The porphyry copper deposit exposed in the Liberty open-pit mine near Ely, Nevada; Part 1, Syngenetic Formation

Abstract

Detailed mapping of the Liberty open-pit copper mine indicates that a monzonite porphyry complex was emplaced into a folded sedimentary sequence during active faulting in Cretaceous time. The upper portion of the intruded igneous material was the first to solidify and at times mobile mushes of plagioclase, hornblende, and biotite crystals in a viscous groundmass were injected along lines of rupture into already solid and altered upper portions of the same igneous body. Alteration of the upper part of the porphyry complex took place simultaneously with the continued injection and crystallization of magma from deeper in the system.First, K-feldspar partly replaced plagioclase and later, muscovite replaced most of the remaining plagioclase. Phlogopitic biotite containing rutile replaced hornblende and iron-rich biotite. This alteration preceded the final emplacement of a second, comagmatic portion of the rock which in turn exhibits an alteration suite mainly composed of iron-rich biotite, montmorillonite, and the zeolites, chabazite and stilbite.The distribution of sulfides in igneous rocks and immediately adjacent sediments suggests that the sulfides originated in the porphyries and locally escaped into the surrounding rock as the porphyries crystallized. Most chalcopyrite, the main primary copper-bearing mineral, is distributed as grains in the groundmass with no trace of alignment along earlier cracks. The groundmass which contains the chalcopyrite is composed mainly of aplitic-textured K-feldspar and quartz, and it formed at the time of the original crystallization of the rock and not as a result of later alteration processes.In some rocks the groundmass K-feldspar contains a greater proportion of potassium than does coexisting phenocryst K-feldspar. If a dense aqueous pore fluid had been present during the cooling history of the rock these feldspars should have equilibrated by base exchange of Na (super +) and K (super +) . It is postulated that the formation of hydrous alteration products after feldspar and hornblende may effectively have kept the fugacity of water low in the porphyry as it cooled so that only an attenuated gas phase existed in the pores of the rock.