IN THE "WAKE" OF MIGRATED EXTENSION: THE TIMING OF ORE FORMATION IN ACTIVELY EVOLVING STRIKE-SLIP FAULT SYSTEMS

Hydrothermal mineral deposits are commonly localized along strike-slip fault systems, most typically in zones of transtension, particularly in extensional bends and stepovers. Structural geologic studies of selected mining districts of orogenic gold, epithermal vein, and porphyry copper deposits localized in extensional stepovers reveal a correlation between the timing of high-grade ore formation and concurrent intra-stepover transpression related to a progressive spatial migration of the locus of extension within the stepover. At Muruntau, Uzbekistan, and Virginia City and Goldfield, Nevada, the highest-grade ores were deposited within the vein systems during this period of transpressional deformation. Transpression occurred in the “wake” of extension that had migrated 1-2 km away from the focus of active hydrothermal fluid flow. A similar relation is observed in porphyry copper deposit-localizing strike-slip fault systems. For example, at Sierrita-Esperanza, Arizona, the porphyry-style mineralized rock occurs in a right-lateral strike-slip releasing bend into a stepover. The porphyry copper-molybdenum deposit formed after the emplacement of a granodiorite stock within the stepover had accommodated extension within the stepover.

The nature of the transpressional deformation is manifested in different ways in the different districts studied. Behind the locus of migrated extension, but still in its wake, the styles of transpressional deformation may include reverse faulting, fault-slip inversion, anticlinal folds, structural anticlines (horst blocks), and vertical-axis folding of vein structures including boudinaging. Within such features, hydrothermal fluid flow is strongly focused, constrained flow situations that apparently favor high-grade ore formation. The relation of intra-transtensional transpressive deformation to ore formation may contribute to why some districts became economically world-class.