Hydrothermal alteration associated with ore veins in a large part of the Front Range mineral belt was studied in an attempt to correlate it with the mineralogic, petrologic, and structural features of the ore deposits. Six patterns of alteration were found; they range from the simplest type with a single zone of hydromica to the most complex type with five zones that are characterized by phlogopite carbonate (fringe zone), montmorillonite, kaolinite, orthoclase, and hydromica (innermost zone). A regular mixed-layer illite-montmorillonite mineral may substitute for hydromica either in the whole or in the outer part of the innermost zone, thus producing two subpatterns. Phlogopite and siderite represent the alteration of biotite; they extend to and suddenly increase at the hydromica zone. The remaining zones are based on the alteration of plagioclase. Microcline and quartz are stable in all zones.

Generally, the simplest alteration patterns are found in the southwestern part and along the western side of the belt. They are closely related to Laramide stocks, and most of the associated mineralization is of lead and zinc sulfides. The most complex patterns predominate along the eastern side and are concentrated in the middle to northeastern part of the belt; they are associated preferably with tungsten, pyritic gold, and telluride gold deposits.

The origin of the complex patterns is explained in terms of chemical fronts developed as the altering fluids migrated away from the conduit. The altering fluids were probably gaseous and are believed to have been derived by fractional distillation from the main body of hydrothermal liquids. The remaining silica-rich, less volatile fraction filled the veins after wall-rock alteration. The fluids changed in composition through reaction with the wall rock and successively reached the stability fields of the various secondary minerals. The critical changes responsible for the front (and zone) formation are increase of Si/Al ratio, decrease of K concentration, and increase of pH. No evidence of mutual replacement among hydrothermal minerals was found. The amount of matter transferred during alteration was small. The formation of other patterns is explained on the same principles, assuming slightly different compositions of the altering fluids. An attempt is made to correlate the patterns of alteration with the distribution of the intrusive bodies.

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