Sodium-calcium metasomatism; chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit

Amax, Alloy Div., Empire, CO, United States

Tilting associated with extensional tectonics has exposed 1,800 m of continuous palcovertical relief through the Jurassic-age Yerington porphyry copper deposit. The rarely observed root zones of a porphyry system are characterized at Yerington by sodium-calcium metasomatism. The principal hydrothermal reactions involve the replacement of primary K-feldspar by oligoclase and of primary biotite by actinolite. Copper has been removed. Sodic-calcic alteration developed at deep intrusive contacts nearly simultaneously with potassic alteration and copper mineralization at shallow intrusive contacts. The reactions accompanying potassic alteration are the reverse of those accompanying sodic-calcic alteration. Plagioclase reacted to form K-feldspar; hornblende reacted to form biotite. The potassic and sodic-calcic alteration couplet principally accompanied each of two separate intrusive events: (1) Luhr Hill granite stock and associated porphyry dikes and (2) Walker River granite porphyry dikes. The younger Walker River intrusion was emplaced at slightly deeper levels than the older Luhr Hill complex. Vertically stacked and partially superimposed alteration and mineralization patterns resulted from differences in depth and in timing of emplacement.At lower temperatures, late sodic alteration was superimposed on early sodic-calcic alteration at deeper levels in the system, whereas late quartz-sericite-pyrite alteration was superimposed on potassic alteration at higher levels. Sodic alteration is characterized by the replacement of K-feldspar by albite and of biotite by chlorite. Copper deposited during potassic alteration was remobilized. The transition to lower temperature alteration is associated only with the Walker River intrusion. The hydrothermal system associated with the emplacement of the Luhr Hill complex had not evolved to lower temperature assemblages prior to emplacement of the Walker River dikes.Phase equilibria and geologic reconstruction of depth of emplacement suggest that temperatures during early sodic-calcic alteration were limited to the range of 360 degrees to 480 degrees C at pressures between 300 and 800 bars. Late sodic alteration took place at similar pressures but at temperatures less than 360 degrees C.Nearly simultaneous development of potassic and sodic-calcic alteration resulted from the flow of fluids away from intrusions at shallow levels (potassic alteration) and from the flow of fluids toward intrusions at deeper levels (sodic-calcic alteration). Because K/Na and Na/Ca activity ratios of a fluid in equilibrium with granitic rocks decrease with decreasing temperature, cooling of high-temperature fluids can generate potassic alteration and heating of fluids to high temperatures can generate sodic or sodic-calcic alteration.

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