Geological map (1:50,000 scale) of the Ogilvie Mountains Breccias, Coal Creek Inlier, central Yukon (NTS 116B/11, 13, 14) including geological cross sections and mineral occurrences, as well as marginal notes on regional setting and decription and classification of Ogilvie Mountains Breccias.

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In the Ogilvie Mountains, west-central Yukon, the upper Proterozoic to Lower Cambrian Mount Harper Group (informal name) contains strata equivalent to both basal Windermere Supergroup and Lower Cambrian rocks in other areas of the North American Cordillera. Strata of the Mount Harper group are directly time constrained: the basal Windermere equivalents by a ca. 750 Ma U-Pb age from a volcanic complex which both conformably overlies and intertongues with the sedimentary rocks; the disconformably overlying units by the presence of Lower Cambrian trace fossils. The lower Mount Harper Group (LMHG) unconformably overlies a thick succession of dolostones informally named the Fifteenmile Group. A breccia layer is discontinuously preserved on the unconformity surface. This breccia contains silcretes and calcretes that record several episodes of subaerial exposure. Rare interbedded debris flows suggest that the most recent of these episodes was coincident with initial deposition of the LMHG conglomerates. Silcretes and calcretes in this succession suggest that, at the onset of Windermere deposition in this area, a temperate to equatorial, probably semi-arid to arid climate regime prevailed. Elsewhere, basal breccias in Windermere-equivalent strata generally have been interpreted as fault-related, but some contain features compatible with a karstic origin. A synsedimentary normal fault forms the southern margin of an asymmetric, east-trending half- graben basin which contains the lower Mount Harper Group. Basin fill appears to have been derived exclusively from source areas to the south. Proximal facies consist of fault-talus breccia and up to 1100 m of debris-flow conglomerates that were deposited in coalescing alluvial fans. Intermediate and distal facies include mid-to-lower fan conglomerates and sandstones deposited by sheetfloods, distal debris flows and braided channel streamflows. A basin-fill coarsening-upward megasequence in the eastern part of thc study area records a change from lacustrine redbeds to lower alluvial fan sandstones and conglomerates. The LMHG half-graben basin was formed, and sedimentation was controlled, by normal faulting along the southern margin. Synsedimentary faulting and minor back-stepping of the fault controlled development of the internal sequences. Volcanism conformably postdates this sedimentation, but northerly derived coarse clastic rocks of the upper Mount Harper group (UMHG) .

An ~2-km-thick stratigraphic section measured through three consecutive shale-carbonate sequences documents the previously undescribed upper Fifteenmile Group in the Coal Creek inlier. These descriptions provide the basis for correlation with Proterozoic strata of adjacent inliers in eastern Alaska, as well as in the eastern Ogilvie Mountains. The lowest unit contains interbedded limestone and mudstone with distinctive maroon-weathering layers. Similar strata are present in unit D of the Pinguicula Group exposed in the Hart River inlier. In that area however, the middle sequence containing massive dolostone, that is the most prominent unit of the upper Fifteenmile Group, is missing beneath an angular unconformity. The Callison Lake dolostone is above this surface and is lithologically indistinguishable from the uppermost, stromatolitic carbonate of the upper Fifteenmile Group. Both the middle and upper dolostone units are preceded by black shale, indicating abrupt transgressions. In contrast, the carbonate units contain abundant evidence of shallow water and periodic emergence. We interpret the upper Fifteenmile Group to comprise three shallowing-upward cycles in this area.

The study area is underlain by four stratigraphic successions ranging in age from Middle Proterozoic to Early Paleozoic. From oldest to youngest, they are: Middle Proterozoic Wernecke Supergroup; Middle to Upper Proterozoic Pinguicula Group; Upper Proterozoic Windermere Supergroup; and Uppermost Proterozoic to Lower Paleozoic sandstone and carbonate. Together, they represent about a billion years of intermittent sedimentation punctuated by processes such as deformation, uplift, erosion, magmatism and mineralization. Rocks in the study area record eight phases of contractional and extensional deformation, some of which may be related to strike-slip faulting. Two phases of southwest-verging folds and thrust faults may be related to dextral transpression on the Snake River Fault. Mineral enrichments occur in two general forms:: breccia-related (Middle Proterozoic), and veins (Mesozoic to Tertiary). The breccia-related occurrences have enrichments of Cu ± U, Co, Au and Ag, as dissemminations and veinlets in and near intrusive breccia zones (Wernecke breccia). The vein occurrences comprise Zn-Pb-Ag ± Cu and Au, in veins and related lenses and irregular replacements of carbonate.

Volcanic rocks in the Dawson map-area occur as isolated lenses within the early Paleozoic Selwyn Basin assemblage and as sets of flows and complexes within Proterozoic carbonate rocks near the edge of the Mackenzie Platform. Mount Harper complex, an example of the latter group, has been studied in greater detail than the others because it is more completely exposed, and contains two mafic to felsic volcanic cycles. Unlike the sedimentary rocks, whose correlation with established units in the Wernecke Mountains and Alaska is now well advanced, understanding of the stratigraphic position and internal relations of the volcanic piles is less definite. This report, based on two field mapping seasons, has two parts. Characteristics of the two volcanic groups and correlative occurrences are discussed in the first part; the second presents interpretations from the stratigraphy of the Mount Harper complex as an illustration of the style of volcanism in the region.

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