Abstract Sediment-hosted Zn-Pb and Cu deposits in China include Mississippi Valley-type (MVT) deposits, clastic-dominated (CD) deposits (also historically called sedimentary-exhalative [SEDEX] deposits by some workers), sandstone-hosted (SSH) Zn-Pb deposits, a few large magmatic-related carbonate-replacement deposits (CRD), and volcanic-hosted massive sulfide (VHMS) deposits that have been mistakenly classified as nonmagmatic-related MVT or CD deposits. There are also areas of China that contain important sediment-hosted copper (SHC) deposits. China is exceptionally endowed with MVT deposits with three of the five largest MVT deposits in the world (Huoshaoyun, Jinding, and Changba-Lijiagou). In contrast, China has one CD deposit (Dongshengmiao) in the top 30 CD deposits in the world. The few SHC deposits are small relative to world-class examples. The largest SHC deposits are located in the Yangtze and the North China cratons and hosted in Proterozoic rocks with indications of massive halokinetic features like those observed in the African copper belt. The MVT ores are most abundant in the Yangtze block, Qinling orogen, and the central and eastern Himalayan-Tibetean orogen. There are many other carbonate-hosted deposits in the North China craton and the Cathaysia block that have been widely classified as MVT or sedimentary-exhalative deposits. These are better classified as CRD or skarn deposits based on their proximity to intrusions, alteration assemblages, trace and minor element signatures, and, in some deposits, the presence of skarns minerals. Numerous sediment-hosted Zn-Pb deposits in China have been traditionally classified as SEDEX or syngenetic deposits based on laminated ore textures and stratiform ores that we interpret to reflect deformation and selective replacement processes rather than synsedimentary ore processes. Only two of these sediment-hosted deposits can be unequivocally classified as CD deposits: Dongshengmiao and Tanyaokou in the Langshan area of the North China craton. They are hosted in a siliciclastic-dominated sequence of a Proterozoic passive margin. The location and genesis of many MVT and SHC deposits in China are directly controlled by evaporites and evaporite facies. Evaporite and evaporite facies had an extremely important role in determining the location of the MVT deposits. The second largest sediment-hosted Zn-Pb deposit in China and fifth largest in Asia, Jinding in the Himalayan-Tibetan orogenic belt, is hosted in a hydrocarbon-reduced sulfur reservoir that formed because of salt diapirism. Other large sediment-hosted Zn-Pb MVT deposits in China that are interpreted to be controlled by structures produced by evaporite diapirism are Daliangzi and Tianbaoshan in the western Yangtze block. The largest Zn-Pb deposit in China is the newly discovered oxidized Huoshaoyun Zn-Pb MVT deposit, also in the Himalayan-Tibetan orogenic belt that is hosted in an evaporite-bearing sequence. The third largest Zn-Pb resource in China is at the Changba-Lijiagou deposit and, together with numerous smaller deposits, define a belt of metaevaporites in a carbonate platform sequence of the northern Yangtze platform. Other evaporite-related MVT ores include the Huize deposits that are hosted in a former Carboniferous evaporite-bearing hydrocarbon reservoir and the extensive Sinian dolostone-hosted Zn-Pb deposits that reflect evaporite dissolution breccias in the Yangtze block. The Tarim craton in northwestern China contains the only significant SSH deposit at Uragen. The ore zone lies in the footwall of an evaporative unit that may have served as a hydrocarbon and reduced sulfur trap. Furthermore, the most significant SHC deposits are hosted in Proterozoic rocks in the North China craton and the Yangtze block that contain extensive halokinetic breccias and structures.

Abstract Some sediment-hosted base metal deposits, specifically, the clastic-dominated Zn-Pb deposits, carbonatehosted Mississippi Valley-type (MVT) deposits, sedimentary rock-hosted stratiform copper deposits, and carbonate-hosted polymetallic (“Kipushi-type”) deposits, are or have been important sources of critical elements including Co, Ga, Ge, PGEs, and Re. Cobalt is noted in only a few clastic-dominated and MVT deposits, whereas sedimentary rock-hosted stratiform copper deposits are major producers. Gallium occurs in sphalerite from clastic-dominated and MVT deposits. Little is reported of germanium in clastic-dominated deposits; it is more commonly noted in MVT deposits (up to 4,900 ppm within sphalerite) and has been produced from carbonate-hosted polymetallic deposits (Kipushi, Tsumeb). Indium is known to be elevated in sphalerite and zinc concentrates from some MVT and clastic-dominated deposits, produced from Rammelsberg and reported from Sullivan, Red Dog, Tri-State, Viburnum Trend, Lisheen, San Vincente, and Shalipayco. Platinum and palladium have been produced from sedimentary rock-hosted stratiform copper deposits in the Polish Kupferschiefer. Sedimentary rock-hosted stratiform copper deposits in the Chu-Sarysu basin are known to have produced rhenium. Although trace element concentrations in these types of sediment-hosted ores are poorly characterized in general, available data suggest that there may be economically important concentrations of critical elements yet to be recognized.

Abstract Sediment-hosted Pb-Zn deposits contain the world’s greatest lead and zinc resources and dominate worldproduction of these metals. They are a diverse group of ore deposits hosted by a wide variety of carbonate andsiliciclastic rocks that have no obvious genetic association with igneous activity. A range of ore-forming processes in a variety of geologic and tectonic environments created these deposits over at least two billion years of Earth history. The metals were precipitated by basinal brines in synsedimentary and early diagenetic to low-grade metamorphic environments. The deposits display a broad range of relationships to enclosing host rocks that includes stratiform, strata-bound, and discordant ores. These ores are divided into two broad subtypes: Mississippi Valley-type (MVT) and sedimentary exhalative (SEDEX). Despite the “exhalative” component inherent in the term “SEDEX,” in this manuscript, direct evidence of an exhalite in the ore or alteration component is not essential for a deposit to be classified as SEDEX. The presence of laminated sulfides parallel to bedding is assumed to be permissive evidence for exhalative ores. The distinction between some SEDEX and MVT deposits can be quite subjective because some SEDEX ores replaced carbonate, whereas some MVT deposits formed in an early diagenetic environment and display laminated ore textures. Geologic and resource information are presented for 248 deposits that provide a framework to describe and compare these deposits. Nine of the 10 largest sediment-hosted Pb-Zn deposits are SEDEX. Of the deposits that contain at least 2.5 million metric tons (Mt), there are 35 SEDEX (excluding Broken Hill-type) deposits and 15 MVT (excluding Irish-type) deposits. Despite the skewed distribution of the deposit size, the two deposits types have an excellent correlation between total tonnage and tonnage of contained metal (Pb + Zn), with a fairly consistent ratio of about 10/1, regardless of the size of the deposit or district. Zinc grades are approximately the same for both, whereas Pb and Ag grades are about 25 percent greater for SEDEX deposits. The largest difference between SEDEX and MVT deposits is their Cu content. Three times as many SEDEX deposits have reported Cu contents, and the median Cu value of SEDEX deposits is nearly double that of MVT deposits. Furthermore, grade-tonnage values for MVT deposits compared to a subset of SEDEX deposits hosted in carbonate rocks are virtually indistinguishable. The distribution of MVT deposits through geologic time shows that they are mainly a Phanerozoic phenomenon. The ages of SEDEX deposits are grouped into two major groups, one in the Proterozoic and another in the Phanerozoic. MVT deposits dominantly formed in platform carbonate sequences typically located within extensional zones inboard of orogenic belts, whereas SEDEX deposits formed in intracontinental or failed rifts, and rifted continental margins. The ages of MVT ores are generally tens of millions of years younger than their host rocks; however, a few are close (<~5 m.y.) to the age of their host rocks. In the absence of direct dates for SEDEX deposits, their age of formation is generally constrained by relationships to sedimentary or diagenetic features in the rocks. These studies suggest that deposition of SEDEX ores was coeval with sedimentation or early diagenesis, whereas some deposits formed at least 20 m.y. after sedimentation. Fluid inclusion, isotopic studies, and deposit modeling suggest that MVT and SEDEX deposits formed from basin brines with similar temperatures of mainly 90° to 200°C and 10 to 30 wt percent NaCl equiv. Lead isotope compositions for MVT and SEDEX deposits show that Pb was mainly derived from a variety of crustal sources. Lead isotope compositions do not provide criteria that distinguish MVT from SEDEX subtypes. However, sulfur isotope compositions for sphalerite and galena show an apparent difference. SEDEX and MVT sulfur isotope compositions extend over a large range; however, most data for SEDEX ores have mainly positive isotopic compositions from 0 to 20 per mil. Isotopic values for MVT ores extend over a wider range and include more data with negative isotopic values. Given that there are relatively small differences between the metal character of MVT and SEDEX deposits and the fluids that deposited them, perhaps the most significant difference between these deposits is their de-positional environment, which is determined by their respective tectonic settings. The contrasting tectonic setting also dictates the fundamental deposit attributes that generally set them apart, such as host-rock lithology, deposit morphology, and ore textures. Brief discussions are also presented on two controversial sets of deposits: Broken Hill-type deposits and a subset of deposits in the MVT group located in the Irish Midlands, considered by some authors to be a distinct ore type (Irish type). There are no significant differences in grade tonnage values between MVT deposits and the subset that is described as Irish type. Most features of the Irish deposits are not distinct from the family of MVT deposits; however, the age of mineralization that is the same as or close to the age of the host rocks and the anomalously high fluid inclusion temperatures (up to 250°C) stand out as distinctly different from typical MVT ores. The dominance of bacteriogenic sulfur in the Irish ores commonly ascribed as uniquely Irish type is in fact no different from several MVT deposits or districts. A comparison of SEDEX and Broken Hill-type deposits shows that the latter deposits contain significantly higher contents of Ag and Pb relative to SEDEX deposits. In terms of median values, Broken Hill-type deposits are almost three times more enriched in Ag and one and a half times more enriched in Pb compared to other SEDEX deposits. Metamorphism is a characteristic feature but not a prerequisite for inclusion in the Broken Hill-type category, and known Broken Hill-type examples appear to occur in Paleo- to Mesoprotero-zoic terranes. Broken Hill-type deposits remain an enigmatic grouping; however, there is sufficient evidence to support their inclusion as a separate category of SEDEX deposits.

Abstract Approximately 30 million oz (Moz) of gold has been recovered from metamorphosed terranes in Alaska. One-third of the production has come from lodes in rhe southern half of the state, predominantly from mines in the Juneau gold belt, Chichagof district, and Willow Creek district. The majority has been taken from placers in northern and interior Alaska, including those near Nome and Fairbanks, where many of the lode sources have been completely eroded. Host terranes for gold-bearing quartz veins are dominated by pelitic rocks and range in age from Precamblian(?) to Tertiary. Ore formation, however, was reshicted to a 60-m.y. interval between Albian and Eocene time. In northern Alaska, ore-forming hydrothermal systems were active in the southern Brooks Range during Albian time, on the Seward Peninsula at 109 Ma, and in the Yukon-Tanana upland between 92 and 77 Ma. In southern Alaska, seaward of the late Mesozoic to early Cenozoic Alaska Range-Coast Mountains magmatic arc, ore formation was younger; veining occurred at 63 to 57 Ma in the Valdez Creek disttict, 66 Ma in the Willow Creek district, 55 ± 2 Ma along the Juneau gold belt, and 57 to 49 Ma in the Chugach accretionary prism. Calc-alkaline plutonic rocks occur within a few tens of kilometers of most of the gold deposits in Alaska and generally were emplaced coevally with hydrothermal activity. Ore-bearing veins are composed predominantly of quartz and show brittle and/or ductile features depending on the district. Carbonate minerals, chlorite, and white mica are common gangue phases. Sulfide minerals, most commonly pyrite or arsenopyrite, compose less than 3 percent of the vein material. Silicification, carbonization, and sericitization characterize wall rocks within a few meters of most veins. This alteration is best developed adjacent to veins in igneous host rocks. Ore fluids are H 2 O dominant, with approximately 5 to 15 mole percent CO 2 and significant amounts of CH 4 , N 2 , and H 2 S. Relatively high nonaqueous gas contents in parent fluids responsible for the Alaska-Juneau deposit, deposits in the Fairbanks disttict, and many or the veins in the Valdez Creek district resulted in episodic immiscibility accompanying gold getnesis. Fluid salinities are less than 8 wt percent NaCI equiv. Vein formation temperatures rnnged from 225° to 375°C, with emplacement depths of about 3 to 10 km. In the Fairbanks district, however, some ores were more shallowly emplaced. Ore fluid values of δ 18 O (7-12‰) and δD (-15 to -55‰) for most deposits are consistent with a fluid in isotopic: equilibrium with the metamorphic terranes and one that was produced during greeuscltist to amphibolite facies devolatilization reactions. Values of δ Dchlorite between -135 and - 187 per mil for veins from the Nome disttict provide a notable exception and may be the product of postcrystallization recquilibmtion between the micas and circulating metelogic waters. A sedimentary rock origin for sulfur in most veins is favored because δ 34 S sulfide values vary with the age of host terrane according to a pattern approximating that of matine sulfate through time. Lead isotope compositions of sulfide minemls are a function of vein host lithologies and provide little input into the understanding of gold ore genesis. Gold vein formation was related to the internction among the Kula, Farallon, and North America plates. Middle to Late Cretaceous veining in northem and interior Alaska postdated initial terrane collision and obduction of oceanic rocks by 100 m.y. The veining might be a product of rising crustal temperatures associated with continued collision, accompanied by development of a major continental margin trench and the eventual subduction of the Famllon plate. Alternatively, a shift to trench retreat and slab rollback may have caused asthenospheric upwelling, crustal thinning, plutonism, and Cretaceous hydrothermal activity above the subducting Farallon plate. Veining in southern Alaska in the early Tertiaty is hypothesized to be a product of crustal thickening and shear heating in the inner fore arc and Farallon-Kula tidge subduction in the outer fore arc.

Abstract The stable carbonate platform of the U.S. midcontinent is host to the largest concentration of Mississippi Valley-type (MVT) zinc-lead mining districts in the world. Of these districts, the Southeast Missouri Lead District which includes the main Viburnum Trend and Old Lead Belt subdistricts, is unique in several ways. Lead dominates over zinc and the ores have significant amounts of copper, silver, cobalt, and nickel. Hosted in the Upper cambrian Bonneterre Formation, above the basal Cambrian Lamotte Sandstone which is underlain by felsic Precambrian basement, the Viburnum Trend is situated very low in the carbonate section when compared to the other districts of the region. Of the Ozark districts, the Southeast Missouri Lead District is also unique in its proximity to the Reelfoot Rift which is concealed beneath the Mississippi Embayment. The possible importance of the Reelfoot Rift as a pathway for brine migration from a southerly source has recently been demonstrated by Farrand Land (1985) and Farr (1987). Detailed descriptions of the mines of the Viburnum Trend are given in a special issue of ECONOMIC GEOLOGY (V. 72, NO. 3, 1977). Other informative background papers on the ore deposits of the Southeast Missouri Lead District include those by snyder and Gerdemann (1968), Gerdemann and Meyers (1972), Davis (1977) and Sverjensky (1981).

Abstract Microthermometry, laser Raman spectroscopy, and mass spectrometry were used to study fluid inclusions in gold-bearing quartz veins from the mines of the Juneau gold belt, Unmixing of a CO 2 -rich parent fluid led to the contemporaneous trapping of H 2 O-dominant and CO 2 -dominant inclusions during gold deposition at the Alaska-Juneau, Reagan, and Ibex mines. Ore fluids at all other mines were trapped as homogeneous, H 2 O-dominant fluids, with less than 10 mole percent CO 2 . Both N 2 and CH 4 are present at the percent level within the volatile phases in all deposits; H 2 S makes up one-third of the volatile phase and 2 mole percent of the total ore fluid at the Sumdum Chief mine. The ore fluids contained less than 5 equiv wt percent NaCl. Gold deposition occurred at temperatures above 250°C and at depths of at least 5 km. The gold-forming fluids are believed to have been derived from devolatilization reactions associated with prograde metamorphism of dominantly pelitic, subducted crust.