Allard Stock, La Plata Mountains, Colorado; an alkaline rock-hosted porphyry copper - precious metal deposit.

Abstract

Epizonal Laramide pluton. The stock and associated mineralization evolved by a complex interaction of igneous and metasomatic processes as follows: stage 1, forceful emplacement of syenite plutons; stage 2, intense hydrothermal argillization of stage 1 syenites; stage 3, intrusion of mafic syenite plutons, some of which are layered, along shear zones; stage 4, violent escape of volatiles, producing intrusive breccia pipes; stage 5, intense K-metasomatic alteration of the older stock units with production of a fenitelike alteration halo extending 1000 m beyond the stock and simultaneous intrusion of carbonate-bearing alkali feldspar dikes; stage 6, emplacement of trachyte, syenite pegmatite, and mafic pegmatite dikes; stage 7, deposition of metallic minerals, sanidine, quartz, calcite, and fluorite as veinlets and disseminated replacement masses from hydrothermal solutions. The stock shows enrichment of volatiles CO sub 2 , F, and S, high concentrations of Ba and Sr, low concentrations of Rb, high concentrations of Cu, Ag, Au, Bi, Te, Pt, and Pd, with little or no enrichment in Nb, U, Y, and Zr.--Modified journal abstract.

50 samples from this reference