Dolores Creek, Yukon; a disseminated copper mineralization in sodic metasomatites

Abstract

The Dolores Creek mining area situated in the Proterozoic core of the Wernecke Mountains (northeastern Cordillera, Yukon) contains various types of copper, cobalt, and uranium mineralization in altered metasediments and diorite dikes.The 4,000 to 5,000-m-thick succession of Proterozoic rocks consists of two major sequences separated by an angular unconformity. These sequences crop out in a complex northeast-trending, northeast-plunging anticline.The lower part of the older, lower to middle Proterozoic sequence underlies most of the map area and consists of a thick, monotonous pile of marine siltites intergraded with argillaceous quartzites, silty argillites, and impure carbonates. These grade upward into argillites, impure arenites, and at the top into thick bedded, spectacularly orange-weathering dolomites. The metamorphic grade reaches the lower greenschist facies near the base and diminishes upward. Late Proterozoic (?) diorite or gabbro dikes are common in places.The younger, upper Proterozoic sequence occurs as erosional remnants in the immediate vicinity of the Dolores Creek area. Basal purple and green shales are its characteristic lithology.A conspicuous feature of the Dolores Creek area and of the Proterozoic terrane elsewhere in the Wernecke and Ogilvie Mountains is the widespread presence of altered breccias and pseudoconglomerates in Proterozoic metasediments. The breccias are both semiconformable and discordant, gradational to sharply outlined, and occur as elongate masses and lenses along faults or on strike continuations of diorite dikes.Most breccias are hydrothermally altered. In the Dolores Creek alteration-mineralization center, a roughly elliptical area 6 X 4 km has been altered into a central core of albite-(chlorite) metasomatites of syenitic appearance grading outward into albite-chlorite metasomatites with relict breccia fabrics; to fine albitized metasiltites and metacherts; to (albite)-paragonite + or - sericite, carbonate metasomatites; to quartz-sericite rocks; and finally spotted phyllites.Widespread, but so far uneconomic, mineralization within and around the Dolores Creek alteration-mineralization center is dominated by chalcopyrite which occurs with specularite as pervasive disseminations and fracture fillings in sodic metasomatites associated with breccias (Porphyry prospect), hair-thin bedded replacements in altered metasediments adjacent to diorite dikes, veins, and replacement masses in carbonate rocks. Cobaltite occurs in short discontinuous veins, and a pitchblende occurrence along fractures in altered breccias has been explored recently.The Porphyry occurrence resembles in some respects porphyry copper deposits, but the alteration zonality is reversed: the core of Na-feldspathization grades outward into Na-phyllic, K-phyllic, propylitic, and carbonatization zones. The geochemistry of mineralization at Dolores Creek shows many parallels with mineralized spilites, particularly with the homomineralic spilites described by Amstutz. The diorite dikes and their parent magmatic body are considered to have been the source of most metals and of the mineralizers.