The Nonsulphide zinc mineralization of Reef Ridge, Alaska

The Reef Ridge (RR) prospect is located in the McGrath District, Yukon-Koyukuk region of west central Alaska, USA. It represents a “Nonsulphide” zinc deposit, currently under exploration by Doyon, Ltd. Reef Ridge belongs to the Farewell Terrane (FT), long regarded as a piece of the Paleozoic passive margin of Western Canada [1]. More recent studies suggest RR was a micro-continent between the Siberian and Laurentian (North American) cratons during the early Paleozoic [2]. The RR area occurs in one of the FT subterranes, known as the Nixon Fork subterrane (NFST) [3]. The NFST represents a sequence consisting of 3 different stratigraphic units separated by unconformities: 1) Precambrian metamorphic rocks; 2) Lower Paleozoic (Early Ordovician-Late Devonian) platform carbonate rocks; and 3) Upper Paleozoic and Mesozoic clastic rocks. The Reef Ridge nonsulphide zinc deposit is hosted in the Lower Paleozoic (unit 2), which mainly consists of Lower-Middle Devonian highly brecciated shallow water dolomites. The most significant style of Reef Ridge mineralization consists of nonsulphide zinc ores with minor remnant sulphides. Breccia cement contains zones of patchy sphalerite, with minor amounts of pyrite and/or marcasite, as well as traces of galena. The area has been intensively oxidized, with both goethite and hematite present in surface gossans and in core specimens. Highly brecciated dolomites containing goethite and hematite are characteristic of core and surface samples containing 10% to a high of 48% zinc as primarily smithsonite. A second minor sulphide style of zinc mineralization was noted in two minor drill intercepts. Sulphide zinc mineralization consists of a moderately to strongly silicified lithotype comprised of a sphalerite rich breccia matrix and clasts of strongly silicified dolomite with variable sphalerite mineralization. Drill intercepts containing sphalerite mineralization yielded zinc assays of 8% to 24% zinc. The Reef Ridge area is still an active exploration project. New chemical data (soil sampling) have been collected during the summer 2012 and a new field activity will be planned in summer 2013. A complete petrographic and mineralogical characterization of both core and surface samples from the Reef Ridge prospect has been carried out in collaboration with the Naples University. Samples were examined using a range of techniques: XRD, chemical analysis, SEM-EDS. Preliminary results indicate that most of the assayed zinc in the nonsulphide ore occurs as smithsonite (with up to 75% weight in a few samples). Smithsonite replaced sphalerite, and precipitated as cement in the micro-macroporosity of the carbonates, as well as in veinlets of various sizes. The highest enrichment, however, is associated with replacement fronts in the dolomite host rock. The unmineralized dolomite contains only traces of iron and rare zinc. However, dolomite fragments in the breccias appear non-mineralized in hand samples yet contain diffuse smithsonite mineralization (as micro crystals <1 µm) in thin section. Small amounts of zinc also occur in Fe-(hydr)oxides, which are present both in core and surface samples (especially in the gossanous specimens). Additionally, zinc traces have been identified in clay minerals (illite/muscovite and kaolinite).