Volcanogenic Mineralization Research Articles

Vertical variations of minerals in clayey sedimentary rocks in the cores of two-deep exploration wells from the Kozlu coalfield (Zonguldak, NW Türkiye), with emphasis on tonstein (schieferton) formation

The majority of Turkish bituminous coal resources are located in the Zonguldak Basin. The Carboniferous coal seams in the Alacaağzı, Kozlu, and Karadon formations have received interest. However, the detailed mineralogical compositions of clayey sedimentary rocks in coal-bearing sequences in the basin have been limited. This study aims to investigate vertical variations of mineralogical compositions of Carboniferous and Early Cretaceous clayey sedimentary rocks cored in the two-deep exploration wells (K20/K and K20/H) from the Kozlu coalfield using XRD and SEM-EDX analyses data. In addition, a special interest is also given to tonstein layers identified within the Kozlu and Karadon formations. The XRD analyses revealed that the quartz and clay minerals are the main components, and the latter is generally the dominant phase in all studied formations. Besides, clay fraction minerals (chlorite, illite/mica, and kaolinite) are common, while illite/smectite mixed-layer clay mineral is only identified in the two samples. The tonstein samples display similar mineralogical compositions to other samples, while several accessory minerals (e.g., apatite, baddeleyite, monazite, and zircon) are also present. Furthermore, calcite, dolomite, and hematite are mainly observed from the Early Cretaceous samples of the Zonguldak Formation. The vertical distributions of quartz and clay fraction minerals are variable throughout the studied two exploration wells. The vertical variations of clay fraction mineral illite/mica could be mainly related to changes in clastic influx from the basement, while kaolinite seems to be controlled by synchronous volcanic inputs during the Carboniferous. This could also be evident by the predominance of kaolinite and associated volcanogenic mineral grains in tonstein samples. The common presence of carbonate minerals in the Zonguldak Formation is clearly related to shallow marine depositional conditions, while hematite in the reddish mudstone and claystone samples of this formation could be related to short-term terrestrization during the Cretaceous or afterwards.

Why do calcareous microfossils preferentially occur in tuffaceous sandstones of the Santa Marta formation (James Ross Island, Antarctica)?

The Santa Marta Formation (James Ross Island, Antarctica) presents one of the most complete Santonian-Campanian high-latitude marine records in the Southern Hemisphere. Recent micropaleontological studies pointed to an unusual preferential preservation of calcareous microfossils, such as foraminifera and nannofossils, in tuffaceous sandstone levels of the Santa Marta Formation. Here we aim to characterize depositional and early diagenetic conditions that influenced preservation of calcareous microfossils within the succession of the Santa Marta Formation (Alpha Member). We used X-ray fluorescence (FRX)-derived elemental ratios, as well as carbonate (CaCO3), total organic carbon (TOC), and total sulfur (TS) contents to characterize sediment properties, as well as X-ray diffraction (XRD)-derived mineralogical analysis. Elemental ratios suggest hydrothermal influence during diagenesis throughout the stratigraphic succession, as indicated by (Fe + Mn)/Ti values > 20. This interpretation is supported by the occurrence of typical low-temperature hydrothermal minerals, such as garronite and heulandite. Contribution of volcanic ashes in the tuffaceous sandstone levels is evidenced by high values of the log (Zr/Ti) ratio, and the presence of volcanogenic minerals. Since stratigraphic levels characterized by the highest abundances and richnesses of calcareous microfossils also present high log (Zr/Ti) values and CaCO3 contents, we hypothesize that the input and subsequent alteration of volcanogenic material (e.g., ashes) acted as a buffer, increasing porewater alkalinity during early diagenesis. Such a chemical mechanism was already described for modern marine sediments deposited under the influence of significant input of volcanogenic materials, and can explain the preferential preservation of calcareous microfossils in tuffaceous sandstones of the Upper Cretaceous Santa Marta Formation.

The Intrusive Phase in the Evolution of a Volcanic Structure and of Au–Ag Low Sulfidation Mineralization: The Nevenrekan Deposit, Magadan Region, Russia

This paper provides information on the structural setting and ore composition of the Nevenrekan deposit. We provide an account of the regional and local factors of mineralization control. The ore bodies are confined to the junction between regional faults and annular ruptures of a circular subsidence. The geochemical zoning of the ore field is disturbed. We show how geochemical associations are distributed in the ore zones of this deposit. The structural geological and geochemical data, as well as results from a comparison among the Au‒Ag low sulfidation (LS) deposits at the Okhotsk–Chukchi, Uda–Murgali, and Central Kamchatka volcanogenic belts, shows that the mineralization occurred in two phases. The first phase involved the generation of epithermal Au–Ag mineralization, while the second, plutogenic, phase produced intrusion-related gold system that is seen as epidotization and silicification of rocks in the ore field, as well as in the metamorphism of volcanogenic mineralization. The presence of the later mineralization phase associated with granitoids is typical of the Nevenrekan deposit as distinguished from the other Au–Ag (LS) deposits in the Evensk ore region, and makes it similar in this respect to the Karamken, Nyavlenga, and Cirotan deposits.

K-bentonites in Ordovician–Silurian transition from South China: implications for tectonic evolution in the northern margin of Gondwana

Numerous K-bentonites from the Ordovician–Silurian (O–S) transition in South China record important information about the geodynamics of volcanic activity in the northern margin of Gondwana. A series of K-bentonite beds have been identified in the Zhoujiaxi Group (Early Silurian) in foreland basins from central Hunan Province, China. They are dominantly composed of illite with minor kaolinite. Volcanogenic minerals include quartz, feldspar, biotite, and lesser apatite and zircon. U–Pb zircon ages from three beds, 442.8 ± 1.8, 442.2 ± 1.9 and 441.6 ± 2.0 Ma are compatible with the age of the O–S global boundary. Geochemical results indicate calc-alkaline felsic magmas derived from continental crust and erupted in a subduction-related collisional environment. Isopach schemes and grain size reveal that the volcanic ashes were sourced from a volcanic event likely with a volcanic explosivity index of 8, and transported 300 – 1000 km away from the palaeo SSE. The volcanism was associated with northwestern subduction of the Zhenghe-Dapu Ocean beneath the southeastern South China Block (SCB) on the northern margin of Gondwana. The Zhenghe-Dapu fault might be a suture zone, implying the consumption of the Zhenghe-Dapu Ocean during collision between the Nanhai terrane and the SCB. This study supports that the Wuyi-Yunkai Orogenic Belt is a collision-type orogen rather than an intraplate belt. Supplementary material: Plots and geochemical data are available at https://doi.org/10.6084/m9.figshare.c.5027096

Geochemical Characteristics of Volcanogenic Deposits and Exhalation Mineralization in the Crater Part of the Active Kudryavy Volcano (Iturup Island of the Kuril Arc)

Geochemical Characteristics of Volcanogenic Deposits and Exhalation Mineralization in the Crater Part of the Active Kudryavy Volcano (Iturup Island of the Kuril Arc)

A geochemical and mineralogical update on two major tonsteins in the UK Carboniferous Coal Measures

Samples of two prominent tonsteins in the UK Carboniferous coal-bearing strata, representing kaolinised mafic ash and felsic ash have been comprehensively analysed by ICP-MS, laser ablation ICP-MS, SEM/EDX, XRD and XRF. Compositions have been compared with possible parent ashes in order to establish the extent to which elements are lost or retained during the alteration process. This is more readily achieved for the mafic tonsteins because they can be directly linked to the local, contemporaneous volcanics. The volcanic source of the felsic tonsteins is not known with certainty and the geochemical comparisons are made with modern volcanics from what is thought to be a similar tectonic/magmatic setting. Evidence of element migration during the tonstein formation has also been obtained by analysing the adjacent coals. Some mobility of U, Th, Nb, Y, Zr Hf and Ta is demonstrated, which imposes some constraints on the use of discrimination diagrams for the interpretation of original ash composition and tectonic setting. Notwithstanding some mobility of the incompatible elements, the discrimination diagrams are of use in providing broad igneous groupings. The TiO2/Al2O3 ratio is useful in that these two elements show the least mobility and the ratio is diagnostic of ash composition and sediment inclusion in the ash if the quartz content is also taken into account. The sediments associated with the felsic tonstein do not have a constant value for this ratio, which increases with grain size and quartz content and enables ash with included sediment to be recognized. Based on SEM/EDX and LA ICP-MS element locations have been identified. Zircon is a diagnostic volcanogenic mineral which makes an important contribution to the trace element budget, but there are other sources, notably a range of phosphate minerals, some of igneous origin and others diagenetic. This is also demonstrated from the whole rock geochemistry which shows there is an excess of U and Th over that which can be accommodated within the calculated zircon content.The REE profiles for the two types of tonstein differ significantly to each other. One is comparable with the profiles recorded from the associated basalts and partial melting of the upper mantle is thought to be responsible. The felsic tonstein, on the other hand, shows a profile common to many rhyolites with evidence of significant fractional crystallization. Again, in spite of some mobility during the alteration, the REE profiles appear to reflect the original ash composition, which is consistent with other aspects of the geochemistry.

The Geochemistry of Volcanogenic Mineralization in the Northwestern Segment of the Pacific Ore Belt: Northeast Russia

This paper considers the geochemistry of volcanogenic mineralization in the northeastern segment of the Pacific Ore Belt (Northeast Russia). We give new evidence for the compositions and concentrations of trace and rare-earth elements (REE) in the ores of volcanogenic fields: Au-Ag epithermal (of various types and ages), Cu-Mo-Au porphyritic, Au-Bi related to granitoidal intrusions, Sn-Ag subvolcanic and kies polymetallic enriched in Au and Ag, as well as REEs in alkaline volcanic rocks. Geochemical signatures have been compiled for 17 formation types of volcanogenic fields. It was found that the ore-forming fluids in most fields belonged to an NaCl-H2O hydrothermal system that was enriched in Cl relative to F; the values of Y/Ho in the ores of nearly all types correspond with the interval of ratios characteristic for present-day hydrothermal fluids in backarc basins; most of the ores that we studied had near-chondrite spectra with configurations similar to those of the REE spectra in volcanic rock sequences of the andesite-diorite series. Comparative analysis of REE spectra in the distribution of trace elements over classes of gold concentration shows synchronous enrichment of ores in similar sets of trace elements. The high Co/Ni ratio in volcanogenic ores probably reflects the superposition of a later magmatic fluid upon an earlier mineralization. Samples from ores of volcanogenic fields, except for Kuroko, show δCe and δEu varying from negative to mildly positive values, thus indicating low-oxidizing conditions during deposition. It was found for Au-Ag epithermal ores that they are enriched in a wide range of trace elements; they have low concentrations of REEs, the light REEs are more abundant than the heavy ones, and the Eu anomalies vary considerably from small negative to low and high positive values. The results provide evidence of an exhalation hydrothermal origin of the Khotoidokh field. It has been shown that the REEs in the ores of the Bol’shoe field are of the type that is most valuable to industry. The results can be used to deal with practical problems: determining the formation type and evaluating the industrial value of a field; detecting accessory components in ores; and discriminating between the types of geochemical anomalies (in rocks or in soil) and stray fluxes as to the potential of a field.

Evaluating soil salinity and water management in Chaco Canyon, New Mexico

Previous studies in Chaco Canyon, New Mexico suggested that water management systems constructed during periods of increased aridity resulted in elevated salinity levels to the point that soils were no longer viable for growing cultigens. Salinity, pH, powder X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy analyses of sediments and water collected from Chaco Canyon between the years 2013 and 2015 demonstrate conclusively that soils were suitable for the cultivation of maize by Ancestral Puebloans. Our findings clearly indicate that the salts are non-deleterious sulfate minerals. All of the cations and anions needed to form these minerals occur in the water of Chaco Canyon. Thus, increased soil salinity was not a critical factor in the abandonment of Chaco Canyon by Ancestral Puebloans. Sulfate and volcanogenic minerals increased soil fertility that allowed for the development and maintenance of an agricultural urban center in this dryland environment. Water management of sulfate and volcanic mineral rich soils created an environ ideal for maize agriculture. The occurrence of non-local Ancestral Puebloan maize in Chaco Canyon can be explained in terms of kinship mobility, the distance that goods and services move between extended families.

Major and Trace Element Geochemistry of Coals and Intra-Seam Claystones from the Songzao Coalfield, SW China

Silicic, mafic and alkali intra-seam tonsteins have been known from SW China for a number of years. This paper reports on the geochemical compositions of coals and tonsteins from three seam sections of the Songzao Coalfield, SW China, and evaluates the geological factors responsible for the chemical characteristics of the coal seams, with emphasis on the influence from different types of volcanic ashes. The roof and floor samples of the Songzao coal seams mostly have high TiO2 contents, consistent with a high TiO2 content in the detrital sediment input from the source region, namely mafic basalts from the Kangdian Upland on the western margin of the coal basin. The coals from the Songzao Coalfield generally have high ash yields and are highly enriched in trace elements including Nb, Ta, Zr, Hf, rare earth elements (REE), Y, Hg and Se; some variation occurs among different seam sections due to input of geochemically different volcanic ash materials. The geochemistry of the Songzao coals has also been affected by the adjacent tonstein/K-bentonite bands. The relatively immobile elements that are enriched in the altered volcanic ashes also tend to be enriched in the adjacent coal plies, possibly due to leaching by groundwaters. The coals near the alkali tonstein bands in the Tonghua and Yuyang sections of the Songzao Coalfield are mostly high in Nb, Ta, Zr, Hf, Th, U, REE and Y. Coal samples overlying the mafic K-bentonite in the Tonghua section are high in V, Cr, Zn and Cu. The Datong coal, which has neither visible tonstein layers nor obvious volcanogenic minerals, has high TiO2, V, Cr, Ni, Cu and Zn concentrations in the intervals between the coal plies affected by mafic and alkaline volcanic ashes. This is consistent with the suggestion that a common source material was supplied to the coal basin, derived from the erosion of mafic basaltic rocks of the Kangdian Upland. Although the Songzao coal is generally a high-sulfur coal, most of the chalcophile trace elements show either poor or negative correlations with total iron sulfide contents. The absence of traditional pyrite-metal associations may reflect wide variations in the concentrations of these elements in individual pyrite/marcasite components, or simply poor retention of these elements in the pyrite/marcasite of the relevant coals.

Controls on clay minerals assemblages in an early paleogene nonmarine succession: Implications for the volcanic and paleoclimatic record of extra-andean patagonia, Argentina

The distribution of the clay minerals of the Banco Negro Inferior-Río Chico Group succession (BNI-RC), a middle Danian–middle Eocene mainly continental epiclastic–pyroclastic succession exposed in the Golfo San Jorge Basin, extra-Andean Patagonia (∼46° LS), is assessed in order to determine the possible origin of clay and specific non-clay minerals using X-ray diffraction and scanning electron microscopy analyses. The control over the clay mineralogy of the sedimentary settings, contemporary volcanism, paleoclimate and weathering conditions is considered. A paleoclimatic reconstruction is provided and correlated with the main global warming events that occurred during the early Paleogene.Mineralogical analyses of BNI-RC demonstrate that smectite and kaolin minerals (kaolinite, halloysite and kaolinite/smectite mixed layers) are the main clay minerals, whereas silica polymorphs (volcanic glass and opal) are common non-clay minerals. Throughout the succession, smectite and kaolin minerals are arranged in different proportions in the three clay–mineral assemblages. These show a general vertical trend in which the smectite-dominated assemblage (S1) is replaced by the smectite-dominated assemblage associated with other clays (S2) and the kaolinite-dominated assemblage (K), and finally by S2 up-section. The detailed micromorphological analysis of the clay and non-clay minerals allows us to establish that the origins of these are by volcanic ash weathering, authigenic and pedogenic, and that different stages in the evolution of mineral transformations have occurred.The supply of labile pyroclastic material from an active volcanic area located to the northwest of the study area could have acted as precursor of the authigenic and volcanogenic minerals of the analyzed succession. Diverse fine-grained lithological facies (muddy and tuffaceous facies) and sedimentary settings (coastal swamp and transitional environments, and different fluvial systems) together with variable climate and weathering conditions controlled the mineralogical transformations and the arrangement of clay–mineral assemblages. The paleoclimatic reconstruction suggests a general warm and humid climate. However, the temporal trend of the clay–mineral assemblages, the ratios between smectite and kaolinite and the micromorphological analysis of clay minerals contrasted with evidence from sedimentological analyses suggest a warm and seasonal climate for the basal part of the unit, a warm and humid climate with a relatively more perennial rainfall regime in the middle part of the unit, and a warm and less humid, probably subhumid, climate up-section. Such a reconstruction makes it possible to establish a correlation with some of the hyperthermal events of the Early Paleogene Global Warming (EPGW) and, consequently, constitute one of the most complete time records of the EPGW in South America.

Geochronology and geochemistry of granitoid rocks from the Zhibo syngenetic volcanogenic iron ore deposit in the Western Tianshan Mountains (NW-China): Constraints on the age of mineralization and tectonic setting

Several large syngenetic volcanogenic iron ore deposits including the Chagangnuoer Fe–Cu, the Zhibo Fe and the Beizhan Fe deposits occur in the Awulale Fe–Cu Metallogenetic Belt (AMB) which is located in the southeastern corner of the Yili block in the Western Tianshan Mountains (NW-China). In the present study, we report geochronological and geochemical data for granitoid rocks associated with the Zhibo iron ore deposit in order to constrain the age of the stratiform Fe mineralization and tectonic setting of this deposit. Laser ablation-ICPMS (LA-ICPMS) U–Pb zircon ages of 318.9±1.5Ma and 304.1±1.8Ma were obtained for a diorite and a granite pluton, respectively, both of which intruded the volcanic–volcaniclastic rocks of the Early Carboniferous Dahalajunshan Formation. Additional LA-ICPMS U–Pb zircon ages of 320.3±2.5Ma and 294.5±1.6Ma were obtained for two crosscutting granite dikes, respectively. The ca. 319Ma diorite has a high-potassium calc-alkaline, weakly peraluminous composition and is characterized by large ion lithophile element (LILE) and light rare earth element (LREE) enrichment and flat heavy rare earth element (HREE) patterns. It further shows distinct negative anomalies of Nb, Ta and Ti. In contrast, the ca. 304Ma granite classifies as A2 type granite and is characterized by an enrichment of LILE and LREE, flat HREEs and distinct negative anomalies of Ba, Nb, Ta and Ti. The ca. 320Ma granite dike has a low-potassium and weakly peraluminous composition and is characterized by LILE and LREE enrichment and a distinct depletion of Nb, Ta, Ti and HREE. The ca. 295Ma granite displays a high-potassium calc-alkaline and weakly peraluminous composition and is characterized by a slight enrichment of LILE and LREE, flat HREEs and a distinct depletion of Nb, Ta and Ti. The data presented here in conjunction with previously published data suggest a submarine, syngenetic volcanogenic mineralization age of ca. 320Ma for the Zhibo iron ore deposit. The Zhibo and other submarine volcanogenic iron ore deposits of the AMB are inferred to have formed in a continental arc-setting related to subduction tectonics. Furthermore, post-collisional granitic magmatism from 304 to 295Ma was superimposed on the entire AMB.

Identifikasi Gunung Api Purba Karangtengah di Pegunungan Selatan, Wonogiri, Jawa Tengah

DOI: 10.17014/ijog.v4i4.85Karangtengah area in the southeastern part of Wonogiri Regency, is part of the Southern Mountains which is wholly composed of Tertiary volcanic rocks. Nevertheless, the eruptive centre of these volcanic rocks is still unknown. Based on an integrated study that comprises geomorphology, stratigraphy, structural geology, petrology, alteration, and mineralisation the existence of Karangtengah paleovolcano can be identified. The paleovolcano was formed below sea water, basaltic in composition, and it was part of a volcanic island arc during the time. Volcanogenic minerals are found in the central facies containing Fe, Cu, Pb, and Zn.

An accurately delineated Permian-Triassic Boundary in continental successions

The Permian-Triassic Boundary Stratigraphic Set (PTBST), characteristic of the GSSP section of Meishan and widespread in marine Permian-Triassic Boundary (PTB) sequences of South China, is used to trace and recognize the PTB in a continental sequence at Chahe (Beds 66f–68c). Diversified Permian plant fossils extended to the PTBST, and a few relicts survived above which they are replaced by gymnosperm pollen of Triassic aspect. In the nearby Zhejue Section, the continental PTBST is characterized by the fungal ‘spike’ recorded in many places throughout the world. The boundary claybeds (66f and 68a,c) of the PTBST are composed of mixed illite-montmorillonite layers analogous with those at Meishan. They contain volcanogenic minerals such as β quartz and zircon. U/Pb dating of the upper claybed gives ages of 247.5 and 252.6 Ma for Beds 68a and 68c respectively, averaging 250 Ma. In contrast to the situation in Xinjiang and South Africa, the sediment sequence of the Permian-Triassic transition in the Chahe section (Beds 56–80) become finer upward. Shallowing and coarsening upward is not, therefore, characteristic of the Permian-Triassic transition everywhere. The occurrence of relicts of the Gigantopteris Flora in the Kayitou Fm. indicates that, unlike most marine biota, relicts of this paleophytic flora survived into the earliest Triassic. It is concluded that Bed 67 at Chahe corresponds to Bed 27 at Meishan, and that the PTB should be put within the 60-cm-thick Bed 67b④, now put at its base tentatively. This is the most accurate correlation of the PTB in continental facies with that in the marine GSSP.

Mineral and organic components of the buried paleosols of the Nevado de Toluca, Central Mexico as indicators of paleoenvironments and soil evolution

Results of earlier studies of Quaternary tephra–paleosol sequences in Central Mexico revealed contradictions between paleopedological and lacustrine records. To settle the contradictions, selected quantitative characteristics of mineral and organic components of the paleosols PT1–PT7 from the Nevado de Toluca sequence were studied as independent paleoclimate proxies. Mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation, both being climate-dependent processes. Stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation. In all paleosols, the sand fractions consist mainly of unstable volcanogenic minerals, although the grains have weathering features. Short durations of pedogenesis are responsible for limited weathering status. The clay fraction is dominated by halloysite and dehydrated halloysite in units PT1–PT4, which suggests drier periods during paleosol formation, whereas major amounts of kaolinite in the units PT5–PT7 indicate extended humid pedogenesis. δ13C values of PT2–PT4 humus are intermediate between those characteristic of C3 and C4 plants. Phytolith assemblages which include both forms of C3 (dominant) and C4 (minor quantities) grasses agree with carbon isotope data. These data indicate plant cover changes driven by wet/dry paleoclimate oscillations. The new results, showing drier stages during the formation of Late Pleistocene paleosols PT2 and PT3, correct the primary interpretation of these units as indicating only humid pedogenesis, and settles the contradiction between paleopedological and lacustrine records.

Origin of Gold and Emerald Mineralization in the Murchison Greenstone Belt, Kaapvaal Craton, South Africa

The Murchison greenstone belt (MGB) represents one of a number of Archaean volcano-sedimentary successions within the Kaapvaal Craton of South Africa (Fig. 1, inset). It is situated approximatively 200 km north of the 3.5 to 3.2 Ga Barberton greenstone belt. The belts form part of the cratonwide Thabazimbi-Murchison lineament which represents a major suture along which assembly of the northern Kaapvaal Craton took place (Mc Court, 1995). Gold, antimony, arsenic, tungsten and mercury are the most frequent metals found in strncturaly controlled, hydrothermal ore deposits on the central portion of the belt. In addition, volcanogenic copper-zinc massives sulphide and pegmatite-related emerald mineralization are located in the northern and southern parts of the belt, respectively. This study presents new U-Pb isotope determinations on single zircon grains from key formations in the MGB, as well as Pb-Pb isotopic determinations of pyrites associated with gold, antimony and emerald mineralization.

Volcanic rock-hosted gold and base-metal mineralization, associated with Neoproterozoic-Early Paleozoic, back-arc extension in the Carolina terrane, southern Appalachian Piedmont

Volcanogenic mineral deposits in the Carolina terrane, southern Appalachian Piedmont,include Kuroko-type polymetallic massive sulfide deposits and disseminated gold-pyrite deposits associated with propylitic, silicic, argillic, and advanced argillic alteration. Host rocks are metavolcaniclastic and metaepiclastic rocks of a Neoproterozoic-Early Cambrian magmatic arc. The favorable gold horizon is the transition from a lower succession of andesitic and rhyolitic pyroclastic rocks with basal mafic lavas to an upper sequence of epiclastic sedimentary units and minor lava and ash flows. Kuroko-type deposits are associated with mafic to bimodal volcanic rocks in the upper sequence. Whole-rock oxygen isotope analyses indicate that gold mineralization is associated with a transition from hydrothermal systems dominated by isotopically relatively light (δ18O = -6‰ to -10‰) waters, typical of high-latitude subaerial systems, to seawater (δ18O = 0‰). Plots of δ18O vs. SiO2 of the host rocks show a compositional gap associated with mineralization at the subaerial to submarine transition. Values of δ18O for the hydrothermal waters, lithostratigraphic analyses, and tectonic models of the Carolina terrane demonstrate that mineralization coincided with extension in a rifted arc.

Metallogenic evolution in Wales: constraints from lead isotope modelling

Galena has been collected from different styles of mineralization throughout Wales. Galenas from the Ordovician volcanogenic mineralization at Parys Mountain, north Wales, give highly variable lead isotope ratios, which could have been produced either by a two-stage history or by the mineralizing fluids interacting and mixing with radiogenic lead derived from uranium-rich upper crust. The variation in the lead isotopic ratios of galena from veins cutting lower Palaeozoic sediments is considered to be due to separate periods of mineralization during the early Devonian and early Carboniferous. The most likely source for this mineralization is the thick sequence of lower Palaeozoic basinal mudstones. Both the disseminated galena in Mesozoic sedimentary rocks and galena from veins within the underlying Carboniferous Limestone give Triassic and Jurassic model lead ages, which agree with mineralization ages deduced from geological considerations. It is suggested that the source for this lead mineralization is the underlying Carboniferous shales. These and other galenas from Wales have significantly different isotope ratios to those from the Irish carbonate-hosted ore deposits, which would indicate that the mineralizing fluids had either different sources or evolutionary histories.

Early proterozoic ore deposits and tectonics of the Birimian orogenic belt, West Africa

Abstract The Early Proterozoic of the West African craton comprises a series of volcanic troughs and sedimentary basins (the Birimian) with granitic terranes accreted on Archean nuclei in the Man and Reguibat shields. Studies of the Birimian of the Man Shield indicate a model of polycyclic evolution, with a major collision event (D 1 ; 2.1 Ga) thrusting part of the Proterozoic terrane over the Archean before individualization of numerous volcanic troughs and clastic-infill basins. The proposed evolution for the Birimian orogenic belt comprises: (1) deposition of the sedimentary Lower Birimian (B1) with minor tholeiitic volcano-sedimentary intercalations (containing chert and/or Mn-formations), and with most of the detritus being derived from Early Proterozoic sources, apart from contamination near the Proterozoic/Archean contact; (2) pre-B2 crustal thickening related to D 1 thrusting; (3) formation, over about 40 Ma, of the Upper Birimian (B2) with numerous volcanic troughs of different composition (tholeiitic and rare komatiitic, bimodal tholeiitic to calc-alkaline, volcano-plutonic) and Tarkwaian clastic-infill basins; and (4) major transcurrent tectonics with sinistral (D 2 ) and dextral (D 3 ) strike-slip faults. Such a tectonic evolution from a collision (D 1 ) phase to a transcurrent (D 2 , D 3 ) phase is typical of collision belts, and in the present case the evolution of the Birimian orogenic belt can be extended into Guyana. The metallogenic history of the Birimian shows a three-phase evolution coinciding with the orogenic evolution, and extends over almost 150 Ma from the Perkoa massive (ZnAg) sulfides (2.12 Ga) with a clear mantle affinity to the latemesothermal Au quartz veins (∼2 Ga) with (according to lead isotopes) a high crustal participation. The economic mineralization of belt thus consists of: 1. (1) “Pre-orogenic” (pre-D 1 ) deposits related to early extension zones. This was diverse with stratiform Au tourmalinite (type 1 Au: Loulo in Mali; Dorlin in Guyana), stratiform Fe (Cu) (Faleme in Senegal) and Mn (Nsuta in Ghana; Tambao in Burkina Faso), and a single massive ZnAg sulfide deposit (Perkoa in Burkina Faso) associated with regional volcanosedimentary (variably tholeiitic) stratigraphic marker beds; 2. (2) “Syn-orogenic” (post-D 1 to syn-D 2 /D 3 ) deposits with disseminated Au-sulfides (type 2 Au: Yaoure in the Ivory Coast) in extensional zones of the B2 followed by auriferous paleoplacers (type 3 Au) in B2 extensional zones (Tarkwaian Banket conglomerate) or syn-D 2 transtensional zones (debris flow of Orapu in Guyana). 3. (3) “Late-orogenic” (post-peak D 2 /D 3 ) deposits with mesothermal Au mineralization evolving from a “disseminated gold-bearing arsenopyrite and Au-quartz lode” type (type 4 Au: Ashanti in Ghana) to a “quartz-vein” type with free gold and CuPbZnAgBi paragenesis. Most of the gold in West Africa comes from this phase. Finally, the metallogeny of the Birimian appears as rich in Au- and Mn-formations and poor in volcanogenic mineralization and BIF. It differs from the Archean metallogeny of West Africa and other regions, through the presence of certain deposit types (such as Au-stratiform tourmalinite, gold-bearing conglomerate with Au and FeTioxides but no uranium or sulfides, and Au-arsenopyrite-rich shear-zone deposits) that are very common in, but not exclusive to, the Proterozoic and Paleozoic.

Redistribution of rare earth elements during diagenesis of turbidite/hemipelagite mudrock sequences of Llandovery age from central Wales

Abstract Geochemical studies of recently mapped Upper Llandovery turbidites in central Wales indicate localized rare earth element (REE) mobilization and fractionation in mudrock-dominated sequences. Closely associated turbidite sandstones, turbidite mudstones and laminated anoxic hemipelagites have generally similar major elemental compositions but display strongly differentiated REE distributions. Shale-normalized REE distribution patterns show that anoxic hemipelagites are consistently enriched in the light and middle REE, with greatest enrichment occurring for La-Eu. This enrichment reflects a concentration of millimetre-scale zoned REE-rich monazite nodules in the hemipelagites. In contrast, the REE patterns of the associated turbidites mirror those of the anoxic hemipelagite, being relatively depleted in the light and middle REE, with Nd and Sm generally showing greatest differentiation. The relationship between the concentration of nodules and degree of enrichment of REE in anoxic hemipelagite and turbidite unit thickness imply upward migration of REE into the overlying hemipelagite. However, mass balance considerations suggest that the turbidite-hemipelagite couplets were not necessarily closed systems. Within the turbidites, REE were initially held in unstable volcanogenic minerals and adsorbed on to clay minerals and iron-manganese hydroxides. The REE were liberated into the sediment porewaters during early diagenesis and fractionated in the migrating porewaters as a result of the greater solubility of heavy REE relative to light REE. Expulsion of porewaters from the turbidite sediment during burial carried REE in solution through the intervening hemipelagic layers where REE were precipitated, probably in the presence of organic matter, and possibly initially as the hydrous phosphate rhabdophane which later transformed to monazite. Fractionation of the REE during this process led to preferential enrichment of the light and medium REE within the hemipelagic sediment. The use of REE as indicators of sediment provenance in the Llandovery turbidite-hemipelagite mudrock sequences of central Wales must be treated with caution since significant REE element migration and fractionation is shown to have occurred during their diagenesis. Similar effects might be expected in other sequences comprising interbedded organic-rich/organic-poor mudrock and detailed petrographic analyses should be undertaken to evaluate the possibility of diagenetic REE fractionation and redistribution in conjunction with the use of REE patterns or Sm-Nd isotopes in provenance studies of these rocks.

Proterozoic Volcanogenic Mineralization Styles

Summary The Proterozoic contrasts with the Archaean and Phanerozoic in the apparent relative lack of volcanogenic mineral deposits. However, analogues of the three distinct volcanogenic mineralization styles recognized in the Recent record can be traced back into the Proterozoic and probably into the Archaean. These are: (1) massive sulphide deposits, (2) iron ores and (3) epithermal deposits. The Proterozoic is characterized by sedimentary basins and stable cratons, and Proterozoic ore styles tend to reflect these trends. However, volcanogenic mineralization appears to be at least as abundant in suitable successions in the Proterozoic as in other eras. Although analogues can be found in the Proterozoic for the Phanerozoic volcanogenic ore styles, no satisfactory Recent analogue of the Broken Hill-type has been identified. Similar processes in the mid-Proterozoic and Phanerozoic record appear to have resulted in SEDEX Zn-Pb deposits of Mt Isa and Irish types. The Broken Hill-type was formed in a shallow aqueous/sub-aerial environment and, as with the coeval sedimentary ores, may have been modified by non-actualistic, early Proterozoic surface environmental conditions. In contrast, the major metal geochemistry of Proterozoic massive sulphide ores differs little from their Archaean or Phanerozoic counterparts, possibly reflecting their deep ocean genesis. Formation of Proterozoic volcanogenic massive sulphide deposits was concentrated around 1.9 Ga. The ore districts are restricted to a linear zone, about 1000 km wide and some 8000 km long, on the eastern margin of the reconstructed Proterozoic supercontinent. It is suggested that the massive sulphide ore districts trace the scar of an early Proterozoic Wilson cycle. In contrast, Proterozoic Broken Hill-type ores and their sub-varieties display a wide geographic spread, consistent with their essentially continental character.