STRATIGRAPHIC AND STRUCTURAL CONTROLS ON ARCHEAN LODE GOLD DEPOSITS (Invited Presentation)

Detailed mapping and high resolution geochronology allow increasingly precise insights into the evolution of Neoarchean greenstone belts and associated lode gold deposits. Rapid fault-controlled sedimentation and multiple episodes of deformation led to an architecture of abundant planar discontinuities, including unconformities that overprinted early syn-volcanic mineralization phases, and focused late stage orogenic deformation. These discontinuities may have been reactivated in a variety of stress fields if they were weak, or they may have provided seals to prevent hydrothermal fluid circulation and promoted local fracturing, brecciation and mineral deposition. Several large Archean lode gold deposits yield evidence for multiple episodes of mineralization and fluid flow. We illustrate this dynamic picture by recent work in the Kalgoorlie and Yalgoo-Singleton greenstone belts of the Yilgarn craton, which shows that thick sequences of clastic rocks were deposited in time frames of a few million years – at the limits of the resolution of the commonly used SHRIMP U-Pb method on zircon. Previously unrecognized unconformities subdivide some of the clastic sedimentary sequences, and are spatially associated with gold mineralization. An important structural configuration in some lode gold deposits is the intersection of two or more structures at oblique or high angles. This geometry may be important for gold mineralization because the intersection of the structures allows channelized fluid flow if both structures are permeable. Alternatively one of the structures may be permeable and the other may act as a seal to form a trap. The possibility of reactivation is also inherently greater with structures in different orientations. Similar mechanical considerations may apply to lode gold deposits of other ages, but the concentration of lode gold mineralization in the Neoarchean may indicate that a combination of rates of sedimentation, magmatism, and deformation lead to the establishment of hydrothermal systems at particularly auspicious times and places for gold mineralization.