- © 2009 Society of Economic Geologists, Inc.
The crustal continuum model has been the dominant model for formation of orogenic gold deposits for more than 25 years. This model is based partly on the timing of mineralization at the Griffin’s Find gold deposit, located in the southwestern Yilgarn Block in Western Australia, which was previously interpreted to have formed via influx of gold-bearing hydrothermal fluid under granulite facies conditions at 700° to 750°C and 500 to 700 MPa. In this study, new petrogenetic constraints are placed on the timing of mineralization at Griffin’s Find. Peak metamorphism is here redefined to have involved conditions of 820° to 870°C and at least 550 MPa. This metamorphism caused significant partial melting of silicate assemblages within and surrounding the deposit, initially through dehydration melting and then decompression melting under fluid-absent conditions. Typical orogenic hydrothermal fluids (which have X(CO2) from 0.04 to 0.30) cannot have been added to the rock under these conditions. This statement is supported by the quartz-calcite assemblages preserved within the deposit, as these would have been completely destroyed if typical orogenic fluids had been added at temperatures beyond ~750°C. Infiltration of a high X(CO2) or X(CH4) fluid during peak metamorphism is precluded by the ubiquitous presence of biotite and cordierite throughout the deposit. Gold sulfide textures are consistent with solid-state prograde and retrograde metamorphic reactions in some samples, and with development of a gold-rich polymetallic melt in others. The presence of gold and sulfides in textural equilibrium (well-rounded and subspherical morphologies) with peak metamorphic minerals, particularly cordierite, indicates that extensive fluid influx cannot have occurred after peak metamorphism, as these silicates would have been completely retrogressed to hydrous phases. Thus mineralization at Griffin’s Find must have been introduced prior to granulite facies metamorphism. Textures in mineralized microcline-rich gneiss imply original mineralization temperatures within the greenschist facies, similar to the conditions of formation for other orogenic gold deposits. The results of this study have thus removed the high-temperature end of the crustal continuum model. Given the difficulty of transmitting hydrothermal fluids through rocks beyond 600° to 650°C without causing partial melting (migmatization), we suggest that gold deposits cannot form at temperatures beyond these. For the same reasons, mineralizing fluids cannot be sourced from high-grade metamorphic rocks. Thus, we find that gold deposits cannot form over a metamorphic continuum. Orogenic gold deposits should thus be thought of as a mesothermal phenomenon.