Alunite in the Marysvale volcanic field forms two (three are described) different types of deposits which contrast in appearance and conditions of origin: 1) Replacement deposits are generally fine-grained and formed by near-surface replacement of intermediate-composition volcanic rocks. The deposits form a bead necklace around a monzonite stock. Each deposit is zoned horizontally from alunitic cores to kaolinitic and propylitic envelopes and zoned vertically from pyrite/propylite upward through alunite/jarosite/hematite to a silica cap. Alunite does not extend below 100 m. Sulphur isotope ratios agree with derivation from underlying Mesozoic evaporites. 2) Natroalunite of 14-m.y. age crosscuts replacement-type alunite deposits. Its S-isotope ratios are comparable with those of pyrite in the volcanics. The Na may be from underlying Mesozoic halites. 3) Veins of coarse-grained alunite of 14-m.y. age filled extension fractures above a postulated stock. S-isotope ratios indicate a probable magmatic source. The contrasting properties of the Marysvale alunite deposits preclude any simple relation to ore deposits, but serve to refine interpretations based on other geological considerations. The replacement deposits are a logical near-surface result of skarn forming processes at depth around the monzonite stock. The vein- type deposits are a logical near-surface result of porphyry metallization in an underlying stock. -G.J.N.
Additional Publication Details
Origins and exploration significance of replacement and vein-type alunite deposits in the Marysvale volcanic field, west central Utah.