Fluid-Fluid Interactions in Magmatic-Hydrothermal Ore Formation

Abstract

Hydrothermal ore deposits have provided some of the first geological evidence for the interaction of multiple fluids in the Earth’s interior, including fluid mixing as well as fluid phase separation. Physical evidence comes from veins and breccias documenting the impact of fluids in breaking rocks and creating fracture permeability for ore formation, as a result of the fluid expansion that is commonly associated with phase separation between melts and multiple fluids. Chemical evidence of fluid–fluid interaction includes the precipitation of minerals in veins as a result of fluid boiling or mixing between chemically contrasting fluids. Fluid inclusions record interactions of multiple fluids, especially in magmatic-hydrothermal ore deposits and associated igneous rocks (Fig. 1⇓). In recent years, fluid inclusions have contributed to a more quantitative understanding of the role of fluid–fluid interactions, since new microanalytical techniques allow determination of element concentrations in all types of fluids, from low-density vapor through aqueous and hypersaline liquids to hydrous salt melts (Roedder 1984; Samson et al. 2003). Figure 1. Geological evidence for fluid phase separation in ore-forming magmatic-hydrothermal systems. (a) ‘Bonanza grade’ gold (∼1 wt%) that was precipitated together with fine-grained pyrite in a fluidized breccia resulting from instantaneous boiling and expansion of magmatic fluids ascending from an underlying andesite porphyry intrusion. The breccia consists of adularia–sericite altered clasts of sedimentary wall rock (s) and nodules of (originally colloidal?) pyrite with bright specks of gold (p+g), both cemented by roscoelite (r; ∼ vanadium-rich muscovite) and late calcite (c) (Zone VII ore body, Porgera, Papua New Guinea; Ronacher et al. 2004). (b) Bladed manganocalcite, the characteristic habit of calcite precipitated from aqueous epithermal fluids as a result of the pH change following loss of CO2 to a separating vapor phase (Madan, Bulgaria; Kostova et al. 2004). (c) Growth band in a euhedral quartz …