COMB QUARTZ LAYERS IN THEPORPHYRY COPPER DEPOSIT AT YERINGTON, NEVADA

Comb quartz layers have been frequently observed in porphyry molybdenum, but only rarely in porphyry copper deposits. At Yerington, Nevada, we mapped a thin zone, 10-20cm thick,which has an apparently hemispherical shape at the apex of a late stock 75m in diameter on several levels 3950-4000 ft elev. The zone consists of up to four layers of quartz, magnetite and chalcopyrite crystals up to several cm across. The quartz crystals, with prismatic and rhombohedral faces, all terminate downward in aplitic porphyry and the layers are separated by aplitic porphyry. The stock truncates mineralized stockwork quartz veins of two earlier intrusions, but contains no quartz veins itself. Significantly, it contains abundant small 2-5mm miarolytic cavities lined with quartz, pyrite, chalcopyrite, chloritized biotite and K-feldspar indicating that the magma was saturated with water at the time of crystallization, supporting an idea expressed earlier by others that the comb quartz layers were crystallized from an exsolved aqueous phase. Stockwork veins, comb quartz layers, miarolytic cavities and mineralized breccia pipes all represent features due to the exsolution of an aqueous phase from mineralizing porphyries under somewhat different conditions and at different stages of development of porphyry deposits. The Yerington comb quartz contrasts greatly with that at the classic Henderson porphyry molybdenum deposit, where as many as hundreds of zones each consist of dozens of quartz layers. Thus the rarity of reports of comb quartz in porphyry copper, as opposed to molybdenum, deposits, suggests a strong control of magma compositions.