Volcanic massive sulfide deposit (VMS) refers to a deposit containing a large amount of pyrite and a certain amount of copper, lead and zinc, which is related to submarine volcanism. In the west, this kind of deposit is often referred to as "volcano or volcanic massive sulfide" (VMS) or "volcanic hosted massive sulfide" (VHMS), which was called "pyrite deposit" by geologists in the former Soviet Union. According to their main ore-forming elements and associated rock types, R. W. Hutchinson (1973) of Canada divided these deposits into three types, namely pyrite-sphalerite-chalcopyrite deposits (zinc-copper type) occurring in differentiated mafic to felsic volcanic rocks, with the Archean as the dominant era; The pyrite-galena-sphalerite-chalcopyrite deposit (Pb-Zn-Cu type) occurring in acidic calc-alkaline volcanic rocks is dominated by Phanerozoic era; Pyrite-chalcopyrite deposit (copper type) occurs in mafic and ophiolite suite volcanic rocks in Phanerozoic era. F. J. Sawkins (1976) classified VMS-type deposits into four main types according to their tectonic environment and host rock series: ① sunspot type, which occurs at the convergence edge of ocean plates and exists in Archaean-Tertiary felsic calc-alkaline volcanic rocks; (2) Cyprus type, which occurs at the discrete edge of ocean plate and exists in the low-potassium basaltic volcanic rocks on the upper part of Paleoproterozoic-Tertiary ophiolite suite; (3) Bieshi, which has no clear plate tectonic environment, may occur in the extended continental margin rift environment or in the pre-arc trench environment, and exists in the strongly deformed clastic sedimentary rocks and mafic volcanic rocks from Paleoproterozoic to Tertiary; (4) Sullivan type, developed in the intraplate active zone, related to the extensional activity in the early stage of continental division (graben basin), and produced in the terrigenous sedimentary rock series with considerable thickness from Proterozoic to Paleozoic, and hardly related to the volcanic activity horizon. In this book, Sullivan-type deposits are classified as jet sedimentary deposits.
In recent years, it has been classified into another subtype, called Au-VMS) (B. Dube et al., 2006). It is defined that the concentration of gold (10-6) is greater than the total mass percentage of base metal (Zn+Cu+Pb,%), and it is Fe, Cu and Zn containing a lot of silver and gold.
VMS type deposit is one of the main sources of copper, lead, zinc, gold and silver in the world. By 2002, it is estimated that VMS-type deposits in the world have provided more than 50 × 108t sulfide ore, of which zinc production accounts for 22%, copper production accounts for 6% and copper production accounts for 9. 7% lead production, 8. 7% of silver production and 2. 2% of gold production. At the same time, VMS deposits are also important sources of Co, Sn, Se, Mn, Cd, In, Bi, Te, Ga and Ge, and some deposits also contain a large amount of As, Sb and Hg.
Such deposits are widely distributed, and there are as many as 800 such deposits in the world with the scale exceeding 20×104t (total metal reserves of Cu+Pb+Zn). Spain, Portugal, Canada, Australia, Russia, Kazakhstan, Japan and India produce such super-large deposits (table1; Figure 1). There is Gansu Baiyin Factory (copper 1 15) in China. 22 × 104t, copper grade 1. 48.6%), Xiaotieshan, Gansu (Pb+Zn 105. 54 × 104t, lead+zinc grade 8. 9%), Ashele, Xinjiang (copper 108. 56 × 104t, copper grade 2. 49%), Xinjiang Cocotale (lead+zinc 283. 44 × 104t, lead+zinc secondary. 46% ~ 6.95%), Xitieshan, Qinghai (Pb+Zn 33 1. 24 × 104t, lead+zinc grade 9. 02%) and Yunnan Dahongshan (copper 152.5438+0×).
Table 1 World Super-large VMS-type Deposits *
* copper metal reserves > 500× 104t or lead+zinc metal reserves > 500× 104t.
Second, the geological characteristics
1. Regional tectonic background
VMS-type deposits are usually formed near the plate edge, including the discrete plate edge of mid-ocean ridge or back-arc extensional basin, the convergent plate edge or continental edge of island arc, the islands within the plate and the tectonic environment represented by Archean greenstone belt. The deposit is a kind of "jet" deposit near the crater, which is produced at or near the seabed in the underwater volcanic environment and is formed by the concentrated discharge of metal-rich hydrothermal fluids.
2. Geological characteristics of the deposit
(1) host rock
VMS deposit is related to marine volcanism, and it is a massive sulfide deposit with volcanic rocks or volcanic-sedimentary rocks as host rocks. The deposit is the product of submarine volcanic activity in the early stage of geosyncline development. The direct host rocks of most ore bodies are acidic volcanic rocks, especially acidic pyroclastic rocks (Figure 2). Agglomerates, coarse tuffs and occasionally massive dacite to rhyolite dacite flow constitute the ore-bearing beds of many massive sulfide deposits. About 50% of VMS-type deposits are spatially related to felsic volcanic rocks, and these deposits are often related to rhyolite domes or felsic clastic rocks.
(2) occurrence of ore bodies and ore minerals
VMS-type deposits often occur in groups. For example, about half (69) of the 65,438+050 such deposits in Canada occur in six metallogenic areas, with an average of 65,438+02 deposits in each metallogenic area. The Spanish-Portuguese ore belt contains 88 VMS-type deposits. There are more than 100 such deposits in the eastern Urals of Russia. These mineral deposits are located near the center of volcanic eruption.
Fig. l Schematic diagram of the distribution of VMS-type deposits (areas) in the world (resource source: Tao Bingkun, etc., 1994).
Fig. 2 Comprehensive profile of neves-corvo mining area (quoted from Tao Bingkun et al., 1994).
The upper part of the ore body is layered and lenticular, mostly integrated with strata, controlled by strata, and the ore is massive. The lower part of the ore body is a breccia-like recharge zone, and the ore is in the form of reticular veins or disseminated, accompanied by a wide range of lava alteration zones (Figures 3 and 4). This network vein represents the near-surface channel of submarine hydrothermal system, while the massive sulfide lens represents the sulfide accumulation of hydrothermal deposits above and around the submarine outlet. In many cases, there is a thin layer of pyrite, hematite or siliceous jet rock above sulfide deposits, which forms a cover and extends outward from the deposit and can be used as a stratigraphic indicator. This kind of sedimentary layer is considered to represent the chemical deposition in the attenuation stage of hydrothermal activity during the volcanic quiet period.
Fig. 3 graphic profile of typical sulfide deposits in Cyprus (quoted from R. W. Hutchinson et al., 197 1).
The main metal minerals in the ore are iron sulfide, especially pyrite or pyrrhotite, chalcopyrite, sphalerite and galena, and sometimes bornite, tetrahedrite and magnetite. Gangue minerals precipitated together with sulfides include quartz, chlorite, barite, gypsum and carbonate.
Fig. 4 schematic diagram of basic characteristics of idealized volcanic massive sulfide deposits (quoted from J. W. Lydon, 1984).
(3) hydrothermal alteration
Hydrothermal alteration of host rocks is obvious, and the most common ones are silicification, timely sericitization, sapropertization and mudstone. Timely sericitization is mainly related to copper-type deposits, and mudstone is the characteristic of lead-zinc-copper-type deposits. The alteration zone is developed in the footwall rock of the ore body and is strongly metasomatized by magnesium. However, the overlying rocks of the ore body are formed after mineralization, so the alteration is weak or non-existent. Silicification occurs along the lateral direction of the ore bed, and the sodium deficiency zone may be widely distributed in the footwall sequence of the formation, and sodium may be added to the footwall of the formation directly above the deposit. Regional sericite chlorite alteration can be seen in roof volcanic rocks.
(4) Metallogenic age
VMS-type deposits occur in Archean to modern seabed. In the world, there are important deposits in Archean, Proterozoic, Caledonian, Hercynian, Chimir and Alpine periods. However, the main metallogenic ages are different in different regions, such as Archean and Proterozoic deposits in Canada, Hercynian deposits in Urals of Russia and Altai of Kazakhstan, and Tertiary deposits in Japan. The massive sulfide deposits currently forming on the ocean floor were first discovered in the mid-ridge of the East Pacific near 2 1 north latitude in 1978, and later discovered in the mid-ridge of the Galapagos in 198 1 year by the United States, and in 1982.
(5) Gold-rich volcanic massive sulfide deposit (Au-VMs)
The geological characteristics of gold-rich volcanic massive sulfide deposits are basically the same as those of VMS deposits, and some of them are regarded as gold deposits because of their high gold content. According to statistics, there are only about 30 world-class Au-VMs deposits (gold reserves+output ≥30t) in the world (Table 2), which are distributed in Canada, Australia, Sudan, Sweden, Kazakhstan and the United States. The world gold reserve+output of Au-VMs deposit is about 1453t, which is equivalent to 1. World gold reserves+2% of output (world gold reserves+output is about 120689t). The gold scale of Au-VMs type deposits ranges from 2t to 300t. The gold grade is generally > 4g/t, and the largest known deposit is Horn, Canada, with a gold content of 331t; The most abundant deposit is Eskay Creek, with a gold grade as high as 44g/t. The gold in the deposit mainly exists in the form of natural gold, silver-gold ore and telluride of gold. Gold particles are very fine, generally1~ 5 microns, and pyrite is the main inclusion.
Table 2 Main types of gold deposits in the world *
Source: B. Dube et al., 2006.
* gold reserves+output > 30t.
Three. Genesis of ore deposits and prospecting criteria
1. Genesis of the deposit
Because people can witness the formation of modern submarine massive sulfide deposits, the understanding of the causes of such deposits has become increasingly consistent. Most scholars believe that it is caused by volcanic eruption of syngenetic deposits, and the main points of this genetic model are shown in Figure 5.
Fig. 5 schematic diagram of genetic model of VMs type deposit (quoted from J. W. Lydon, 1988)
Fig. 5 shows three viewpoints on the genesis of VMS-type deposits. These three possible genetic models are all related to faults, which bring ore-forming solutions to the seabed and deposit minerals on the seabed. The left and right models in the figure are called convection ring models, indicating that hydrothermal system is a pair of fluids dominated by seawater. The basic concept of convection ring model is that groundwater, mainly seawater, is convected under the action of magmatic heat source, and ore-forming components are leached from rocks along the way, forming VMS-type deposits. However, this convection hypothesis cannot satisfactorily explain that in many mining areas, most massive sulfide deposits only exist in a relatively narrow stratigraphic interval compared with the time spanned by volcanic activity, nor can it explain why many submarine volcanic accumulations obviously do not contain such deposits. However, if the model in the middle of Figure 5 is used, these phenomena can be satisfactorily explained. This model is the mechanism of earthquake pumping, also known as aquifer model. As shown in the figure, seawater is stored in permeable rocks and surrounded by impermeable layers; The cold water solution stored in permeable rocks is heated to about 400℃ by the intrusion of the sub-volcano below; The metals in volcanic rocks are leached out by heated primary water; When tectonic activity (earthquake fault) cuts the impermeable layer, the metal-rich hydrothermal solution rises along the fault; When the hydrothermal solution containing metal is mixed with cold seawater, metal sulfides quickly precipitate and accumulate to form ore bodies.
2. Prospecting signs
Summarizing the formation and distribution of such deposits can provide an important basis for general survey and prediction. For a long time, many countries have studied and summarized the general survey of such deposits, and obtained many special geological, geophysical and geochemical prospecting indicators and methods that are conducive to finding such deposits.
(1) Geological exploration standard
1) The tectonic environments such as mid-ocean ridge, discrete plate margin, convergent plate margin, continental margin, intraplate island, intra-arc and inter-arc rift basin are favorable for the formation of VMS-type deposits.
2) VMS-type deposits may occur in sodium-rich or potassium-sodium differentiated spilite porphyry areas, ophiolite areas, ancient greenstone belts and sedimentary rocks related to volcanic jet deposits.
3) VMS-type deposits often occur near the eruption center of acid volcanoes in the above volcanic areas; Volcanic facies (rich lead-zinc ore) is located near the center of volcano (rich copper ore) or far away.
4) Various acidic pyroclastic rocks (acidic agglomerate, acidic pyroclastic breccia, acidic tuff, etc.). ) is a common host rock of VMS-type deposits. Rhyolite is the most common floor rock, and sedimentary rocks and/or basic volcanic rocks are the most common roof rocks.
5) The ore occurs in favorable horizons between volcanic rocks, which can be iron-rich spouting rocks, sulfide-bearing supergene clastic rocks, shale or carbonate rocks. The contact zone of various lithology and lithofacies in volcanic rock series, especially the contact zone between basic or neutral volcanic rocks and acidic volcanic rocks, and the interface between acidic or basic volcanic rocks and overlying sedimentary rocks, is often the location of VMS-type deposits; Thin-bedded siliceous, Fe-Mn sedimentary rocks are special lithologic signs in the upper part of VMS-type deposits.
6) It is particularly important to distinguish the volcanic cycles in different periods and study the metallogenic ages in various regions, especially to determine the main metallogenic ages. Because the deposit is related to volcanic activity in a certain period and is mostly integrated with rock strata, attention should be paid to the stratigraphic control of mineralization in the general survey.
7) Pay attention to the intersection of various faults, the intersection of faults and folds and complex structural areas, as well as various volcanic structures (such as acid volcanic domes and crater distribution areas, etc. ), pay special attention to rift faults in the same volcanic period.
8) Hydrothermal alteration of the deposit includes regional sericite chlorite alteration, magnesium metasomatism of footwall rocks, silicification along the horizontal horizon of the ore, etc. , are all useful exploration indicators.
9) For gold-rich VMS-type deposits, the existence of aluminum-bearing mineral assemblage is a useful prospecting indicator for this kind of deposits, and garnet containing andalusite, kyanite, staurolite and manganese is the main mineral assemblage of Au-VMS-type deposits in ancient metamorphic terrane.
10) These deposits often appear in groups with diameters ranging from 20 to 40 kilometers. Therefore, in areas with such deposits, we should continue to search for new deposits in favorable areas or parts according to ore-controlling factors and using prospecting signs.
(2) Geophysical exploration standards
Geophysical prospecting method is an important means to explore VMS-type deposits, and commonly used methods include electromagnetic method, electrical method, magnetic method, gravity method and resistivity method. Different countries have different geological conditions, and different methods have different effects.
1) In the Precambrian shield area, electromagnetic method is very effective to find such deposits, such as canadian shield area. Many mineral deposits were discovered by electromagnetic method or surface electromagnetic method, which has been confirmed by work in India, Africa, South America, Southeast Asia and Australia.
2) In the Spanish-Portuguese ore belt, due to the extremely rugged terrain of the ore belt, the airborne electromagnetic method is invalid. The most effective geophysical methods in this area are gravity method, DC earth resistivity method and electromagnetic method (Thuram method). In this ore belt, the resistivity method is usually used to explore the new area first, and then the gravity method is used to check the anomaly.
3) Electrical method (induced polarization method, transition field method, natural electric field method, etc.). ), magnetic survey and high-precision gravity survey are widely used in the countries of the former Soviet Union to investigate some anomalies and identify some geological structures in detail. Generally, the whole mineralization area is delineated by induced polarization method, and the concealed ore bodies in these areas are identified by transition field method.
4) Regional magnetic survey can determine the main volcanic layers, structures and alteration, and induced polarization method can determine the ore (stone) zone and pyrite alteration halo.
(3) Geochemical prospecting criteria
The scale of geochemical anomalies of VMS-type deposits is many times larger than that of ore bodies, so various geochemical methods are widely used in prediction and general survey, including primary halo, secondary halo, hydrochemical halo and biogeochemical methods.
Primary halo method is an effective method to find blind ore bodies in VMS type deposits. The main indicator elements of primary halo in VMS-type deposits are copper, zinc, lead, silver, arsenic, molybdenum, cobalt and barium, and sometimes bismuth, mercury, selenium and tellurium are used as auxiliary indicator elements.
2) The scale of secondary halo is much larger than that of ore body and primary halo of ore body, which is more conducive to geochemical survey. The typomorphic elements of secondary halo in VMS deposit are copper, silver, lead, barium, zinc, molybdenum, tin, cobalt and mercury. Among these elements, only zinc and copper often form meaningful secondary aggregation. Ba, Mo and Ag are inactive, so they appear in the secondary halo, indicating that the ore source is nearby.
3) The mark of water geochemical survey is the low pH value of water; The content of mineralized components in water is relatively high. The main characteristic elements in water are iron, sulfur, copper, lead and zinc, and the secondary elements are chromium, mercury, gold, arsenic, antimony, barium, bismuth and indium.
4) The vertical zoning of metals (upward along the stratum) is: Cu, Au→Pb, Zn, Ag, Au→Ba.
5) Trace elements in zinc mine are arsenic, antimony, magnesium and thallium, and trace elements in copper mine are bismuth, tellurium, molybdenum and cobalt. ..
6) Most deposits have obvious soil lead anomalies, and Zn and Pb show scattered soil anomalies.
7) Trace elements of iron cap are gold, selenium, tellurium, arsenic, antimony, bismuth, cadmium, indium, thallium, mercury, tin and barium.
(Dai Zixi)