Abstract

The Big Hurrah mine, located near Nome, Alaska, is an example of gold-bearing quartz vein mineralization in an uplifted metamorphic terrain. The host rocks are sheared carbonaceous metasedimentary and metavolcanic rocks that underwent Jurassic regional blueschist metamorphism. Penetrative deformation of the host rocks includes shearing and later faulting.Five types of veins can be distinguished at the mine based upon structural style, mineralogy, and fluid inclusion populations. From oldest to youngest they are: type I, early quartz lenses; type II, tabular veins; type III, ribbon gold-bearing quartz veins; type IV, quartz-albite + or - arsenopyrite veins; and type V, late carbonate-quartz veins. Type I and II veins are strongly sheared and are premineralization. Gold mineralization occurs predominantly in.the type III veins, which are discontinuous quartz lodes occupying northwest-striking fissures. These fissures are thought to be conjugate faults developed in the late stages of shearing. Wall-rock alteration is limited to minor silicification, carbonatization, and quartz stockwork near type III veins. Type V veins postdate mineralization and are not sheared. Mineralization consists of coarsegrained native gold associated with arsenopyrite and minor scheelite. Much of the gold occurs with sheared carbonaceous ribbons in the type III veins. A geochemical arsenic anomaly extends beyond mineralized gold-bearing quartz veins and can be used as a guide for exploration.Fluid inclusion studies indicate the presence of multiple generations of fluids which have evolved from early CO 2 -CH 4 -bearing fluids to later aqueous fluids with salinities of 2.1 to 6.6 equiv wt percent NaCl. Fluid inclusion homogenization temperatures range from 390 degrees to 90 degrees C, decrease from early veins to late veins, and decrease with time in a single vein type. Pressure and temperature estimates from secondary fluid inclusions in type I veins indicate that CO 2 -CH 4 -H 2 O immiscibility may have occurred at 250 degrees C and 0.8 kb. Gold precipitation in type III veins may have been triggered by a change in pH associated with CO 2 -CH 4 -H 2 O immiscibility, by a drop in temperature, or by reduction of the ore fluids by wall-rock carbon. The Big Hurrah deposit is broadly similar to other gold-bearing quartz vein deposits which occur in deformed metamorphic rocks and which are generally considered to have formed as a result of regional metamorphism.

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