Self-ordering and complexity in epizonal, mineral deposits

Epizonal base and precious metal deposits makeup a range of familiar deposi t styles including porphyry copper-gold, epithermal veins and stockworks, c arbonate-replacement deposits, and polymetallic volcanic rock-hosted (VHMS) deposits. They occur along convergent plate margins and are invariably ass ociated directly with active faults and volcanism. They are complex in form , variable in their characteristics at all scales, and highly localized in the earth's crust. More than a century of detailed research has provided an extensive base of observational data characterizing these deposits, from their regional setti ng to the fluid and isotope chemistry of mineral deposition. This has led t o a broad understanding of the large-scale hydrothermal systems within whic h they form. Low salinity vapor, released by magma crystallization and disp ersed into vigorously convecting groundwater systems, is recognized as a pr incipal source of metals and the gases that control redox conditions within systems. The temperature and pressure of the ambient fluid anywhere within these systems is close to its vapor-liquid phase boundary, and mineral dep osition is a consequence of short timescale perturbations generated by loca lized release of crustal stress. However, a review of occurrence data raises questions about ore formation t hat are not addressed by traditional genetic models. For example, what are the origins of banding in epithermal veins, and what controls the frequency of oscillatory lamination? What controls where the phenomenon of mineraliz ation occurs, and why are some porphyry deposits, for example, so much larg er than others? The distinctive, self-organized characteristics of epizonal deposits are sh own to be the result of repetitive coupling of fracture dilation consequent on brittle failure, phase separation ("boiling"), and heat transfer betwee n fluid and host rock. Process coupling substantially increases solute conc entrations and triggers fast, far-from-equilibrium depositional processes. Since these coupled processes lead to localized transient changes in fluid characteristics, paragenetic, isotope, and fluid inclusion data relate to c onditions at the site of deposition and only indirectly to the characterist ics of the larger-scale hydrothermal system and its longer-term behavior. T he metal concentrations (i.e. grade) of deposits and their internal variati on is directly related to the geometry of the fracture array at the deposit scale, whereas finer-scale oscillatory fabrics in ores may be a result of molecular scale processes. Giant deposits are relatively rare and develop where efficient metal deposi tion is spatially focused by repetitive brittle failure in active fault arr ays. Some brief case histories are provided for epithermal, replacement, an d porphyry mineralization. These highlight how rock competency contrasts an d feedback between processes, rather than any single component of a hydroth ermal system, govern the size of individual deposits. In turn, the recognit ion of the probabilistic nature of mineralization provides a firmer foundat ion through which exploration investment and risk management decisions can be made.