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ECONOMIC GEOLOGY

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Ashutosh Singh
University of Delhi (DU), New Delhi

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Porphyry Copper Deposits Porphyry-type ore bodies are low-grade, high-tonnage accumulations of mineralized rock associated with intrusive magmatic bodies. They are most commonly mined by open-pit methods, and are by far the biggest single orebodies exploited at present. They can range from hundreds to thousands of millions of tonnes of ore Example: Giant Chuquicamata deposit in Chile In India Malanjkhand Deposit in Madhya Pradesh. The intimately associated igneous rocks with these deposits are normally porphyritic (that is, they contain large crystals) and range from intermediate to felsic in composition. They include diorites, monzonites, granodiorites, tonalites alkalic intrusions 1 Almost all porphyry deposits form within volcanic arcs, both in continental arcs (e.g. the Cordillera of western North and South America) and in intra-oceanic island arcs (e.g. the 'Southwest Pacific' arcs of Papua New Guinea, Solomon Islands, etc.) The majority of porphyry deposits are relatively young ( < 75 Ma). The ore bodies are continuous bodies, hundreds of metres to a couple of kilometres in diameter and depth extent, centred on one or more of the small intrusions of the magmatic centre. In most economic porphyries, ore grade is uniform or varies gradually with position except where the body is cut by syn- or post-mineralisation intrusions. Copper is the most common commodity but they are also important sources of molybdenum and gold. Smaller quantities of other metals, including Ag, Sn, W, and Pd can also be refined from porphyry copper ores. Mineralization in porphyry deposits is widely disseminated throughout the host rocks and ore grades are very low, but because of their large size, the orebodies can be mined profitably. 2 Porphyry mineralization commonly encompasses large volumes of the surrounding host rocks to the intrusion. These can include igneous, metamorphic, and sedimentary rocks ranging in age from Precambrian to Phanerozoic. The emplacement of porphyry bodies is usually controlled by regional fault structures and zones of fractured rock. The intrusive bodies themselves may be composed of a single intrusion or of multiple intrusions. The intrusions are passively emplaced into the surrounding host by stoping and assimilation. Porphyry ores were emplaced at relatively shallow levels in the crust (less than 4 km) and that they may have provided the magma source for the generation of large volcanoes on the surface which have since been eroded away. Where several intrusions of magma are present, it is common for mineralization to be related to the latest intrusions, which tend to be most differentiated. The phenocryts in the intrusions indicate that their magmas were partially crystalline when emplaced and that crystallization of the remaining melt occurred rapidly. Two types of orogenic belts host porphyry deposits: those created by the subduction of oceanic crust beneath continental crust along a continental margin (e.g., west coast of South America) 3 and those found along island arcs where two oceanic plates are colliding. Most deposits are of Mesozoic or Cenozoic age, but some are Palaeozoic. Older Precambrian deposits are difficult to recognize because of later deformation and erosion, but some rare examples have been found. The Porphyry Cu deposit may be - totally within the host rock partially in the stock and partially within the country rocks - in the country rocks only The mineralization tends to occur in concentric zones (the ore minerals themselves are alteration minerals and are formed by the same chemical processes that resulted in the hydrothermal alteration) From Centre onwards - Barren or low grade zone with minor chalcopyrite and molybdenite, pyrite a few to 10% (disseminated ore) - Molybdenite increases - Chalcopyrite increases (Veinlet ore) 4 - Pyrite mineralization increased to form peripheral pyrite rich halo (10 -15% pyrite) and minor chalcopyrite and molybdenite. The magmas that form porphyry copper deposits are thought to be generated by the melting of subducted oceanic crust. This crust contains a high concentration of water because of the presence of water bearing minerals. When oceanic crust is heated, it dehydrates and then melts to produce magma into which the water can dissolve. The magma is less dense than the surrounding mantle rocks; it rises and penetrates the lower crust, where further melting and assimilation of crustal rocks can occur. These modified magmas can rise up to high levels in the crust. During their ascent, the pressure drops, causing the water dissolved in the magmas to separate a process referred to as first boiling . The rising magma may also begin to crystallize as it ascends, generating crystals of minerals such as plagioclase. These crystals eventually form the phenocrysts found in porphyritic rocks. The exsolution of water cools the remaining magma and induces more rapid crystallization before it can reach the surface. Further crystallization results in more water being expelled in a process called second boiling . 5 The outer surface of the intrusive body cools more rapidly, forming a carapace of essentially solid rock at temperatures much lower than its centre. This confines the remaining partially molten interior and any water exsolving from it, resulting in a large increase of pressure (Fig. a). Early-stage crystallization of the outer regions of the intrusion confines partially crystallized magma and its exsolved brine. This causes a build-up of pressure in the intrusion, which is released by fracturing of the carapace and its surrounding host rocks. When the internal pressure builds up to a high enough level, the carapace fractures and the high-temperature fluids are released upwards into the solidified porphyry and its surrounding host rocks (Fig. b). 6 High-temperature brines are released into the fractured rocks, resulting in potassic alteration and low-grade copper mineralization. The release of pressure and loss of fluid cause further crystallization deeper into the intrusion. The release of these fluids and the concomitant drop in pressure induces crystallization deeper in the intrusive mass, and the cycle begins anew. This cyclic process continues and the carapace and the fractures generated within it migrate downward to greater depths (Fig. c). As the influx of magmatic fluids wanes, meteroic waters invade the solidified intrusion, become heated, and react with the rock, causing phyllic alteration. These circulating fluids can also generate argillic alteration of the periphery of the phyllic zone as well as propylitic alteration further from the intrusion. Eventually, confining pressures, caused by the overlying rocks, rise to the point where fracturing cannot occur, and the process ceases. 7 Although water is an important component released from the magma as it crystallizes, other components can also be expelled from the melt. Trace metals in the magmas may also partition into the brines, causing enrichment of valuable metals in these high-temperature fluids. When the brines are expelled during fracturing, they pass through rock with which they are no longer in chemical equilibrium. This results in an exchange of chemical components between rock and fluid, a process known as hydrothermal alteration . This exchange causes changes to the fluid as well as to the wall rock. At the same time, cooling of the fluid can occur. Taken together, these two processes induce gradual changes in the physical and chemical properties of the fluid. These changes bring about the precipitation of ore minerals in the fractures, together with other gangue minerals such as quartz and potassium feldspar. The hydrothermal alteration generated during the formation of porphyry deposits is one of their distinctive features. 8 In the idealized model, alteration zones centred on the porphyry intrusion consist of the potassic, phyllic, argillic, and propylitic zones. The central potassic zone is characterized by the presence of potassiumbearing minerals such as orthoclase and biotite. Anhydrite, chlorite, and sericite may also be present. Veins in the potassic zone are filled with minerals similar to those found in the altered wall rock. It is caused by brines which are composed of a high proportion of magmatic fluid and have very high temperatures (400 700 C). In the early stages of development these magmatic fluids are expelled upwards and outwards into fractured rock which, at the same time, prevents the incursion of more dilute fluids at lower temperatures from the surrounding wall rock (Fig. 1b). Phyllic alteration surrounds the potassic core and has the mineral assemblage quartz sericite pyrite. During the formation, most of the original silicate minerals in the rock are broken down by the hydrothermal fluids and replaced by sericite or clay minerals, or both. Excess silica is generated which forms quartz. Iron is released from the alteration of iron bearing minerals as well as it is added by the fluid form pyrite. 9 This alteration is due to incursion of cooler meteoric water into the porphyry environment. This water gets heated, rises and reacts at the same time with the host rocks. Due to continuous supply of meteoric water convection is set around the periphery of the intrusion overprinting the pre-existing potassic assemblage. Argillic alteration is normally found on the periphery of the phyllic zone, and is characterized by the presence of clay minerals like kaolinite and montmorillonite. These minerals typically form when hydrothermal alteration is caused by acidic heated fluids. Most minerals are unstable when in contact with such fluids and break down, releasing most of the metals in their structures. During this process, the rock acts as a neutralizer of the acidic fluid, and metals are exchanged for hydrogen ions. Metals such as silicon and aluminum are less soluble in these fluids and remain as quartz and clay minerals. Isotopic evidence shows that argillic alteration is caused by hydrothermal fluids composed mainly of meteoric water. The propylitic alteration zone extends outward from the intrusion into less altered host rocks. 10 It is identified by the common occurrence of chlorite and calcite as alteration products of biotite and hornblende in the host. Other minerals that are present in this zone include pyrite and epidote. Propylitic alteration represents the weakest alteration found in porphyry coppers. It is caused by heated convecting meteoric water. Ore minerals in porphyry deposits are commonly found in concentric zones around the intrusion, much like the alteration zones. The central potassic core normally contains low-grade mineralization consisting of minor chalcopyrite, molybdenite, and pyrite. These minerals occur in dense microfractures in the altered host intrusion. Along the contact between the potassic and phyllic alteration zones, higher-grade ore is found; this consists of chalcopyrite, molybdenite, and pyrite hosted in microfractures and larger fracture networks. The total sulphide mineral content may be as high as 10 15 per cent; copper concentrations vary from 5 to 10 kg per tonne of ore. Mineralization in the argillic and propylitic alteration zones is typically of low grade and uneconomic. 11 Generalized model for porphyry Cu deposits showing relation of ore minerals, alteration zoning, supergene enrichment and associated skarn, replacement, and vein deposits. Some Important points on Porphyry Cu deposit May be - Totally within the host stock - Partially within the stock and partially within the country rock - In the country rocks only They - Occur in subvolcanic environment - Associated with some high level stocks - Associated with subaerial calc-alkaline volcanics - Zones of alteration close upwards - Potassic zones die out - As the upper limit of economic mineral is reached silicic and argillic alteration important - Downwards porphyry changes to equigranular - Intrusions have low silica/alkali ratio - Host pluton may be Syenite Monzonite Diorite Or alkali intrusions - Biotite may be the most prominent K mineral when orthoclase may not develop, plagioclase is main feldspar - Significant gold may occur and Mb/Cu is usually low - Quartz less chlorite, epidote, and albite fairly common Accessory metal Either Mb continental crust Or Au island arc Dimensions are enormous which means 12 very large volume of rock (both country rock and parent plutons) has been permeated by hydrothermal solution Crackle brecciation - Brecciation occurs due to expansion as volatiles are released from the magma Fractures are filled with the mineralization to form stockwork Its zone is circular in outline Larger than the ore body Fades out in propylitic zone The host magma of the deposit reaches about 0.5 to 2 km of the surface. This leads to equigranular crystallization in its outer portion. As crystallization proceeds, anhydrous minerals form and liquid magma becomes richer in volatiles. So vapour pressure increases. If vapour pressure exceeds confining pressure then retrograde boiling occurs. If retrograde boiling occurs in largely consolidated rock tensile strength of the rock acts against the vapour pressure to rise above the confining pressure. So expansion takes place and extensive and rapid brecciation takes place. Crystallization and bubble formation causes absorption of heat so that in the central part of the pluton T of the system falls. So rapid cooling takes place at a later stage. This results in increase in nucleation sites and rapid nucleation takes place. 13 So a fine grained groundmass is formed. That is why the intrusion is porphyrytic in nature. As retrograde boiling takes place Aquous phase (hydrothermal solution) is produced. Fugacity of oxygen (fO2) controls fractionation of S into the aquous phase Fugacity of oxygen (fO2 ) in a magma is largely determined by the Fe3+/Fe2+ ratio of the magma Fe3+/Fe2+ ratio depends on the source rock that has generated the magma So Aquous fluid separated from I type magma with higher fO2 tend to produce S rich porphyry Cu mineral Aquous fluid from S type magma may deposit S poor Sn oxide mineralization S isotope studies show that S in porphyry Cu deposit is largely of upper mantle or remelted oceanic crust origin Water associated with sericites from the phyllic zone of alteration are depleted in O18 relative to Bt of the potassic zone Connate waters from the country rocks are involved in the sericitization Whereas Meteoric water is involved in phyllic alteration by hydrothermal fluids Field and microscope data show that Potassium silicate and propyllitic alteration Phyllic and argillic alteration (later) 14 Porphyry body cools, solidifies and generates magmatic hydrothermal fluid - This fluid reacts with the porphyry and the surrounding rocks - This gives rise to central potassic alteration. At this stage much of metals and S are probably introduced. - Away from the intrusion convective circulation of water takes place due to thermal gradient & Propyllitic alteration takes place - As the intrusion cools, meteoric connate hydrothermal system mineral and lower T minerals sericite, propyllite, clay minerals form. - They replace part of potassic alteration zone minerals such as feldspar and biotite. - Economic deposit of porphyry Cu deposit forms if the parent magma is enriched in Cu. - Cu prefers octahedral sites in both melt and crystalline phases - Magmas with more octahedral sites will contain more Cu in the melt at the retrograde boiling stage and more likely to give rise to Porphyry Cu Deposits - Octahedral sites are directy proportional to alumina : alkali ratio & Mineralized intrusions have higher ratio than barren intrusions - If Cu behaves as an incompatible element during early stages of crystallization due to poor development of magnetite and augite Then The melt becomes enriched in Cu - Metal distribution in porphyry deposits strongly correlates with the regional Time Space distribution of host intrusions And not the Host of the intrusion itself - Porphyry Cu deposits are subduction controlled The final concentration of Cu into stockworks is an upper crustal process 15 Porphyry Mo - Have many features common with Porphyry Cu deposits Average grades 0.1 0.45 MoS2 Sometimes Sn W may be produced as a by product Stockworks are more important than disseminations Shape of the ore body may be like Inverted cup hollow cylinder Tabular inverted bowl Porphyry Sn & W - Both these deposits have many features similar to the porphyry Cu deposits. Large volume of rocks contain 0.2 0.3% Sn Potassic zone of alteration is absent The stockworks have inverted cone shape Sericitic alteration grades outwards into propylitic alteraion and pyrite halos may be present In porphyry W hydrothermal alteraion zones do not show a regular concentricity Grades ~ 0.2 W with Mo 16

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