Volcanic Massive Sulfides

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1 Volcanic Massive Sulfides This text is based on the review papers of Gibson et al. (2007) and Galley et al. (2007). Additional literature is listed in the references. Definition. The ore deposits of this type are called volcanogenic, volcanic-associated, or volcanichosted massive sulfide deposits, abbreviated as VMS or VHMS deposits. These terms are used in the literature as synonyms. The VMS deposits are syngenetic, stratabound to stratiform accumulations of massive to semi-massive sulfides. The term syngenetic means that they are formed as a part of a volcano-sedimentary sequence, at the same time as the neighboring parts of the sequence. The overlying rocks are then younger. The term stratiform refers to the strata-like (layer-like) shape of parts of the VMS deposits. The term stratabound also implies a layer-like form but additionally says that the mineralization was spatially controlled by a pre-existing layer. Even though this may seem to contradict the syngenetic origin of the VMS deposits, it is not so. As we will see later, the rich accumulations of massive sulfides may have formed in the uppermost, perhaps unconsolidated sediments at the bottom of the ocean. These VMS deposits are then bound to this layer (stratum) and therefore called stratabound. The last important piece of information is that the VMS deposits form always at or near the seafloor in submarine volcanic environments. Tectonic settings. As mentioned above, the VMS deposits form in submarine volcanic settings. That does not mean, however, that any submarine volcanic activity can develop a VMS accumulation or a deposit and does not explain why in some cases, VMS are present and in others, they are not. The VMS deposits form preferentially during episodic rifting of oceanic and continental volcanic arcs, fore arcs, and in back-arc extensional environments. Note that these are all divergent, extensional environments, but VMS accumulations are rare at the mid-ocean ridges where the rifting is permanent over longer geological periods of time. The episodic nature of rifting appears to be very important. Rifting causes crustal thinning and crustal thinning, in turn, causes depressurization of the upper mantle. A pressure drop in the upper mantle induces partial melting and rise of the buoyant basaltic magmas upwards. These magmas reach the base of the thinned crust and pond (accumulate) there. If the mantle delivers a sufficient amount of basaltic magmas and the rifting event is longer-lived, the basaltic magma may cause melting in the lower crust. These melts, however, have normally intermediate or acidic character and are able to rise through the crust to shallow crustal levels. The other vital contribution of crustal thinning to the origin of VMS deposits is the development of an extensive fault system along which the fluids may circulate. Faulting is easy to understand in an extensional environment. Faults penetrate all members of the pre-existing volcano-sedimentary sequence and permit efficient hydrothermal fluid circulation. Geographic and age distribution: The present-day geographic distribution of the VMS deposits reflects the existence and post-formation tectonic history of divergent settings in which the VMS deposits formed (see above). Therefore, when plotted on a global map, the VMS deposits appear to be scattered over all continents and no clear pattern can be discerned. In contrast to the other types of deposits discusses in this course, however, there are also known VMS deposits, albeit only of small size, in the oceans. They originate now in the vicinity of the mid-ocean ridges and can be studied by all modern techniques during the course of their formation. Here, it must be pointed out that similar deposits of geological, older age are usually not preserved because they were subducted. Deposits preserved in the geological record formed in oceanic and continental nascent arcs, rifted arcs and back arc settings (as

2 described above). In terms of the age, the VMS deposits range from 3.4 Ga old in Pilbara Block in Australia until currently active black smokers fields, for example in the Atlantic ocean. Geometry. VMS deposits consist of two essential parts. One of them is a concordant lens of massive sulfides, commonly with > 60 % of sulfide minerals. The other one is a discordant vein-type sulfide mineralization called the stringer or stockwork zone. The size, grade, and importance of the two parts of the VMS deposits is variable. In some of them, the massive sulfides are mined and the stringer zone is of little relevance. In others, larger quantities of ores are also extracted from the stringer zone. For sketches of the VMS deposits with the two parts shown, see the slides below. There may be also additional geological units associated with the VMS deposits; these units are of no economic interest and they are more closely described in the section about the genesis of these deposits. Mineralogy of the VMS deposits. The ore mineralogy of the VMS deposits is usually very simple. The dominant minerals are pyrite, pyrrhotite, chalcopyrite, galena, and sphalerite. From the sulfate minerals, anhydrite and barite may be present. Gangue minerals include quartz, sheet silicates, iron oxides, and a variety of alteration products of the host rocks; the nature of these minerals will of course depend on the chemical composition of the host rocks. Size and grade. There were about 800 VMS deposits known in the world several years ago, this number probably growing slowly. Their size varies from 200,000 tons of ore (small) to giant deposits. One of the largest VMS deposits is Neves Corvo in Spain, within the famous Iberian Pyrite Belt. This belt alone contains 88 VMS deposits, with estimated 62.6 Mt of Zn+Cu+Pb. Most of the VMS deposits are small, only a few are large (so-called giant deposits ), and even fewer still larger (so-called supergiant deposits ). The massive sulfide lenses contain > 60 % sulfides which secures in some deposits their economic viability. Therefore, the VMS deposits are commonly smaller and rich, not the ideal combination of properties for the modern mining industry. The currently forming VMS accumulations in the black-smoker fields are all too small for exploration. Classification. The VMS deposits are commonly classified according to their base metal content, gold content, and host-rock lithology. The base-metal division is the simplest one, recognizing Cu-Zn, Zn-Cu, and Zn-Pb-Cu groups according to the ratio of these elements. Au-rich VMS deposits are arbitrarily defined as those where the gold content (in ppm) exceeds the Pb+Zn+Cu content (in wt%). A classification which takes into account the geological factors is that developed by Barrie and Hannington (1999) and presented in a significantly simplified form in Table 1. Here, the deposits are defined based on the dominant host-rock lithology. This may include mafic volcanic rocks, intermediate to acidic volcanic rocks, and siliciclastic sedimentary rocks. Note that the volcanic rocks may be present as effusive bodies flows, domes, sills, and dikes. Alternatively, the volcanic rocks may occur in the form of explosive, pyroclastic rocks such as tuffs and tuffites. A combination of effusive and explosive forms is called bimodal and is of great importance for the VMS deposits (see Table 1). The siliciclastic sediments include the usual and common rocks such as shales and greywackes.

3 Table 1. VMS deposit types, facies, tectonic settings. Simplified after Gibbson et al. (2007), with additional data from Galley et al. (2007). type typical facies tectonic settings (inferred for Archean examples) examples mafic Dominantly mafic flow and 10 % felsic mature intra-oceanic South Urals, Russia; Troodos, flows/domes. Mafic sills and dikes common, minor shale and chert (ophiolite assemblage). backarcs Cyprus; Pontides, Turkey maficsiliciclastic Mafic sills, subordinate flows with shale, sedimented mid-ocean Outokumpu, Finland; Besshi carbonaceous shale, minor chert, trace to ridges, transforms or district, Japan absent felsic volcanic facies. backarcs bimodal-mafic Dominantly mafic flows with up to 25 % felsic rifted oceanic arcs Abitibi, Flin Flon, Canada; flows/domes and subordinate felsic/mafic Tambo Grande, Peru volcaniclastic rocks and terrigenous sedimentary rocks (greywackes, shales). bimodal-felsic Felsic flows/domes, volcaniclastic rocks dominant with % mafic flows and sills and <10 terrigenous sediments (greywackes, shales). Some portions of succession may be subaerial or shallow water. highsulfidation Sub-type of bimodal-felsic deposits. See above. bimodal felsic felsicsiliciclastic Siliciclastic rocks with up to 75 % felsic volcaniclastic rocks with subordinate flows/domes with < 10 % mafic flows and sills and minor Fe- and Mn-rich sedimentary rocks (iron formation). Some portions of succession may be subaerial or shallow water. continental margin arcs and related backarcs mature backarcs epicontinental Skellefte and Bergslagen, Sweden; Hokuroko, Japan Iberian Pyrite Belt, Spain and Portugal; Jibelt and Guemessa, Morocco Genetic model. As mentioned above, the VMS deposits form at the bottom of the oceans, in episodic rifting settings. These deposits form from hot aqueous fluids, that is, hydrothermal fluids, which are discharged at the ocean bottom. Generation of a sufficient amount of ore-bearing hydrothermal fluids requires a source of heat, a source of the fluids, and a source of metals. For the VMS deposits, each source is different. Heat source is usually a large, sill-like, synvolcanic hypabyssal intrusion. Such intrusion can drive and sustain a long-lived hydrothermal system. Note that hypabyssal means that the intrusion is emplaced at shallow depths (i.e., similar to some volcanic rocks) but solidifies in the depth (i.e., similar to plutonic rocks). Hypabyssal rocks are usually considered to be more closely related to volcanic rocks, although they do not reach the surface. Fluid source is largely seawater, although isotopic studies show also magmatic signature in some of the fluids. Seawater penetrates into the volcano-sedimentary sequence along deep, synvolcanic faults which form readily in the extensional environment. Metals are leached from the volcano-sedimentary rock sequence by the hot circulating fluids. In other words, the intrusion itself delivers little or no metals. This is a marked difference from other magmatic deposits, for example, the porphyry-cu deposits. The fluids not only leach metals but cause extensive, massive alteration of the host rocks. The massive sulfides form at or near the seafloor. Sulfides precipitate when the hot, reduced, and slightly acidic hydrothermal solutions mix with cold, oxidized, and slightly basic seawater. Sulfide precipitate where the hydrothermal solutions reach the seawater, with the most spectacular mixing sites being the black or the white smokers. However, mixing of the two types of solutions and formation of rich ores is much more effective within the uppermost layer of sediments. These sediments are highly

4 porous and prone to replacement by newly formed minerals (e.g., sulfides). Therefore, the richest massive sulfide accumulation originate probably from the systems where the hydrothermal solutions where discharged in a diffused manner through the porous sediments, not in a localized manner through the smokers. Metals can re-distributed by later hydrothermal action (see zone-refining in this text). Far-away, so-called distal, products represent a hydrothermal contribution to the background sedimentation. They are called exhalites in this case and are an important exploration feature. From an economic point of the view, exhalites have no value. They are silica and iron-rich sediments and may be reminiscent of banded-iron formation in the field. Note that in terms of their origin, they have nothing common with the banded-iron formation (as far as we believe that we can explain the formation of BIF). Zonation of the VMS deposits: There are two types of zonation of the VMS bodies. One of them is the metal zonation in the massive sulfide lens which will be described in this section. The other type is the alteration zonation which will be described later. Progressive deposition of metal sulfides at or near the ocean bottom results in the formation of a complexly textured, semi massive to massive sulfide mound. The flow of hydrothermal fluid through the mound structure commonly results in remobilization of previously deposited metals along a chemical and temperature gradient perpendicular to the seawater interface. This process is referred to as zone refining and results in a chalcopyrite-rich core and a sphalerite±galena-rich outer zone. In extreme cases, much of the base and precious metals can be remobilized out of the sulfide mound and carried into the seawater column by venting hydrothermal fluids. Massive pyritic cores and thin, base- and preciousmetal enriched outer margins are a characteristic of VMS deposits that have had a protracted thermal history. Alteration. Alteration around the bodies of the VMS deposits is extensive and comprises distinct zones. The character of alteration depends not only on temperature, that is, the distance from the magmatic body, but also largely on the rock/water ratio. In the deeper portions of the hydrothermal system, there is more rock than water on the volume basis and the fluids are hot (up to 400 ⁰C) and acidic. In the upper portions, on the other hand, the fluids are cooler (up to 300 ⁰C), only slightly acidic, and, most importantly, there is more water than rock on the volume basis. The distribution of the resulting alteration mineral assemblages mimic that of regional metamorphic facies. The high-temperature alteration zone is characterized by Fe-Ca-rich amphibole, clinozoisite, Caplagioclase, and magnetite and corresponds roughly to the amphibolite facies. Above this zone, a zone with albite, quartz, chlorite, actinolite, and epidote is found, corresponding approximately to the greenschist facies. Closer to the seafloor are zeolite-clay and related sub-greenschist mineral assemblages characterized by K-Mg-rich smectites, mixed-layer sheet silicates and K-feldspar. In addition to the mineralogical changes, the zones can be also mapped and described by geochemical or isotopic work. Environmental impact. The VMS deposits may have unusually negative impact on the environment, owing to two factors. First of all, they are very rich in sulfides. The parts where pyrite is the only sulfide mineral are not interesting from the economical point of view and are disposed of. Pyrite weathers quickly in contact with the atmosphere, releases divalent iron, sulfate, and protons into water, hence decreasing the ph of these solutions very markedly. This environmental problem is called acid-mine drainage (AMD) and is, in fact, most developed in the former mining districts of VMS deposits, such as the Rio Tinto district in the Iberian Pyrite Belt (Spain) or at the Iron Mountain site (California, USA). At the latter site, ph of the waters attained a record value of 3.6 (Nordstrom and Alpers 1999). The second factor are the rocks in which these deposits occur. As shown above (Table 1), these are mafic or

5 felsic volcanic rocks and siliciclastic sediments. Note that these rocks contain no carbonates and offer no possibility to neutralize, at least partially, the acidic waters. In other words, the buffering capacity of these rocks is zero and the generated acidity is simply released into the environment. In addition, the extremely acidic solutions are capable of leaching metals (e.g., Pb, Zn, Cu, Cd) from the other sulfides, transport these metals, and make them available for the plants or animals. Exploration for VMS deposits. There are exploration criteria which are relevant to all VMS deposits. These include: 1. The VMS deposits occur near or at the vent area of a volcanic center. The near (so-called proximal) areas are found where felsic flows, domes, swarms of mafic or felsic dikes or sills occur. Hence, careful geological mapping and the recognition of the past geological forms can be very helpful in locating new VMS deposits. The recognition of the proximal parts of a volcanic structure is commonly more difficult in mafic volcanic centers. 2. The VMS deposits occur in fault-bounded basins, depressions or grabens of deposit (tens of kilometers) size scale. Such depressions or basins may be characterized by an abrupt change of the lithology, for example, a sudden appearance of thick flows or volcaniclastic strata. The depressions or basins may be a part of larger extensional basins, sometimes including calderas. Another feature to look for are large volcano-tectonic subsidence structures which are essential for an effective fluid circulation near the magmatic heat source. 3. The VMS deposits within a cluster of deposits (so-called camp) are connected with one or two stratigraphic intervals. These intervals usually mark the hiatus in the volcanic activity and may be signaled by the exhalites, thin clastic-chemical sedimentary units. The presence of exhalites is a good reasons to continue with exploration. 4. The VMS deposits can be signaled by regional semiconformable alteration zones. Within these zones, the rocks are weakly but pervasively altered by hydrothermal fluids. The type and identity of the alteration minerals is mentioned in the section Alteration within this text. An increasing grade of the alteration (metasomatism) indicates closer distance to the central vent and therefore, possibly, a closer distance to a deposit. 5. The VMS deposits occur often in clusters, so-called camps. Therefore, a find of one deposit means with a high probability that other, similar, or perhaps even larger, VMS deposits may be located in its vicinity. 6. The VMS deposits are accompanied by strong geochemical signals. The geochemical variations are associated with the rock alteration patterns. In addition, some systems may show enrichment in elements such Sb, As, and Tl in the central portions. Further on, exhalites are enriched in Fe, Si, and Eu. Portions of the mound with sulfides and their vicinity may also carry elevated contents of Ba and Sr. References Barrie, C.T., Hannington, M.D., 1999: Introduction: Classification of VMS deposits based on host rock composition. In Barrie, C.T., Hannington, M.D. (eds.): Volcanic-Associated Massive Sulfide Deposits: Processes and Examples in Modern and Ancient Settings. Reviews in Economic Geology 8, Barrie, C.T., Ludden, J.N., Green, A.H., 1993: Geochemistry of volcanic rocks associated with Cu-Zn and Ni-Cu deposits in the Abitibi subprovince. Economic Geology 88, Brauhart, C.W., Groves, D.I., Morant, P., 1998: Regional alteration systems associated with volcanogenic massive sulfide mineralization at Panorama, Pilbara, Western Australia. Economic Geology 93, Franklin, J.M., Lydon, J.W., Sangster, D.F., 1981: Volcanic-associated massive sulfide deposits. In: Skinner, B.J. (ed.) Economic Geology 75 th Anniversary Volume,

6 Galley, A., Hannigton, M., Jonasson, I., 2007: Volcanogenic massive sulphide deposits. In: Goodfellow, W.D. (ed.) A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p Gibson, H.L., Allen, R.L., Riverin, G., Lane, T.E., 2007: The VMS model: Advances and application exploration targeting. In: Ore Deposits and Exploration Technologies, Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Exploration, B. Milkereit (ed.), pages Nordstrom, D.K., Alpers, C.N., 1999: Negative ph, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California. Proceedings of the National Academy of Sciences of the USA, 96,

Chapter 4 Rocks & Igneous Rocks

Chapter 4 Rocks & Igneous Rocks Rock Definition A naturally occurring consolidated mixture of one or more minerals e.g, marble, granite, sandstone, limestone Rock Definition Must naturally occur in nature,