Injection-driven swarm seismicity and permeability enhancement: Implications for the dynamics of hydrothermal ore systems in high fluid-flux, overpressured faulting regimes - An invited paper

Many types of hydrothermal ore deposits form at overpressured conditions during high fluid flux through fault zones in the continental seismogenic regime. These include many orogenic gold deposits, some types of Fe oxide Cu-Au systems, and a variety of intrusion-related deposits, including some high-sulfidation epithermal systems. The internal structures of these deposits indicate formation while faults were active, in a regime involving episodic slip. Partial to complete sealing of fault zones by precipitation of hydrothermal minerals occurs between slip episodes. Such ore deposits occur typically within low displacement faults, or networks of such faults. Lodes seldom occupy areas much greater than 1 km² of fault surfaces, and net slip accumulated during ore formation is typically less than 150 m. Fluid injection experiments in low-permeability rocks, and seismicity styles in hydrothermally active settings, provide new insights about the dynamics of ore formation in high fluid-flux faults. In particular, they indicate that swarm seismicity is the characteristic response to injection of large volumes of overpressured fluids into intrinsically low-permeability rock. Injection-driven swarm seismicity and related permeability enhancement involves repeated sequences of thousands of ruptures, mostly with moment magnitude M_w in the range -2< M_w< 4. High seismicity rates are sustained over periods of days to months, and relatively quiescent interswarm periods last years to decades. Ruptures within swarms mostly have diameters much less than 100 m, and slip less than 1 mm. Cumulative rupture areas during a single swarm seldom exceed several km²; maximum cumulative slip is usually less than a few centimeters. Diffusion-like migration of a seismicity front away from the injection source at rates up to hundreds of meters per day is a key characteristic of injection-driven seismicity, and correlates with migration of a fluid pressure front along activated faults. By analogy with injection experiments and natural injection-driven swarm seismicity, the formation of fault-related ore deposits in overpressured, high fluid-flux regimes is interpreted to involve swarm seismicity, rather than mainshock-aftershock sequences. Recurrence intervals of swarms in contemporary injection-driven swarm sequences indicate that the total slip associated with formation of fault-hosted lodes can accumulate in periods as short as 10⁴ to 10⁵ years, during thousands of swarm episodes, and commonly involve>10⁶ ruptures with -2< M_w< 4. Relationships between injected fluid volumes and cumulative moment release during injection experiments indicate that swarm sequences, in faults of dimensions comparable to those of fault-related orogenic Au deposits, for example, are driven by injection of 10⁴ to 10⁵ m³ of fluid at injection rates of at least several tens of L.s^-1. Up to tens of kg Au are deposited per swarm. Cascades of thousands of microseismic ruptures and associated permeability enhancement, coseismic dilatancy, and fluid pressure changes during individual injection-driven swarm sequences create very dynamic hydrologic and reaction regimes. Ore formation involving transitory bursts of rapid flow, abrupt fluctuations in flow rates and fluid pressures, and possibly also fluid temperatures, is likely to promote severe chemical disequilibrium. This has implications for controls on ore deposition and grade distribution in many fault-related ore systems.