The mobility of zirconium and other “immobile” elements during hydrothermal alteration

Abstract

Development of zircon and other Zr phases in hydrothermal deposits indicates 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 peralkaline through metaluminous to peraluminous. A few examples are neither alkalic nor F rich. Three locations in the Trans-Pecos Magmatic Province, Texas, U.S.A., demonstrate this hydrothermal Zr mobility. All three igneous systems are alkalic and F rich but vary in alkali/Al ratios. Peralkaline rhyolites and trachytes in the Christmas Mountains contain as much as 2100 ppm Zr, mostly in aegirine or arfvedsonite; zircon is rare or absent. Fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38,000 ppm Zr, occurring as small, disseminated zircons. The deposits also are enriched in a variety of incompatible elements, including Be, rare-earth elements (REE), Y, Nb, Mo, Hf, Pb, Th and U. The Sierra Blanca intrusions, a series of mildly peraluminous, F-rich rhyolite laccoliths, contain as much as 1000 ppm Zr, mostly as zircon. Hydrothermal zircon occurs as overgrowths on magmatic grains, as veinlets connected to overgrowths, and in fluorspar replacement bodies in adjacent limestone. The highest Zr concentrations in fluorspar are ∼200 ppm. Metaluminous quartz monzonite from the Infiernito caldera contains 400–600 ppm Zr, mostly as zircon. Euhedral zircon in quartz-fluorite veins in the quartz monzonite indicates mobility of Zr. Zirconium concentrations in the veins are unknown, but the paucity of zircon suggests little Zr enrichment relative to the host. Zircon and, more rarely, zirconolite, occur in skarn in the Ertsberg District of Irian Jaya, Indonesia. Unlike in Texas, related 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, volcanogenic massive-sulfide deposits and mylonites. We propose that differences in Zr mineralogy of igneous source rocks is an important factor in determining the availability of Zr to hydrothermal fluids. Although Zr concentrations in the Sierra Blanca and Christmas Mountains rhyolites are similar, Zr enrichment in fluorspar was much greater in the Christmas Mountains. We suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release Zr, but that zircon was only moderately attacked. Therefore, far more Zr was available for transport and subsequent deposition in the Christmas Mountains than at Sierra Blanca. Availability of other trace elements probably is also governed by their mineral host. Although Zr mobility is most common in F-rich hydrothermal systems related to alkalic and F-rich igneous systems, mobility at Ertsberg may have been promoted by sulfate complexing.