Acid Metavolcanic Rocks Research Articles

New insights on the Escoural Orogenic gold district (Ossa-Morena Zone, SW Iberia): Geochemistry, fluid inclusions and stable isotope constraints from the Monfurado gold prospect

The Escoural gold district belongs to the Montemor-Ficalho metallogenic belt which is part of the Portuguese section of Ossa-Morena Zone (OMZ), at the SW of Iberia. The Escoural gold district includes twelve gold prospects and/or deposits largely controlled by the NW-SE Montemor-o-Novo Shear Zone (MNSZ) and associated fault zones, extending for approximately 30 km. Ubiquitously, gold-arsenopyrite-loellingite assemblages hosted in quartz-sericite-chlorite veins are found in most deposits, although, in the Monfurado prospect, the gold-bearing assemblages are more complex. This prospect is located in the vicinity of a Cambrian SEDEX-VMS iron deposit, from which massive and disseminated iron-ores hosted in marbles and calcsilicate rocks, were exploited. The interplay of the gold mineralizing processes with the iron-rich host rocks has favored gold deposition at the Monfurado prospect. Selected samples from six drill cores allowed to define two mineralizing events: the pre-ore and ore stages. Two types of gold mineralization characterize the ore-stage: i) massive sulfide horizons in which gold (Au = 85.6–86.3 wt%; Ag = 13.1–13.6 wt%) is hosted in arsenopyrite and pyrite or, seldomly, gold particles (Au = 91.8 wt%; Ag = 7.1 wt%) found in an arsenopyrite-rich layer; and ii) quartz-chlorite-pyrite veins crosscutting acid metavolcanic rocks with rhyolite-rhyodacite affinities, in which gold (Au = 80.5–82.9 wt%; Ag = 16.8–18.7 wt%) is found as fracture filling in pyrite, sometimes accompanied by Bi-Te phases. Arsenopyrite geothermometer suggests that for type i the overall deposition temperature falls within the range of 188 °C to 372 °C. Type ii mineralization lacks arsenopyrite, and for this reason, thermodynamic constraints were gathered from fluid inclusions and chlorite geothermometer. CH4-rich fluid inclusions are ubiquitous in transgranular fluid inclusion planes, suggesting that reduced fluids percolated the rocks that host type ii mineralization. The reduced fluids support the transport of gold in sulfide complexes, such as AuHS- and Au(HS)-2. Furthermore, secondary H2O-NaCl fluid inclusions (Lw2) were found, with mean salinities of 6.0 eq. w(NaCl) and mean homogenization temperature of 226 °C, with corresponding pressures of 3.0 MPa, thus suggesting late hydrostatic regimes. Chlorite geothermometer results are in the range of 229 °C and 309 °C, agreeing with the fluid inclusion homogenization temperatures for Lw2 fluids.Sulfur isotope (δ34S) analysis of representative sulfide phases collected from both mineralization types, revealed signatures ranging from 8.5 ‰ and 10.6 ‰, indicating a single sulfur source. The gathered results suggest that although fluid transport is structurally controlled by MNSZ activity, the sulfidation reactions promoted by fluid-rock interactions are the main control on gold deposition from type i mineralization. It is further suggested that a coeval gold-event can lead to the deposition of two different types of mineralization, related to distinct gold deposition mechanisms.The tectonic and geodynamic settings in which the Escoural gold district developed correlate it to worldwide Palaeozoic orogenic gold deposits, with the Monfurado prospect being an example of the complexity of such geological settings.

New geochronologic and geochemical constraints for the Pedro Pereira metavolcanosedimentary sequence: Evidence for a 2.77 Ga oxygen oasis record in the São Francisco-Congo paleocontinent

Exposed within the reworked basement of the Araçuaí orogen, eastern Brazil, the Pedro Pereira metavolcanosedimentary sequence (PPMS) comprises a lower unit dominated by metaultramafic/mafic rocks, an intermediate unit dominated by metarhyolites and a predominant upper unit of acid metavolcanites associated with chemical and clastic metasedimentary rocks. This sequence has been defined and investigated in the ’80s when it was considered as a Mesoarchean greenstone belt. Since then, few studies have addressed these rocks, and due to the lack of reliable geochronological data, a Paleoproterozoic age has also been suggested for the sequence. To reevaluate the age and evolution of the unit, new whole-rock geochemical and zircon U–Pb (LA-ICPMS) geochronologic data were obtained for the iron formation and acid metavolcanic rocks, respectively. When normalized to post-Archean Australian Shale standard, BIF samples can be divided into three groups. Group 1 BIF samples are marked by significant enrichment of the LREE, very high ∑REE + Y, positive La anomaly, extreme negative Ce anomaly, negative Eu anomaly, and subchondritic Y/Ho ratio. In contrast, Group 2 BIFs are marked by the enrichment of HREE, low ∑REE + Y, positive La anomaly, negative Eu anomaly, slight negative Ce anomaly, and predominantly super- and subchondritic Y/Ho ratios. The massive iron formation (MIF) group shows very high middle-to-heavy REE + Y enrichment, strong negative Ce anomaly, negative Eu anomaly, very high ∑REE + Y, and variable Y/Ho ratios. The modification of the iron formation by post-depositional hydrothermal alteration might have disturbed the original basin water signature. However, Group 2 is more compatible with pure chemical precipitates and its slight negative anomalies suggest an oxic environment. A PPMS metarhyolite from the upper portion of the sequence interlayered with the BIFs yielded the Concordia Age of 2771 ± 8.4 Ma. This age rules out a Paleoproterozoic origin for the PPMS, as suggested/considered by recent papers. The results imply that PPMS possibly represents one of the oldest records of oxygenation previous to the GOE in the geologic archives and could provide information to understand the early evolution of localized pockets of oxygen before the Great Oxidation Event.

Aluviální zlato v oblasti Dlouhá Ves - Řepová (zábřežské krystalinikum)

A detailed panning prospection of the Řepovský potok, Míroveček and Ospitský potok creeks (northern part of the Zábřeh Upland) proved that trace amount of alluvial gold occurs in the whole studied area. The collected gold sheets are 0.2 to 3 mm in size and mostly have shapes only weakly modified by transport in the stream. Although the chemical composition varies widely between Au-rich silver and pure gold (28.1 - 99.8 at. % Au), individual sheets are typically (few exceptions exist) compositionally homogeneous. Less than half of them has the narrow high-fineness rim, which was formed by leaching of silver in supergene conditions. In lower reach of the Řepovský potok creek there were also found gold sheets containing elevated mercury (0.1 - 5.1 at. %), which most probably originated from neighbouring vein-type Ag-Zn-Pb deposit Řepová. In contrast, the source of Hg-free gold sheets was not unambiguously established. The first possibility includes small vein-type mineralizations similar to the Řepová ore deposit and containing trace amount of gold (an occurrence of this type was newly recognized in the vicinity of Dlouhá Ves village). Second, one cannot exclude the trace occurrence of gold sheets directly in underlying metasiltstones or acid metavolcanic rocks of the Zábřeh Crystalline Complex [with respect to repeatedly observed intergrowths of gold with polymineral aggregates of phyllosilicates (muscovite, chlorite, biotite, clay minerals), feldspars (K-feldspar, albite, K-Na feldspar) and minor quartz in the alluvial gold sheets]. The particles of metallic Cu-Zn-Sn alloys with admixture of Ni, Sb and In, panned from the Řepovský potok creek, could contain metals originated from smelting of polymetallic ores from the deposit Řepová, which comprise the same minor elements.

Geologia, petrografia e geoquímica das rochas metavulcânicas ácidas da Estrada Real, Rio de Contas (BA)

Ao longo da Estrada Real, no município de Rio de Contas (BA), afloram rochas metavulcânicas ácidas representativas da Formação Novo Horizonte, Supergrupo Espinhaço, que se formaram na fase sin-rifte do Espinhaço. Estas rochas foram submetidas a intensos processos de deformação e metamorfismo, que modificaram as suas feições originais. Este trabalho visa compreender a gênese do magmatismo ácido sin-rifte do Espinhaço e os processos metamórficos e hidrotermais que afetaram a região, com base em dados de campo, petrográficos e geoquímicos de rocha total. As metavulcânicas ácidas possuem mineralogia constituída por blasto-pórfiros de quartzo, porfiroblastos de andaluzita e cianita, pseudomorfos de granada, feldspatos, biotita, muscovita, sericita, clorita, epidoto, monazita, zircão e minerais opacos imersos em uma matriz quartzo-feldspática. Os processos de sericitização e muscovitização estão presentes nas rochas e indicam a ocorrência de alterações hidrotermais. De acordo com as características petrográficas e geoquímicas, os protólitos das rochas metavulcânicas ácidas foram classificadas como riolitos. Os diagramas de Harker mostram valores dispersos dos elementos maiores Ca, Na e K e dos LILEs, indicando uma provável mobilidade desses elementos. No diagrama multielementar de elementos-traço e ETRs são visualizadas anomalias negativas de Nb-Ta, Sr, P e Eu, além de enriquecimento de ETRL em relação aos ETRP. A formação dessas rochas em um ambiente de rifte continental é sugerida, a partir de um processo de fusão crustal com a interação de um manto previamente modificado por zona de subducção (manto metassomatizado).

Ordovician and Cretaceous tectonothermal history of the Southern Gemericum Unit from microprobe monazite geochronology (Western Carpathians, Slovakia)

The Southern Gemericum basement in the Inner Western Carpathians experienced a polyphase regional deformation. Differences in the pre-Alpine and Alpine events have been constantly discussed. To address this, monazites from metapelites and acid metavolcanic rocks were dated using the Th–U–Pb electron microprobe method. Three monazite generations, such as Precambrian, Early Paleozoic, and Alpine, have been recognized in the greenschist facies pelites and acid metavolcanic rocks of the Southern Gemericum basement. Both inherited magmatic monazite grains in metavolcanites and rare relics of detrital monazites within the polyphase monazite grains in metapelites yielded the Precambrian age in the time span of 550–660 Ma. They prove the provenance and derivation from deeper crustal Cadomian fragments. High-Y magmatic monazites of Early Paleozoic age (444 ± 13 and 477 ± 7 Ma) have been recorded in the acid metavolcanites and their metavolcaniclastics. These ages roughly fit within the previously published magmatic zircon age determinations (at 494 ± 1.7 and 464 ± 1.7 Ma) that clearly indicate two-phase volcanic activity in the Early Paleozoic Southern Gemericum basin. The Early Paleozoic magmatic monazites were partly overprinted by the low-Y Alpine monazites (133 ± 5 and 184 ± 16 Ma) at their rims. In Al-rich metapelites, the newly formed low-Y monazites of Alpine age commonly occur, reflecting the polystage compression geodynamic evolution with three distinct peaks at 100 ± 8, 133 ± 5, and 190 ± 16 Ma, respectively. No data as the evidence of the pre-Alpine metamorphic events were observed in metapelites. Only some monazites yield the age indications for the Permian extensional thermal re-heating (260–290 Ma). The monazite age data from the Southern Gemericum basement indicate the strong overprinting due to the polyphase Alpine deformation at least in the greenschist facies conditions.

Provenance of the Sumian basal schists and age of the Lopian metavolcanic rocks at the Archean-Proterozoic boundary in the Kukasozero structure, North-Karelian zone of the Karelides, Baltic Shield

This paper presents the results of a study of the Paleoproterozoic basal garnet-kyanite-staurolite-two-mica paraschists from the Kukasozero structure of the Karelides of Northern Karelia, Baltic Shield, underlying Neoarchean acid metavolcanic rocks, and schists with quartz, phengite, kyanite, staurolite, garnet, and tschermakite located in the Paleoproterozoic rocks and considered to be metasomatic in origin. It was established that the sedimentary protolith of the Paleoproterozoic paraschists contains detritus of Neoarchean igneous rocks as follows from detrital 2737 ± 11 Ma zircons with oscillatory magmatic zoning. Metavolcanic 2757 ± 13 Ma rocks, close in age and composition, are known directly in the framework of the Kukasozero structure and are considered to be the most likely source of the sedimentary schist protolith. The coincidence of the Nd-model ages of paraschists (t DM is 2.73–2.76 Ga) with the age of detrital zircons indicates no contribution of older rocks to the protolith composition. The age of magmatic crystallization of metavolcanic rocks directly underlying the Paleoproterozoic paraschists is 2681 ± 18 Ma and coincides with the age of porphyry granites in the western framework of the structure (2680.3 ± 3.6 Ma). No detrital zircons of similar age were found in basal paraschists, but the restricted amount of dated zircons does not allow us to draw a final conclusion about the absence of detritus of the underlying metavolcanic rocks in the paraschist protolith. It was confirmed that phengite-bearing schists are the products of acid metasomatism of the Paleoproterozoic amphibolites and amphibole schists (metavolcanic rocks). The metasomatic features were revealed in garnet-kyanite-staurolite-two-mica paraschists, so the strict identification of their sedimentary protolith is impossible. The paraschists do not represent metamorphosed weathering crust, because acid metasomatism gives a false impression of the greater maturity of the primary sedimentary rocks.

Metaconglomerados e rochas associadas do Grupo São Roque a norte da cidade de São Paulo, Brasil

The São Roque Group is composed of rocks deposited in marine environment with coeval volcanic activity, the Morro Doce Formation is dominated by meta-arkose and feldspatic meta-sandstone with expressive metaconglomeratic lenses, which form a sequence regarded as the basal unit of São Roque Group. Metavolcanic acidic and basic rocks interspersed in this sequence are an important tectonic and geochronologic marker (1.75-1.79 Ga). The petrographic study of the granite pebbles from the Morro Doce Formation metaconglomerates allowed the identification of four petrographic varieties: porphyritic biotite monzogranite, inequigranular monzogranite, equigranular monzogranite and inequigranular leucogranite. The comagmatic character of these pebbles is confirmed by petrographic and geochemical data. Acid metavolcanic rocks interlayered with meta-arkose and metaconglomerates in the Morro do Polvilho region correspond to trachydacite and porphyritic meta-rhyolite. The meta-arkose shows geochemical affinities with metaconglomerate granitic pebbles, and differs from the acid metavolcanic rocks both in their geochemical signature and in its sedimentary fabrics. Their geochemical characteristics are typical of within-plate magmatism, especially the low mg # (~ 20), high Zr (560-730 ppm), Y, Nb, and low Sr (70-120 ppm), and is similar to the acid metavolcanics from the on Espinhaço Supergroup. The Jaraguá amphibolite body, still of unknown age, seems to be intrusive into the Morro Doce Formation, and has geochemical characteristics akin to ocean-floor magmatism, with low concentrations of incompatible elements such as Rb, Nb and Th, and are similar to supracrustal metabasics from the Pirapora do Bom Jesus region

Review of isotope data for Precambrian rocks from the Disko Bugt region, West Greenland

Pb-Pb and Rb-Sr isotope data yield whole-rock isochron ages of c. 2800 Ma for two localities of granitoid rocks in the Disko Bugt region, with little evidence of strong later disturbance. Rb-Sr isotope data on Archaean metasediments from two localities, however, were strongly disturbed during the early Proterozoic. Sm-Nd whole-rock data for acid metavolcanic rocks within Archaean supracrustal sequences yield model ages of c. 2800 Ma. The early Proterozoic age of a younger sequence of supracrustal rocks (the Anap nunâ Group) is confirmed by Sm-Nd data, and Rb-Sr whole-rock data for albitised siltstones show that albitisation took place hundreds of millions of years after the peak of early Proterozoic orogenic activity.

The petrochemistry of the auriferous, volcanosedimentary Riacho dos Machados Group, Central-eastern Brazil: geotectonic implications for shear-hosted gold mineralization

The Ouro Fino Gold Deposit at the Riacho dos Machados Mine is located in the Araçuaí Fold Belt at the eastern margin of the São Francisco Craton. The gold is shear zone-hosted and associated principally with a sulphide-bearing, quartzmuscovite schist, derived from the hydrothermal alteration of pelitic and quartzofeldspathic schists of the Riacho dos Machados Group (RMG). Along the shear zones, mineral associations typical of the amphibolite facies are progressively altered to assemblages typical of the greenschist facies. It has become evident from the studies that the process of segregation and concentration of chemical elements and minerals is the very process of mineralization, involving the concentration of Au. The RMG comprises ultramafic, mafic and acid metavolcanic rocks intercalated in a sequence of metapelites. The metapelites have textures and geochemistry compatible with greywackes. The dacitic and mafic/ultramafic rocks are of calc-alkaline and tholeiitic subalkaline association, respectively. The RMG probably formed within a continental volcanic arc. The thrust faults that establish the regional architecture, and which host the Riacho dos Machados gold mineralization, are related to a continental collision of unknown age between the precursor to the São Francisco Craton and the Guanambi-Correntina Block.

Early Palaeozoic Acid Magmatism in the Saxothuringian Belt: New Insights from a Geochemical and Isotopic Study of Orthogneisses and Metavolcanic Rocks from the Fichtelgebirge, SE Germany

rocks range from 316 to 298 Ma for muscovites and from 306 to Orthogneisses and acid metavolcanic rocks from the Fichtelgebirge, 280 Ma for biotites, demonstrating thermal influences from Late NE Bavaria, Germany, are predominantly chemically evolved (monVisean to Late Stephanian (325–290 Ma) granite intrusions. The zo)granites and rhyodacites–dacites, respectively. The metavolcanic involvement of dominantly crustal-derived melts is considered to rocks are variably tectonized and include samples with anomalously account for the peraluminous character (A/CNK > 1·08), high low CaO, Na2O and MnO ( 80 initial Sr/Sr (> 0·709) and negative eNd(500 Ma) (–2·9 to –6·4) wt %) and K2O (>6 wt %) concentrations, implying selective of the intrusive and volcanic rocks. The generation of the melts is element mobility during post-eruptive events. Sm–Nd isotope data assumed to have taken place within an overall extensional geodynamic for samples from three main orthogneiss units (Wunsiedel, Selb, setting. The wider range in eNd(500 Ma) for the metavolcanic rocks Waldershof) yield a composite Sm–Nd whole-rock isochron cor(–3·8 to –6·4; orthogneisses: –2·9 to –4·0) suggests that differential responding to an age of 560 ± 45 Ma. This age estimate is admixture of a pelagic sedimentary component during emplacement constrained, however, to be less than ~560 Ma by the presence of may have occurred. Nd model ages range from 1·5 to 1·7 Ga and detrital Cadomian zircons in the country rocks. Fifteen specimens are consistent with those of granitoids from the Erzgebirge and the from the Wunsiedel orthogneiss give an Rb–Sr whole-rock isochron Lausitz but completely different from those of metavolcanic rocks of 480 ± 4 Ma with an initial Sr/Sr ratio of 0·7095 ± from the Thuringer Wald. In addition, Nd model ages of the Early 0·0007 (MSWD = 2·7). Rb–Sr isotope data from the WalPalaeozoic granitoids in the Fichtelgebirge are consistent with those dershof orthogneiss and the metavolcanic rocks suggest, however, of the Late Carboniferous granitoids from the same area. This that, in general, Sr isotopic equilibrium, if ever reached, was suggests similar source material for the pre-Variscan and the late significantly modified during later events. Taking recent geoVariscan Fichtelgebirge granitoids. chronological literature data into account, it now appears that Early Palaeozoic acid magmatism in the Fichtelgebirge commenced with the intrusion of the orthogneiss precursors during the Early and Mid Ordovician and ended with the eruption of the volcanic

Project SUPRASYD 1993 - granitic rocks and shear zones with possible gold potential, Julianehåb batholith, South Greenland

In 1992 the Geological Survey of Greenland (GGU) initiated the project SUPRASYD in order to carry out an economic assessment of the Ketilidian orogen in South Greenland, especially the supracrustal rocks in the southern and eastern parts of the orogen (see Dawes & Schønwandt, 1992). Geological investigations in the area east of Nanortalik and along the east coast of South Greenland as far as 62°N had previously indicated that acid metavolcanic rocks were an important constituent of the supracrustal rocks, and it was therefore expected that the region might have a significant potential for sulphide deposits.

Subseafloor hydrothermal alteration during the Early Proterozoic at Garpenberg, Central Sweden

The Early Proterozoic sulfide deposit at Garpenberg is located in the metallogenetic province of central Sweden. It is a strata-bound massive sulfide deposit contained in a supracrustal sequence of mainly acid metavolcanic rocks. Stratiform Zn-Pb-Cu mineralization is underlain by Cu-bearing stockwork ore and an extensive alteration zone. The sulfide ores and their altered wall rocks were formed by subseafloor hydrothermal activity. The alteration pattern observed in the wall rocks of this deposit is consistent with a hydrothermal system where the fluid consists mainly of seawater and a high water/rock mass ratio predominates. The hydrothermal activity caused the destruction of the primary mineralogy, mainly feldspars, and a general redistribution of the chemical elements in the altered wall rocks which were principally depleted in Ca, Na and Eu and enriched in Mg. Eu was redeposited with the ore metals near or at the seafloor and Ca was deposited as limestone. Most of the major and trace elements show large mobility during the alteration; only Ti, Zr, Y and REE (excluding Eu) behaved as relatively immobile elements.

Rb–Sr and U–Pb ages of volcanism and granite emplacement in the Michipicoten belt—Wawa, Ontario

The Michipicoten greenstone belt at Wawa, Ontario is typical of Archean volcanic belts in the Superior Province. The supracrustal rocks are divisible into lower, middle, and upper metavolcanic sequences, which are separated by iron formation and clastic metasedimentary rocks. These are intruded by granitic stocks and embayed by granitic batholiths.This study reports whole rock Rb–Sr and zircon U–Pb ages for the lower and upper metavolcanics, for the granitic rocks that are physically within the greenstone belt (internal granites), and for the granitic rocks that embay the greenstone belt (external granites). The apparent Rb–Sr ages for the lower metavolcanics are 2530 ± 90, 2285 ± 70, and 2680 ± 490 Ma. The U–Pb ages are 2749 ± 2 and 2744 ± 10 Ma. The internal granites give an Rb–Sr age of 2560 ± 270 Ma and a U–Pb age of 2737 ± 6 Ma. The external granite at Hawk Lake indicates an Rb–Sr age of 2550 ± 175 Ma and a U–Pb age of 2747 ± 7 Ma. It is possible that this unit contains elements older than 2812 Ma as it contains xenocrystic zircons. The upper volcanics give a U–Pb age of 2696 ± 2 Ma, which indicates that the belt evolved over a time period in excess of 53 Ma. The Rb–Sr ages are significantly younger than the U–Pb zircon ages and have very large uncertainties in age; hence it is unlikely that they have any stratigraphic significance. They probably reflect the Kenoran orogeny at about 2560 Ma. The 2285 ± 70 Ma Rb–Sr isochron age has an initial ratio of 0.7275 ± 0.0052, which is interpreted as a rotational isochron defining a younger post-Kenoran event in the area. The zircon ages appear to be correct chronostratigraphically. Furthermore, it appears that the granitic rocks are coeval and may also be cogenetic with the lower acid metavolcanic rocks.

Precambrian Weathering in Acid Metavolcanic Rocks from the Superior Province, Villebon Township, South-Central Québec

Chloritoid- and kyanite-bearing acid metavolcanic rocks of the Abitibi Greenstone belt have acquired an aluminum surplus by weathering prior to metamorphism. The weathering increases from the top of the volcanic unit downwards, as shown by increasing values for both Niggli-t and the Zr/P ratio. The depositional environment of these rocks is postulated to be either shallow marine or terrestrial.