Hydrothermal Zebra Dolomite in the Great Basin, Nevada—Attributes and Relation to Paleozoic Stratigraphy, Tectonics, and Ore Deposits

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.

The spectrum of zinc-lead deposits formed in basinal mineral systems encompasses VMS (volcanogenic massive sulphide), SHMS (shale-hosted massive sulphide), Irish-type and MVT (Mississippi valley Type) deposits. The platform carbonate-hosted part of that spectrum, the Irish-type and MVT deposits, has created the greatest challenges to pigeonholing approaches and the Irish Midland deposits have been variably considered as unique, “SedEx” variants, or MVT variants. In fact, the Irish-type spectrum of deposits can be considered as a global diaspora of diverse deposits that, nonetheless, show a number of distinct and economically significant characteristics in style and setting. For this reason, they warrant consideration as a discrete deposit type, though not in a neat pigeonhole, that is best considered in a mineralizing system context. The distinguishing features of Irish-type mineral systems can be considered in terms of source, trigger, pathway, trap, and preservation. The key features that distinguish Irish-type from ‘typical’ MVT and SHMS deposits are related to basin type and setting, timing of the mineralization event, mineralization style and chemistry, and deposit geometry. Empirically, these characteristic basin to deposit scale features overlap both MVT and SHMS but, together, are unique to Irish-type systems. This gives rise to criteria that can be applied to determine prospectivity of basins for Irish-type deposits and to target deposits within these basins. It is important to distinguish Irish-type from MVT systems because their economic characteristics are different. However, it is also important to recognise that there is great variability within the broad basinal carbonate-hosted zinc-lead deposit family and that each basin, and indeed each trend and deposit, are to some extent unique. It is therefore extremely important to avoid model-driven exploration and to develop a targeting understanding that acknowledges the model framework but is based on actual observations and data. To understand this diversity and targeting context, it is pertinent to consider the wide range of carbonate-hosted deposits that do not fit into the published MVT pigeonhole, including the Irish Midlands deposits; the Alpine deposits; deposits in the Basque-Cantabria Basin; deposits on the Gondwana margin including a number of deposits in North African,, southeast Turkey, Iran, and Duddar in Pakistan; the Early Cretaceous deposits on the Atlantic margin in Gabon; the Ordovician of the Sibumasu terrane (Tibet to Southeast Asia); Polaris in the Franklinian Basin; Nanisivik in the Borden Basin; and the Devonian Lennard Shelf deposits of Western Australia. All of these deposits occur in rift-sag basins with carbonate platforms, in some cases with multiple rift-sag cycles or with successor basins and, where constrained, the mineralization event is syn-basinal and typically related to early extension or inversion events. The deposits are stratabound and mostly tabular and continuous, often show strong direct control by extensional structures, are typically dominated by replacement, and commonly have significantly higher grades than ‘typical’ MVT deposits such as those in the mid-continent US, Silesia (Poland) and Pine Point (Canada).

From the first issue in 1905 onward, Economic Geology has been the main publication for those who study mineral deposits; indeed, it is now difficult to imagine economic geology without Economic Geology. It is interesting to ask, therefore, Who were the farsighted people who founded the journal, and Why did they think a specialized publication devoted to mineral deposits was needed?Let us first address the question, Who were the founders? They were the 12 men who collectivelydecided a new publication was needed, who then planned the financial structure to support the venture, and who served as the original editorial group. All were employed by, or associated with, the U.S. Geological Survey. Josiah Edward Spurr suggested the need for a journal sometime in November or December 1904. After informal discussions, nine of the founders met in the office of Waldemar Lindgren in the headquarters of the U.S. Geological Survey in Washington, D.C., on May 16, 1905, and founded the Economic Geology Publishing Company. The sole purpose of the company was the publication of a journal '...devoted primarily to the broad application of geologicprinciples to mineral deposits of economic value, and to the scientific description of such deposits, and particularly to the chemical, physical, and structural problems bearing on their genesis.' Initial financing for the new company was raised by the sale of 80 shares at a cost of $25 per share.Eight of the men at the founding meeting formed the first board of directors; Spurr was president, Frederick L. Ransome, secretary, and George O. Smith, treasurer. Other members were Arthur H. Brooks, Marius R. Campbell, Walter H. Weed, Waldemar Lindgren, and a young academic from Lehigh University in Pennsylvania, John D. Irving. Theninth man at the meeting was H. Foster Bain. Irving was appointed editor. Lindgren, Ransome, and Campbell from the U.S. Geological Survey, together with three academics, James F. Kemp of Columbia University, Heinrich Ries ofCornell University, and Charles K. Leith of the University of Wisconsin, were appointed associate editors. The initial board members, the editor, and associate editors are the people we now recognize as the founders of Economic Geology. Two others, Frank D. Adams, of McGill University in Canada, and John. W. Gregory, of Glasgow University in Scotland, were subsequently added as associate editors, and a third person, W. S. Bayley of the University of Illinois, was appointed as business editor, but

More than 2800 published fluid inclusion data (primary inclusions) from eighteen typical Mississippi Valley-type (MVT) Zn-Pb deposits and districts were examined to study possible relationships between MVT fluid properties and metal tonnages and ore grades. The mean Th and salinity values for fifteen MVT deposits and districts are 122°C ± 21°C and 20.7 ± 2.6 wt% CaCl 2 equivalent, respectively. In general, temperatures increase from pre-ore to ore stages, and salinities decrease from ore to post-ore stages. From post-ore to late calcite stages, salinity and temperature decrease with various trends within specific districts. Several conclusions are drawn: first, results show that Hanor’s (1996) salinity threshold for metal-rich basinal fluids applies to typical MVT ore fluids (~16 wt% CaCl 2 equivalent). Second, the lower limit of mean Th values of measured MVT deposits and districts (n = 15) of ~75°C is similar to Hanor’s (1996) observed temperature threshold for metal-rich basinal brines of ~60°C (n = 224). Third, there is no statistically significant relationship between fluid inclusion homogenization temperatures (Th) and metal tonnages or ore grades. Fourth, there seems to be a tendency for large tonnage districts/deposits to be associated with mean fluid salinities that are relatively low (~16 to 21 wt% CaCl 2 equivalent) within the overall range of salinity means (~16 to 26 wt% CaCl 2 equivalent). There is also a correlation between higher ore grades and higher salinities. Finally, comparison with other sedimentary rock-hosted Zn-Pb deposit types shows a trend of salinity decrease and temperature increase from MVT, sandstone-hosted and diapir-related Pb-Zn deposits, to vein-type and Irish-type Pb-Zn deposits, and then to sedex deposits. However, more data are needed for sedex deposits. The salinity threshold for significant Zn-Pb transport decreases to ≥~10 wt% CaCl 2 equivalent at ~250°C. This study suggests that fluid inclusion analysis during exploration may be useful in assessing ore-forming potential (threshold salinity) and possible ore grades (salinity).

Low-temperature, carbonated-hosted, strata-bound, Zn-Pb±fluorite±barite deposits are generally referred to as Mississippi Valley-type (MVT) deposits in recognition of the occurrence of many such deposits within the drainage basin of the Mississippi River in the central United States, where they were first studied in detail. MVT deposits contain a substantial proportion of the world’s reserves of zinc and lead. They are the main source of these metals in the United States and contribute significantly to the production of lead and zinc in Canada and Europe.

Na-Cl-Br systematics of fluid inclusions from Mississippi Valley-type deposits, Appalachian Basin: Constraints on solute origin and migration paths

This paper combines petrography with in situ laser-ablation inductively coupled plasma mass spectrometry to document trace-element variations in pyrite (Py) from Mississippi Valley-type (MVT) and fracture-controlled replacement (FCR) deposits in the Kootenay Arc, British Columbia. Three generations of pyrite are Py 1, Py 2, and Py 3. Pyrite 1, the earliest (occurring in MVT deposits only), has higher Ag, Ba, Cu, Ge, Pb, Sb, Sr, Tl, and V than adjacent Py 3. It has higher Ag, Au, Ba, Cu, Ge, Pb, and Tl than Py 2. Pyrite 2 occurs in MVT and FCR deposits. Relative to FCR Py 2, MVT Py 2 is enriched in Co, Ni, Mo, Ba, Tl, and Pb and depleted in other elements. The FCR Py 2 has growth-related compositional banding, which is absent in MVT Py 2. The FCR Py 2 has Ag, Cu, Ga, Ge, In, Sn, and Zn enriched cores, intermediate Au- and As-rich bands, and Co- and Ni-rich rims. Pyrite 3, the latest occurring pyrite, present in MVT and FCR deposits, is enriched in Co and Ni near overgrowths or infillings of sphalerite. Variations in composition of Py reflect mineralogy, characteristics of ore-forming fluids, and differences in physicochemical conditions between MVT and FCR deposits at the time of ore deposition.

Northeastern Mexico hosts numerous epigenetic stratabound carbonate-hosted low-temperature hydrothermal deposits of celestine, fluorite, barite and zinc-lead, which formed by replacement of Mesozoic evaporites or carbonate rocks. Such deposits can be permissively catalogued as Mississippi Valley-type (MVT) deposits. The deposits studied in the state of Coahuila are associated with granitic and metasedimentary basement highs (horsts) marginal or central to the Mesozoic Sabinas Basin. These horsts controlled the stratigraphy of the Mesozoic basins and subsequently influenced the Laramide structural pattern. The Sabinas Basin consists of ~6,000-m-thick Jurassic to Cretaceous siliciclastic, carbonate and evaporitic series. The MVT deposits are mostly in Barremian and in Aptian-Albian to Cenomanian formations and likely formed from basinal brines that were mobilized during the Laramide orogeny, although earlier diagenetic replacement of evaporite layers (barite and celestine deposits) and lining of paleokarstic cavities in reef carbonates (Zn–Pb deposits) is observed. Fluid inclusion microthermometry and isotopic studies suggest ore formation due to mixing of basinal brines and meteoric water. Homogenization temperatures of fluid inclusions range from 45°C to 210°C; salinities range from 0 to 26 wt.% NaCl equiv., and some inclusions contain hydrocarbons or bitumen. Sulfur isotope data suggest that most of the sulfur in barite and celestine is derived from Barremian to Cenomanian evaporites. Regional geology and a compilation of metallogenic features define the new MVT province of northeastern Mexico, which comprises most of the state of Coahuila and portions of the neighboring states of Nuevo Leon, Durango and, perhaps extends into Zacatecas and southern Texas. This province exhibits a regional metal zonation, with celestine deposits to the south, fluorite deposits to the north and barite and Zn–Pb deposits mostly in the central part.

Paleozoic platform carbonate rocks of the Rocky Mountains host Mississippi Valley-type (MVT), magnesite, barite, and REE-barite-fluorite deposits. Farther west, platform carbonate rocks of the Kootenay Arc host MVT and fracture-controlled replacement (FCR) deposits. This is the first systematic LA-ICP-MS study of carbonates in MVT and FCR deposits. We investigated seven MVT deposits in the Rocky Mountains, and five MVT deposits in the Kootenay Arc. None of the post-Archean Australian shale (PAAS)-normalized REE profiles show light REE (LREE) depletion and strong negative Ce anomalies characteristic of modern seawater: some profiles are nearly flat; others show depletion in LREE similar to seawater but without negative Ce anomalies; others are middle REE enriched. Carbonates with a strong positive Eu anomaly precipitated from or interacted with different fluids than carbonates with flatter profiles without a strong positive Eu anomaly. REE signatures reflect crystallization conditions of primary carbonates, and crystallization and re-equilibration conditions of carbonates with ambient fluids during diagenesis, deep burial, and/or metamorphic recrystallization. Chemical evolution of fluids along their migration path, fluid-to-rock ratio, fluid acidity, redox, and temperature also influence REE profile shape, which helps establish genetic and timing constraints on studied deposits and improves knowledge of the metallogeny of the Kootenay Arc and Rocky Mountains.

Remarkable advances in age dating Mississippi Valley-type (MVT) lead–zinc deposits provide a new opportunity to understand how and where these deposits form in the Earth's crust. These dates are summarized and examined in a framework of global tectonics, paleogeography, fluid migration, and paleoclimate. Nineteen districts have been dated by paleomagnetic and/or radiometric methods. Of the districts that have both paleomagnetic and radiometric dates, only the Pine Point and East Tennessee districts have significant disagreements. This broad agreement between paleomagnetic and radiometric dates provides added confidence in the dating techniques used. The new dates confirm the direct connection between the genesis of MVT lead–zinc ores with global-scale tectonic events. The dates show that MVT deposits formed mainly during large contractional tectonic events at restricted times in the history of the Earth. Only the deposits in the Lennard Shelf of Australia and Nanisivik in Canada have dates that correspond to extensional tectonic events. The most important period for MVT genesis was the Devonian to Permian time, which corresponds to a series of intense tectonic events during the assimilation of Pangea. The second most important period for MVT genesis was Cretaceous to Tertiary time when microplate assimilation affected the western margin of North America and Africa–Eurasia. There is a notable paucity of MVT lead–zinc ore formation following the breakup of Rodinia and Pangea. Of the five MVT deposits hosted in Proterozoic rocks, only the Nanisivik deposit has been dated as Proterozoic. The contrast in abundance between SEDEX and MVT lead–zinc deposits in the Proterozoic questions the frequently suggested notion that the two types of ores share similar genetic paths. The ages of MVT deposits, when viewed with respect to the orogenic cycle in the adjacent orogen suggest that no single hydrologic model can be universally applied to the migration of the ore fluids. However, topographically driven models best explain most MVT districts. The migration of MVT ore fluids is not a natural consequence of basin evolution; rather, MVT districts formed mainly where platform carbonates had some hydrological connection to orogenic belts. There may be a connection between paleoclimate and the formation of some MVT deposits. This possible relationship is suggested by the dominance of evaporated seawater in fluid inclusions in MVT ores, by hydrological considerations that include the need for multiple-basin volumes of ore fluid to form most MVT districts, and the need for adequate precipitation to provide sufficient topographic head for topographically-driven fluid migration. Paleoclimatic conditions that lead to formation of evaporite conditions but yet have adequate precipitation to form large hydrological systems are most commonly present in low latitudes. For the MVT deposits and districts that have been dated, more than 75% of the combined metal produced are from deposits that have dates that correspond to assembly of Pangea in Devonian through Permian time. The exceptional endowment of Pangea and especially, North America with MVT lead–zinc deposits may be explained by the following: (1) Laurentia, which formed the core of North America, stayed in low latitudes during the Paleozoic, which allowed the development of vast carbonate platforms; (2) intense orogenic activity during the assembly of Pangea created ground preparation for many MVT districts through far-field deformation of the craton; (3) uplifted orogenic belts along Pangean suture zones established large-scale migration of basin fluids; and (4) the location of Pangea in low latitudes with paleoclimates with high evaporation rates led to the formation of brines by the evaporation of seawater and infiltration of these brines into deep basin aquifers during Pangean orogenic events.

A combined Sr, O and C isotope study has been carried out in the Pucara basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucara basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucara basin. The San Ramon granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucara Group is accompanied by only a slight decrease in δ 18O values and essentially no change in δ 13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ 13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ 18O values in carbonate rocks from the lower part of the Pucara Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucara sequence and cooler, dilute brines possibly representing local fluids within the Pucara sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucara Group.

Constraints on the sources of ore metals in Mississippi Valley-type deposits in central and east Tennessee, USA, using Pb isotopes

Roy D. Merritt Coal exploration, mine planning, and development 1986 Noyes Publication Park Ridge, New Jersey 464 ISBN 0-8155-1070-5

Mississippi Valley-type (MVT) deposits represent enrichments of base metals and other elements up to 1000's of times greater than their average concentrations in the Earth's crust. These enrichments most commonly consist of Zn and Pb as the minerals sphalerite and galena, but Ba and F, as the minerals barite and fluorite, can be abundant and even predominate over Zn and Pb in some deposits. Key to understanding how MVT deposits become enriched in the above elements is knowledge of the concentrations of these elements in the mineralizing fluids and how the fluids interact with host rocks. This dissertation consists of three studies that address this knowledge gap. The first was a case study of the Hansonburg, New Mexico MVT district, in which barite and fluorite are the principal ore minerals. The aim of the study was to test the hypothesis that anomalously F-rich fluids formed fluorite-rich MVT deposits by analyzing the composition of fluid inclusions in ore stage minerals. The study showed that the Hansonburg mineralizing fluids were very F-rich, with F concentrations of 100's to 1000's of ppm. The fluids would have been very acidic, which would have suppressed metal sulfide mineral precipitation. The second was a numerical reactive transport modeling study of the Illinois-Kentucky district, a fluorite dominant atypical MVT district. The aim of the study was to assess the importance of F-rich ore fluids in producing the features of the deposits observed in the field. These models showed that silica-armoring of conduits along which the F-rich fluids ascended was critical for allowing the fluids to retain their F-rich, acidic profile until the fluids entered the limestone host rocks. Further, the models showed that high ore fluid F concentrations of 100's to 1000's of ppm were necessary to form the spatial distributions of fluorite mineralization and limestone host rock dissolution observed in the field, and to form the fluorite mineralization within a geologically reasonable time period. The final experimental geochemistry study was undertaken with the goal of producing a tool for determining aqueous Zn concentrations from solid solution Zn concentrations in dolomite using element partitioning theory. Dolomite easily incorporates Zn into its crystal lattice and is a common ore-stage mineral in MVT deposits, thus it has a high potential for this purpose. However, experimental distribution coefficients (D) for the partitioning of Zn between dolomite and aqueous solution are unknown. A series of dolomite precipitation experiments was performed at temperatures between 125 and 200 degrees C, 10 MPa pressure, and aqueous Zn concentrations from about 10 to 1000 ppm for periods of time ranging from 10 to 80 days. The D values calculated from these experiments have applications in hydrothermal ore formation, sedimentary diagenesis, and low-grade metamorphism.

Geochemistry of fluid inclusion brines from Earth's oldest Mississippi Valley-type (MVT) deposits, Transvaal Supergroup, South Africa