ALLOCHTHONOUS ROCKS OF THE COSMOS HILLS, WESTERN BROOKS RANGE, ALASKA AND THEIR CONSTRAINTS ON STRUCTURAL, SEDIMENTOLOGICAL, AND ORE DEPOSITIONAL HISTORIES

1:5000 mapping in Bornite district, Cosmos Hills, W Brooks Range, AK, found critical unreported rock relations: (1) Rock units, incl. Anirak schist, Bornite carbonate sequence, and Beaver Creek phyllite (A-B-B), are allochthonous, bound by zones of mylonite with top-S(±20°) stretching lineations that require retrodeforming rocks northward ~10s of km. Mid-Silurian or older depositional substrate of Bornite seq is unknown but was not Anirak. (2) Bornite seq stratigraphic-up proceeds SSW to NNE, from siliciclastic to carbonate (grades up from dolomite to calcite) with assoc’d graphitic organic C: a) mylonitic siliciclastic siltst, 25m thick; b) mylonitic phyllite-clast dolowackest interlayered with laminated dolomudst (local debrite), 250m thick; phyllite clasts from proto-Brookian highland with older metamorphic history; c) fossilif dolowackest with interlayered carbonaceous calcitic mudst (likely debrites), 250m thick; d) carbonaceous calcitic mudst (phyllite) with interlayered micritic limest (mbl) and dolo-clast lime wackest, 200m thick. Units ‘b’ and ‘c’ contain Cu. A 100m-thick zone of calcmylonite with dolost boudins and greenst sills on Aurora Mtn may underlie ‘a.’ Total thickness of Bornite ~800m. North younging of Bornite on Anirak indicates early faulting akin to Jr detachment faults in Err nappe, Switzerland. (3) Anirak has upper "member" of mbl with quartzite bands and irreg white qz veins (AQM); overlain by metapelites and 2 greenst sills, 1 thick (10-30m), 1 thin (3-5m); “AQM-thick-thin” shows hundreds of m of missing Anirak section. Largest omission is where Bornite Cu deposit is faulted against middle Anirak. (4) 3 thrust sheets repeat the A-B-B tectonostratigraphy on Cosmos, Inerevuk Mtns. One add’l thrust puts Bornite on Beaver Creek. Thrust faults contain slivers of Anirak; therefore thrusts postdate earlier LANF(?) of Bornite on Anirak. (5) "P7" phyllite in Bornite deposit is 50m-thick mylonite that contains albite lime phyllite, a marker unit in W Cosmos Hills. P7 separates upper (#1 Orebody) from lower (Lower Reef) ore zones and is here interpreted to be a top-S thrust that places a northerly dolost onto a southerly dolost. Hitzman (1986) attributes changes in paragenesis from Lower Reef to #1 Orebody to thick vertical plumbing; here, changes in paragenesis are attributed to thin horizontal plumbing, stacked and reworked during Jr-K orogeny.