Robert Moritz

Fluorite 87Sr/86Sr and REE constraints on fluid–melt relations, crystallization time span and bulk DSr of evolved high-silica granites. Tabuleiro granites, Santa Catarina, Brazil

Abstract The evolved high-silica Tabuleiro granites within the Early Paleozoic Santa Catarina Composite Massif, Pelotas Batholith, southern Brazil are characterized by the presence of euhedral to subeuhedral accessory fluorite and geochemical features typical of topaz–rhyolites and related A-type granites. Sr isotopes and REE data of the Tabuleiro granites and their accessory fluorite are used to constrain fluid–melt relations, crystallization time span and bulk crystal–melt Sr partition coefficient D Sr .Correlations involving REE, Eu/Eu*, Rb/Sr, Sr and 87 Sr / 86 Sr in fluorite and fluorite-host granites show that fluorite records the differentiation trend of the host Tabuleiro granites. REE-normalized patterns and Eu/Eu* relations in fluorite-bearing granites indicate that fluorite forms after the crystallization of the quartzo-feldspathic framework in residual melts. The Tabuleiro accessory fluorites yield high and variable 87 Sr / 86 Sr ratios between 0.72334 and 0.8192. These ratios are neither the result of fluorite precipitation from a fluid nor Sr isotopic resetting. They result from 87 Rb decay in fluorine-rich high-Rb/Sr melts evolved by fractional crystallization in a magmatic system with a long crystallization time-span. Melt residence times of 300 to 700 ka and D Sr of 4.7 to 6.0 are necessary to yield the high fluorite 87 Sr / 86 Sr ratios. These results are compatible with those deduced elsewhere from high-silica rhyolitic volcanic equivalents.

Paleozoic orogenic gold deposits in the eastern Central Andes and its foreland, South America

Abstract In the eastern Central Andes and its foreland (6°–34°S), abundant quartz veins emplaced along brittle–ductile deformation zones in Ordovician to Carboniferous granites and gneisses and in saddle-reefs in lower Paleozoic turbidites represent a coherent group of middle to late Paleozoic structurally hosted gold deposits that are part of three major Au (±Sb±W) metallogenic belts. These belts, extending from northern Peru to central Argentina along the Eastern Andean Cordillera and further south in the Sierras Pampeanas, include historical districts and mines such as Pataz–Parcoy, Ananea, Santo Domingo, Yani–Aucapata, Amayapampa, Sierra de la Rinconada and Sierras de Cordoba. On the basis of the available isotopic ages, two broad mineralization epochs have been identified, with Devonian ages in the Sierras Pampeanas Au belt (26° to 33°30′S), and Carboniferous ages for the Pataz–Maranon Valley Au-belt in northern Peru (6°50′ to 8°50′S). The absolute timing of the southeastern Peruvian, Bolivian and northwestern Argentinian turbidite-hosted lodes, which form the Au–Sb belt of the southern Eastern Andean Cordillera (12° to 26°S), is poorly constrained. Field relationships suggest overlap of gold veining with Carboniferous deformation events. The northernmost belt, which includes the Pataz province, is over 160-km-long and consists of sulfide-rich quartz veins hosted by brittle–ductile shear zones that have affected Carboniferous granitic intrusions. Gold mineralization, at least in the Pataz province, occurred a few million years after the emplacement of the 329 Ma host pluton and an episode of molassic basin formation, during a period of rapid uplift of the host units. The two southern belts are associated with syn- to post-collisional settings, resulting from the accretion of terranes on the proto-Andean margin of South America. The Au–Sb belt of the southern Eastern Andean Cordillera presumably formed in the final stages of the collision of the Arequipa–Antofalla terrane and the Sierras Pampeanas Au belt is considered concurrent with the late transpressional tectonics associated with the accretion of the Chilenia terrane. The three Devono–Carboniferous Andean belts are the South American segments of the trans-global orogenic gold provinces that were formed from Late Ordovician to Middle Permian in accretionary or collisional belts that circumscribed the Gondwana craton and the paleo-Tethys continental masses. A paleogeographic map of the Gondwana supercontinent in its Middle Cambrian configuration appears as a powerful tool for predicting the location of the majority of the Paleozoic orogenic gold provinces in the world, as they develop within mobile belts along its border. The three South American belts are sited in the metallogenic continuation of the Paleozoic terranes that host the giant eastern Australian goldfields, such as Bendigo–Ballarat and Charters Towers, with which they share many features. When compared to deposits in the French Massif Central, direct counterparts of the Andean deposits such as Pataz and Ananea–Yani are respectively the Saint Yrieix district and the Salsigne deposit. Considering the ubiquity of the Au (±Sb±W) vein-type deposits in the Eastern Cordillera and Sierras Pampeanas, and the relatively little attention devoted to them, the Devonian and Carboniferous orogenic gold deposits in the eastern section of the Central Andes constitute an attractive target for mineral exploration.

Geochemistry, tectonics, and crustal evolution of basement rocks in the Eastern Rhodope Massif, Bulgaria

Orthogneisses derived from granitoids with Variscan protolith ages dominate the lower unit of high‐grade metamorphic basement of the Eastern Rhodope Massif in south Bulgaria. We present whole‐rock geochemistry and Sr–Pb isotopic composition of these orthogneisses, which are compared with Pb isotopes of parametamorphic rocks, and hydrothermal ore deposits and associated rocks, to better constrain their composition, origin, and contribution to late Alpine hydrothermal processes. The igneous mineral assemblage is partly preserved, and the field textures and microstructures of the orthogneisses are consistent with a ductile, amphibolite‐grade tectono‐metamorphic overprint during Alpine time, when they were involved in the metamorphic nappe stack. Whole‐rock geochemistry revealed compositions of the orthogneisses largely unaffected by the amphibolite‐grade metamorphism, displaying a magmatic differentiation trend of the igneous protoliths. The protoliths are peraluminous medium‐K calc‐alkaline S‐type granitoids, whose tectono‐magmatic setting discrimination consistently indicates a continental volcanic arc origin. The orthogneisses present trace element and rare‐earth elements (REE) patterns based on which a group of high‐field strength elements‐depleted and REE fractionated orthogneisses and a group of LREE‐enriched orthogneisses can be distinguished. Both geochemical groups show compositions similar to the bulk and upper continental crust and its sedimentary counterparts. Crustal Pb isotope ratios (206Pb/204Pbi = 18.24–18.66) of the orthogneisses are comparable to the paragneisses (206Pb/204Pbi = 18.31–18.93) and uniform in both (207Pb/204Pbi = 15.64–15.72) and 208Pb/204Pbi ratios in the paragneisses (38.23–38.60) and the orthogneisses (38.32-38.56). The trace element data and 87Sr/86Sri isotopes of the orthogneisses (0.7050–0.7117) overlap those of the parametamorphic rocks (0.7039–0.7144), and confirm the supra‐crustal origin of the igneous precursors. A heterogeneous crustal source region is suggested in which melting and crustal contamination during magma genesis with subsequent fractional crystallization was involved in the petrogenesis. Comparative Pb isotope systematics suggests that a significant crustal Pb input to ore‐forming hydrothermal fluids was derived primarily from the metamorphic basement, implying that the brittlly deformed basement during crustal extension acted as an immediate environment for fluid leaching during late Alpine hydrothermal ore‐forming processes.

Sr, C and O isotope systematics in the Pucará basin, central Peru

A combined Sr, O and C isotope study has been carried out in the Pucará basin, central Peru, to compare local isotopic trends of the San Vicente and Shalipayco Zn-Pb Mississippi Valley-type (MVT) deposits with regional geochemical patterns of the sedimentary host basin. Gypsum, limestone and regional replacement dolomite yield 87Sr/86Sr ratios that fall within or slightly below the published range of seawater 87Sr/86Sr values for the Lower Jurassic and the Upper Triassic. Our data indicate that the Sr isotopic composition of seawater between the Hettangian and the Toarcian may extend to lower 87Sr/86Sr ratios than previously published values. An 87Sr-enrichment is noted in (1) carbonate rocks from the lowermost part of the Pucará basin, and (2) different carbonate generations at the MVT deposits. This indicates that host rocks at MVT deposits and in the lower-most part of the carbonate sequence interacted with 87Srenriched fluids. The fluids acquired their radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Pucará basin. The San Ramón granite, similar Permo-Triassic intrusions and their clastic derivatives in the Mitu Group are likely sources of radiogenic 87Sr. The Brazilian shield and its erosion products are an additional potential source of radiogenic 87Sr. Volcanic rocks of the Mitu Group are not a significant source for radiogenic 87Sr; however, molasse-type sedimentary rocks and volcaniclastic rocks cannot be ruled out as a possible source of radiogenic 87Sr. The marked enrichment in 87Sr of carbonates toward the lower part of the Pucará Group is accompanied by only a slight decrease in δ18O values and essentially no change in δ13C values, whereas replacement dolomite and sparry carbonates at the MVT deposits display a coherent trend of progressive 87Sr-enrichment, and 18O- and 13C-depletion. The depletion in 18O in carbonates from the MVT deposits are likely related to a temperature increase, possibly coupled with a 18O-enrichment of the ore-forming fluids. Progressively lower δ13C values throughout the paragenetic sequence at the MVT deposits are interpreted as a gradually more important contribution from organically derived carbon. Quantitative calculations show that a single fluid-rock interaction model satisfactorily reproduces the marked 87Sr-enrichment and the slight decrease in δ18O values in carbonate rocks from the lower part of the Pucará Group. By contrast, the isotopic covariation trends of the MVT deposits are better reproduced by a model combining fluid mixing and fluid-rock interaction. The modelled ore-bearing fluids have a range of compositions between a hot, saline, radiogenic brine that had interacted with lithologies underlying the Pucará sequence and cooler, dilute brines possibly representing local fluids within the Pucará sequence. The composition of the local fluids varies according to the nature of the lithologies present in the neighborhood of the different MVT deposits. The proportion of the radiogenic fluid in the modelled fluid mixtures interacting with the carbonate host rocks at the MVT deposits decreases as one moves up in the stratigraphic sequence of the Pucará Group.

Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens

Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by upper-crustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ∼30 m.y., comprising Middle Eocene (48.9–43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 ± 0.8 m.y., followed by postsubduction Late Eocene–Middle Oligocene (37.8–28.1 Ma) shoshonitic magmatism over 9.7 ± 0.9 m.y., and Late Oligocene–Early Miocene (26.6–21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 ± 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (eHfzircon = +8 to +11.3) and oxygen (δ18Ozircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5–10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatic-hydrothermal systems, and especially giant PCDs.

Fluid evolution in the El-Sid gold deposit, Eastern Desert, Egypt

Abstract Auriferous quartz (±carbonate) veins in the El-Sid mine cut through the western margin of the Fawakhir granitic intrusion and the immediate country ophiolites. Gold mineralization is spatially and temporally associated with ENE–WSW fault/shear zones developed late in the deformational history of the area. Field and microscopic studies suggest two distinct ore stages; namely an early gold-Fe–As-sulphide, and a late gold-base metal mineralization. New microthermometric and Raman data suggest gold deposition as a result of a complex history of fluid immiscibly, dilution of low-salinity aqueous-carbonic fluids in the early mineralization stage, while wall-rock alteration and pressure loss precipitated Au from intermediate-salinity aqueous ore fluids during the late stage. Fluid inclusion isochoric reconstructions, combined with oxygen and sulphur isotope data, indicate conditions of 320±20 °C and 1.3±0.2 kbar for the early gold-Fe–As-sulphide mineralization, and c. 200±15 and 0.6±0.9 kbar for the late gold-Zn–Pb–Cu-sulphide stage. The clockwise evolution path in pressure-temperature space likely documents gold mineralization under post-peak metamorphic conditions. The calculated sulphide δ34SH2S equilibrium values −9.04‰ to –4.75‰, may refer to a variable redox state of sulphur in the ore fluids from the early to late mineralization stages. The stable isotope signature of the vein quartz and calcite suggest mixed magmatic and metamorphic fluid sources (δ18O H2O=+4.9‰ to +7.4‰). Unusually low δ13C values of calcite in the late mineralization (−13.9‰ to −14.7‰) may reflect input of magmatic CO2 and/or oxidized carbonaceous material in the infiltrating fluid.

Peri-Gondwanan Ordovician crustal fragments in the high-grade basement of the Eastern Rhodope Massif, Bulgaria: evidence from U-Pb LA-ICP-MS zircon geochronology and geochemistry

Field, geochemical, and geochronologic data of high-grade basement metamafic and evolved rocks are used to identify the nature and timing of pre-Alpine crustal growth of the Rhodope Massif. These rocks occur intrusive into clastic-carbonate metasedimentary succession. Petrography and mineral chemistry show compositions consistent with Alpine amphibolite-facies metamorphism that obliterated the original igneous textures of the protoliths. Bulk-rock geochemistry identifies low-Ti tholeiitic to calc-alkaline gabbroic-basaltic and plagiogranite precursors, with MORB-IAT supra-subduction zone signature and trace elements comparable to modern back-arc basalts. The U-Pb zircon dating revealed a mean age of 455 Ma for the magmatic crystallization of the protoliths that contain inherited Cambrian (528–534 Ma) zircons. Carboniferous, Jurassic, and Eocene metamorphic events overprinted the Ordovician protoliths. The radiometric results of the metamorphic rocks demonstrate that Ordovician oceanic crust was involved in the build-up of the Rhodope high-grade basement. Dating of Eocene-Oligocene volcanic rocks overlying or cross-cutting the metamorphic rocks supplied Neoproterozoic, Ordovician and Permo-Carboniferous xenocrystic zircons that were sampled en route to the surface from the basement. The volcanic rocks thus confirm sub-regionally present Neoproterozoic and Paleozoic igneous and metamorphic basement. We interpret the origin of the Middle-Late Ordovician oceanic magmatism in a back-arc rift-spreading center propagating along peri-Gondwanan Cadomian basement terrane related to the Rheic Ocean widening. The results highlight the presence of elements of Cadomian northern Gondwana margin in the high-grade basement and record of Rheic Ocean evolution. The eastern Rhodope Massif high-grade basement compared to adjacent terranes with Neoproterozoic and Cambro-Ordovician evolution shares analogous tectono-magmatic record providing a linkage among basement terranes incorporated in the Alpine belt of the north Aegean region.

Trace element diffusion and incorporation in quartz during heating experiments

Heating of quartz crystals in order to study melt and high-temperature fluid inclusions is a common practice to constrain major physical and chemical parameters of magmatic and hydrothermal processes. Diffusion and modification of trace element content in quartz and its hosted melt inclusions have been investigated through step-heating experiments of both matrix-free quartz crystals and quartz crystals associated with sulfides and other minerals using a Linkam TS1500 stage. Magmatic and hydrothermal quartz were successively analyzed after each heating step for Cu, Al, and Ti using electron probe micro-analyzer. After the last heating step, quartz crystals and their hosted melt inclusions were analyzed by laser ablation inductively coupled plasma mass spectrometry and compared to unheated samples. Heated samples reveal modification of Cu, Li, Na, and B contents in quartz and modification of Cu, Li, Ag, and K concentrations in melt inclusions. Our results show that different mechanisms of Cu, Li, and Na incorporation occur in magmatic and hydrothermal quartz. Heated magmatic quartz records only small, up to a few ppm, enrichment in Cu and Na, mostly substituting for Li. By contrast, heated hydrothermal quartz shows enrichment up to several hundreds of ppm in Cu, Li, and Na, which substitute for originally present H. This study reveals that the composition of both quartz and its hosted melt inclusions may be significantly modified upon heating experiments, leading to erroneous quantification of elemental concentrations. In addition, each quartz crystal also becomes significantly enriched in Cu in the sub-surface layer during heating. We propose that sub-surface Cu enrichment is a direct indication of Cu diffusion in quartz externally sourced from both the surrounding sulfides as well as the copper pins belonging to the heating device. Our study shows that the chemical compositions of both heated quartz and its hosted inclusions must be interpreted with great caution to avoid misleading geological interpretations.

Sedimentary-rock-hosted epithermal systems of the Tertiary Eastern Rhodopes, Bulgaria: new constraints from the Stremtsi gold prospect

Abstract Precious metal epithermal, sedimentary-rock-hosted prospects constitute a new class of ore deposits recently described in the Tertiary Eastern Rhodopes of southeastern Bulgaria. The Stremtsi prospect investigated in this contribution is located in a distal location with respect to the main cluster of sedimentary-rock-hosted Ada Tepe and Rosino gold prospects of the Eastern Rhodopes. The Stremtsi prospect is hosted by a Priabonian clastic sedimentary rock sequence, overlying metamorphic rocks of the Central Rhodopean dome. The eastern part of the Stremtsi prospect contains high gold grades, and is characterized by a strongly silicified zone, including adularia and silicified dolomite blades, diagnostic for boiling conditions during ore formation in such low-sulphidation epithermal systems. The western part of the Stremtsi prospect consists of a barite, sphalerite and galena mineralization, associated with silicification, and illite and carbonate alteration. Both parts are underlain by subvertical quartz-carbonate-pyrite veins. Primary and secondary fluid inclusions, respectively, in dolomite and barite yield homogenization temperatures ranging between 90 and 247 °C. The salinity of primary inclusions in dolomite falls between 1.9 and 5.1 wt% NaCl equivalent, whereas the one of secondary fluid inclusions in barite ranges between 0.0 and 3.1 wt% NaCl equivalent. The variable homogenization temperatures reflect post-entrapment re-equilibration of the fluid inclusions, whereas the salinities were preserved and the inclusions in dolomite are interpreted in terms of dilution of a saline fluid in the western part of the Stremtsi prospect. The sulphur isotope compositions of sulphides from Stremtsi range mainly between −4 and +4‰. They are not diagnostic and can be attributed to magmatic, metamorphic, and sedimentary sources. They overlap with the main compositional range of sulphides from other sedimentary-rock-hosted epithermal systems and reveal the existence of hydrothermal fluids with common characteristics during ore formation throughout the Eastern Rhodopes. In addition, at Stremtsi, negative δ34S values between −42.6‰ and −8.8‰ combined with framboidal pyrite and elevated δ34S values of +7.0‰ to +19.5‰ support locally derived sulphur generated, respectively, by bacterial and thermochemical sulphate reduction. Modelling of O, C, and Sr isotope data of dolomite support the above described ore-forming processes. A positive correlation between δ18O (+12.7‰ to +19.7‰ V-SMOW) and δ13C (−2.8‰ to +1.5‰ V-PDB) values for dolomite from the eastern, silicified and gold-enriched zone of the Stremtsi prospect is satisfactorily modelled by boiling between 140 and 180 °C of a deeply circulating fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively, and radiogenic strontium leached from the metamorphic basement rocks or its clastic counterparts in the Priabonian host rocks. By contrast, negative correlations of δ18O values (+13.4‰ to +23.3‰ V-SMOW) with δ13C values (−0.6‰ to −3.9‰ V-PDB) and 87Sr/86Sr ratios of dolomite from the western, barite and base metal-rich zone are adequately modelled by a shallow, low temperature (70 °C), intra-formational fluid recharged by meteoric water, which interacted with organic matter, that is, coal layers, and carbonate rocks from the Priabonian host sequence, mixing with a deep, moderate temperature (190 °C), 87Sr-enriched fluid characterized by δ18O and δ13C values of +5.5‰ V-SMOW and −1.5‰ V-PDB, respectively. Disequilibrium conditions revealed by sulphur isotope thermometry of two galena-barite pairs yielding discrepant temperatures of 190 and 306 °C are consistent with fluid mixing. A plateau age of 37.57±0.31 Ma obtained by 40Ar/39Ar dating of adularia from Stremtsi is interpreted as a maximum age because of the saddle-shaped age spectrum. Combined with 40Ar/39Ar age data from previous studies, it reveals that the sedimentary-rock-hosted epithermal prospects constitute an independent, regional and older ore-forming hydrothermal system, distinct from the younger volcanic-rock-hosted epithermal deposits of the Bulgarian and Greek Eastern Rhodopes.

Source of lead in the gold-bearing quartz-fuchsite vein at the Dome mine, Timmins area, Ontario, Canada

The quartz-fuchsite vein at the Dome mine, Timmins area, Canada, is characterized by a consistent spatial association of gold and galena. A lead isotope study was carried out to trace the source of the lead in the galenas, and to evaluate possible source reservoirs for the gold. Although other sources cannot be entirely ruled out, the lead isotope composition of most of the galenas and of local quartz-feldspar porphyries suggests that most of the lead in the vein-galenas was derived from the Dome mine porphyries. However, lead isotope systematics do not allow unique constraints to be placed on the gold source. Conceptual genetic models commonly advocate gold source reservoirs such as shallow to deep seated igneous intrusions, the middle to lower crust or the mantle for Archean lode gold deposits. A likely scenario is that ore-bearing hydrothermal fluids originated at depth, and were preferentially channeled along the porphyries to the site of ore deposition. The hydrothermal fluids leached lead from the porphyries, and deposited gold and galena together in the quartz-fuchsite vein. Two galena samples have “anomalous” lead compositions and are colinear with the main galena cluster suggesting one or more contamination episodes, broadly between 1250 and 2480 Ma, resulting in further addition of radiogenic lead to the galenas.