Characterization of the nonsulfide zinc prospects in the Reef Ridge-type (Alaska): a possible new genetic model?

The Reef Ridge-type (RR) prospects, located in the Doyon Land, McGrath District, in the Yukon-Koyukuk region of west central Alaska (USA) represent supergene “nonsulfide” zinc concentrations. From a geological point of view the RR prospects belong to the Farewell Terrane (FT), which was long regarded as a piece of the Paleozoic passive margin of Western Canada. More recent studies suggest that FT was a micro-continent between the Siberian and Laurentian (North American) cratons during the early Paleozoic [2]. The RR area occurs in the Nixon Fork subterrane (NFST) [3], whose stratigraphy mainly consists of 3 different Units separated by unconformities: 1) Precambrian metamorphic rocks; 2) Lower Paleozoic (Early Ordovician-Late Devonian) platform carbonate rocks; 3) Upper Paleozoic and Mesozoic clastic rocks. The RR deposit is hosted by Lower-Middle Devonian shallow water dolomitized limestone. The host rock is strongly brecciated and the mineralization occurs in the breccia cement. It mainly consists of nonsulfide zinc ores with remnant sulfides (sphalerite, with minor amounts of pyrite and/or marcasite, as well as traces of galena). The whole area has been intensively oxidized, possibly in Late Tertiary, with both goethite and hematite present in surface gossans and in core specimens. Although the occurrence of Zn sulfides has been noted in two minor intercepts at the Reef Ridge prospect, the most economic Zn phase is the carbonate smithsonite. Core and outcrop samples were examined using XRD, SEM-EDS and QEMSCAN®. Preliminary analytical results indicate that most smithsonite has replaced sphalerite, and precipitated as cement in the macro-microporosity of the carbonates, as well as in veinlets of various sizes. The highest enrichment, however, is associated with irregular replacement fronts in the dolomite host rock. Smithsonite is usually zoned and contains about 1% CaO and MgO. FeO can occur in the smithsonite lattice as well. Small amounts of zinc have been detected in the Fe-(hydr)oxides, both in core and surface samples (mainly in the gossanous specimens), and additional zinc traces have been identified in clay minerals (illite/muscovite and kaolinite). Both core and surface samples where smithsonite prevails may contain from 10% up to 48% zinc. Primary ores at Reef Ridge have been interpreted to belong to the MVT class of deposits, possibly related to the Late Paleozoic orogeny [3] [4]. However, the results of a recent soil sampling campaign have identified some important Cu, As, Sb, and Mo anomalies around the RR area: this is a geochemical association not common for typical Mississippi Valley-type deposits. The RR mineral belt is surrounded by several Cu-Au skarn ores associated with Late Cretaceous magmatites (i.e. Nixon Fork mine), and the anomalies detected in the area could be rather associated with a distal polymetallic mineralization around deeply set magmatic intrusions. The Reef Ridge area is still an active exploration project and new field activity is planned in summer 2013. The above-mentioned new genetic concept will be tested and, if proven sound, may bring a change to the exploration strategy.