|Cumulative Factors in the Generation of Giant Calc-Alkaline Porphyry Cu Deposits|
Jeremy P Richards, Department of Earth and Atmospheric Sciences, University of Alberta, Canada
in - Porter, T.M. (Ed), 2005 - Super Porphyry Copper & Gold Deposits - A Global Perspective; PGC Publishing, Adelaide, v. 1, pp 7-25.
The formation of porphyry Cu deposits in calc-alkaline magmatic arcs is considered to be the cumulative product of a wide range of processes beginning with dehydration of the subducting oceanic slab. No single process is key to the formation of large deposits, but the absence or inefficient operation of any contributory process, or the action of a deleterious process, can stunt or prevent deposit formation.
A starting premise is that normal calc alkaline arc magmas have the potential ultimately to form a porphyry Cu deposit (i.e., arc magmas are inherently "fertile"). This characteristic is ascribed to the relatively high oxidation state and high H2O, Cl, and S contents of typical arc magmas (metal contents do not need to be anomalously enriched). Given the availability of such magma, the next most important factor in the formation of large porphyry Cu deposits is the flux of this magma reaching the upper crust. The supply of magma must be sufficiently voluminous and localised to maintain an active upper crustal magma chamber of >100 cubic km, in order for enough Cu (and S) to be available for extraction by magmatic hydrothermal fluids. These requirements imply a long-lived magmatic system rooted in the supra-subduction zone mantle wedge, with the formation of an extensive lower-crustal melting and assimilation (MASH) zone. Compressional tectonic regimes are thought to favour the formation of such magma bodies as sill complexes deep in the lithosphere. Relaxation of compressional stress permits the voluminous rise of buoyant, evolved magmas to upper crustal levels, and explains the common occurrence of porphyry Cu deposits at the end of protracted tectono-magmatic events. Pre-existing zones of structural weakness in the crust facilitate magma ascent, and dilational volumes at transpressional jogs and step-overs in strike-slip fault systems provide optimal conditions for focused flow and emplacement. The geometry of the upper crustal magma chamber so formed includes a cupola zone (commonly <2 km depth) into which bubble-rich, buoyant magma rising from depths of >5 km convectively circulates, releasing its volatile load into the overlying carapace. This fluid dynamic mechanism enables efficient partitioning of metals from a large volume of magma into the exsolving hydrothermal fluid, and achieves focused delivery of that fluid into the carapace zone. Cooling of the fluid and wall rock reactions result in efficient precipitation of metals in association with potassic and, in some deposits, phyllic alteration.
Ore-forming potential may be spoiled by tectonic conditions and histories that do not focus magma generation and emplacement, crustal conditions (such as the presence of reduced lithologies in the deep crust) that cause early sulphide saturation and segregation, or catastrophic explosive volcanism that destroys the magmatic-hydrothermal ore-forming process by venting fluids directly to the surface.
Exploration indicators for large porphyry Cu deposits include the development of a well-established magmatic arc with concentrations of sub-volcanic plutonic centres, localised by large-scale structural features.
Note: This article was fully peer reviewed by two experts in the field.