Magmatic and Magma-Hydrothermal Ore-Forming Processes

Using microbeam analytical techniques, Lowenstern et al. [1991] and Lowenstern [1993] discovered Cu sulfides in CO- and Cl-bearing vapor bubbles in melt inclusions within phenocrysts in pantellerites and rhyolites, thus demonstrating that melt Cu could be strongly partitioned into an early magmatic vapor phase in
[4] phenocryst-poor magmas. The possibility of strong partitioning of Cu into an early vapor phase, prior to extensive crystallization of phases that would otherwise remove Cu from the melt, means that crystallization-induced volatile saturation (second boiling) is not necessary for the creation of metal-rich fluids in shallow HO- or CO-rich silicic magma chambers. They also argued that volcanic contributions of Cu to the atmosphere may be more significant than previously thought. Meeker et al. [1991] identified crystalline elemental gold and gold chloride particles being emitted from Mount Erebus in Antarctica. This plus consistent Au/Cl ratios of aerosols from the volcano suggested that the gold is transported as a chloride gas species. Transport of trace metals in volcanic gases from Mount St. Helens was modeled by Symonds and Reed [1993], who likewise concluded that most were volatilized from shallow magma as simple chlorides and deposited as sublimates upon cooling as oxides, sulfides, halides, tungstates and native elements.

Rye [1993] summarized the evolution of magmatic-hydrothermal ore-forming fluids based on many years of stable isotopic research on such ore deposits. He reviewed evidence for high-level interactions of deep magmatic components with shallow wall-rock and meteoric waters, and emphasized the episodic, successive input of deep magmatic volatiles and evolved brine into shallower crustal levels to generate acid alteration and ore deposition. His paper and the review papers by Giggenbach [1993] and Hedenquist and Lowenstern [1994] are perceptive, complementary evaluations of the processes that form magma-hydrothermal ore deposits.