THE MOBILITY OF ZIRCONIUM AND OTHER IMMOBILE ELEMENTS DURING HYDROTHERMAL ALTERATION

Development of zircon and other Zr phases in hydrothermal deposits ind icates that Zr can be highly mobile in these systems. Mobility is most common in, but not restricted to, F-rich hydrothermal systems related to alkalic, F-rich igneous suites; these suites can range from peralk aline through metaluminous to peraluminous. A few examples are neither alkalic nor F rich. Three locations in the Trans-Pecos Magmatic Provi nce, Texas, U.S.A., demonstrate this hydrothermal Zr mobility. All thr ee igneous systems are alkalic and F rich but vary in alkali/Al ratios . Peralkaline rhyolites and trachytes in the Christmas Mountains conta in as much as 2100 ppm Zr, mostly in aegirine or arfvedsonite; zircon is rare or absent. Fluorspar replacement deposits in limestone at cont acts with the rhyolites contain as much as 38,000 ppm Zr, occurring as small, disseminated zircons. The deposits also are enriched in a vari ety of incompatible elements, including Be, rare-earth elements (REE), Y, Nb, Mo, Hf, Pb, Th and U. The Sierra Blanca intrusions, a series o f mildly peraluminous, F-rich rhyolite laccoliths, contain as much as 1000 ppm Zr, mostly as zircon. Hydrothermal zircon occurs as overgrowt hs on magmatic grains, as veinlets connected to overgrowths, and in fl uorspar replacement bodies in adjacent limestone. The highest Zr conce ntrations in fluorspar are similar to 200 ppm. Metaluminous quartz mon zonite from the Infiernito caldera contains 400-600 ppm Zr, mostly as zircon. Euhedral zircon in quartz-fluorite veins in the quartz monzoni te indicates mobility of Zr. Zirconium concentrations in the veins are unknown, but the paucity of zircon suggests little Zr enrichment rela tive to the host. Zircon and, more rarely, zirconolite, occur in skarn in the Ertsberg District of Irian Jaya, Indonesia. Unlike in Texas, r elated igneous rocks are metaluminous, and the hydrothermal system was F poor. Worldwide, hydrothermal zircon, other Zr phases, and Ti- and Al-bearing phases occur in skarn, epithermal precious metal veins, vol canogenic massive-sulfide deposits and mylonites. We propose that diff erences in Zr mineralogy of igneous source rocks is an important facto r in determining the availability of Zr to hydrothermal fluids. Althou gh Zr concentrations in the Sierra Blanca and Christmas Mountains rhyo lites are similar, Zr enrichment in fluorspar was much greater in the Christmas Mountains. We suggest that hydrothermal solutions could easi ly break down aegirine and arfvedsonite to release Zr, but that zircon was only moderately attacked. Therefore, far more Zr was available fo r transport and subsequent deposition in the Christmas Mountains than at Sierra Blanca. Availability of other trace elements probably is als o governed by their mineral host. Although Zr mobility is most common in F-rich hydrothermal systems related to alkalic and F-rich igneous s ystems, mobility at Ertsberg may have been promoted by sulfate complex ing.