Abstract-The solubility of gold was measured in aqueous KCl (0.5 m) solutions under oxygen, sulfur, and slightly acidic pH buffered conditions between 350 and 450°C at a constant pressure of 500 bars. Two buffer assemblages were used to constrain f O 2 , f S 2 , and consequently f H 2 and aH 2 S: respectively, pyrite-pyrrhotitemagnetite (Py-Po-Mt) and pyrite-magnetite-hematite (Py-Mt-Hm). The measured solubility of gold at equilibrium with Py-Po-Mt and Qtz-KF-Mus is 52 Ϯ 8 ppb at 350°C, 134 Ϯ 17 ppb at 400°C and 496 Ϯ 37 ppb at 450°C. With Py-Mt-Hm and Qtz-KF-Mus the solubility of gold is increased to 198 Ϯ 9 ppb at 400°C and 692 Ϯ 10 ppb at 450°C. These results are consistent with the aqueous complex AuHS°being the dominant gold-bearing species. The equilibrium constants (log K R10 ) for the reaction:have been determined at 350, 400, and 450°C and are, respectively, Ϫ5.20 Ϯ 0.25, Ϫ5.30 Ϯ 0.15, and Ϫ5.40 Ϯ 0.15. These values are similar to those suggested by Zotov (written pers. commun.) and those obtained by recalculating the experimental data of Hayashi and Ohmoto (1991). They are significantly higher than those derived by Benning and Seward (1996) and the possible causes of the discrepancies are discussed. The equilibrium constant for AuHS°shows that this species plays an important role in the deposition of gold in natural environments. Cooling, H 2 S loss, pH change, and oxidation seem to be effective mechanisms for gold precipitation, depending on the local ore forming conditions.

The melilite-bearing skarns of Cornet Hill (CH) and Upper Cerboaia Valley (UCV), in the Apuseni Mountains of Romania, occur at the contact between monzodiorite bodies of Ypresian age (Paleocene) and Neojurassic calcitic marbles. Typical wollastonite -grossular -diopside endoskarns are separated from exoskarns (tilleyite and spurrite or wollastonite at CH, wollastonite only at UCV), at most places, by a melilite-rich rock, in which veins and vein-like zones of recrystallization are composed only of idiomorphic melilite crystals reaching 15 cm across. Titanian garnet and wollastonite are the principal minerals associated with melilite (and also monticellite, perovskite, vesuvianite, cuspidine, spurrite, tilleyite, calcite, hydroxylellestadite, hydrogrossular and other minor alteration-induced minerals). A different association that includes aluminian diopside and grossular occurs (1) as veinlets in the marble close to the skarns and (2) as relict inclusions in endoskarns. From the geometrical relationships of the zone sequences and the veins, the textural features of the mineral associations and the inferred conditions of fluid-mineral equilibrium, these mineralogical peculiarities are interpreted as resulting from the superposition of two main stages. Firstly, there was circulation of a comparatively CO 2 -rich fluid formed the early aluminian diopside -grossular endoskarns, with depletion in Si (and Fe, Na, K) and inert behavior of Mg, Al, Ti. Then, a high-temperature (750°C) fluid circulated on both sides of the contact between marble and endoskarns, and developed the melilite-rich, titanian-garnet-bearing rocks partly at the expense of previously formed endoskarns, and spurrite or wollastonite (CH) or wollastonite (UCV) exoskarns at the expense of marble. The pressure of CO 2 was very low, less than 26 bars at UCV and 16 bars at CH, with a H 2 O pressure less than 750 bars. Not only Si and Ca were mobilized, but also Mg, Al and Ti, leached from the endoskarns and deposited in the veins and the nearby part of exoskarns. This stage, which occurred in the temperature range corresponding to the end of the crystallization of plagioclase in the monzodiorite, has pegmatitic chemical and textural features. The main flow of fluid ended with the development of tilleyite partly at the expense of spurrite and wollastonite at CH, and local high-temperature (about 710°C) recrystallization of the zonation, mostly in veins, especially in the endoskarn-exoskarn boundary, but also within the endoskarns. A monticellite -gehlenite association appeared in the melilite-rich rocks, later followed by vesuvianite, whereas in the endoskarn, vesuvianite developed together with coarse-grained wollastonite and grossular.

International audienc

We describe the occurrence of high-temperature, spurrite-, tilleyite-and gehlenite-bearing skarns from Cornet Hill, part of the Metaliferi Massif, Apuseni Mountains, Romania, and the main mineral species developed in these rocks. The host skarns are developed at the contact between a quartz monzonitic -monzodioritic body of Paleocene -Ypresian age and Tithonian limestones. The primary mineral assemblage mainly consists of tilleyite, spurrite and gehlenite, with various amounts of garnet and wollastonite; perovskite, monticellite and hydroxylellestadite are present but scarce. The skarns have clearly undergone a late metasomatic event, which produced, for example, diopside veins cross-cutting tilleyite, spurrite, and gehlenite, and small masses and veins of vesuvianite replacing gehlenite. Subsequent hydrothermal and weathering overprints on the primary assemblages resulted in the formation of three secondary parageneses: (1) an early hydrothermal one that includes scawtite, xonotlite and hibschite, (2) a late hydrothermal one that includes 11 Å tobermorite, riversideite, thomsonite, gismondine, aragonite, and calcite, and (3) a weathering paragenesis that includes plombièrite, portlandite, and allophane. The main properties of these mineral species, as revealed using chemical, optical and X-ray powder analyses, are reported here. We document the first occurrence of plombièrite, tobermorite, riversideite, portlandite and allophane in Romania.

No abstract