Epigenetic Veins Research Articles

Geology and geochemistry of volcanogenic massive sulfide deposits and related igneous rocks, Prince William Sound, south-central Alaska

The Prince William Sound region is located approximately 100 km east-southeast of Anchorage, Alaska, and contains the geologically diverse Late Cretaceous to Eocene Valdez and Orca Groups, which comprise a portion of the accretionary prism complex of south-central Alaska. The rocks host more than 600 known massive sulfide deposits and prospects. Historically, the deposits have been described as epigenetic vein, replacement, and shear zone-hosted ores. We define the Prince William Sound massive sulfide deposits in terms of two genetically related, yet geochemically distinct, types of deposits, both related to submarine volcanism. The first type is hosted in submarine pillow basalt, massive basalt flows, and mafic tuffs and is analogous to Cyprus-type deposits. The second type is hosted in intercalated slate and graywacke and is analogous to Besshi-type deposits. Both types are similar morphologically and are characteristically stratiform to strata bound. The deposits vary in size from less than 1 million tons to greater than 5 million tons, with the majority of the largest deposits forming in sediment-dominated Besshi-type environments.The massive sulfide deposits are Fe-Cu-Zn systems dominated mineralogically by pyrite, pyrrhotite, chalcopyrite, and sphalerite. Minor phases include cubanite, arsenopyrite, tetrahedrite, and secondary marcasite. Silicate gangue is comprised predominantly of quartz and chlorite, with minor sericite, talc, and calcite. Primary sedimentary features preserved within massive sulfide are ubiquitous within the deposits and include planar bedding, crossbedding, and graded bedding.Fluid inclusion microthermometry of quartz gangue from the massive sulfide deposits is consistent with hydrothermal fluid temperatures found in genetically related modern seafloor hydrothermal environments. Minimum trapping temperatures of fluid inclusions in quartz from the stockwork feeder zone of the volcanic-hosted deposits generally range from 210 degrees to 260 degrees C. Fine-grained anhedral quartz gangue in massive sulfide layers also contains sparse primary fluid inclusions; typically the homogenization temperature is lower in the sediment-hosted deposits. Hydrothermal fluids in the volcanic-hosted settings exited the volcanic pile directly into seawater and were rapidly quenched, thus fluid inclusions only record high-temperature fluid conditions. Similar hydrothermal fluids in the sediment-hosted settings exited the volcanic pile into warm, seawater-saturated sediments and were cooled more slowly, in an environment where lower temperature fluid inclusions were formed and preserved.Sulfur and oxygen isotope data support a submarine origin for the deposits. Sulfur isotope values range from delta 34 S GDT = 0.8 to 5.9 per mil for volcanic-hosted deposits, and from 2.7 to 9.2 per mil for sediment-hosted deposits. The higher values for sediment-hosted ores reflect increased amounts of reduced seawater sulfate in the hydrothermal fluid, with sulfate reduction occurring in the seawater-hydrothermal fluid mixing zone within the sediment pile below the seawater-sediment interface. Oxygen isotope values for quartz gangue associated with massive sulfide also reflect the lower temperature deposition that is characteristic of the sediment-hosted deposits: delta 18 O quartz = 7.0 to 7.8 per mil (SMOW) for volcanic-hosted deposits and delta 18 O quartz = 9.6 to 16.9 per mil for sediment-hosted ores.Whole-rock geochemical analyses of the basalts and related igneous rocks from both the Valdez and Orca Groups show that these rocks are consistent with formation in midocean ridge and/or seamount environments. The geochemistry of the Valdez Group rocks suggests that sediment contamination of basaltic magma has occurred, imparting a primitive arclike signature. Discrimination diagrams show that the rocks from both groups have normal midocean ridge basalt, enriched midocean ridge basalt, and low K arc tholeiite affinities, and plot along the tholeiitic-calc-alkaline boundary.The fluid inclusion, stable isotope, and whole-rock data collected in this study are all consistent with the hypothesis that the massive sulfide deposits in the Prince William Sound district are sea-floor volcanogenic massive sulfide deposits that formed in both sediment-starved and sedimented midocean ridge settings.

Geochemical alteration associated with the Selbaie copper-zinc-silver deposit

The object of this study is to establish the nature of geochemical alteration in the wall rocks of the Selbaie Cu-Zn-Ag deposit, and to ascertain whether bedrock geochemistry could be applied in exploration for deposits of this type. The Selbaie Cu-Zn-Ag deposit, in northwestern Quebec, is atypical of base-metal deposits in the Canadian Shield, as mineralization occurs in epigenetic veins and as disseminations in volcanic wall rocks, rather than as syngenetic massive sulfide bodies which have been the focus of extensive comparable studies in the past. The deposit comprises three mineralized zones in felsic tuffs and quartz porphyry. Two of these, the A1 and A2 zones, which are the focus of the present study consist of quartz-carbonate veins containing chalcopyrite, sphalerite, and pyrite, and accessory sulfide minerals. The B zone is not considered in this study. Samples of drill core were collected from sections crossing the A1 and A2 zones and analyzed for total Si, Al, Fe, Mg, Ca, Na, K, Ti, Mn, P, and S by X-ray fluorescence, and for aqua-regia-extractable Cu, Zn, Ni, Pb, Fe, and Mn by atomic absorption. Although the distribution of individual element concentrations is clearly related to bedrock type, rocks proximal to mineralization have higher SiO 2, Fe 2O 3, Cu, Zn, Fe, and S contents, and lower Al 2O 3 and K 2O contents, than rocks more distal from mineralization. MgO content shows an irregular distribution and may also increase toward mineralization. CaO and Na 2O contents are very low in all pyroclastic rocks. In order to take into account the effect of bedrock type, linear regression analysis was performed on the data to estimate the normal concentration of individual elements using SiO 2 contents as the predicted variable. Spatial distributions of residual values of element concentrations indicate that rocks close to mineralization have higher residual Fe 2O 3, Cu, Zn, Pb, and Fe contents, and lower residual Al 2O 3 and K 2O contents, compared to rocks more remote from mineralization. These distribution patterns are clearer than in the case of the single-element data but variations between element responses still exist in some instances. In an attempt to overcome the variation in patterns revealed by some element distributions, discriminant analysis was undertaken to see whether mineralized areas could be distinguished on the basis of an element association. Discriminant functions were calculated for data from areas adjacent to mineralization, and from areas remote from mineralization. Areal distribution of discriminant scores outlined an anomalous area extending up to 200 m beyond the mineralization. Petrographic studies of selected samples correlated high SiO 2 and low Al 2O 3 content with silicification in the most altered rocks adjacent to mineralization. High Fe 2O 3, MgO, and extractable Fe concentrations were found to occur in chlorite- and carbonate-rich zones. Samples rich in S, Cu, Zn, and b, and to a lesser extent Fe, were characterized by the presence of sulfides. High concentrations of Al 2O 3 and K 2O relative to that of SiO 2 are typical of sericitization. On this basis, the results demonstrate that anomalously high or low concentrations of element associations, revealed by discriminant analysis, occur in large coherent zones exceeding the size of the deposit. The procedure is potentially useful as an exploration guide for epigenetic disseminated and vein-type base-metal styles of mineralization as well as the syngenetic massive sulfide type of deposits that have been extensively studied in the past.

Boron in graphite: Content, speciation, and significance

The mean boron content of syngenetic graphite in metasedimentary rocks is significantly higher than that of epigenetic vein graphite or igneous graphite. The boron commonly found in chemical analyses of natural graphites does not occur in solid solution with the carbon but rather is contained in a mechanically included borosilicate phase.

The gold content of some Archaean rocks and their possible relationship to epigenetic gold-quartz vein deposits

Gold mineralization in Archaean granite-greenstone environments, especially gold-quartz veins, contributes considerably to the world's gold production. The formation of epigenetic gold mineralization in greenstone belts is generally explained by the metamorphic secretion theory. This theory is based on the assumption that the source of the gold may be komatiitic or tholeiitic lavas, pyritic chemical or clastic sediments and even granitic rocks from which, as a result of regional metamorphic overprinting, gold was extracted and concentrated in suitable structures. It has been shown that in proposed potential source rocks, gold is predominantly associated with sulfide minerals and thus relatively easily accessible to secretion and reconstitution processes. A large number of various rock types originating from granite-greenstone terranes of the Kaapvaal and the Rhodesian cratons were geochemically investigated, and the following ranges for gold determined: volcanic rocks (komatiitic and tholeiitic): 0.1–372 ppb granitic rocks of the basement: 0.3–7.8 ppb iron-rich chemical sediments: 1.0–667 ppb Statistical treatment of the data reveals that volcanic rocks as well as iron-rich chemical sediments are favorable sources for epigenetic gold mineralization formed by metamorphic secretion, while the granitic rocks make less suitable primary gold sources. This finding explains the close spatial relationship which is common between gold-quartz veins and greenstone belts. The conspicuous abundance of epigenetic gold mineralization in the Archaean, however, is attributed to the unique geologic and metamorphic history of the granite-greenstone terranes.

Proterozoic copper deposits in NW Queensland, Australia: Sulfur isotopic data

Sulfur isotopic disequilibrium is commonly observed between associated pyrite and copper sulfides in NW Queensland. A sulfur isotopic study of copper mineralization in dolomites at Paradise Valley and arenites at Mammoth has allowed the significance of such disequilibrium to be evaluated. Copper mineralization at Paradise Valley is characterized by a greater enrichment in 34S, with δ34S values often greater than +30‰, for both copper sulfides and associated syngenetic/diagneetic pyrite. At Mammoth, copper sulfides have isotopic compositions (δ34S=−15.9 to −0.3‰) transitional between disseminated syngenetic/diagenetic pyrite (δ34S=−5.7 to −1.7‰) and epigenetic vein pyrite (δ34S=−17.9 to −7.1‰) suggesting progressive reaction and replacement of syngenetic/diagenetic pyrite by a copper-bearing mineralizing fluid under oxidizing conditions. The isotopic data, within the constraints imposed by geological and geochemical factors, support a model of reaction between copper-bearing mineralizing fluids and pre-existing syngenetic/diagenetic pyrite for both the carbonate- and arenite-hosted deposits.

Metallogenic analysis of the Stewart area, British Columbia; a statistical approach

A statistical analysis of mineral deposit data from the Stewart, British Columbia, area contained in MINFILE (a computerized mineral inventory), based on X 2 probabilities derived from contingency tables, shows that it is possible to define significant associations between the metal content of deposits, the lithostratigraphy of their host rocks, and the presence or absence of granitoid intrusions at each deposit. In volcanic and volcaniclastic units of the Hazelton assemblage, mineralization is predominantly Cu-Pb-Zn (+ or -Ag, Au) and shows no significant association with intrusions. These deposits are interpreted as syngenetic-volcanogenic in origin. Later intrusions may cause remobilization but do not alter the metal content of these deposits. In clastic sedimentary units of the Hazelton and Bowser assemblages, mineralization is predominantly Pb-Zn-Ag and significantly associated with granitoid intrusions. These deposits are interpreted as epigenetic veins. The statistically derived associations are in good agreement with field-based classifications of the deposits and demonstrate the possibilities of classifying mineral deposits based on the information contained in computerized data files. The noted lithostratigraphic associations also suggest that, in the Stewart area, the source of the metals in the deposits may be their adjacent wall rocks rather than the granitoid intrusions. The role of the intrusions may be only to provide a heat source to promote hydrothermal circulation and leaching of metals from favorable wall rocks to form mineral deposits.

Precambrian coal or anthraxolite; a source for graphite in high-grade schists and gneisses

Argues that without considering data for igneous carbon (carbonatites) or for ultrahigh metamorphic calcareous gneisses such as are found in the Grenville province of North America, the conclusion that the source of carbon for vein graphite is magmatic or carbonate carbon is not justified. Points out that marbles and calc-silicate gneisses occur with the graphite-bearing granulite facies rocks in Sri Lanka. Calcite may also be seen in the veins with the graphite. Concludes that whether the graphite in epigenetic veins in high grade schists and gneisses has variable carbon isotope ratios depends on whether it was derived from organic material in carbonate or noncarbonate metasediments.

Kuroko-type deposits in Vanua Levu, Fiji

Abstract Minor Kuroko-type deposits occur within a group of acidic volcanic rocks in northeast Vanua Levu, Fiji. Six prospects are described and their relationship to each other and the associated volcanic activity discussed. Variations exist between deposits formed at or near the surface around exhalative centres and those occurring at depth as epigenetic vein and stockwork deposits. Fijian volcanism and ore deposition seem to be closely associated with a tensional—transform regime of regional dimensions rather than subduction. The ore fluids are thought to have separated from cooling felsic magma and mixed with seawater on their way to the surface.

The lead-zinc and associated ores of La Union, Sierra de Cartagena, Spain

The Pb-Zn-Fe ores of La Union, Spain, occur in an area of about 50 square km in three facial types of parageneses: (1) sulfides-carbonates-silica, (2) sulfides-carbonates-silica-greenalite-magnetite, and (3) oxides-hydroxides-sulfates-carbonates-silica. The distribution of the ore deposits shows a central zone with parageneses (3) and (1) in direct association with late-Tertiary subvolcanic rocks and breccia pipes, an intermediate zone characterized mainly by paragenesis (2), and an outward zone characterized mainly by paragenesis (1). The occurrence of paragenesis (3) in the three zones at higher levels than parageneses (2) and (1) suggests a vertical zoning. A geochemical zoning with, from the central zone outward, Sn, Cu, Be, and Ba richer zones is also apparent. Mineralogical data indicate formation temperatures largely below 250 degrees C and, within range of probable values of f (sub S 2 ) and f (sub CO 2 ) , an increasing importance of f (sub O 2 ) toward the central and higher zones. The absence of crosscutting relations between deposits of different paragenetic type suggests a monascendant or facial zoning related to a center of Late Tertiary subvolcanic rocks and superimposed on the pre-Pliocene geology of the region. The ore deposits are syngenetic fissure veins in Late Tertiary volcanic rocks and epigenetic veins and peneconcordant replacement orebodies in older formations. Geological relations and the structure of the ore deposits indicate strong structural and lithological controls and shallow depths of formation within the zone of ground-water movement.

Mineralogy and Genesis of the Manganese Oxide and Silicate Rocks in Kajlidongri and Surrounding Areas, Jhabua District, Madhya Pradesh, India

Manganese silicate rocks interbedded with manganese oxide ores occur enclosed in phyllites of Precambrian Aravalli Group at Kajlidongri, Madhya Pradesh, India. The phyllites are metamorphosed up to the biotite zone of regional metamorphism. In the bedded deposits the ore minerals are bixbyite, braunite and hematite of two generations and jacobsite associated with spessartite and alurgite. In the epigenetic veins cutting across bedded deposits and in the wall rocks surrounding them, hollandite, micaceous hematite, cryptomelane and pyrolusite are present associated with brown manganese pyroxene, blanfordite, winchite, tirodite, alurgite, manganophyllite and piemontite. The origin of the bedded deposits is attributed to metamorphism of original manganese-rich sediment interbedded with pelitic and psammitic rocks. The problems of selective formation of bixbyite and braunite, the appearance of bixbyite in the biotite zone and jacobsite in the chlorite zone are discussed. The unusual mineral assemblage in and around the veins is explained as due to reconstitution of manganiferous sediments by processes associated with the epigenetic veins.

Geology of the Area around Kajlidongri, Jhabua District, Madhya Pradesh

Abstract Manganese silicate rocks interbedded with manganese oxide ores occur enclosed in phyllites of the Precambrian Aravalli Group in and around Kajlidongri, Jhabua District, Madhya Pradesh. The phyllites are metamorphosed up to the biotite zone of regional metamorphism. The manganese oxide minerals in the bedded ore deposits are braunite and hematite of two generations, bixbyite and jacobsite associated with spessartite and alurgite. A granite body and a few small calcareous bodies present in the area are younger than the phyllites and the enclosed manganese deposits. Epigenetic veins cutting across all the formations are important in this area. Though the mineral composition of the veins are in general simple, being constituted by quartz and calcite, a number of minerals are present in them and the surrounding wall rocks where they intersect bedded manganese deposits. Their unusual mineral composition containing aegirine-augite, micaceous hematite, richterite, zoisite, cummingtonite, scapolite, barite, fluorite, hollandite, blanfordite, brown manganese pyroxene, winchite, tirodite, alurgite, piedmontite, manganophyllite, tilasite, pyrolusite and cryptomelane is explained from field and textural evidences as due to reconstitution of the sedimentary manganese by the effect of the epithermal veins. An interesting feature of the phyllites is the presence of fluid inclusions in quartz which have a definite pattern of abundance in relation to the granite and the carbonate bodies. The petrographic characters of the carbonate bodies indicate their replacement origin apparently due to the activity of fluids derived from granite.

Value of domestic production of minerals from various classes of rocks

Fluids and sedimentary rocks accounted for 48 and 36 percent, respectively, of the total value of the 1946 domestic mineral production. The epigenetic (vein and replacement) and metamorphic deposits each contributed 6 percent of the total, and residual and igneous deposits accounted for 3 and 1 percent, respectively. Fuels were the most valuable of all the minerals produced--77 percent of the total. Petroleum, natural gas, and similar fluid fuels accounted for 47 percent of the total production and 98 percent of that from fluids; bituminous coal made up 24 percent of the total production and 67 percent of that from sedimentary rocks; anthracite coal was valued at 6 percent of the total production and 92 percent of that from metamorphic rocks. Exclusive of fuels, the bulk of the production of nonmetallics, and of the total mineral production too, was from sedimentary rocks (74 and 50 percent, respectively) and from veins and replacements (8 and 28 percent, respectively). Most of the production of metals, however, was from veins, etc. (62 percent) and residuum (29 percent).

A native copper deposit near Jefferson City, Montana

Native copper occurs as a cement placer material within a restricted area near Jefferson City, Montana. The deposit is situated in a shallow canyon near the headwaters of the major Prickly Pear Creek drainage system where an intimate association of peat muck, bog-iron oxide, and alluvial gravels combine to establish a unique set of natural reducing conditions that attend the resulting deposition of secondary copper from percolating sub-surface waters. The copper is derived by oxidation of narrow epigenetic veins that occur up-stream in rocks of the Boulder batholith. It is carried downward as copper sulphate in solution until it reaches the reducing environment.Although both carbonaceous peat muck and iron oxide may be reducing agents, it appears that, in this instance, the iron oxide deposit plays the dominant role in reduction. The peat muck, in fact, seems to have even kept the copper in solution in most cases, and thus has permitted its localization on or near the limonitic bog material.