated to igneous intrusions at depth. Rapid expansion of the fluids shatter ed large volumes of country rock, mainly sedimentary rocks of the Wernecke Supergroup, and dioritic to syenitic rocks of the Bonnet Plume River intrus ions. In the central parts of the breccia zones, fragments underwent consid erable motion, and in some cases became rounded from abrasion. Venting of b recciated rock and fluid is considered likely, but surface deposits are now here preserved. At one locality, large blocks of country rock foundered int o 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, chl orite, feldspar, mica, and other minerals. In most cases, clasts and wallro cks were hydrothermally altered, leading to metasomatic growth of secondary minerals including flecks of hematite or rhombs of dolomite. Widely dissem inated earthy hematite and local potassic alteration in the breccia clasts resulted in color changes from original drab hues of gray and brown to stri king pink and red. Clasts with embayments rimmed with secondary minerals su ch as specular hematite are evidence for the dissolution of clasts or their diagenetic cements by hydrothermal fluids. The main phase of brecciation a nd metasomatism occurred at ca. 1.6 Ga, as indicated by a 1595 +/- 5 Ma U-P b date on titanite. Subsequent minor hydrothermal events related to emplace ment of the Hart River intrusions and Bear River dykes occurred at 1382.8 /- 7.4 Ma (U-Pb rutile) and less than or equal to ca. 1270 Ma (U-Pb baddele yite), 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 provinc es developed from related systems of hydrothermal activity, and provide add itional evidence for models linking the cratons of North America and Austra lia in Proterozoic time. (C) 2001 Elsevier Science B.V. All rights reserved .