Felsic Metavolcanics Research Articles

RE-APPRAISAL OF THE SANTA RITA GREENSTONE BELT STRATIGRAPHY, CENTRAL BRAZIL, BASED ON NEW U-PB SHRIMP AGE AND SM-ND DATA OF FELSIC METAVOLCANIC ROCKS

The Santa Rita greenstone belt represents one of the supracrustal belts of the Archaean terranes of Goiás, central Brazil. The stratigraphic sequence of this greenstone belt comprises a lower unit made of komatiites and basalts and an upper metasedimentary unit made of carbonaceous schist, chert, iron formation and marble, unconformably overlain by clastic metasedimentary rocks. Felsic metavolcanics occur at the interface between the metabasalts and the upper metasedimentary pile. U-Pb SHRIMP age for zircons from the felsic metavolcanics reveal that it is not part of the Archaean sequence, but represents the product of a Mesoproterozoic (1580 ± 12 Ma) magmatic event. Sm-Nd isotopic data (initial e CHUR values between -10.5 and –14.9) and T DM values of 3.0 and 3.2 Ga, within the range of the surrounding TTG terranes, indicate that the original felsic magmas were produced by re-melting of Archaean crust. The data demonstrate that the Goiás greenstone belt contains infolded and imbricated Proterozoic rocks, as previously suggested by Sm-Nd isotopic analyses of some of the upper detrital metasedimentary rocks.

RE-APPRAISAL OF THE SANTA RITA GREENSTONE BELT STRATIGRAPHY, CENTRAL BRAZIL, BASED ON NEW U-PB SHRIMP AGE AND SM-ND DATA OF FELSIC METAVOLCANIC ROCKS

The Santa Rita greenstone belt represents one of the supracrustal belts of the Archaean terranes of Goias, central Brazil. The stratigraphic sequence of this greenstone belt comprises a lower unit made of komatiites and basalts and an upper metasedimentary unit made of carbonaceous schist, chert, iron formation and marble, unconformably overlain by clastic metasedimentary rocks. Felsic metavolcanics occur at the interface between the metabasalts and the upper metasedimentary pile. U-Pb SHRIMP age for zircons from the felsic metavolcanics reveal that it is not part of the Archaean sequence, but represents the product of a Mesoproterozoic (1580 ± 12 Ma) magmatic event. Sm-Nd isotopic data (initial e CHUR values between -10.5 and -14.9) and T values of 3.0 and 3.2 Ga, within the range of the surrounding TTG terranes, indicate that the original felsic magmas were produced by re-melting of Archaean crust. The data demonstrate that the Goias greenstone belt contains infolded and imbricated Proterozoic rocks, as previously suggested by Sm-Nd isotopic analyses of some of the upper detrital metasedimentary rocks.

Isotopic age data of single zircons from the Archaæan Sukumaland Greenstone Belt, Tanzania

This study presents precise U Pb single zircon age data for three rhyolitic metavolcanic units: one trachyandesitic porphyry, presumably of extrusive origin; one palæosome of a migmatitic gneiss; and one lamprophyric dyke cross-cutting banded iron formation (BIF) of the Upper Nyanzian Supergroup in the Archæan Sukumaland Greenstone Belt, Tanzania. The obtained ages partly contradict the expected field stratigraphical relationships. The isotopic ages of the felsic metavolcanics give a good age bracket for the deposition of the Nyanzian Greenstone Belt units. The older felsic metavolcanics yield ages of 2808±3 Ma and 2780±3 Ma, respectively, whereas the younger felsic metavolcanics provide a minimum age of 2654+15 −13 Ma for the greenstone belt. The porphyritic trachyandesite, intercalated with oxide BIF at Geita, yield an age of 2699±9 Ma and the lamprophyre dyke an age of 2644±3 Ma, respectively. The data for the lamprophyre imposes a maximum age on the Au mineralisation at Geita Mine, since the dyke has been mineralised together with adjacent BIF. The palæosome of a migmatitic gneiss, possibly a basement flanking the greenstone belt to the south at Kahama, has been dated at 2680±3 Ma. Previous models advocating the pre-existing, significantly older, Dodoman basement gneiss within the Archæan Sukumaland Greenstone Belt of northern Tanzania cannot be supported. Emplacement or complete reworking of the granitic protolith coeval with greenstone belt volcanism appears more likely.

U‐Pb Ages for Zircon and Titanite from the Ramagiri Area, Southern India: Evidence for Accretionary Origin of the Eastern Dharwar Craton during the Late Archean

The north‐south‐trending Ramagiri Schist Belt is trident shaped, with three prongs separating three granitic terranes. Whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite, the central prong includes felsic metavolcanics and volcanogenic metasediments. The U‐Pb zircon age for the pyroclastics in the central prong of the Ramagiri Schist Belt is $$2707\pm 18$$ Ma, considered as the time of emplacement of the felsic volcanics. To the east of the belt, migmatitic Chenna gneisses yielded a U‐Pb zircon age of $$2650\pm 7$$ Ma, inferred as the minimum age of their magmatic precursors. These gneisses had experienced a thermal event at least 100 m.yr. later, as evidenced from the U‐Pb titanite age of $$2545\pm 1$$ Ma. From the central Ramagiri Complex, a granodiorite has identical U‐Pb zircon and titanite ages of $$2613\pm 6$$ and $$2614\pm 4$$ Ma, respectively, and a quartz diorite has a titanite age of $$2595\pm 1$$ Ma. These intrusives with roof‐pendants of pillowed basalts are inferred to have been emplaced as high‐level plutons that cooled rapidly. From the western Gangam Complex, U‐Pb zircon and titanite ages for a granodiorite were $$2528\pm 1$$ and $$2516\pm 3$$ Ma, respectively, and a monzodiorite has yielded a titanite age of $$2510\pm 2$$ Ma. A postkinematic granitic vein intrusive into the Gangam Complex yielded a titanite age of $$2468\pm 4$$ Ma. The three granitoid terranes separated by a <3‐km‐wide schist belt do not have common magmatic or metamorphic histories, as evidenced by their distinct titanite and zircon ages. Intrusion of voluminous western Gangam Complex at 2528 Ma did not alter the titanite ages of granitoids in either the central Ramagiri Complex or the eastern Chenna Gneiss just a few kilometers away. Therefore, the Ramagiri Schist Belt, consisting of highly sheared rocks, could represent a terrane boundary between three disparate granitoid terranes juxtaposed after 2516 and before 2468 Ma. Available geological and geochronological information from Kolar, Ramagiri, and adjoining areas suggests that growth of Dharwar continental crust could have been accomplished by both accretion of a series of arcs onto and collision of a large, 2530 Ma continent with an older (<3000 Ma) continent at around 2500 Ma.

Neoproterozoic bimodal volcanism in the central Ogcheon belt, Korea: age and tectonic implication

UPb zircon ages were determined for a metavolcanic rock from the Munjuri Formation in the central Ogcheon belt, Korea. Four zircon fractions were analyzed of which three are nearly concordant; the data define a discordia with an upper intercept at 755.8 ± 1.3 Ma and a lower intercept at 160 ± 19 Ma (MSWD = 0.72). The upper intercept age is interpreted to indicate the timing of bimodal volcanism, and the lower intercept age is interpreted to reflect Jurassic thermal metamorphism. This is the first definite evidence supporting the existence of a formation in the Ogcheon belt that is at least in part late Precambrian. Nd isotope and trace element geochemistry of the amphibolites and felsic metavolcanics suggests that they originated from a mantle source in a continental rift environment. The rift does not appear to have matured enough to develop oceanic crust, since rocks indicating deep sea environments are absent. The possibility that the Ogcheon belt is an extension of the east Chinese collision belt, as suggested by several authors, is not supported by the available evidence.

SmNd ages of Archaean metavolcanics of the Dharwar craton, South India

Well constrained SmNd arrays of whole rocks are reported for the first time from three important stratigraphic units of the Dharwar volcano-sedimentary sequence of south India. If interpreted as isochrons, these arrays will correspond to the following ages and initial Nd ratios relative to CHUR: 2.911±0.049 Ga and −0.20 ± 0.40 for the Kalasapura mafic basal metavolcanics of the Bababudan greenstone belt; 2.848±0.070 Ga and −0.88 ± 0.82 for the Santaveri Formation comprising mafic to felsic metavolcanics overlying the Kalasapura rocks; and 2.747±0.015 Ga and +1.72 ± 0.10 for the Ingaldhal mafic-felsic metavolcanic suite of the Chitradurga greenstone belt. The possibility that these SmNd arrays are mixing lines between unrelated components depleted and enriched, respectively, in light rare earth elements has been discussed, leading to the conclusion that the ages closely correspond to their time of eruption: the older suites from the Bababudan belt from a mantle source of CHUR-like composition, the younger suite from the Chitradurga belt from one with a prior history of LREE depletion.

The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga

Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform 207Pb/ 206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵ Nd(t) values between + 1.1 and − 1.1 and Nd T DM model ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.

The stable isotope signature of kilometre‐scale fracturedominated metamorphic fluid pathways, Mary Kathleen, Australia

Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre‐scale fluid transport during amphibolite facies metamorphism. These 10‐ to 100‐m‐scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10‐km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcite values of –2 to –7% and δ18Ocalcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole‐rock values of 3.5 to 7%. and unaltered banded calc‐silicate and marble wallrocks have δ13Ccalcite of –1.6 to –0.6%, and δ18Ocalcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc‐silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc‐silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100‐ to 500‐m‐scale isotopic exchange with the surrounding metadolerites and calc‐silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc‐silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc‐silicates, or thermal fluctuations attending deformation and metamorphism. The overall L‐shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc‐silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc‐silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.

Hydrothermal alteration in volcanic rocks, eastern part of the Lukavice Group, ?elezn� Hory Mountains, Czech Republic

Many rocks mapped as felsic metavolcanics in the eastern part of the Lukavice Group are shown to be altered mafic metavolcanics, similar to those in the Noranda and Flin Flon-Snow Lake mining districts, Canada. The relatively fresh rocks of the Lukavice Group are rhyolite, dacite-andesite, and andesite-basalt of calcalka-line character. Assuming no substantial volume change during alteration, Ti, P, La, Ce, Yb, Lu, Th (partly), Sc and V contents remained unchanged. Altered rocks are enriched in (Fe + Mg), K and Si and depleted in Na, Ca and Zr. Some elements show both increased and decreased contents in altered rocks (Mg, Ba, Sm, eu, Tb and Hf). Although hydrothermal alteration in the Lukavice Group is of large extent, it is of the proximal Kuroko style and not of regional ‘Amulet Rhyolite’ style. Implications for a large hydrothermal system within a volcanic pile are discussed in relation to the Ordovician Lukavice Group and its mineral deposits and to some other parts of the Bohemian Massif with volcanosedimentary sequences of the same age.

A SEQÜÊNCIA VULCANOSSEDIMENTAR DO GRUPO NOVA LIMA NA REGIÃO DE PIEDADE DO PARAOPEBA, BORDA OESTE DO QUADRILÁTERO FERRÍFERO, MINAS GERAIS

Piedade do Paraopeba is located at western Quadrilatero Ferrifero, where a 15 km long tectonic slice of Nova Lima Group rocks are stacked between the gneissic rocks of the Bonfim Complex and the basal Minas Supergroup formation (the Moeda Quartzite). The southern part of the Nova Lima sequence consists of ultramafic and mafic metavolcanics with local preserved pillow structures, minor felsic metavolcanics, metachert and thin carbonate and oxide iron formation. This sequence grades northwards to an interlayered sequence of carbonaceous phyllites, quartz-mica schists, volcanoclastic metarenites, and iron formation. The metavolcanic rocks underwent intense metassomatic alteration. Chemical analyses show that the ultramafic rocks are probably former komatiitic peridotites while the upper mafic rocks are tholeitic basalts. A polyphasic structural evolution has been distinguished in the area. The main deformation phase (Dn) led to basal thrusts with a W-NW tectonic transport.

U-Pb zircon age and Nd isotopic composition of granitoids, charnockites and supracrustal rocks from Heimefrontfjella, Antarctica

Heimefrontfjella and Mannefallknausane, in Dronning Maud Land, Antarctica, comprise an amphibolite-facies terrain and a granulite terrain, separated by a major mylonite zone. The amphibolite terrain is made up of mafic to felsic metavolcanics and metasediments, intruded by granitoid plutons: the granulite terrain has supracrustal rocks with similar lithologies, intruded by felsic plutonic rocks that crystallized as charnockites.

The role of water-rock interaction and fluid evolution in forming the porphyry-related Sisson Brook W-Cu-Mo deposit, New Brunswick

Sisson Brook is a subeconomic W-Cu-Mo deposit associated with Devonian porphyritic intrusions and hosted by metagabbro, mafic metavolcanics, felsic metavolcanics, and metasediments. Four stages of veining and alteration have been recognized. Early mineralization consisted of disseminated and vein-bearing molybdoscheelite accompanied by pervasive amphibole alteration. Subsequent scheelite and molybdenite deposition was restricted largely to quartz veins and was associated with biotitization. Both stages of mineralization were concentrated in metagabbro and mafic metavolcanics. The final mineralization stage consisted of wolframite and chalcopyrite in quartz vein stockworks associated with phyllic alteration and developed mainly in felsic metavolcanics and metasedimentary rocks.Analyses of amphibole and mass balance calculations show that Na (super +) was added to the rock and that Si (super +4) , Mg (super +2) , and Ca (super +2) were leached during molybdoscheelite mineralization. A relatively high f (sub O 2 ) fluid is indicated by increases in the Fe (super +3) /Fe (super +2) ratio of the alteration amphibole and a low f (sub S 2 ) by the associated iron enrichment. Mass balance calculations show that Mg (super +2) and Ca (super +2) continued to be leached during scheelite-molybdenite mineralization and Si (super +4) , which had previously been leached, was added to the rock with K (super +) and Na (super +) . Fluid inclusion data suggest that W-Cu-Mo mineralization was deposited from a comparatively low-salinity aqueous liquid at temperatures between 330 degrees and 430 degrees C. The highest temperatures were attained during the second stage of mineralization, at which time minor hypersaline fluids also circulated in the hydrothermal system.A model is proposed in which hydrothermal activity was initiated by the circulation of meteoric formational waters in metagabbro adjacent to moderately high level porphyry intrusions. This stage was marked by low fluid-rock ratios which controlled mineralization by fixing molybdoscheelite with Ca supplied by the host rocks. An increase in temperature during subsequent molybdenite-scheelite mineralization was the result of the introduction of orthomagmatic fluids from the porphyries. The major change in mineralizing conditions was an increase in f (sub S 2 ) which permitted deposition of molybdenite and gave rise to scheelite instead of molybdoscheelite. Scheelite deposition was caused by the high Ca contents of the host rocks and/or destabilization of KWO (super -) 4 ion pairs as a result of biotitization. Fluid-rock ratios reached a maximum during deposition of wolframite and chalcopyrite, reflecting the influx of meteoric waters through large, open fracture systems. The wolframite and chalcopyrite were deposited as a result of decreasing temperature or, in the case of chalcopyrite, and/or decreasing pH and/or increasing f (sub S 2 ) . The study demonstrates the importance of fluid-rock interaction and the nature of the host lithologies in controlling this style of W-Cu-Mo mineralization.

Geochemical variability in the Greenstone Belts of Goiás (Brazil): the Hidrolina and Crixás Sequences

The greenstone belts of Crixas and Hidrolina of Goias, Brazil, are dismembered components of the same body or of originally adjacent belts. Their tectono-metamorphic history and stratigraphy is similar, as well as their igneous components (meta-komatiites, meta-basalts and felsic meta-volcanics). Chemically, they differ for the CaO and Al2O3 versus MgO trends and for the behaviour of many incompatible and compatible trace elements (Cr, Ni, Ti, Sc, Y, Zr, P). The difference in the Ca and Al behaviour, wich is anomalous at Crixas, is tentatively attributed to an unconstrained igneous crystallization process. This process should imply the growth of clinopyroxene into an olivine-dominated crystal mush, and has acted at Crixas and not at Hidrolina. The difference in the trace element behaviour has been mainly attributed to a compositional and modal heterogeneity of the mantle source, which was probably spinel-richer at Crixas than at Hidrolina.

Regional trends in mineral assemblages of Swedish Proterozoic granitic pegmatites and their geological significance

Abstract In the Svecokarelian rocks of Sweden the pegmatites can be divided into two main types, both of late orogenic affiliation: (1) The tourmaline ± Li-mineral type occurring in lower medium- to medium-grade metasediments. Typical minerals are tourmaline, garnet, beryl, columbite-tantalite, triphylite, primary Ca-Fe-Mn-phosphates, Li-aluminosilicates. (2) The Y-REE-mineral type comprising three subgroups: a) those in migmatitic environment, b) those with beryl and garnet in areas dominated by mainly felsic metavolcanics in lower medium- to medium-metamorphic grade, and c) those associated with granites of the Transscandinavian Granite-Porphyry Belt. In the Svecononvegian rocks of SW Sweden the rare-mineral bearing pegmatites, contemporaneous with the late- to postoro-genic Bohus granite, show similar trends with schorl, columbite and beryl in lower-grade rocks and Y-REE-minerals in higher-grade rocks. The trends in this region are not obvious because of the polymetamorphic environment. Li-minerals and primary Ca-Fe-Mn-phosphates are absent or very rare. The differences in mineralogy of the late erogenic pegmatites can be related to the dominant type of surrounding and underlying regional lithologies, which significantly affected the compositions of the parent granitic melts.

Genesis of an early Proterozoic zinc deposit in high-grade metamorphic terrane, Saxberget, central Sweden

The Saxberget strata-bound Zn-Pb-Cu sulfide deposit of early Proterozoic age (ca. 1.9 Ga) is located in central Sweden. It is hosted by a supracrustal sequence of quartz-feldspar and quartz-mica gneisses, with minor intercalations of amphibolite, marble, and calc-silicate gneiss (skarn). The area has undergone several metamorphic events subsequent to syndepositional hydrothermal alteration: regional high-temperature and moderate- to low-pressure metamorphism in the upper amphibolite facies up to granite melting; medium-grade, low-pressure metamorphism associated with granite intrusions; and retrograde metamorphism in the greenshist facies probably related to a burial metamorphic event. The metamorphism was essentially isochemical and bulk chemical changes to the host rocks were premetamorphic and probably prelithification.Geochemical and petrographic evidence indicates a volcanic protholith for the gneisses and amphibolite. They are the metamorphic equivalents of mainly felsic pyroclastic rocks, whereas amphibolites are metamorphosed mafic volcanic rocks of basaltic composition. The quartz-mica gneisses represent the metamorphic equivalents of hydrothermally altered felsic volcanic rocks. The intercalation of metavolcanoclastic and metasedimentary rocks (marble, skarn, and reworked volcanics) indicates a shallow-water submarine environment for this ore-bearing sequence. Geochemically the present composition of the metavolcanic rocks displays calc-alkaline affinity, and their bimodal character suggests a tensional environment of deposition.The Zn-Pb-Cu sulfide mineralization at Saxberget is mainly bounded to a calc-silicate horizon which laterally grades into dolomitic marble. The orebodies trend in a general east-west direction and dip 35 degrees to 45 degrees S, with a horizontal extension on the surface of about 500 m and more than 1,500 m in the pitch direction (southeast); the thickness can reach up to 40 m. From the footwall to the hanging-wall side, the ores include stratiform massive ball ore, semi-massive to disseminated strata-bound Zn-Pb ores in calc-silicate gneiss (skarn ore), and disseminated Cu-rich ore in quartz-mica gneiss. Large-scale zoning is observed in the mineralization, the massive Zn-Pb-rich ball ore dominates in the western part of the mine and disseminated skarn ore and Cu-rich ore are more common in the eastern part of the mine. The present footwall rocks are banded felsic metavolcanics which are normally less altered and deformed than the hanging-wall rocks. The contact between the footwall rocks and the ores is partly defined by the ball ore, which appears to represent a tectonically deformed sulfide body along a significant thrust. Upward, the ball ore grades into or is in contact with the skarn ore, which shows intense small-scale folding. The hanging wall of the ore-bearing horizon consists of strongly altered metavolcanic and metasedimentary rocks. Geochemically, the hanging-wall rocks are depleted in Na and Ca and are enriched in Mg and Fe, displaying a geochemical zoning relative to the orebodies. This geochemical zoning is mineralogically expressed in the hanging-wall rocks by orthoamphibole-bearing and spinel-bearing assemblages close to the ore and followed outward by the garnet-bearing quartz-mica gneisses which in turn are succeeded by the cordierite-bearing quartz-mica gneisses.The geochemistry and interrelationships of the various petrologies strongly suggest that the stratigraphic sequence at Saxberget is inverted and that the ore represents a proximal exhalite located on a subsurface alteration pipe. The deposit is classified with the Broken Hill type of middle Proterozoic stratiform sulfides, which is represented in the Phanerozoic by the familiar sedimentary exhalative group of stratiform zinc-lead-copper deposits in sedimentary rocks.

The Taivaljarvi Ag-Zn deposit in the Archean greenstone belt, eastern Finland

The Taivaljaervi Ag-Zn deposit is associated with felsic metavolcanics in the Archean green-stone belt of eastern Finland. The deposit is stratiform and consists of four ore layers, the A, B, C, and D orebodies. The deposit contains 3.45 million metric tons of ore with 1.27 percent Zn, 0.57 percent Pb, 160 g/metric ton Ag, and 0.37 g/metric ton Au. The content of sulfide ore minerals is low, only 5 percent on average. The main ore minerals are sphalerite, galena, chalcopyrite, pyrite and pyrrhotite, dyscrasite, freibergite, electrum, and native Ag. The deposit displays disseminated and vein-type ore textures. The volcanic host rocks are deformed and metamorphosed in high amphibolite facies, and the metamorphic mineral assemblages reflect the variation in chemical compositions of the rock sequence. The intensive hydrothermal alteration associated with the mineralization predated the regional metamorphism, but it can still be recognized in the chemical compositions of the rock sequence. The variation in main and trace element compositions in a traverse of the volcanic formation is presented in tables and diagrams. The delta 34 S values fluctuate from 2.0 to 3.8 per mil, indicating volcanic origin of sulfur. The ore deposit is a product of hydrothermal activity associated with volcanoclastic felsic eruption, and the type of deposition is assumed to be epithermal in character.

Geologic setting and genesis of gold deposits of the Dahlonega and Carroll County gold belts, Georgia

Most gold deposits of the northern Piedmont of Georgia are within the Dahlonega gold belt and the Carroll County gold belt. Together these belts can be traced across Georgia from Alabama to North Carolina and consist of at least 217 abandoned mines and prospects. The overwhelming majority of the gold deposits occurs within the New Georgia Group, with a small number occurring within the western belt of the Sandy Springs Group. Important host rocks for the gold deposits include mafic and felsic metavolcanics of the Pumpkinvine Creek Formation, metasediments and metavolcanics of the Canton Formation, metavolcanics of the Mud Creek Formation, meta-igneous rocks of the Kellogg Creek mafic complex, and metavolcanics and metasediments of the Univeter Formation. Most gold deposits of the Dahlonega and Carroll County belts can be grouped into six districts: the Villa Rica district, the Burnt Hickory Ridge district, the South Canton district, the Auraria district, the Dahlonega district, and the White County district. Gold was mined intermittently from the Dahlonega and Carroll County belts from 1829 to 1935. Production from these belts accounted for the majority of the recorded statewide production of just over one-half million ounces.An extrapolation of metamorphism dates from the southern Piedmont suggests that rocks in the Dahlonega and Carroll County gold belts were metamorphosed to at least greenschist facies (biotite subfacies) and as high as amphibolite facies (kyanite subfacies) in the Late Devonian approximately 365 million years ago and were affected by four fold generations. Rocks of the New Georgia Group are interpreted as the oldest rocks in the Dahlonega and Carroll County belts. The New Georgia Group is composed predominantly of metavolcanic rocks and grades upward into the overlying Sandy Springs Group, a predominantly metased-imentary sequence.Whole-rock and trace element chemistry of mafic metavolcanic host rocks and the presence of abundant interlayered metasediments strongly suggest that rocks of the Dahlonega and Carroll County gold belts were deposited in a back-arc basin environment. Contemporaneous with the deposition of the rocks within the back-arc basin, convecting thermal waters leached gold from underlying rocks. Upon reaching the sea floor in the vicinity of a vent, gold was precipitated in siliceous horizons (exhalites) or was incorporated into contemporaneous tuffs, flows, and sediments. Subsequently, during metamorphism and deformation of the host rocks, gold and other elements were remobilized and redeposited in structurally controlled ore shoots in the siliceous horizons. Gold originally incorporated into contemporaneous tuffs, flows, and sediments migrated to structurally controlled sites (fold hinges, shear zones, cleavages, etc.) in these host rocks, thus giving rise to two types of deposits: ore shoots in metamorphosed, auriferous siliceous horizons, and gold-bearing quartz bodies (sweat-outs) derived from metamorphosed auriferous host rocks. Exposure and weathering of the gold deposits resulted in a supergene enrichment of gold in the weathered horizon (saprolite) and a mechanical and chemical concentration of gold in placers.

Ion microprobe ages of zircons from early Archaean granite pebbles and greywacke, barberton greenstone belt, Southern Africa

We report single-grain and grain-domain U-Pb zircon ages for three granitoid clasts from the Moodies conglomerate and for detrital zircons from a Fig Tree greywacke from clastic sediments in the early Archaean Barberton greenstone belt of southern Africa. The crystallization ages obtained for the Moodies clasts vary between 3570 ± 6 Ma and 3518 ± 11 Ma, while the Fig Tree zircons show a surprisingly uniform source terrain age of 3453 ± 9 Ma that may date the stratigraphically underlying Onverwacht Group felsic metavolcanics. The discordancy patterns are ascribed to a combination of variable Pb loss at unspecified times in the past and new zircon growth and/or recrystallization < 3300 Ma ago. The SHRIMP data record details of isotopic variation, even within single zircons, that result in precise definition of upper concordia intercept ages. Our results suggest an age range between ∼ 3460 and ∼ 3570 Ma for the source region of the Barberton belt clastic metasediments, presumably the Ancient Gneiss Complex of Swaziland and felsic volcanics of the Overwacht Group. This source included geochemically evolved granitoids that predate the oldest greenstones by some 100 Ma.

Constraints on the age of the Bulawayan group metavolcanic sequence, Harare Greenstone Belt, Zimbabwe

Renewed attempts to date the bimodal felsic-mafic-felsic Archaean metavolcanic sequence of the Harare Greenstone Belt in Zimbabwe by Rb:Sr and Sm:Nd methods show that the Rb/Sr isotope systems within the greenstones have been extensively disturbed. However isotopic data from the surrounding granitic terrain do constrain the age of the intervening greenstone succession, which rests on gneissic “basement”, exposed only in a small area, giving an age of 2865±135 Ma, but was intruded by tonalitic and injection gneisses yielding analytically indistinguishable ages of 2688±138 Ma and 2622±168 Ma, respectively. A felsite from the Bulawayo Belt gives 2615±28 Ma, which is approximately consistent with the constraints deduced for the Harare Belt. Sm:Nd model ages suggest that the two main felsic metavolcanic formations were extruded in the period around 2.74 to 2.62 Ga. Thus the felsic metavolcanics overlap in age with the tonalite-granodiorite plutonics; they are geochemically similar and may well be co-eval. Both the bimodal greenstone succession and these early tonalitic intrusives were strongly deformed by a major tectonic event at about 2.62 Ga. Emplacement of post-tectonic, unfoliated, potassic granite plutons or sheets, at about 2.60 Ga, marked the last major Archean thermal event in the region and the stabilisation of the Zimbabwe Craton.

Rift-related igneous activity and metallogenesis in SW Bergslagen, Sweden

The Bergslagen ore belt in central Sweden comprises a mid-Proterozoic complex of broadly coeval volcano-sedimentary and plutonic rock units, showing overlapping ages between 1.9 and 1.84 Ga, and formed in a rift tectonic setting. The volcano-sedimentary rock unit of this complex is referred to as the Bergslagen supracrustal sequence and divided into the lower leptite, the middle leptite, and the upper leptite and slate groups. In SW Bergslagen, the development of the plutono-volcano-sedimentary complex began about 1.9 Ga ago with the early volcanic phase of extrusion of lower leptite group felsic volcanics. In the subsequent initial rift phase, branching basins were formed in which felsic volcanics and iron-formations of the middle leptite group were deposited. The succeeding rift phase was marked by the deposition of felsic volcanics, volcaniclastic and other sediments of the upper leptite and slate group in rift phase basins. The rift phase is characterized by bimodal mafic-felsic volcanism and plutonism. Metabasic flows are found between the felsic metavolcanics and metasediments in the rift basins, whereas metabasic sills and dykes are associated with granite intrusions in the horsts between the rift basins. The metabasic rocks are referred to as the Bergslagen metabasic rock suite and the granites as the Bergslagen older granite suite. The alkaline to subalkaline chemistry and anorogenic suite trace element enrichments of the older granites, their close affinities to associated silicic volcanics, the continental tholeiitic character of the metabasite rock suite, and the outspoken bimodality of the rift phase magmatism, all support the contention of a rift tectonic setting. Manganese and sulfide ores are found as syngenetic exhalative-sedimentary deposits in the rift basins and epigenetic replacement deposits in older formations below the floor of and in the horsts between the rift basins. These ore deposits and associated metasomatic alterations are related to widespread sub-seafloor hydrothermal metamorphism in connection with hydrothermal convection systems driven by the heat of the various rift phase mafic and felsic intrusions and extrusions. The rift phase terminated with cessation of volcanism and was followed by the post-rift phase of tectonic compression and the intrusion of about 1.84 Ga old hybride rocks of the Hyttsjö gabbro-tonalite suite.