The original Sixteen To One gold-quartz vein, Alleghany, California

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Extremely rich gold shoots are scattered in the Sixteen To One vein in otherwise almost barren quartz. The main result of this study is the recognition of some structural features that might guide exploration for ore shoots.The Alleghany gold-quartz veins occupy a conjugate system of reverse faults which may have formed due to lateral compression from intrusion of ultramafics and mafics into the schistose country-rock. Alteration on the Sixteen To One vein apparently began before serpentinization of the ultramafics had ended. Early chloritization, sericitization, and especially, carbonatization, are suggested to have released silicon from adjacent and deeper wall-rock to be deposited in vein and wall-rock quartz, vein quartz being a concentrated alteration product.The ore minerals are thought to have deposited during the latter part of vein quartz deposition while the vein and wall-rock were cooling. Ore minerals show little replacement of each other or of quartz. The early ore minerals formed bulky grains, ascribed mainly to open-space filling or replacement of carbonate; as vein quartz increased, the later ore minerals, ending with gold, formed increasingly tenuous patterns as fracture-fillings and a great variety of marginal deposits on grains of earlier ore minerals or quartz.The positions, production, and some characteristics of practically all ore shoots mined were ascertained and compared with other features of the vein and wall-rock. Correlations of these features with gold were reduced to numerical statements termed association indices. The most significant findings are: (1) high-grade commonly follows late shears on or near the vein walls, especially the foot-wall, or crosses the vein diagonally up-dip on intravein shears and ribbons; (2) the high-grade distribution accords with that of openings expectable from the reverse movement indicated: high-grade is most abundant where the vein is flatter than adjacent areas along both strike and dip, and virtually lacking in correspondingly, steep areas; (3) serpentinite and cross-faults partly localized by serpentinite steepened the vein shear-zone: much high-grade occurs in complementary flats and near cross-fault intersections.