Abstract

All of the many known alunite deposits in the Marysvale region of Utah are confined to the Tertiary volcanics and more specifically to the Bullion Canyon Volcanics of early Tertiary age. The alunite in these rocks occurs both in veins and in irregular replacement bodies. The White Horse deposit, about 4 1/2 miles northeast of Marysvale, is a replacement deposit in a calcic quartz latite of the upper part of the Bullion Canyon Volcanics. This latite flow is divided into an upper vesicular and brecciated layer and a lower more massive layer which has a clearly developed flow structure. Locally, while the flow was still in motion, the upper vesicular layer and the flow structure of the lower layer were conformably folded. In the main these folds are isoclinal, trend north, are slightly overturned to the west, and plunge gently south.At the White Horse deposit the alunite occurs in three main ore bodies, each of which consists of several smaller bodies whose long dimensions are about parallel and trend northerly. At the surface the main bodies are roughly lenticular, several hundred feet long, and less than 100 feet wide at their widest part. For the most part they dip steeply east and plunge gently south. Surrounding each of the bodies is a thick envelope of partly kaolinized latite.The alunite bodies are mostly restricted to the upper vesicular and brecciated layer of the latite and are most continuous and reach their greatest depth along the axial planes of the primary fold troughs. The infolding of the upper vesicular and brecciated layer apparently produced porous troughs of rock along which the mineralizing solution could circulate. Concentrated in these troughs, the solution was able to replace the porous rock thoroughly. Subsequent erosion has left parts of the infolded and replaced porous layer.During the alunitization of the latite 5-10 per cent of the silica and most of the sodium, calcium, and iron were removed. The amount of aluminum and potassium was not appreciably changed. The only addition during the replacement was 15 to 23 per cent of SO 3 . These data indicate that the alunite replacement was accomplished by a sulphate-bearing solution. At the deposit there is now no evident surface or near-surface source of sulphur and therefore the sulphur added during alunitization is believed to have been in part at least of magmatic origin. It is suggested that the quartz monzonite stock exposed north of the mine area may overlie the source.

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