Abstract

The Pewabic mine is in the southeastern part of the zone of pyrometasomatism which surrounds the intrusive quartz monzonite at Hanover, New Mexico. Extensive development has proved that the south lobe of the intrusive is floored and that the sedimentary rocks, chiefly limestones, shales, and intercalated quartz diorite sills, were thrust aside laterally by the magma. The overthrust, a minimum of 500 feet, was expressed by close folding accompanied by rock flowage of the bedded rocks in the upper plate of a flat thrust fault.

The less intense effects of the exomorphism resulted in marbleization of the limestone and hornstonization of the shales; the stronger effects were epidotization of the aluminous rocks and andraditization of the limestones. The lower plate was mildly metamorphosed. The sphalerite ore bodies, essentially uncontaminated by lead and copper, are localized by the intersection of the thrust fault with nearly vertical post-silicate northeast fault zones resulting in flatly pitching chimneys and ore blankets.

The facts indicate that the rocks were metamorphosed without volume change and that large amounts of silicon, iron, and zinc were brought in. Much of the silica can be accounted for by epidotization and chloritization of the aluminous rocks, and a question discussed is whether the silica, largely older than the sphalerite, may not have been derived from, alterations by gaseous volcanic emanations.

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