CHEMICAL COMPOSITIONS OF MAGMATIC STEAM ALUNITE VEINS FROM DIVERSE PORPHYRY-EPITHERMAL SYSTEMS–IMPLICATIONS FOR MAGMATIC VOLATILE CONTENTS AND DEGASSING PROCESSES

Alunite veins from 10 well known mining districts throughout the world (Alunite Ridge, UT; Cactus, CA; El Indio and Tambo, Chile; Goldfield, NV; Lake City, Red Mountain Pass, and Rico, CO; , Questa, NM; and Tolfa, Italy) are currently being studied by EMP and LA-ICPS to profile compositional variations in a wide spectrum of elements. Previous stable isotopic studies indicate these veins precipitated from magmatic steam discharged from underlying stocks. They occur at epithermal levels of "porphyry systems” in arcs (Andes, Cascades), continental rifts (Basin and Range, Rio Grande), and continental volcanic fields (San Juan Mountains).

Most of the veins are comprised of coarse banded alunite, which provides a detailed record of successive steam discharges. Several veins have individual bands with Na concentrations that ramp up from base to rim, however, sawtooth patterns are also common. Trace metals analyzed by LA-ICPMS vary considerably between districts. Banded veins from the productive high sulfidation (HS) Au/Ag deposit at Tambo have relatively high correlated concentrations of As, Bi, Pb, and REE’s. Similar veins from rich HS Au/Ag deposits at El Indio and Goldfield also contain relatively high Pb, but low As and Bi. A vein overlying the Questa Mo-porphyry deposit contains relatively high Pb, low Bi, and trace Sn and Sb. In contrast, samples from two “barren” acid-sulfate systems (Lake City, CO and NG, UT) are generally devoid of trace metals. F content as determined by EMPA exhibits sawtooth patterns that show no obvious correspondence to homogeneous growth bands. Veins from Tolfa, El Indio, and Alunite Ridge have low F abundance (0.15-0.3%), those at Tambo, Lake City, and Rico (0.4-0.8%) are moderate, and Questa is high (1.9%). Preliminary data indicates that there is a positive correlation between halogens in magmatic biotite and vein alunite.

The vein alunites record magmatic processes in single brief episodes and reflect variations in the steam component as the magma is tapped. Variations in Na and other elements in banded veins record individual degassing events. Halogens and metals in magmatic steam alunite may vary with magma composition and tectonic setting; possibly providing an indication of what lies below.