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Location: 130 km N from St.George, Utah, United States
5213 Durie RdMississaugaOntario, CanadaL5M 2C6
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There is no known formal industrial mineral ore deposit model for alunite. The characteristics for a model and some exploration criteria are derived from three publications: Hall (1978), Hall and Bauer (1983), and Hofstra (1984). The local alunite deposit has been described, in the above-mentioned publications, as hydrothermal alteration of calc-alkaline volcanic rocks. Alunite mineralization is found on four ridges that occur within the Blawn Mountain Project. Acid sulfate alteration associated with a shallow, possibly laccolithic intrusion altered the silicic-alkalic rhyolite porphyries, flows and tuffs belonging to the Miocene Blawn Formation and the Oligocene Needles Range Group. Alteration tends to be in linear bodies reflecting the role of normal faults in controlling the mineralization. Alteration is zoned away from the point of hydrothermal fluid upwelling. The mineralized ridges are erosional remnants of a once larger altered area. Krahulec (2007) described the appearance of rocks from the silica cap and quartz-alunite zone as follows, “The Silica Cap is a zone of intense silicification believed to be the near-surface manifestation of the hydrothermal channel-ways. The silica is typically buff, dense, and massive but may be quite porous and vuggy locally and resemble a siliceous sinter… On the surface the Quartz-Alunite alteration zones are composed of white to cream to buff to gray to pink, generally fine grained, punky to dense, intermixed alunite and silica with only minor amounts of other impurities, mainly iron… Alunite also occurs locally as coarse (>0.5in.), lathy, typically pink crystals in veins. Kaolinite becomes increasingly important, at the expense of alunite, in the Quartz-Alunite zone near the boundary with the Hematite-Clay zones and also where the QuartzAlunite zones are cut by faults (Walker, 1972). Dickite (a high-temperature member of the kaolinite group) is reported by Whelan (1965) and Thompson (1991) in the Quartz-Alunite zone”. Krahulec gives the following description of the two geometries, “The cone-shaped (narrow end at the base) zones are interpreted as the primary area of strong hydrothermal upwelling . . . and the adjoining flat-bottomed zones are recognized as permeability-controlled areas above the paleoground-water table where steam-heated H2S is oxidized to H2SO4. Only the central portion of Area C (Area 1) at Blawn Mountain is clearly a funnel-shaped zone. The other flat bottomed alunite zones are strongly controlled by the higher porosity and permeability of the host volcanic rocks, while the hydrothermal cones are largely independent of these factors (Hofstra, 1984)”. Krahulec continues this discussion by quoting Hofstra, “…The control of permeability on the degree of alteration intensity is most important near the margins of Quartz-Alunite altered zones. Alteration is pervasive and unaffected by variations in the permeability of the host rocks”. The alteration zones tend to be thicker in cone-shaped areas than in flat-lying areas. It is possible that there were more cone-shaped feeder zones but they were eroded or are buried under valley fill.
