The Reef Ridge supergene nionsulphide zinc mineralization, Alaska

The Reef Ridge (RR) deposit is a typical supergene "Nonsulphide" zinc prospect, located in the McGrath area in the Yukon-Koyukuk region of west central Alaska (USA). The area has been intensively oxidized, with both goethite and hematite occurring in surface gossans and in core specimens. The potentially economic mineralization is in the oxidation zone; it consists mainly of Zn carbonates mixed with Fe-(hydr)oxides. The deposit was discovered in 1975. Since then, several exploration activities (Doyon Ltd. and other companies such as Patino Inc. and Pasminco Exploration Ltd.) have been carried out around Reef Ridge; this led to the discovery of a larger area with similar showings of ore similar to Reef Ridge (i.e. Saddle, Midway, Beaver Pass, Soda Creek etc.) [1]. The RR deposit has been the only one further investigated for its high Zn grade 460 Mt at 17.4% Zn. RR is located in sedimentary rocks of the Farewell Terrane (FT), which represents, according to recent studies, a micro-continent between the Siberian and Laurentian (North American) cratons [2]. The mineralization is hosted in Lower-Middle Devonian shallow water brecciated dolomite of a Paleozoic carbonate platform sequence belonging to the Nixon Fork subterrane (NFST) [3]. The mineralization consists of oxidized minerals (mainly Fe-hydroxides and zinc-nonsulphides), associated with minor remnant sulphides. Locally, the cores of the breccias are cemented by sphalerite, with minor amounts of pyrite and/or marcasite concretions. The zinc sulphides, hosted in silicified lithotypes and detected in two minor drill intercepts, are not considered economic. A complete petrographic and mineralogical characterization of both surface specimens and core samples has been conducted in collaboration with Doyon Ltd. The samples were analyzed by XRD, chemical analysis (ICP-ES and ICP-MS), and SEM-EDS. Preliminary investigation with QEMSCAN has also been carried out at the Camborne School of Mines. The 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 replaces sphalerite and dolomite precipitates as cement in the macro- and microporosity of the carbonates and occurs as veinlets of various sizes. Small amounts of zinc have also been identified in Fe-(hydr)oxides, which are abundant both in drill core and surface samples (especially in the gossanous specimens). Zinc traces have been identified in clay minerals (illite/muscovite and kaolinite). In order to detect the nature of the fluids that deposited the nonsulphide ores, we have carried out stable isotope analyses on smithsonite and carbonate host rocks. The data suggest that C originated from the Devonian carbonates, while the O precipitated from local meteoric waters at very low temperatures. To further enlarge the prospecting area, new geochemical data (soil sampling) were collected by Doyon during the summer 2012 around three magnetic anomalies SE of RR. The survey detected anomalous concentrations of Cu and As in the soil samples, that could suggest a link to a polymetallic, skarn-related mineralization similar to the Au-Cu Nixon Fork deposit.