投影片 1 - 國立臺灣大學

投影片 1 - 國立臺灣大學

Sedimentary Exhalative Deposits Sedimentary exhalative deposits (abbreviated as SEDEX from SEDimentary EXhalative) are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluid s into a water reservoir (usually the ocean), resulting in the preci pitation of stratiform ore. SEDEX deposits are the most important source of lead, zinc an d barite, a major contributor of silver, copper, gold, bismuth and tungsten. Stratiform vs. Stratabound Classification

SEDEX deposits are distinctive in that it can be shown that the ore minerals were deposited on the bed of an ocean or marine envir onment in a second-order basin environment, related to discharge of metal-bearing brines into the seawater. This is distinct from ot her Pb-Zn-Ag and other deposits which are more intimately associat ed with intrusive or metamorphic processes or which are trapped w ithin a rock matrix and are not exhalative. Genetic model Source of metals is sedimentary strata which carry metal ions tr apped within clay and phyllosilicate minerals and electrochemical ly adsorbed to their surfaces. During diagenesis, the sedimentary pile dehydrates in response to heat and pressure, liberating a hig

hly saline formational brine, which carries the metal ions within th e solution. Transport of these brines follows stratigraphic reservoir pathwa ys toward faults which isolate the buried stratigraphy into recogn izable sedimentary basin; the brines percolate up the basin boun ding faults and are released into the overlying oceanic water. Trap sites are lower or depressed areas of the ocean topograp hy where the heavy, hot brines flow and mix with cooler sea water, causing the dissolved metal and sulfur in the brine to be d eposited as sulfide layers. Mineralization types SEDEX mineralisation is best known in Pb-Zn ore deposit clas

sification schemes as the vast majority of the largest and most important deposits of this type are formed by sedimentary-exh alative processes. However, other forms of SEDEX mineralization are known; The vast majority of the world's barite deposits are consider ed to have been formed by SEDEX mineralization processes The scheelite (W) deposits of the Erzgebirge in Czechoslov akia are considered to be formed by SEDEX processes Deposition The mineralizing fluids are conducted upwards within sedi mentary units toward basin-bounding faults. The fluids mo ve upwards due to thermal ascent and pressure of the und

erlying reservoir. Faults which host the hydrothermal flow c an show evidence of this flow due to development of mass ive sulfide veins, hydrothermal breccias, quartz and carbo nate veining and pervasive ankerite-siderite-chlorite-sericit e alteration. Fluids eventually discharge onto the seafloor, forming exte nsive, stratiform deposits of chemical precipitates. Dischar ge zones can be breccia diatremes, or simple fumarole co nduits. Black smoker chimneys are also common, as are seepage mounds of chert, jaspilite and sulfides. A black smoker

A white smoker in the 9 oN EPR The interior of a recovered black chimney Part of a 360C black smoker chimney with the Main Endeavour hydrot hermal field on the Juan de Fuca Ridge in the N.E. Pacific ocean. Vibra nt colonies of tube worms with red gills thrive on this large edifice, whic h is predominantly composed of iron- and sulfur-bearing minerals. Fig. a The Mid-Atlantic Ridge hosts numerous hydrothermal fields (colored dots). Both Logatchev and Rainbow host high-temperature black smokers. Fig. c. One of four pinnacles that form the summit of the 200-foot tall

carbonate chimney called Poseidon in the Lost City hydrothermal field. The white chimney in the foreground is actively venting 55C fluids. As the chimneys age they turn grey to brown in color, such as the one shown towards the back. Image courtesy of University of Washington. Fig. d. This beautiful, actively venting carbonate structure resembling a snow-covered Christmas tree is about 3 feet high. Specific examples of deposits Sullivan Pb-Zn mine The Sullivan Pb-Zn mine in British Columbia, Canada was worked for over 150 years and produced in excess of 100 Mt of ore grading in ex cess of 5% Pb and 6% Zn.

Earlier, deeply buried sediments devolved fluids into a deep reservoir of sandy siltstone and sandstone Intrusion of dolerite sill into the sedimentary basin raised the geother mal gradient locally Raised temperatures prompted overpressuring of the lower sediment ary reservoir which breached overlying sediments, forming a breccia diatreme Mineralising fluid flowed upwards through the concave feeder zone o f the breccia diatreme, discharging onto the seafloor Sediments were deposited in an extensional second-order sediment ary basin during extension Ore fluids debouched onto the seafloor and pooled in a second-orde r sub-basin's depocentre, precipitating a stratiform massive sulfid

e layer from 3 to 8 m thick, with exhalative chert, manganese an d barite. Hydrothermal circulation around a mid-ocean ridge Volcanogenic massive sulfide ore deposit Volcanogenic massive sulfide ore deposits or VMS are a typ e of metal sulfide ore deposits, mainly Cu-Zn which are associat ed with and created by volcanic-associated hydrothermal eve nts. They are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the seafloor, in a wide range of ancient and modern geological settin gs. They occur within volcano-sedimentary stratigraphic succ

essions, and are commonly coeval and coincident with volcan ic rocks. As a class, they represent a significant source of the w orld's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga and Ge as co- or by-products. VMS deposits are forming today on the seafloor around unders ea volcanoes, mid ocean ridges and trench systems, notably th e Tongan Arc. Mineral exploration companies are exploring for S eafloor Massive Sulfide deposits. Classification The close association with volcanic rocks and eruptive centers sets VMS deposits apart from similar ore deposit types which share similar source, transport and trap

processes. Volcanogenic massive sulfide deposits are distinctive in that ore deposits are formed in close temporal association with submarine volcanism and are formed by hydrothermal circulation and exhalation of sulfides which are independent of sedimentary processes, which sets VMS deposits apart from SEDEX deposits. Genetic model The source of metal and sulfur in VMS deposits is a combination of incompatible elements which are concentrated in the fluid ph ase of a volcanic eruption, and metals leached from the hydr othermal alteration zone by hydrothermal circulation.

Transport of metals occurs via convection of hydrothermal fluids, the heat for this supplied by the magma chamber which sits belo w the volcanic edifice. Cool ocean water is drawn into the hydr othermal zone and is heated by the volcanic rock and is then expelled into the ocean, the process enriching the hydrothermal fluid in sulfur and metal ions. The ore materials are trapped within a fumarole field or a black smoker field when they are expelled into the ocean, cool, and pr ecipitate sulfide minerals as stratiform sulfide ore. Metal zonation Most VMS deposits show metal zonation, caused by the changing p hysical and chemical environments of the circulating hydrothermal fl

uid. Ideally, this forms a core of massive pyrite and chalcopyrite around the throat of th e vent system, with a halo of chalcopyrite-sphalerite-pyrite grading into a distal sphalerite-galena and galena-manganese and finally a chert-managanese-hematite facies. The mineralogy of VMS deposits consists of over 90% iron sulfide, mainly in the form of pyrite, with chalcopyrite, sphalerite and galena also being major constituents. Distribution In the geological past, the majority of VMS deposits were formed i n forearc and arc environments by intermediate and felsic volca

nic edifices, and appear to form more readily in the Phanerozoi c than in the Proterozoic and Archean. This is probably due to the predominance of basaltic rift-related volcanism in the Archean, an d the gradual trend towards more felsic, cooler volcanism as th e Earth ages. Most VMS deposits are associated strongly with convergent margi ns, for example the Besshi and Kuroko type deposits from the co llisional arc setting of Japan. VMS deposits can be found associat ed with thick sedimentary sequences intruded by volcanic edifices, and some VMS deposits may be found considerably up-section fro m their source magma chamber, with no distal volcanic facies, by f luids traveling through overlying blankets of sediments.

VMS deposits are currently being formed by hydrothermal p rocesses along submarine divergent margins: mid-ocean ri dges and back arc rifts. The sulfurous plumes called blac k smokers deposit a variety of metal sulfides as the hot hyd rothermal solutions meet and mix with deep ocean water. Recent examples are the still-active Red Sea massive sulfi des. The majority of world deposits are small, with about 80% of known deposits in the range 0.1-10 Mt. Examples of VMS d eposits are Kidd Creek, Ontario; Flin Flon, Manitoba; and Rio Tinto, Spain. Atlantis II Deep

Montmorillonite Sulfide Goethite Carbonate Basalt Cross section of the Atlantis II Deep Upper sulfide zone (4 m) Atlantis II Deep metalliferous

sediments Lower sulfide zone (2.5 m) Mississippi Valley-type (MVT) deposits Simplified definition Mississippi Valley-type (MVT) deposits are epigenetic strat abound carbonate-hosted sulfide bodies composed predom inantly of sphalerite and galena. These deposits account fo r 35 percent of the world's lead and zinc resources. The y are so-named because several classic MVT districts are l ocated in carbonate rocks within the drainage basin of the Mississippi River in the central United States (US). Importa nt Canadian districts include Pine Point, Cornwallis, Nanisi

vik, Newfoundland Zinc, Gays River, Monarch-Kicking Hors e, and Robb Lake. Scientific definition MVT deposits are stratabound, carbonate-hosted sulphide bodie s, composed predominantly of zinc and lead, bound in sphalerite and galena. The deposits occur mainly in dolostone as open-sp ace fillings, collapse breccias and/or as replacement of the ca rbonate host rock. Less commonly, sulfide and gangue mineral s occupy primary carbonate porosity. The deposits are epigeneti c, having been emplaced after lithification of the host rock. MVT deposits originate from saline basinal brines at temperatur es in the range of 75o-200oC. They are located in carbonate pla

tform settings, typically in relatively undeformed orogenic forelan d rocks, commonly in foreland thrust belts, and rarely in rift zone s. Individual ore bodies are generally <2 million tonnes, are zinc-d ominant, and possess grades which rarely exceed 10% (Pb+Z n). They do, however, characteristically occur in clusters, referr ed to as "deposits" and "districts". For example, the Cornwallis district in Nunavut hosts at least 25 deposits and 75 ore bodies and the Pine Point district in the Northwest Territories hosts two deposits (Pine Point and Great Slave Reef) and more than 90 o re bodies. Other districts may contain a half-dozen to more than 300 ore bodies, which can contain up to several hundred million

tonnes of ore scattered over hundreds to thousands of square k ilometers. Mineral deposit subtypes MVT deposit sub-types include those that are of high-temperat ure carbonate replacement Pb-Zn (+/-Fe, Ag) and the diapir-r elated deposits. The carbonate-hosted F-Ba deposits and the sandstone-hosted lead deposits, and some "Irish-type" depo sits are also included as "sub-types". The "Irish-type" deposits are stratabound, structurally controlled , carbonate-hosted, Pb-Zn deposits that have sedimentary exha lative (SEDEX) and/or MVT characteristics. The MVT-SEDEX r elationship save that a continuum does appear to exist between

the two major deposit-types. Mineralogy MVT deposits have simple mineral assemblages that consist of sphalerite, galena, pyrite, and marcasite. Gangue minerals are dolomite, calcite, and quartz, and occasionally barite and flu orite. These are accessory minerals present in some but absent i n most districts. Chalcopyrite, bornite, and other copper minerals are normally not constituents in MVT deposits and are only abund ant in some deposits/districts, such as the Viburnum Trend of the Southeast Missouri district in the US and the Cornwallis district in Canada. Deposits of the Viburnum Trend have a unique and com

plex mineralogy that is not typical of most MVT deposits/districts and includes siegenite, bornite, tennantite, bravoite, digenite, cov ellite, arsenopyrite, fletcherite, adularia, pyrrhotite, magnetite, mill erite, polydymite, vaesite, djurleite, chalcocite, anilite, and enargit e (Leach et al., 1995). Summary of economic characteristics MVT deposits account for 35 percent of the world's lead a nd zinc resources, and they are dispersed throughout the world (Fig. 1). A large proportion of Pb and Zn production comes from several classic MVT districts located in the dra inage basin of the Mississippi River in the central US. MVT deposits also occur in Canada, Europe (Poland, France, Ir

eland, Spain, Austria, Italy, former Yugoslavia), Australia, China, Peru, Morocco, and South Africa. In Canada, there are 16 districts (Fig. 2), each of which contains 2 to more t han 100 deposits. The Pine Point district, for example, con tains more than 90 deposits distributed over 1,600 square kilometers. Figure 1: Distribution of Mississippi Valley-type deposits and districts worldwide Figure 2: Distribution of Mississippi Valley-type in Canada (Map D1860A). Districts shown are, Cornwallis; Nanisivik, Pine Point, Prairie Creek, Robb Lake, Monarch-Kickin

g Horse, Blende, Bear-Twit, Gayna River, Goz Creek, Gays River, Jubilee, Walt on, Newfoundland Zn, Upton, and Esker. MVT deposits contributed only negligibly to Canadian Zn and Pb production prior to the opening of the Pine Poin t deposit in 1964. Between 1964 and 2002, about 30% of the annual lead and zinc production was derived from MVT deposits. With the closure of Pine Point in 1988, Newfoundland Zinc in 1990, and Polaris and Nanisivik in 2002, however, the proportion of Zn and Pb derived from MVT deposits in Canada dropped to nil. At the time (November 2003), there are no MVT mines in production in Canada.

Nature of sulphide bodies MVT deposits occur in clusters of a few to hundreds of individual ore b odies that vary in character and shape and are often interconnected (F ig. 6). Deposits and ore bodies range from massive replacement zone s to open-space fillings of breccias and fractures, to disseminated clust ers of crystals that occupy intergranular pore spaces (Leach and Sang ster, 1993). Ore-hosting structures are most commonly zones of highly brecciated dolomite; and in some instances (e.g., Pine Point, Robb L ake, and Newfoundland Zinc) these zones are arranged in linear patt erns suggesting a tectonic control. These breccia zones may range fro m more or less concordant tabular structures, controlled by individual s trata, to discordant cylindrical structures within tens of metres of sedim

entary sequences (Fig. 6). At Pine Point, the orebodies are either tabul ar or prismatic structures in interconnected paleokarst networks. The MVT ore bodies therefore, are discordant on the deposit scale, but str atabound on a district scale. Pine Point Dimensions The dimensions of ore bodies can be difficult to measure because of t heir irregular and variable shape. At Pine Point, the L-36 oratory has dimensions of 1450 m in length, 50 to 400 m in width, and 2.5 to 1 0 m in thickness and the X-15 oratory is 800 m in length, 400 m in wi dth, and 20 to 30 m in thickness. At Robb Lake, several bodies exten

d for more than 300 m along bedding and crosscut more than 50 m of stratigraphic section; others are thin and narrow bodies and pods parallel to bedding. At Polaris, the main ore body had dimensions of 8 00 m in length, 300 m in width, and 150 m in thickness. Figure 6: Schematic representation of the Robb Lake breccia-hosted Zn-Pb ore, sho wing textural and mineralogical zoning and stratigraphic controls Figure 7: A. Crackle breccia with disseminated s phalerite crystals in white dolomite cement, Robb Lake, BC.

B. Mosaic breccia with sphalerite crysta ls in white dolomite cement, Robb L ake, BC. C. Rubble breccia consisting of variably altered dolostone fragments and sh ale fragments in white dolomite ce ment. Note the dolostone fragment s with zebra texture, Robb Lake, B C. D. Rock-matrix breccia with dolostone, shale, and white sparry dolomite fra gments in dark grey fragmental mat rix, Robb Lake, BC.

E. Aggregates of massive sphalerite cr ystals and white sparry dolomite alo ng small fractures, Pine Point, NWT . F. Aggregates of colloform sphalerite a nd skeletal galena completely repla cing the carbonate, Polaris, Nunavu t; scale bar is 1 cm. Figure 8: Various ore texture s from Polaris, Nun avut, NWT; scale b

ar is 1 cm. A. Keel ore: sphalerite and skeletal galen a are replacing the carbonate clasts. B. Keel ore: crystalline sphalerite in soluti on collapse brecci a. C. Ocean zone: Massi ve galena and spar ry dolomite in pseu dobrecciated carbo

nate host rock. Conventional models for deposition of sulphides Three models are proposed for the chemical transport and depositio n of sulphides. The mixing model proposes transport of base metals in fluids of low s ulphur content. Mixing of the metal-rich brines with fluids containing hy drogen sulphides (H2S) at the depositional site triggers sulphide precip itation. The sulphate reduction model involves transport of base metals and sulphate in the same solution. Precipitation occurs at depositional sites when sulphate is reduced upon reaction with organic matter or methan e (CH4).

The reduced sulphur model requires that the base metals and the re duced sulphur (S2-) be transported together in the same solution. Prec ipitation occurs either through cooling, mixing with diluted fluids, chang es in pH, or loss of volatiles. Age of MVT ore-forming events Recent advances in age dating of MVT deposits provide new eviden ce that there are important genetic relationships between convergen t orogenic events and the formation of MVT deposits. The most important periods for MVT genesis in the history of the Earth happened during the Devonian-Permian and the Cretaceou s-Tertiary when large-scale contractional tectonic events occurred. Other deposits such as Nanisivik and those of the Lennard Shelf of

Australia are associated to extensional events in Ordovician and E arly Mississippian time, respectively. These may prove to be mor e abundant, but so far little is known about the role of continental ex tension and the formation of MVT deposits. There is a paucity of MVT deposits of Precambrian, Early Paleo zoic, and Mesozoic ages. Distribution of radiometric and paleomagnetic ages of MVT deposit s/districts and their host rocks Geological properties Many of the MVT deposits of the world potentially formed during large contractional tectonic events at specific times in the Earth's history.

The Devonian to Permian period saw a series of continental collis ions that culminated in the formation of the supercontinent Pang ea. Over 70% of the total MVT Pb-Zn metals produced so far worl dwide were formed at that time. The Cretaceous-Tertiary period saw the breakup of Pangea, punct uated by the Alpine and Laramide orogenies affecting the wester n margin of North America and Africa-Eurasia. Districts such as Ro bb Lake, Pine Point, and Monarch-Kicking Horse in North America ma y have formed during the Laramide orogeny in Cretaceous-Tertiary tim e. The Cracow-Silesian deposits in Poland are believed to have forme d during the Outer Carpathian orogeny in the Tertiary period. Tectonic processes

Leach et al. (2001a) stressed the genetic links between MVT minerali zation and regional- and global-scale tectonic processes. It is now c lear that MVT deposits/districts are products of enormous hydrothermal systems that left trace mineralization over a wide area and that the nearl y ubiquitous occurrence of MVT deposits on the flanks of basins reflects focused migration of deep-basin brines into shelf-carbonate sequences. Thus, the regional hydrogeologic framework is of paramount importance in the evaluation of large areas for their potential to contain MVT deposit s or districts. MVT deposits in North America (such as those of the Ozark district) hav e been attributed to large-scale migration of fluids mainly during converg ent orogenic processes. The topographically driven fluid flow model ass ociated with ore fluid migration in compressive tectonic regimes best de

scribes the MVT mineralization in North America. Other deposits such a s the Lennard Shelf in Australia, Alpine deposits in Europe and North Afr ica, and Nanisivik have been attributed to continental extension and ma y require other fluid-driving mechanisms. Supplement-1 Grade and tonnage characteristics The size, grade, and metal ratio parameters of individual MVT depos its are difficult to compare. As mentioned by Sangster (1990, 1995) and Leach and Sangster (1993), several deposits/districts were mine d before accurate data were recorded; and the MVT deposits tend to occur in clusters and form districts, therefore the production and res

erve data are usually presented as district totals, not for each individ ual deposit or ore body. There are over 80 MVT deposits/districts (with grade and tonnage fig ures) worldwide, sixteen of them in Canada. The best geological res ource estimates for most individual Canadian deposits are 1 to 10 mi llion tonnes with 4 to 10% combined Pb and Zn; and the majority of t hese are Zn-rich relative to Pb. The Polaris and Prairie Creek deposi ts are unusually large (22 and 12 million tonnes, respectively) and ha ve anomalously high grades (17% and 22.6% Pb+Zn, respectively). Most individual deposits worldwide yield less than 10 million tonnes of ore with combined Pb+Zn grades seldom exceeding 15%. The siz e of MVT districts is approximately an order of magnitude greater tha

n the size of individual deposits with combined Pb+Zn grades betwe en 2 and 6%. The metal ratios in deposits/districts, expressed as Zn/ (Zn+Pb) values show a weak bimodal distribution. The majority of de posits/districts have a Zn/(Zn+Pb) value around 0.85 with a smaller g roup around 0.45. The Canadian deposits/districts show a similar dis tribution compared to worldwide deposits. In his worldwide compilati on of MVT deposits, Sangster (1990) shows a clear bimodal distribut ion with the majority of deposits around 0.8 and a smaller group at 0. 05. The group at 0.05 corresponds to the deposits of the southeast Missouri district. Our data on the southeast Missouri district are inco mplete and this explains the low value around 0.05. Host rocks

The deposits are hosted in carbonate rocks, usually dolostone and le ss frequently limestone. The dolostone consists of medium to coars e-grained white sparry dolomite that has replaced a fine-grained dolo stone host, which itself has replaced a limestone host. The Pine Poin t orebodies, for example, are enclosed in large, discordant zones of s econdary coarse-grained vuggy dolostone with white saddle dolomite and calcite gangue. The district host rocks are mostly fine-grained cr ystalline dolostone and local limestone. Gays River is an example of a deposit in early diagenetic dolostone without secondary sparry or s addle dolomite. In the East Tennessee, Alpine, and Newfoundland Zi nc districts, the secondary dolomite is only locally developed, wherea s at Jubilee in Nova Scotia, the deposit is exclusively hosted in limest one.

There is sufficient distinction of densities between sulphide and gangue minerals that make gravity surveys successful in geophysical exploration. The degree of porosity can var y significantly in the host-rocks, which may be detectable b y gravity (Lajoie and Klein, 1979). Borehole geophysical su rveys, such as sonic and density logs can differentiate bet ween porous and non-porous horizons. Mineralogy MVT deposits have simple mineral assemblages that consist of sphalerite, galena, pyrite, and marcasite. Gangue minerals are

dolomite, calcite, and quartz, and occasionally barite and flu orite. These are accessory minerals present in some but absent i n most districts. Chalcopyrite, bornite, and other copper minerals are normally not constituents in MVT deposits and are only abund ant in some deposits/districts, such as the Viburnum Trend of the Southeast Missouri district in the US and the Cornwallis district in Canada. Deposits of the Viburnum Trend have a unique and com plex mineralogy that is not typical of most MVT deposits/districts and includes siegenite, bornite, tennantite, bravoite, digenite, cov ellite, arsenopyrite, fletcherite, adularia, pyrrhotite, magnetite, mill erite, polydymite, vaesite, djurleite, chalcocite, anilite, and enargit e (Leach et al., 1995).

The abundance of iron sulphides relative to other sulphide minerals in MVT deposits ranges from dominant to nil. Iron sulphide abundance may vary greatly from district to district and between deposits from the same district (Leach et al., 1995; St. Marie et al., 2001). For example, at Nanisivik, iron sulphides are abundant, whereas in some Appalachi an deposits, only traces to minor amounts of iron sulphides are prese nt. Deposits that contain significant iron sulphides can be detected by induced polarization (IP) surveys and ground electromagnetic method s (EM), whereas those that contain only sphalerite and minor galena a re generally poor conductors and have variable resistivity (Summer, 1 976). At Pine Point, the sphalerite is non-polarizable, however IP prov ed to be successful for locating ore bodies. Sphalerite and gangue mi nerals have widely varying densities, so gravity surveys could prove u

seful in exploring for ore bodies containing mainly sphalerite. In the C ornwallis district and the "Irish-type" deposits, IP and geochemical sur veys have been combined to discover ore bodies (Hallof, 1966). Most deposits/districts are zinc-rich relative to lead and have Zn/(Z n+Pb) ratios greater than 0.5. Some deposits, including many in th e East Tennessee district and in the Newfoundland Zinc district ar e essentially free of lead and have Zn/(Zn+Pb) ratios close to 1.0. Textures Sulphide textures are mostly related to open-space filling of breccias, frac tures, and vugs. Replacement of carbonate host rocks and internal sedim ents, and sulphide disseminations are also observed (e.g., Polaris, Pine P

oint, Robb Lake deposits). The mineralized breccias are of several textur al types: crackle, mosaic, rubble, and rock-matrix ("trash") breccias. Sulp hides and white sparry and saddle dolomite constitute the cement betwee n the fragments. Descriptions of the breccias can be found in Ohle (1959, 1985), Sangster (1988, 1995), Leach and Sangster (1993), Paradis et al. (1999), and Nelson et al. (2002). In these open-space features, the sulphi de and gangue mineral textures are varied. The sulphides are disseminat ed, massive, and banded. Disseminated sulphides occur as fine to coarse crystals of sphalerite and galena overlain by, or intergrown with white, co arse, crystalline sparry dolomite cement. Coarse sphalerite crystals occas ionally coat the tops of fragments or line the bottoms of cavities forming a texture known as "snow on the roof" (Leach and Sangster, 1993; Sangste r, 1995).

Sphalerite also forms massive aggregates of coarse-grained colloform and botryoidal crystals and laminae of fine-grained crystals. Massive s ulphides are found in replacement zones of the carbonate host rocks. At Nanisivik, replacement of the dolostone is the main mechanism of ore deposition. It consists of massive pyrite, sphalerite, and galena th at replace the dolostone along high-angle normal faults and form mant os that shallowly crosscut bedding (Patterson and Powis, 2002). At Po laris, massive, carbonate replacement, breccia-fill and vein sulphide o re form a 10- to 30 m thick, high grade Zn-Pb-Fe, tabular unit hosted i n the upper part of the deposit. Elsewhere, the replacement is selectiv e and follows stylolites, organic-rich layers, fossil-rich bands, and carb onate sand matrix. At Newfoundland Zinc, selective replacement of th

e bioturbated limestone by the hydrothermal dolomite produced a pse udobreccia (Lane, 1984). At Monarch-Kicking Horse, Pine Point, Robb Lake, and Pend Oreille deposits, selective replacement of a variety of primary rock fabrics by the hydrothermal dolomite formed a zebra text ure. Chemical properties Ore chemistry Lead and zinc are the primary commodities recovered from MVT dep osits. Silver, cadmium, germanium, copper, barite, and fluorite, althou gh generally absent in most deposits, are by-products in some deposi ts. A complex suite of trace minerals may be present in some deposit s and may include some or all of the following minerals: arsenopyrite,

bravoite, bornite, chalcopyrite, carrollite, celestite, chalcocite, covellit e, digenite, djurleite, enargite, gallite, germanite, greenockite, linnaeit e, marcasite, millerite, molybdenite, pyrrhotite, renierite, siegenite, te nnantite, tungstenite, and vaesite (Foley, 2002). Elements associated with these minerals are As, Cu, Co, Ni, Cd, Ag, In, Ge, Ga, Sb, Bi, As , Mo, Sn, and Au. Co, Ni, and Co are diagnostic accessory elements i n deposits of the southeast Missouri and Upper Mississippi Valley dis tricts (Foley, 2002). Thallium and As are enriched in sphalerite of the Silesian deposits (Viets et al., 1996). The majority of MVT deposits have essentially no geochemica l signature because of limited primary dispersion of elements bounded in sphalerite and galena into the carbonate rocks (La

very et al., 1994). When weathering of the sulphides occurs a nd minerals such as limonite, cerussite, anglesite, smithsonite , hemimorphite, and pyromorphite are formed, the soil and str eam sediments of the regions surrounding the deposits may c ontain anomalous concentrations of Pb, Zn, Fe, and trace ele ments Sb, As, Bi, Ag, Tl, Cd, Mn, and Cu. In the East Tennes see district, detectable Zn, Fe, and Pb anomalies are found in residual soil and stream sediments (Leach et al., 1995). In the Pine Point district, Pb, Zn and Fe, which are anomalo us in lake sediments, soils and tills, are used for geochemic al dispersion surveys in the exploration of orebodies. Zinc gi ves larger and more contrasting anomalies in lake sediment

s and soils than Pb; however, not all Zn anomalies are asso ciated with an orebody. Shale units in the Western Canadia n Sedimentary Basin, especially organic-rich ones, give high background values in Zn, similar in magnitude to those asso ciated with orebodies. Since the shales have relatively low Pb contents, composite Pb-Zn anomalies are likely minerali zation-related. Relatively poorly defined Pb anomalies are of ten present near orebodies, due to low Pb mobility and the general low Pb/Zn ratios in MVT orebodies. At Pine Point, calcite flooding forms halos around the ore bodie s giving a coarsely granular appearance to the carbonate host r ocks. Fe, Zn, and Pb display pronounced concentric distribution

patterns in the Pine Point and Sulphur Point formations. Iron is t he most widely distributed element, Zn is intermediate, and Pb occurs near the centre of the ore bodies. These anomalous patt erns decrease gradationally from maximum density and high-gr ade prismatic cores to barren country rocks. These anomalies a re widespread in the Pine Point and Sulphur Point formations, n egligible in Watt Mountain Formation, and confined to major sol ution collapse features in the Slave Point Formation. Iron disper sion highs tend to be displaced north of the deposits. Geological properties Many of the MVT deposits of the world potentially formed during large contractional tectonic events at specific times in the Earth's history (Le

ach et al., 2001a). Formation of some deposits, such as Nanisivik in C anada and those of the Lennard Shelf in Australia, are linked to extensi onal events. The most important periods for MVT genesis were during the Devonian-Permian time and the Cretaceous-Tertiary time. The Dev onian to Permian period saw a series of continental collisions that culm inated in the formation of the supercontinent Pangea (Leach et al., 200 1a). Over 70% of the total MVT Pb-Zn metals produced so far worldwi de were formed at that time. Deposits of the Lennard Shelf, Newfoundl and Zinc, Cornwallis, and possibly East Tennessee, Pine Point, and R obb Lake districts are Devonian-Mississippian in age. The Cretaceous-Tertiary period saw the breakup of Pangea, punct uated by the Alpine and Laramide orogenies affecting the western

margin of North America and Africa-Eurasia (Leach et al., 2001a). Districts such as Robb Lake, Pine Point, and Monarch-Kicking Hor se in North America may have formed during the Laramide orogen y in Cretaceous-Tertiary time. The Cracow-Silesian deposits in Pol and are believed to have formed during the Outer Carpathian orog eny in the Tertiary period (Symons et al., 1995). Paleomagnetic, U-Pb, Th-Pb, and Sm-Nd dating of deposits in the Cvennes regio n of France yielded Early to Middle Eocene ages that correspond t o the uplift of the Pyrenees during the closing stages of the Pyrene an orogeny. Furthermore, preliminary paleomagnetic results from t he Reocin deposit in Spain are consistent with a Tertiary age for m ineralization (Lewchuk et al., 1998).

Geological distribution of MVT districts in Canada For the purpose of this compilation and synthesis, a metal logenic district is defined as a continuous area that contai ns the expressions of the geological environment and tect onic events that controlled the formation of MVT deposits. In Canada, 16 MVT districts were identified. Most of these districts consist of several deposits, which comprise two t o more than 100 sulphide bodies. For example, the Gayn a River district contains more than 100 sulphide occurren ces and Pine Point has two deposits (Pine Point and Gre at Slave Reef), which consist of more than 90 individual o re bodies.

The Canadian deposits/districts show a strong concentrati on along an arcuate circum-continental trend. They are ho sted in relatively undeformed and deformed platformal car bonate rocks peripheral to cratonic sedimentary basins. M ost districts are located in western and northern Canada, a nd few are located in the Maritime provinces. The largest g roup of deposits (in terms of number of deposits) is located in the Mackenzie Mountains of the Yukon and the NWT, w here hundreds of small deposits and a few larger ones (Ga yna River, Blende, Bear Twit, Goz Creek, and Prairie Cree k) occur in Proterozoic to Devonian dolostone and limesto ne. Further south, a linear series of MVT deposits occurs i n Cambrian and Silurian-Devonian carbonate rocks of the

Canadian Rocky Mountains within the Robb Lake and Mon arch-Kicking Horse districts. Most of the 16 Canadian deposits/districts are found in defor med carbonate rocks of the foreland thrust belts. Districts suc h as Gayna River, Blende, Goz Creek, Bear Twit, Robb Lake, and Monarch-Kicking Horse are hosted in deformed and thru st-faulted carbonates adjacent to the shelf front in northern an d southern Canadian Cordillera. Only deposits of the Pine Poi nt district occur in weakly deformed carbonate rocks of the or ogenic forelands. Newfoundland Zinc, Gays River, and Upton in eastern Canada are hosted in deformed carbonate rocks of the Appalachian foreland thrust belts. Nanisivik in northern C

anada is in extensional environments associated with east-w est trending normal faults that divide the area into a series of horsts and grabens (Sherlock et al., 2003). Canadian MVT deposits are found in rocks ranging in age from Ea rly Proterozoic to Early Mississippian with the majority of deposits i n rocks of Paleozoic age. The absolute age of mineralization is not known for all Canadian deposits. Table 1 summarizes the most reli able ages for the deposits. Some deposits, such as Nanisivik, Pine Point and Robb Lake show contrasting results between radiometri c and paleomagnetic methods, and the reasons for the discrepanc y are unclear. Radiometric and paleomagnetic data show that min eralization is Paleozoic in age and coincides mainly with periods of

orogenic uplift that occurred in regions adjacent to the respective d eposits/districts. These MVT deposits and those of the Ozark distri ct in the US have been attributed to large-scale migration of fluids during convergent orogenic processes (Leach et al. 2001a). This a ssociation - mineralization and orogenic uplift in a convergent regi me - supports the topographically driven fluid flow model associate d with ore fluid migration in compressive tectonic regimes (see bel ow). Only one deposit so far, Nanisivik, is not associated with a co ntractional tectonic event but is attributed to mid-Ordovician exten sional tectonism (Sherlock et al., 2003). Summary of economic characteristics Seven out of 16 MVT deposits/districts have been mined for

a total of 112.5 Mt of ore. Pine Point was the largest district with close to 10 Mt Zn+Pb metal produced between 1964 an d 1988. An analysis of the Pine Point district showed that mo st deposits contained between 0.18 and 1.8 million tonnes of ore, and the largest (X-15) contained 16 million tonnes of ore (Sangster, 1990). With the closure of Pine Point in 1988, Ne wfoundland Zinc in 1990, and Polaris and Nanisivik in 2002, there are no MVT deposits in production in Canada. Genetic and exploration models Conventional models for fluid transport Recent advances in understanding large-scale fluid flow in th e crust, coupled with new geochemical and geological studie

s of MVT districts, have established that most MVT mineral d istricts are the products of regional or subcontinental-scale h ydrological processes. Deposits formed from hot to warm, sa line, aqueous solutions (similar to oil-field brines) that migrat ed out of sedimentary basins, through aquifers, to the basin periphery and into the platform carbonate sequences. To eff ect this movement of ore-bearing brines, at least three differ ent processes have been proposed: 1. The topographic or gravity-driven fluid flow model (Garven and Fre eze, 1984; Garven, 1985; Bethke and Marshak, 1990; Garven and Raffensperger, 1997). The first model involves flushing of subsurface brines out of a sedime

ntary basin by groundwater flow from recharge areas in elevated r egions of a foreland basin to discharge areas in lower elevated reg ions (Garven and Freeze, 1984; Garven, 1985; Bethke and Marsh ak, 1990; Garven and Raffensperger, 1997). In this model, subsurf ace flow is driven away from an uplifted orogen by the hydraulic he ad produced by tectonic uplift and tends to be concentrated in per meable units of a foreland succession. Considerable geologic evid ence listed by Leach and Sangster (1993) supports this model. Th e model has been proposed for several MVT districts in the world, particularly those of the US mid-continent and the Pine Point distri ct. At Pine Point and the western Canada sedimentary basin (WC SB), Garven (1985) carried out some hydrogeological simulations and demonstrated that Pine Point formed in less than a million yea

rs from circulation of groundwater (rich in Pb and Zn) eastward fro m the elevated thrust belt of the Laramide Orogen through the Mid dle Devonian carbonates of the Keg River Barrier. 2. The sedimentary and tectonic compaction model expulsion of basinal flui ds through sediment diagenesis and tectonic sediment compaction and the episodic fluid release from overpressured aquifers (Jackson and Be ales, 1967; Sharp, 1978; Cathles and Smith, 1983; Oliver, 1986). The second model considers that compaction of sediments in a subsiding b asin drives a continuous outward flow of pore fluids laterally along aquif ers (Jackson and Beales, 1967). Maintaining high initial fluid temperatur es during transport of up to hundreds of kilometers from basin source to platform depositional site could be a problem. A variation of this model,

episodic outward flow, was therefore proposed. The model involves ove rpressuring of subsurface aquifers by rapid sedimentation, followed by r apid and episodic release of basinal fluids (Sharp, 1978; Cathles and S mith, 1983). Another variation of the second model involves tectonic loa ding and compression of sediments during the development of orogenic thrust belts, which may have caused the rapid expulsion of formational f luids outward into the foreland basins with the thrust belts behaving like giant squeegees (Oliver, 1986). Research on the MVT deposits of the L ennard Shelf area, Western Australia has demonstrated that mineralizat ion is associated with compaction-driven dewatering and episodic fluid r elease from overpressured clastic sediments in the nearby Fitzroy Trou gh (Vearncombe et al., 1996).

3. The hydrothermal convection model (Morrow, 1998). The third model involves deep convection circulation of hydrother mal brines due to buoyancy forces related to temperatures an d salinity variations (Morrow, 1998). It supports long-lived flow systems that are capable of recycling subsurface solutions ma ny times through the rock mass. This model has been invoked to explain regional hydrothermal dolomitization in the WCSB (Morrow, 1998), the Manetoe facies of the Northwest Territorie s (Morrow et al., 1990; Aulstead et al., 1988), the Ordovician g as-producing carbonates of the Michigan Basin (Coniglio et al. , 1994), and MVT deposits of northern Canadian Rocky Mount ains (Nelson et al., 2002).

Conventional models for deposition of sulphides Three models involving 1) mixing, 2) sulphate reduction, and 3) reduce d sulphur are proposed for the chemical transport and deposition of sul phides. The mixing model proposes transport of base metals in fluids of low s ulphur content. Mixing of the metal-rich brines with fluids containing hy drogen sulphides at the depositional site triggers sulphide precipitation. Mixing of the ore fluids with a dilute or cool fluid, or reactions with host rocks to change the pH are other variants on mixing. The sulphate reduction model involves transport of base metals and sulphate in the same solution. Precipitation occurs at depositional sites when sulphate is reduced upon reaction with organic matter or methan e.

The reduced sulphur model requires that the base metals and the re duced sulphur be transported together in the same solution. Precipitati on occurs either through cooling, mixing with diluted fluids, changes in pH, or loss of volatiles. Advances in genetic/exploration models of the last decade In the past, MVT deposits were considered to have few connec tions to global tectonic processes. Remarkable advances in ag e dating of MVT deposits in the last 10 years proved to be the best accomplishment in our effort to understand the origin of M VT deposits, their links to global Earth tectonic events, and to i mprove deposit modeling. However, there is still a paucity of in formation on the ages of MVT formation, and in some cases pa

leomagnetic and radiometric age dates show contradictory res ults. MVT deposits that have been dated successfully show a relationshi p to large-scale tectonic events. Most MVT deposits formed during contractional tectonic events associated with the assimilation of Pan gea in the Devonian to Permian, and the collage of microplate assi milation on the western margin of North America and Africa-Eurasia in the Cretaceous to Tertiary. Few deposits correspond to extension al tectonic events in the Ordovician and early Mississippian time (Le ach et al., 2001b). The latter are rare and poorly understood relative to global tectonic events and more research needs to be done on th e subject. Many important questions remain regarding the age datin

g of MVT deposits; some of them are listed below. Tectonic processes Leach et al. (2001a) stressed the genetic links between MVT minerali zation and regional- and global-scale tectonic processes. It is now c lear that MVT deposits/districts are products of enormous hydrothermal systems that left trace mineralization over a wide area and that the nearl y ubiquitous occurrence of MVT deposits on the flanks of basins reflects focused migration of deep-basin brines into shelf-carbonate sequences. Thus, the regional hydrogeologic framework is of paramount importance in the evaluation of large areas for their potential to contain MVT deposit s or districts. MVT deposits in North America (such as those of the Ozark district) hav

e been attributed to large-scale migration of fluids mainly during converg ent orogenic processes. The topographically driven fluid flow model ass ociated with ore fluid migration in compressive tectonic regimes best de scribes the MVT mineralization in North America. Other deposits such a s the Lennard Shelf in Australia, Alpine deposits in Europe and North Afr ica, and Nanisivik have been attributed to continental extension and ma y require other fluid-driving mechanisms. Other questions that address key knowledge gaps also become impor tant in understanding formation of MVT deposits, and indirectly guide t he exploration for undiscovered MVT deposits. They are summarized below: What are the fluid-driving mechanisms for ore formation in extension

al regimes? What is the role of continental extension on the genesis of MVT dep osits? What are the ages for ore formation in extensional regimes? The ne w ages will provide information on MVT genesis in the context of glob al crustal tectonic model and fluid migration? Why do MVT deposits form in some but not all carbonate platforms i n collisional forelands? Why are certain carbonate platforms fertile for MVT deposits while ot hers are barren? What is it about the late stage of some collisions that induces region al-scale fluid migrations? What is the role of paleoclimate in the formation of MVT deposits?

Are evaporites critical to the origin of MVT deposits? Why are MVT deposits mostly associated with dolostones rather than li mestones? Is it due to evidence that many dolomites are formed in evap oritic environments and thus provide sulphates that can be reduced to s ulphides? Or is it simply a physical relationship where dolomites having greater porosity provide an increased probability of deposition of open-s pace filling ore minerals? What type of ground-preparation process is needed for ore deposition and also what governs the location of orebodies in a district? What function do regional tectonic processes such as orogenies, platemargin interactions or eustasy have in the mineralization process? How does the local and regional hydrology and paleohydrology relate t o dolomitization as well as mineralization? What flow paths are involved

and what is the duration of their operation? What controls the hydrology of basins and carbonate platforms? Is it re lated to the distribution of fractures and faults in the basement rocks and overlying sediments? Do some faults serve to recharge or discharge flui ds? Chemical processes Chemical processes that localized deposition of sulphides are critical to the development of models for MVT deposits, yet th e specific chemical reaction that led to sulphide deposition re mains one of the most controversial aspects of MVT deposits . Several questions remain as to: Why do these deposits contain only lead and zinc in econo

mic quantities? What is the mechanism that selects the lead and zinc? What causes metal zoning in some MVT districts as well as in individual orebodies? What is the contribution of organic matter to MVT ore depos ition? Are hydrocarbons critical for mineralization to occur? What controls the chemical composition of ore-forming fluid s? Were evaporates essential for the generation of metallifero us brines? What are the sources of metals? What are the che mical attributes of hydrothermal reaction zones that have gen erated ore-forming fluids? What alteration vectors are most effective in exploring for M

VT deposits? Supplements-2 SEDIMENTARY EXHALATIVE Zn-Pb-Ag BYPRODUCTS: Zn, Pb, Ag (minor Cu, barite). EXAMPLES (British Columbia - Canada): Cirque, Sullivan, Driftpile; Faro, Grum, Dy, Vangorda, Swim, Tom and Jason (Y ukon, Canada), Red Dog (Alaska, USA), McArthur River and Mt. Isa (Australia); Megen and Rammelsberg (Germany). GEOLOGICAL CHARACTERISTICS CAPSULE DESCRIPTION: Beds and laminations of sphal

erite, galena, pyrite, pyrrhotite and rare chalcopyrite, with or without barite, in euxinic clastic marine sedimentary strata.. Deposits are typically tabular to lensoidal in shape and ran ge from centimetres to tens of metres thick. Multiple horizo ns may occur over stratigraphic intervals of 1000 m or mor e. TECTONIC SETTING: Intracratonic or continental margin e nvironments in fault-controlled basins and troughs. Troughs are typically half grabens developed by extension along co ntinental margins or within back-arc basins. DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING : Restricted second and third order basins within linear, fault-c

ontrolled marine, epicratonic troughs and basins. There is ofte n evidence of penecontemporaneous movement on faults bou nding sites of sulphide deposition. The depositional environme nt varies from deep, starved marine to ? shallow water restrict ed shelf. AGE OF MINERALIZATION: The major metallogenic events are Middle Proterozoic, Early Cambrian, Early Silurian and Mi ddle to Late Devonian to Mississippian. The Middle Proterozoi c and Devonian-Mississippian events are recognized worldwid e. In the Canadian Cordillera, minor metallogenic events occur in the Middle Ordovician and Early Devonian. HOST/ASSOCIATED ROCK TYPES: The most common

hostrocks are those found in euxinic, starved basin enviro nments, namely, carbonaceous black shale, siltstone, ch erty argillite and chert. Thin interbeds of turbiditic sandsto ne, granule to pebble conglomerate, pelagic limestone an d dolostone, although volumetrically minor, are common. Evaporites, calcareous siltstone and mudstone are comm on in shelf settings. Small volumes of volcanic rocks, typi cally tuff and submarine mafic flows, may be present withi n the host succession. Slump breccia, fan conglomerates and similar deposits occur near synsedimentary growth fa ults. Rapid facies and thickness changes are found near t he margins of second and third order basins. In some bas ins high-level mafic sills with minor dikes are important.

DEPOSIT FORM: These deposits are stratabound, tabula r to lens shaped and are typically comprised of many bed s of laminae of sulphide and/or barite. Frequently the lens es are stacked and more than one horizon is economic. O re lenses and mineralized beds often are part of a sedime ntary succession up to hundreds of metres thick. Horizont al extent is usually much greater than vertical extent. Indiv idual laminae or beds may persist over tens of kilometres within the depositional basin. TEXTURE/STRUCTURE: Sulphide and barite laminae ar e usually very finely crystalline where deformation is mino r. In intensely folded deposits, coarser grained, recrystalliz

ed zones are common. Sulphide laminae are typically mo nomineralic. ORE MINERALOGY (Principal and subordinate): The principal s ulphide minerals are pyrite, pyrrhotite, sphalerite and galena. Some deposits contain significant amounts of chalcopyrite, but most do n ot. Barite may or may not be a major component of the ore zone. T race amounts of marcasite, arsenopyrite, bismuthinite, molybdenite , enargite, millerite, freibergite, cobaltite, cassiterite, valleriite and melnikovite have been reported from these deposits. These minera ls are usually present in very minor amounts. ALTERATION MINERALOGY: Alteration varies from well develop ed to nonexistent. In some deposits a stockwork and disseminated

feeder zone lies beneath, or adjacent to, the stratiform mineralizati on. Alteration minerals, if present, include silica, tourmaline, carbon ate, albite, chlorite and dolomite. They formed in a relatively low te mperature environment. Celsian, Ba-muscovite and ammonium cla y minerals have also been reported but are probably not common. ORE CONTROLS: Favourable sedimentary sequences, m ajor structural breaks, basins. GENETIC MODEL: The deposits accumulate in restricted second and third order basins or half grabens bounded by synsedimentary growth faults. Exhalative centres occur alo ng these faults and the exhaled brines accumulate in adjac ent seafloor depressions. Biogenic reduction of seawater s

ulphate within an anoxic brine pool is believed to control s ulphide precipitation. ASSOCIATED DEPOSIT TYPES: Associated deposit type s include carbonate-hosted sedimentary exhalative, such a s the Kootenay Arc and Irish deposits (E13), bedded barite (E17) and iron formation (F10). EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: The deposits are typically zoned with Pb found closest to the vent grading outward a nd upward into more Zn-rich facies. Cu is usually found eit her within the feeder zone of close to the exhalative vent. Barite, exhalative chert and hematite-chert iron formation, i

f present, are usually found as a distal facies. Sediments s uch as pelagic limestone interbedded with the ore zone m ay be enriched in Mn. NH3 anomalies have been documen ted at some deposits, as have Zn, Pb and Mn haloes. The host stratigraphic succession may also be enriched in Ba o n a basin-wide scale. GEOPHYSICAL SIGNATURE: Airborne and ground geophysical s urveys, such as electromagnetics or magnetics should detect depo sits that have massive sulphide zones, especially if these are steep ly dipping. However, the presence of graphite-rich zones in the hos t sediments can complicate the interpretation of EM conductors. Al so, if the deposits are flat lying and comprised of fine laminae distri

buted over a significant stratigraphic interval, the geophysical resp onse is usually too weak to be definitive. Induced polarization can detect flat-lying deposits, especially if disseminated feeder zones a re present. OTHER EXPLORATION GUIDES: The principal exploration guideli nes are appropriate sedimentary environment and stratigraphic ag e. Restricted marine sedimentary sequences deposited in an epicr atonic extensional tectonic setting during the Middle Proterozoic, E arly Cambrian, Early Silurian or Devono-Mississippian ages are the most favourable. ECONOMIC FACTORS GRADE AND TONNAGE: The median tonnage for this type of d

eposit worldwide is 15 Mt, with 10 % of deposits in excess of 130 Mt (Briskey, 1986). The median grades worldwide are Zn - 5.6%, Pb - 2.8% and Ag - 30 g/t. The Sullivan deposit, one of the larges t deposits of this type ever discovered, has a total size of more th an 155 Mt grading 5.7% Zn, 6.6% Pb and 7 g/t Ag. Reserves at t he Cirque are 32.2 Mt grading 7.9% Zn, 2.1% Pb and 48 g/t Ag. ECONOMIC LIMITATIONS: The large, near-surface deposits are amenable to high volume, open pit mining operations. Undergrou nd mining is used for some deposits. IMPORTANCE: Sedimentary exhalative deposits currently produ ce a significant proportion of the worlds Zn and Pb. Their large to nnage potential and associated Ag values make them an attractiv e exploration target.

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