Abstract

It has been statistically shown that a great majority of the larger Cretaceous and younger hydrothermal ore deposits of Au, Ag, Cu, Pb, Zn, Mo, Hg, and U in the Basin and Range province are associated with intermediate to acidic intrusive aphanitic or porphyritic rocks of near similar age. Recent work indicates that, except for the Pacific Coast Ranges and the Mother Lode area, similar ore-porphyry relationships apply to deposits over the entire western United States, however, many intrusive porphyries exist that have little or no associated hydrothermal deposits. Some intrusive porphyries, therefore, seem to be genetically related to ore deposits while others are not. These then may conveniently be classified into 2 categories as barren and productive. To aid further the ore finder, an attempt is made here to offer criteria that will serve to distinguish barren from productive porphyries. Among the many suggestive distinctions the following seem to be of most importance. Productive acid porphyry bodies generally are of considerable size and seem to cross cut enclosing rock structures with very little or no disturbance due to intrusive action. On the other hand barren porphyries may be of various sizes but on the whole have a tendency to either be concordant with wall rock structures or, where cross cutting relations exist, show obvious intrusive structural effects such as brecciation, folding, or up arching. Other criteria are tabulated in order of importance.

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