Porphyry Copper Deposits

Transcription

1 Porphyry Copper Deposits This text is based mostly on the Economic Geology Notes by Ron Morton and the review of porphyrycopper deposits by Sillitoe (2010). The latter publication is exhaustive and is available through the ThULB in Jena. Definition: Porphyry copper deposits are large tonnage, low-to medium-grade deposits in which primary (hypogene) ore minerals are spatially and genetically related to felsic to intermediate porphyritic intrusions; the ore is dominantly structurally controlled. The deposits are magmatic hydrothermal with the sulfide and oxide minerals precipitated from saline aqueous solutions at elevated temperatures. They get their name from the porphyritic texture of the plug-like intrusions and dikes spatially associated with the mineralization. Multiphase intrusions are typical and one of these usually has an aplitic texture (sugary groundmass). The ore occurs in narrow, closely spaced veins (hydrofractures) within hydrothermally altered rock. This produces stockwork ore and what has been called disseminated ore, which is actually mineralization along numerous, closely spaced microfractures. To be mineable these deposits must be amenable to bulk mining methods. Geographical Distribution: Porphyry copper deposits occur throughout the world in a series of extensive, relatively narrow, and linear metallogenic provinces (see slides). They are predominantly associated with Mesozoic to Cenozoic orogenic belts in western North and South America and around the western margin of the Pacific Ocean. They also occur, to a lesser extent, in the Tethyan orogenic belt in eastern Europe and southern Asia as well as in Paleozoic orogens in Central Asia and eastern North America. There district location may be controlled by major regional faults such as the Rio Grande Rift in the southwestern United States. This global distribution of porphyry copper deposits is a function of the uneven distribution of magmatism through geological time, which is directly related to plate tectonics and plate configurations. The dominance of phanerozoic (Cenozoic and Mesozoic) deposits over those in older terrains is mostly due to preservation and exposure. Porphyry deposits form relatively near the surface, typically one to four kilometers depth and are subject to subsequent tectonism, erosion, and/or burial. Tectonic Setting: Porphyry copper deposits occur in a variety of tectonic settings and typically represent root zones or bottoms of stratovolcanoes and/or calderas in subduction-related, continental and island-arc settings. In the southwestern United States porphyry copper deposits are associated with granitic rocks emplaced in a continental setting, within or along the margins of calderas that are now largely eroded. In modern islands arcs that border the Pacific Ocean and along the continental arc that is associated with the Andes Mountains the deposits are related to both andesitic stratovolcanoes and calderas. Porphyry copper-gold deposits, such as those associated with Triassic and Lower Jurassic alkali intrusive and volcanic rocks in British Columbia, formed in an island-arc setting, although possibly during periods of extension. Porphyry copper deposits in the Yulong belt of Tibet are related to pull-apart basins in a large, post-subduction strike-slip fault. Size and Grade: Porphyry copper deposits are large and typically contain hundreds of millions of tons of ore, although they do range in size from tens of millions to billions of tons. Copper grades are highly variable ranging from 0.2 to more than 1.6%; they average about 1%. Other common economic metals associated with the deposits include molybdenum (0.005 to about 0.03%), gold (0.004 to 0.35 g/t), and silver (0.2 to 5 g/t). Average deposit size is 750 million tons at a copper grade of 0.75%. With low grades (0.2-greater than 1%) what makes these deposits profitable is their very large size and their shape. Most

2 are relatively near-surface and approximately cylindrical so they lend themselves to large mining schemes. It was actually at the Bingham Canyon porphyry deposit where the first large-scale bulk mining method took place. It started in 1899 and by 1907 there was an open pit mine extracting 6,000 metric tons a day. Today some porphyry copper mines extract more than 100,000 metric tons of ore per day. The maximum production for a 24 hour period is from Morenci in Arizona when 1.3 million tons of ore were removed. Chuquicamata in Chili produces more than 700,000 tons per day from an open pit that is 4.3 km long, 3 km wide and 850 m deep. Ore Minerals: Chalcopyrite is the main ore mined often with minor bornite and chalocite. Molybdenite is commonly present and other, minor minerals are enargite, tetrahedrite, tenentite, electrum, and cassiterite. The main gangue sulfide mineral is pyrite and magnetite is the main oxide mineral. Secondary or supergene minerals associated with porphyry copper deposits (see end of this section) are chalcocite, covellite, cuprite, chrysocolla, azurite, and malachite. It was supergene enrichment of many porphyry coppers that made them economically viable in the first place. Also economically mined at most porphyry copper deposits are gold and silver; most of the gold is micron size. For example Bingham Canyon has produced 23 million ounces of gold since 1900 making it the largest gold producer in the US. Bingham has also produced 190 million ounces of silver. Hydrothermal Alteration and Zoning: Hydrothermal alteration is defined as the chemical and mineralogical changes in rocks brought about by warm to hot aqueous fluids circulating or passing through those rocks. The fluid/rock interaction causes chemical and mineralogical changes in the rock as well as changes in the fluid composition. Obviously such changes depend on 1) the time and duration of the interaction, 2) the permeability of the rocks (alteration will be more intense along more permeable zones), 3) the composition of the fluid (acidic, basic, or neutral), and 4) the temperature of the fluids. Alteration is studied by a) economic petrology-looking at alteration minerals and assemblages present in the rocks and comparing them to unaltered rocks. This is done via hand samples, thin sections, and mineral chemical studies and 2) lithogeochemical studies looking at chemical changes that have occurred in the altered rock relative to an unaltered equivalent rock. In porphyry deposits hydrothermal alteration is widespread (km s) and is typically zoned on a deposit scale, as well as around individual veins and fractures (Fig. 15). Alteration zoning can be defined by distinct mineral assemblages. Typically the upper portions of the host intrusion become extensively fractured due to a) adjustment of the adjacent country rocks as the intrusion cools, and b) high vapor pressure of the fluid (first boiling and volume increase). These fractures are permeable zones along which the ore fluids migrate both laterally and vertically. Alteration zoning in porphyry copper deposits is based on 1) lateral and vertical movement of the hydrothermal fluid, 2) cooling of the hydrothermal fluid as it moves upwards and outwards accompanied by a possible decrease in pressure, and 3) degree of interaction with groundwater. This leads to distinct horizontal and vertical zonal sequences of alteration minerals in the intrusion and adjacent rock. These zones may be grouped into different mineral assemblages based on modal abundances and types of minerals present. Lateral Zonation: 1) Propylitic Alteration Zone: This is the outermost alteration zone and is widespread (300m up to 10km s). It is always present and is composed of chlorite, epidote, carbonate, and pyrite. Chlorite and epidote are the most common and abundant minerals with pyrite varying from trace to as much as 10%. This alteration type forms by conversion of the mafic minerals (hornblende, biotite, and pyroxene) to chlorite-epidote, and plagioclase going to epidote-carbonate and/or recrystallizing to albite. This zone grades outwards into unaltered rocks over a few hundred meters.

3 2) Argillic Alteration Zone: This is an area of clay alteration and is most extensive in rocks rich in plagioclase feldspar; it is not always present. Prominent minerals are kaolinite and montmorillonite both of which replace plagioclase. Pyrite is always present. When it occurs this alteration zone is typically narrow and lense-like. 3) Sericitic or Phyllic Alteration Zone: Sericite, quartz, and pyrite are the dominant minerals in this zone accompanied by minor amounts of rutile, chlorite, and ilmenite, Significant amounts of ore are associated with this alteration type with the ore minerals dominated by chalcopyrite with or without molybdenite and gold. 4) Potassic Alteration Zone: This is an inner or core zone characterized by potassium feldspar and/or biotite with or without amphibole, magnetite, anhydrite, and fluorite. Associated ore minerals may be chalcopyrite, bornite, chalcocite, molybdenite, and gold; pyrite is usually a minor constituent. Alteration mineralogy is controlled in part by the composition of the host rocks. In mafic host rocks with significant iron and magnesium, biotite (and possibly minor hornblende) is the dominant alteration mineral in the potassic zone, whereas potassium feldspar dominates in more felsic host rocks. In carbonate-bearing host rocks, calc-silicate minerals, such as garnet and diopside, may be abundant. Alteration mineralogy is also controlled by the composition of the mineralizing system. In more oxidized environments minerals such as pyrite, magnetite, hematite, and anhydrite are common, whereas pyrrhotite is present in more reduced environments. The alteration styles, their main minerals and their potential as ore contributors are summarized in the table below. alteration type position in system key minerals possible ancillary principal sulfide economic sodic-calcic potassic (Ksilicate) propylitic chlorite-sericite sericitic (phyllitic) advanced argillic (abundance) deep, including below porphyry Cu deposits (uncommon) core zones of porphyry Cu deposits (ubiquitous) marginal parts of systems, below lithocaps (ubiquitous) upper parts of porphyry Cu core zones (common, particularly in Au-rich deposits) upper parts of porphyry Cu deposits (ubiquitous, except with alkaline intrusions) above porphyry Cu deposits, constitutes lithocaps (common) albite/oligocla se, actinolite, magnetite biotite, K- feldspar chlorite, epidote, albite, carbonate chlorite, sericite/illite, hematite quartz, sericite quartz (partly residual, vuggy), alunite, pyrophyllite, dickite, kaolinite minerals diopside, epidote, garnet actinolite, epidote, sericite, andalusite, albite, carbonate, tourmaline actinolite, hematite, magnetite cabonates, epidote, smectite pyrophyllite, carbonate, tourmaline, specularite diaspore, andalusite, zunyite, corundum, dumortierite, topaz, specularite assemblages typically absent pyrite-chalcopyrite, chalcopyrite +- bornite, bornite +- digenite +- chalcocite pyrite (+- sphalerite, galena) pyrite-chalcopyrite pyrite +- chalcopyrite (pyriteenargite +- tennantite, pyritebornite +- chalcocite, pyritesphalerite) pyrite-enargite, pyrite-chalcocite, pyrite-covellite potential normally barren, but locally ore bearing main ore contributor barren, except for subepithermal veins common ore contributor commonly barren but may constitute ore locally constitutes ore in lithocaps and their roots

4 Vertical Alteration: Porphyry copper deposits also exhibit a vertical zonation of alteration minerals that is distinct from the lateral zonation assemblages. Vertical alteration zones are not common at porphyry copper deposits because 1) most of the overlying rocks have been eroded away and 2) they will only form where there has been little interaction between the hydrothermal fluid and ground water.. The alteration assemblages and zonation associated with this alteration were determined by deep drilling in recent volcanic terrains and partial preservation at more recent discoveries, particularly those in active island arc environments. The vertical alteration has been subdivided into 3 zones which are referred to as alsic A, P, and K. Host Rocks: Common Characteristics: a) Associated igneous intrusive rocks range in composition from diorite and quartz diorite to granodiorite and quartz monzonite. Some are alkalic and include monzonite and syenite porphyries. The intrusions are typically cylindrical stocks or broad, domal-shaped near-survace intrusions. b) Multi phase intrusive events or phases are common and range from 2-3 to more than 14 (Henderson); there are 7 at Bingham. Host to ore is often the most differentiated and youngest phase present. c) One or more intrusive phases is porphyritic (sometimes all phases are porphyries). Phenocrysts range from 30-55%. Another phase present is an aplitic rock; one with a fine-grained sugary texture (caused by pressure quenching of the porphyry due to rapid ascent and volatile loss). At depth these porphyritic intrusions merge into the same underlying magma chamber. d) Plagioclase is always present with hornblende common in rocks of intermediate composition and biotite, potassium feldspar, and quartz common in more silicic rocks. Rocks containing plagioclase, potassium feldspar, quartz, biotite and/or hornblende formed at o C with more than >4% water in the magma. In alkalic intrusions the minerals typically present are plagioclase, sodic-rich clinopyroxene, leucite, and garnet. Accessory minerals may include apatite, zircon, magnetite, titanite, fluorite, and monazite e) Breccia pipes (diatremes) are common and may represent hydrothermal explosions caused by rapid fluid rise and, in the near surface, rapid expansion. This cause rocks to fracture and break, and may also lead to the flashing of water to steam leading to phreatic explosions. f) All rocks have been extensively altered. g) Veins in the rocks are ubiquitous, They form throughout the life of the porphyry magmatichydrothermal system (as do breccias). Veins contain a large % of the ore minerals and represent the locus of greatest fluid transport. h) Bottoms of porphyry deposits (areas below the ore deposit) are regions of abundant quartz veins, porphyry dikes, and widespread alteration that may include calcic (garnet, pyroxene, epidote) skarn on the flanks of some systems, and greisens (coarse-grained aggregates) of muscovite-quartz, which occurs directly beneath the ore (SW USA). Ore Geometry: Igneous host rocks constitute the central core of the orebody (or all of it). Ore may then extend outwards for a variety of distances into the country or wall rocks. Distribution of the ore is based on a) composition of wall rocks, b) degree of fracturing and faulting. Because most intrusions are steep-walled cylinders there is a strong tendency for the deposits to exhibit a concentric or shell-like pattern of sulfide and alteration minerals. At first inspection the mineralization appears to be widely disseminated (gives the rock a salt and pepper look; however on a microscopic scale it can be seen that most sulfide minerals are actually located along networks of microfractures (stockworks). This small scale fracturing only extends

5 outwards as far as the sulfide-alteration assemblages and is believed to be due to first boiling of the hydrothermal fluid. The mineralization tends to exhibit an inverted cup shape with: a) A low grade core composed of minor chalcopyrite often accompanied by molybdenite and gold. Bornite and chalcocite may be present in minor mounts. This zone has low amounts of pyrite. b) The main ore zone composed of chalcopyrite (1-4%) and gold with or without silver. Bornite and chalcocite are absent and there are trace amounts of arsenopyrite, and cassiterite. c) A pyrite rich shell (10-20%) with minor chalcopyrite (0.1-1%). d) outward low-pyrite shell (2-5%) with traces of chalcopyrite. Genetic model: Porphyry-Cu systems are typically located in the shallow portions (< 4 km) of the crust. The magma chambers which generate such systems must be large enough (> 50 km 3 ) to liberate enough fluid. The water-rich fluid is released during cooling and crystallization of magma and new pulses of fluids may be delivered to the upper parts of the magma chamber via convection. Fluid loss by explosive volcanism has negative consequences on the potential of a given magmatic system to generate porphyry-cu deposits. The parental magma must be water-rich (> 4 wt % H 2 O) and oxidized to carry the metals. Magmas which are not water-rich do not generate enough fluid and the metals will not be concentrated in the fluids. Reduced magmatic environments may support crystallization of pyrrhotite and concentration of the metals in this mineral rather than in the fluid. In the early stages of evolution, the porphyry-cu systems are dominated by a single, low-salinity (2-10 wt% NaCl) aqueous fluid with X000 ppm of base metals and X ppm Au. Later and at more shallow depths, the fluid splits into two, a hypersaline liquid (brine) and low-density vapor. Coexistence of such fluids has been repeatedly shown by fluid inclusion studies. The brine may contain up to 70 wt %. Most of the metal precipitation is assigned to a later stage when the fluids cooled to ºC and interacted extensively with the wall rock. The ore minerals precipitate in a network of veinlets and as disseminated in the rocks. These structures are the result of hydraulic fracturing. In the latest stages, there is influx of meteoric water into the cooling and aging magmatic system. At this point, epithermal mineralizations may be formed; these are the subject of a separate lecture. References Burnham, C.W., 1997, Magma and hydrothermal fluids, in: Barnes, H.L., ed., Geochemistry of Hydrothermal Ore Deposits, 3rd edition: Wiley Interscience, New York, p Candela, P.A., and Piccoli, P., 2005, Magmatic processes in the development of porphyry-type ore systems, in: Hedenquist, J.W., Thompson J.F.H., Goldfarb, R.J., and Richards, J.R., eds., Economic Geology 100th Anniversary Volume: Society of Economic Geologists, Littleton, Colorado, p Goldfarb, R.J., and Nielsen, R.L., eds., 2002, Integrated Methods for Discovery: Global Exploration in the Twenty- First Century: Society of Economic Geologists, Special Publication 9 Lipman, P.W., and Sawyer, D.A., 1985, Mesozoic ash-flow caldera fragments in southeastern Arizona and their relation to porphyry copper deposits: Geology, v. 13, p Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits: Economic Geology, v. 65, p Schroeter, T.G., ed., 1995, Porphyry Deposits of the Northwestern Cordillera of North America: The Canadian Institute of Mining and Metallurgy, Special Volume 46. Seedorf, E., Dilles, J.D., Proffett, J.M., Jr., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A., and Barton, M.D., 2005, Porphyry deposits: characteristics and origin of hypogene features, in: Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and Richards, J.R., eds., Economic Geology 100th Anniversary Volume: Society of Economic Geologists, Littleton, Colorado.

6 Sillitoe, R.H., 2010, Porphyry Copper Systems. Economic Geology 105, Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious metal deposits, in: Simmons, S.F., and Graham, I., eds., Volcanic, Geothermal, and Ore- Forming Fluids; Rulers and Witnesses of Processes within the Earth: Economic Geology, Special Publication 10, p Sillitoe, R.H., and Bonham, H.F., Jr., 1984, Volcanic landforms and ore deposits: Economic Geology, v. 79, p Sillitoe, R.H., 1993a, Epithermal models: genetic types, geometrical controls and shallow features, in: Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M., eds., Mineral Deposit Modeling: Geological Association of Canada, Special Paper 40, p b, Gold-rich porphyry copper deposits: geological model and exploration implications, in: Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M., eds., Mineral Deposit Modeling: Geological Association of Canada, Special Paper 40, p Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits: Economic Geology, v. 68, p Westra, G., and Keith, S.B., 1981, Classification and genesis of stockwork molybdenum deposits: Economic Geology, v. 76, p