Chapter 5: Creation of Permeability in the Porphyry Cu Environment
Published:January 01, 2020
Richard M. Tosdal, John H. Dilles, 2020. "Chapter 5: Creation of Permeability in the Porphyry Cu Environment", APPLIED STRUCTURAL GEOLOGY OF ORE-FORMING HYDROTHERMAL SYSTEMS, Julie V. Rowland, David A. Rhys
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Porphyry Cu deposits, the major source of many metals currently utilized by modern civilization, form via the interplay between magmatism, tectonism, and hydrothermal circulation at depths ranging from about 2 to as much as 10 km. These crustal-scale features require the deep crustal formation of a hydrous and oxidized magma, magma ascent along extant permeability fabrics to create an upper crustal convecting magma chamber, volatile saturation of the magma chamber, and finally the episodic escape of an ore-forming hydrothermal fluid and a phenocryst-rich magma into the shallow crustal environment. Three general fluid regimes are involved in the formation of porphyry Cu deposits. These include the deep magma ± volatile zone at lithostatic pressure, an overlying zone of transiently ascending magmatic-hydrothermal fluids that breaches ductile rock at temperatures ~700° to 400°C, and an upper brittle zone at temperatures <400°C characterized by hydrostatically pressured nonmagmatic and magmatic fluids. Critical structural steps include the formation of the magma chamber, magmatic vapor exsolution and collection of a hydrothermal fluid in cupola(s), and episodic hydrofracturing of the chamber roof in order to create the permeability that allows a hydrothermal fluid to rise along with a phenocryst-bearing magma. The interplay between stress produced by far-field tectonics and stress produced by buoyant magma and magmatic hydrothermal fluid creates the fracture permeability that extends from the cupola through an overlying ductile zone where temperatures exceed ~400°C into an overlying brittle zone where temperatures are less than ~400°C. As a consequence, during each fluid escape and magma intrusion event, the rising hydrothermal fluid ascends, depressurizes, cools, reacts with wall rocks, and precipitates quartz plus sulfide minerals, which seal the permeability fabric. A consistent vein geometry present in porphyry Cu deposits worldwide is formed by steeply dipping veins that have mutually crosscutting orientations. Two general orientations are common. The principal vein orientation generally consists of closely spaced sheeted veins with orientations reflecting the far-field stress. Subsidiary veins may be orthogonal to the main vein orientation as radial or concentric veins that reflect magma expansion and extensional strain in the wall rocks as they are stretched by ascent of the buoyant magma and fluids. Episodic magmatic-hydrothermal fluid-driven hydrofracturing creates permeability that is commonly destroyed, as well as locally enhanced, by vein and wall-rock mineral precipitation or dissolution and by wall-rock hydrothermal alteration, depending upon fluid and host-rock compositions. The pulsing character of porphyry Cu magmatic-hydrothermal systems, in part produced by permeability creation and destruction, creates polyphase overprinted intrusive complexes, associated vein networks, and alteration mineralogy that reflects temporal temperature fluctuations beginning at magma temperatures but continuing to low temperatures. Temperature oscillations locally allow external nonmagmatic fluids to access principally the marginal areas but also in some cases the center of the porphyry Cu ore zone at ~<400°C between porphyry dike emplacement events. Over time, the upper part of the source magma chamber at depth cools and crystallizes downward and is accompanied by diminishing magmatic fluid input upward, leading to cooling and isothermal collapse of the porphyry system. Cooling permits the access of external circulating groundwater into the waning magmatic-hydrothermal plume. Magmatic-hydrothermal fluids dominate at temperatures >400°C at pressures transient between lithostatic and superhydrostatic. The external, nonmagmatic saline formation waters or meteoric waters dominate the surrounding and overlying brittle crust at temperatures <400°C at hydrostatic pressures, except where they may mix with buoyantly rising magmatic-derived fluids. Exhumation requires substantial topographic relief, precipitation, and time (typically >1 m.y.) and may enhance overprinted relationships and telescope low-temperature on high-temperature hydrothermal alteration assemblages. Synmineral propagation of faults into or out of a porphyry Cu hydrothermal system in the brittle regime at <400°C can provide an escape channel through which a metalliferous fluid may depart, potentially to form lateral quartz-pyrite veins, overprinted polymetallic Cordilleran lode veins, or an epithermal precious metal-bearing deposit at shallow crustal depths.