Geology of Carmacks and Laberge map areas, central Yukon: Incomplete draft manuscript on stratigraphy, structure and its early interpretation (ca. 1986)

Laberge and Carmacks map areas in south central Yukon have three structurally superposed elements. From the base an autocht hon , a ducti lely deformed complex and a sheet cut by strike- slips. The autochthon on the northeast and southwest of the area differs. That on the northeast, Cassiar Platform, includes ancestral North American rocks which are repeated by northeast directed thrusts. That on the southwest, Yukon Crystalline Terrane, includes probable Paleozoic schist and gneiss that are metamorphosed and folded. The ductilely deformed complex in the middle of the structural succession is called Yukon Cataclastic Terrane. It comprises Paleozoic(?) sedimentary, volcanic and granitic rocks partly sheared to mylonite as well as large Mesozoic batholiths and Late Paleozoic basalt. And the strike- slip sheet, Whitehorse Trough, is of Mesozoic volcanic and sedimentary rocks broken by dextral faults. Boundaries between the elements are faults; steep-dipping dextral strike-slips at the top and ductile faults lower down. Cassiar Platform contains late Proterozoic and Paleozoic miogeoclinal strata of North America's ancient margin, repeated on thrusts, folded and metamorphosed during the Jura- Cretaceous. Yukon Crystalline Terrane is underlain by Early Paleozoic? biotite muscovite quartz feldspar gneiss and mica schist with interfoliated amphibolite, marble and serpentinite metamorphosed and de formed during the Late Paleozoic. The rocks, perhaps metamorphic equivalents of coarse schistosity oriented elements those in Cassiar discordantly to Platform, have a that of adjoining elements. The second element, Yukon Cataclastic Terrane, has a ductilely deformed base and a less strained top, each discontinuous, several kilometres thick and separated by faults. The base has a structural succession of three mylonitic units: Paleozoic? immature elastic rocks called Nisutlin Assemblage, Late Paleozoic ophiolite called the Anvil Assemblage and Paleozoic and Mesozoic granite, the Simpson Assemblage. The top includes weakly metamorphosed ophiolite, Atlin Terrane, and a newly named Pennsylvanian basalt called the Semenof Formation. As well it has large, semiconcordant granodiorites, the Tatchun, Granite, Carmacks and Aishihik batholiths. The penetrative flaser of the ductilely deformed rocks dips under Whitehorse Trough from all sides; northeast of the Trough it dips southwest, on the southwest it dips northeast. Metamorphic micas in mylonite last cooled through the argon retention isotherm in the Jurassic indicating ductile strain during the Jurassic. Model K/Ar ages range from 200 to 160 Ma. Radiometric ages from the granodiorite batholiths in upper Yukon Cataclastic Terrane define two intrusive events, one Permian the other Early Jurassic. The older is dated by a K/Ar date of 268 Ma on Aishihik Batholith and by a discordant U/Pb age of 276 Ma on zircon from Selwyn Gneiss, both just outside the project area. The younger is given by a single concordant U/Pb age of 192 Ma on zircon from the Minto Pluton. Most K/Ar ages on the granites range between 165 and 140 Ma. They are thermally reset through tectonic uplift during the ductile s train and dextral strike-slip. Whitehorse Trough, the strike-slip sheet, includes two conformable stratigraphic units: the Upper Triassic Lewes River Group and the Lower Jurassic Laberge Group. They are anandesitic basalt with limestone and an immature volcanic and elastic unit interpreted as volcanic arc bas in deposits. The Takla, Hazelton and Bowser groups of northern British Columbia are time and lithologic equivalents with parallels to Whitehorse Trough strata. Whitehorse Trough strata are not internally strained or metamorphosed. The Lewes River Group is redefined and formally subdivided into a lower basaltic andesite called the Povoas Formation and an upper unit, the Aksala Formation with carbonate and clastic members. The rocks are Carnian and Norian. The Laberge Group is similarly formalized. It has four formations. The lower unit, a few hundred metres thick, of marine shale and slate, is the Hettangian and Sinemurian Richthofen Formation. The middle division has two, laterally equivalent, Pliensbachian units, the Nordenskiold Dacite and Conglomerate Formation, which are 1 or 2 km thick. The upper unit is arkose, the Tanglefoot Formation; it is Toarcian to Bajocian. Nordenskiold Dacite has extensive, thick ash flows, some subaerial, others submarine. The Conglomerate Formation is a set of debris flows of locally derived, detrital alluvium. Several young stratigraphic units overlap boundaries between elements or lie across more than one. The oldest is a JuraCretaceous chert pebble conglomerate, the Tan talus Formation, which fills ex tension basins along the dextral faults around Whitehorse Trough. Two suites of Cretaceous volcanic rocks over lap Yukon Crystalline Terrane and Whitehorse Trough strata. The mid-Cretaceous Mount Nansen Group has areally restricted, dacitic subvolcanic plugs, pipes and dykes. The Late Cretaceous Carmacks Group has a lower thick andesite and an upper flood basalt. The Pliocene Walsh Creek beds and the Pleistocene and recent Selkirk volcanics are local overlap units. Yukon Cataclastic Terrane is interpreted as the detachment zone between the autochthon and the Mesozoic volcanic arc strata. During the Jurassic Whitehorse Trough slipped northwest over the detachment while the autochthon slipped southwest under it. Whitehorse Trough is bounded by dextral faults; on the northeast is the Teslin, Mason, Boswell, Hootalinqua, Semenof set and on the southwest are the Carmacks, Hoochekoo, Ingersoll and Selkirk faults. Internally the Trough is also cut by connected dextral faults. The strike-slips are part of the Teslin Fault system, which may have 1000 km of dextral displacement. In Laberge map area the Teslin Fault's displacement is largely transfer red to two branches, a northeastern one, which follows the edge of White horse Trough, marked by extension basins , and a southwestern one with compressional structures. The northeast branch , Semenof system, incorporates the Mason, Boswell and Hootalinqua faults, which together define two releasing double bends. The southwestern branch, the Chain system follows the Chain, Braeburn and Big Creek faults . Displacement is transferred through the Open and Fairvi ew faults, which are interpreted as restraining bends in t he strike-slip system. Trough Strike-slips along the southwest side of Whitehorse include the Selkirk, Hoochekoo and Carmacks faults, interpreted as dextral faults with Tan talus filled basins at releasing double bends. Other structures in Whitehorse Trough, mainly tight synclines and tighter anticlines are part of the transcurrent slip. The faults and folds are interpreted as products of simple shear, but their orientations are not as expected from such deformation suggesting that stress orientation varied with time. The faults trend northwest and dip steeply, but cannot be traced into the surrounding ductilely deformed rocks. In crosssection they presumably end downward in, or on, the detachment zone and the dextral displacement is gathered along the contact between the top and bottom of the detachment, which must then be a gently dipping strike-slip. Dextral slip was underway by the early Late Jurassic when 164 Ma porphyry dykes were intruded; it continued through 140 Ma when granites in the top of the detachment zone were raised. It had ceased when 118 Ma Mount Nansen volcanics were laid across some faults. Several faults are probably Late Cretaceous "overlap" structures related to the Mount Nansen Group. The Big Creek Fault, the largest of them, apparent ly dropped a block of t he Big Creek Syenite from the detachment zone into the autochthon. Four classes of mineral showings are recognized. Two are hosted by, and genetically related to, the Mount Nansen Group the mid- Cretaceous volcanics that "over lap" Whitehorse Trough's southwest edge. They are gold- silver veins such as those on Mount Nansen and porphyry copper-molybdenite deposits such as those on upper Big Creek. The Mount Nansen Group accounts for the largest number and most interesting showings of the region. The third showing type, coal occurrences, are part of a second "over lap" unit, the Tantalus Formation of the extension basins along copper the dextral strike-slips. Only one type, deposits in schlieren in granodiorite metamorphosed the Minto and Williams Creek occurrences, is fundamentally related to the older rocks. They occur in the top of the detachment zone and were deformed and metamorphosed when these rocks were sheared. Mineral exploration possibilities connected with the Mount Nansen Group are not exhausted. In particular the postulated genetic connection between the Mount Nansen Group and the Big Creek Fault warrants prospecting for unrecognized mineralization. Nordenskiold Dacite is the time equivalent of the Toodoggone volcanics, which hosts epithermal gold showings, and is worth considering in this light. Where the Tantalus-filled extension basins bottom on the detachment zone are possible stratigraphic traps for epithermal gold occurrences which have not been looked for. Whitehorse Trough also warrants study for its hydrocarbon possibilities. The strata fall squarely in the "oil window" and they include potential hydrocarbon source and trap rocks.

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