Calc-silicate Research Articles

The nature and sources of ore-forming fluids in the Bhukia gold deposit, Western India: constraints from chemical and boron isotopic composition of tourmaline

Gold mineralization in the newly discovered Bhukia deposit in northwestern India is hosted in Proterozoic metamorphosed volcano-sedimentary rocks of the Aravalli-Delhi Belt. Three generations of tourmaline occurring in different textural settings are recognized in the host rocks of the deposit. Fine-grained texturally early tourmalines (Tur-I) precipitated during the gold-sulfide mineralization stage. They are dravitic in composition and occur parallel to the S1 tectonic foliation as fine-grained crystals in tourmaline-albite layers within quartz-albite rock and as scattered grains in calc-silicate rocks. Coarse-grained texturally later schorls (Tur-II) are also restricted to calc-silicate and quartz-albite rocks and characterized by sector zoning. Late-stage type III tourmaline (Tur-III), also of schorl composition, replaces Tur-II along fractures and margins. They are interpreted to have formed during a phase of ore remobilization. All tourmalines, especially the schorls, are strongly enriched in Li, Ga, Mn and Zn with Ga concentrations being the highest ever reported in natural tourmalines (up to 1380 ppm). The boron isotope composition is similar in all three tourmaline types with the consistently light δ11B (−10.4‰ to −7.2‰) indicative of a continental source for B. The chemical and B-isotopic composition of tourmaline is suggestive of the involvement of two fluids, a granitic-derived hydrothermal fluid and metapelite-derived metamorphic fluid. The δ11B variations in the tourmalines can be explained by mixing between 11B-poor granitic-derived hydrothermal fluid and 11B-rich metamorphic-hydrothermal fluid. While high V (2110–4247, avg. 3339 ppm) in the early dravitic tourmalines indicate mixing of granitic-derived hydrothermal fluid with pelite-derived metamorphic hydrothermal fluids during gold-sulfide mineralization, the enrichment of Li, Mn, Zn and Ta, and depletion of V in the later schorlitic tourmalines suggest increasing influence of granitic-derived hydrothermal fluids during ore remobilization.

The effects of amphibolite facies metamorphism on the trace element composition of pyrite and pyrrhotite in the Cambrian Nairne Pyrite Member, Kanmantoo Group, South Australia

The trace element composition of pyrite has been used to explore for hydrothermal ore deposits and to understand ore-forming processes. However, the effects of metamorphism on the trace element distribution in pyrite have received relatively limited attention. In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of pyrite and pyrrhotite, along with minor amounts of sphalerite, chalcopyrite, and galena, are used to evaluate the effects of amphibolite facies metamorphism on the trace element distribution and remobilization of metals in iron sulfides in the clastic sediment-hosted Cambrian Nairne Pyrite Member (NPM), South Australia. The NPM and the Mt. Torrens Pb-Zn-Ag prospect, which occur near the base of the Kanmantoo Group, preserve irregularly zoned subhedral to euhedral metamorphic pyrite (Py1) and anhedral pyrrhotite (Po1), along with relatively minor quantities of remobilized anhedral pitted and cataclastic pyrite (Py2a) in quartzofeldspathic rocks and anhedral inclusion-poor pyrite (Py2b) in calc-silicate rocks that armor or cross-cut earlier formed Py1. Rare anhedral secondary melnikovite pyrite (Py3) locally formed on the margins of Py1 and Py2a. Trace element studies show that Py1 in the NPM at Brukunga and Ironstone Ridge contains mean values of 1254 ppm Co, 123 ppm Ni, 2167 ppm As, 16 ppm Se, 10 ppm Cu, 25 ppm Zn, and 15 ppm Pb, whereas Py1 in the Mt. Torrens prospect contains mean values of 2312 ppm Co, 263 ppm Ni, 1835 ppm As, 95 ppm Se, 9 ppm Cu, 6 ppm Zn, and 9 ppm Pb. Rare inclusion-rich cores of Py1 show higher concentrations of trace elements than inclusion-free rims with minor amounts of chalcopyrite, galena, and sphalerite forming along grain boundaries or in fractures within pyrite. This is interpreted to be the result of the release of Cu, Pb, and Zn from pyrite as it recrystallized. Remobilization of these elements then formed discrete sulfides at the millimeter to centimeter scale, with some exceptions at the meter scale where chalcopyrite, sphalerite, and galena, along with other sulfides and sulfosalts, formed in veins and tension gashes. These observations suggest that remobilization of trace metals, including Cu, Pb, and Zn, did not migrate more than a few meters at most. Moreover, despite the recrystallization of pyrite and pyrrhotite and subsequent remobilization of some trace elements (i.e., Co, Ni, As), which were structurally bound in these Fe sulfides, still retain elevated concentrations at amphibolite facies conditions.Remobilization of metals from the NPM during metamorphism to form Cu-Au (e.g., Kanmantoo) and Pb-Zn-Ag-(Cu-Au) (e.g., Angas, Wheal Ellen) deposits in the Tapanappa Formation stratigraphically higher in the Kanmantoo Group seems unlikely. However, it is possible that one source of metals for these deposits could have been leached from the NPM and carried in large hydrothermal cells prior to metamorphism. Such a scenario is consistent with previously published sulfur isotope data for sulfides from the NPM, and Cu-Au and Pb-Zn-Ag-(Cu-Au) deposits, which indicate that sulfur derived from the NPM and pyritic schists in the Kanmantoo Group was a likely source of sulfur for the base and precious metal deposits.

Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (<10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.

Polyphase scheelite and stanniferous silicates in a W-(Sn) skarn close to Felbertal tungsten mine, Eastern Alps

The scheelite exploration target Messelingscharte (Eastern Tyrol, Austria) is located in vicinity of the world-class Felbertal tungsten deposit. W-(Sn) mineralisation occurs in Early Palaeozoic amphibolites (Basal Amphibolite unit) in the Tauern Window of the Eastern Alps. The most important mineralisation type is a Sn-bearing clinozoisite-scheelite skarn of pre-Alpine (Variscan?) age. It occurs as metre-sized irregular pods within amphibolites and amphibole schists. It is composed of major clinozoisite, quartz, plagioclase and scheelite with minor and accessory titanite, calcite and chlorite. Bulk chemical analyses reveal high concentrations of the granitophile elements W (≤7.74 wt% WO3), Sn (≤1254 ppm SnO2), Be (≤41 ppm) and base metals (Cu, Pb, Zn; ∑ ≤ 2500 ppm) in the skarn rock. Three scheelite types are distinguished based on micro-textures, zoning, Mo-content and UV- fluorescence. They show intriguing similarities to scheelite from the Felbertal tungsten deposit where pre-Alpine Mo-bearing scheelite was apparently overprinted by two stages of metamorphism. The unique feature of the investigated W-(Sn) skarn is the association of scheelite with Sn-bearing silicates. Stanniferous clinozoisite and stanniferous titanite were identified as the main Sn carriers (clinozoisite ≤3.00 wt% SnO2; titanite ≤6.48 wt% SnO2); both minerals evidence metamorphic re-crystallisation. Substitution of (Al, Fe)3+ by (Sn, Ti)4+ in clinozoisite is coupled with incorporation of Fe2+. The skarn formed by interaction of fluids of likely magmatic-hydrothermal origin with metabasites. The clinozoisite-dominated calc-silicate rocks are interpreted as a metamorphosed distal W-(Sn) skarn.

Biotite compositional variations – A physicochemical approach to investigate crustal involvement and ore potential in Middle Jurassic plutonic rocks from the Malayer-Boroujerd Plutonic complex, W Iran

Biotite is the most frequent ferromagnesian phase in granitoids from the Malayer-Boroujerd Plutonic complex (MBPC), Zagros Orogen. The abundance of biotite decreases from granitoids towards dioritic intrusions, dioritic and gabbroic dykes to gabbroic intrusions coincident with increase in hornblende/biotite ratio. Primary magmatic biotite composition in the MBPC granitoids (Fe-rich type) changes from annite to siderophyllite, while Fe-depleted secondary biotites from the altered gabbros and gabbroic-dioritic dykes approximate to the phlogopite-eastonite series, and those from least-altered gabbros display average composition of these groups, however, extend towards high-Al end-members. Using Ti-in-biotite geothermometer, the crystallization temperature varies between 550 °C to 750 °C in MBPC granitoids (P 10–15 bars). Mode-III: biotite crystallized at higher oxidizing conditions (f(O2) > 10–10 bars) in gabbroic and dioritic dykes as well as gabbroic intrusions. The progressive decreases in f (O 2)–values and oxidizing conditions from I-type to S-type granites (above HM-to below QFM-buffer) reflect increasing incorporation of less-oxidized upper crustal materials during magmagenesis. In addition, the highest f (O2) in MBPC mafic intrusive rocks is consistent with a modest crustal contamination, if any. There is lack of evidence for porphyry type alteration zoning and subvolcanic porphyritic intrusions associated with porphyry copper system. The small patches of calc-silicate rocks from the NW-MBPC formed in contact with less oxidized and highly fractionated, crustal-derived granular granitoids and are less predisposed to develop Cu-Au mineraliza- tion. However, given the relatively higher f (O2)-values, the Middle Jurassic mafic intrusive rocks originated from the meta- somatised mantle-wedge beneath SaSiZ, appears to be the best candidate for further ore exploration programs in the area.

Perovskites of the Tazheran Massif (Baikal, Russia)

The paper provides details of local geology and mineralogy of the Tazheran Massif, which was the sampling site of perovskite used as an external standard in perovskite U-Pb dating by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively-coupled plasma (LA–ICP–MS) mass spectrometry. The Tazheran Massif is a complex of igneous (mafic dikes, syenite, nepheline syenite), metamorphic (marble), and metasomatic (skarn, calc–silicate veins) rocks. Metasomatites are thin and restricted to the complex interior being absent from the margins. Perovskite has been studied at four sites of metasomatic rocks of three different types: forsterite–spinel calc–silicate veins in brucite marble (1); skarn at contacts between nepheline syenite and brucite marble (2), and skarn-related forsterite–spinel (Fo-Spl) calc–silicate veins (3). Pervoskite from Fo-Spl calc–silicate veins (type 1) is almost free from impurities (<1 wt.% in total: 0.06%–0.4% REE2O3, 0.10%–0.22% Nb2O5, ≤0.1% ThO2). The U contents are from 0.1 to 1.9 wt.% UO2 and are relatively uniform in perovskites from the same vein but differ from vein to vein of this type. Perovskite from the contact skarn (type 2) contains 1.5 to 4.5 wt.% REE2O3 but is poor in other impurities. Perovskite grains from skarn-related Fo-Spl calc–silicate rocks (type 3) belong to two generations that differ in REE, Nb, Th, Fe, and Na concentrations. Early-generation perovskites occurs as compositionally homogeneous octahedral or cubic-octahedral crystals with contents of impurities higher than in other varieties (3.6 wt.% REE2O3, 1.6 wt.% Fe2O3, 1.3 wt.% Nb2O5, 0.7 wt.% ThO2, 0.6 wt.% UO2, and 0.6 wt.% Na2O) but the lowest is at the respective site. Late-generation varieties show highly variable impurity concentrations of 1.5 to 22.7 wt.% REE2O3, 0.4 to 8.4 wt.% Nb2O5, and 0.8 to 4.5% ThO2, while the perovskite component may be as low as 65%. In addition to the lueshite and loparite, components, they contain REEFeO3 and Th0.5TiO3 endmembers which have no natural analogs.

Structural features and age of gold mineralization in the Troia Massif, Borborema Province, NE Brazil: A Paleoproterozoic (∼2029 Ma) hypozonal orogenic gold deposit overprinted by the late Neoproterozoic Brasiliano/Pan–African orogeny

Hypozonal orogenic gold mineralization has been recently recognized in the Troia Massif, an Archean/Paleoproterozoic basement inlier within the Neoproterozoic Borborema Province, NE Brazil. The mineralization occurs in association with quartz veins hosted by the amphibolite facies Paleoproterozoic Serra das Pipocas greenstone belt. The main mineralized areas are located near parallel to lithological contacts, siting on shear zones established at the metavolcanic and metasedimentary boundary. Four deformation events are recognized: (1) Dn was responsible for the early Sn foliation, parallel to the bedding (So) of the greenstone units. (2) Dn+1 is characterized by a pervasive, southeasterly–dipping Sn+1 foliation that is axial–planar to several asymmetric, tight to isoclinal and recumbent folds. (3) Dn+2 represents a strike–slip NE–trending shear zones, and (4) the late Dn+3 event is characterized by ductile–brittle deformation. Evolution from Dn+1 to Dn+3 is interpreted as a progressive deformation, with the maximum compressive stress in the WNW–ESE direction. The main stage of gold mineralization is associated with the Dn event, and comprised by deformed quartz veins and associated high–temperature calc–silicate alteration (diopside, hornblende, titanite) and albitization of the host rocks. Lower temperature gold (±Te, Ag) occurs along late stage brittle structures (Dn+3). The U–Pb titanite age of 2029 ± 28 Ma for the calc–silicate alteration of the early gold mineralization is similar to the age of many Transamazonian/Eburnean gold mineralizations elsewhere. However, the strong Pb loss of titanite grains defines a 574 ± 7 Ma lower intercept age, indicating that early gold mineralization was affected (remobilized) by Neoproterozoic deformational events and metamorphism (Brasiliano/Pan–African orogeny). This is in agreement with the U–Pb zircon age of 575 ± 3 Ma obtained for the syn–tectonic Guaribas dikes, which bracketed the age of Dn+3. Therefore, as Dn+1, Dn+2 and Dn+3 evolved by progressive deformation; they are all of Neoproterozoic age. This two–stage gold mineralization is here interpreted as product of crustal exhumation, with (1) early hypozonal orogenic gold mineralization occurring shortly after the high–grade Paleoproterozoic metamorphism and first exhumation processes of the greenstone pile; and (2) late gold mineralization occurring at shallow levels (second exhumation process), associated with the Neoproterozoic Brasiliano/Pan–African orogeny.

Origin of vesuvianite-garnet veins in calc-silicate rocks from part of the Chotanagpur Granite Gneiss Complex, East Indian Shield: The quantitative P-T-XCO2 topology in parts of the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (+Fe2O3, F)

Origin of vesuvianite-garnet veins in calc-silicate rocks from part of the Chotanagpur Granite Gneiss Complex, East Indian Shield: The quantitative P-T-XCO2 topology in parts of the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (+Fe2O3, F)

New insights into the Precambrian tectonic evolution and continental affinity of the Qilian block: Evidence from geochronology and geochemistry of metasupracrustal rocks in the North Wulan terrane

Abstract The Qilian block, one of the Precambrian terranes in the Qinling-Qilian-Kunlun orogenic system, is a critical region for reconstruction of the overall architecture and tectonic evolution of NW China. This investigation of zircon U-Pb and Lu-Hf isotopes and whole-rock geochemistry of a metasupracrustal sequence in the North Wulan terrane provides new insights into the Qilian block. A Statherian–Calymmian unit (ca. 1.67–1.5 Ga), dominated by Al- and Si-rich gneisses, arkosites, quartzites, and amphibolites with minor calc-silicate rocks and marbles, is interpreted to have been deposited during continental rifting. Detrital zircons show two main age populations of 2685–2276 and 2098–1761 Ma with mostly negative εHf(t) values (–14.0 to +3.6). The sources are characterized by mixed felsic to intermediate igneous rocks as well as recycled components and are interpreted as being derived from the Tarim craton because of the age distribution of their detrital zircons. A Stenian–Tonian unit (ca. 1.1–0.9 Ga) consists mainly of felsic gneisses, quartzites, calc-silicate rocks, marbles, metavolcanic rocks, and amphibolites. The metasedimentary rocks yielded detrital zircon ages clustering at ca. 1.64, 1.43, 1.3–1.2, 1.1, and 0.94 Ga with predominantly positive εHf(t) values (–7.1 to +9.7). One metavolcanic rock has an age of ca. 1110 Ma and εHf(t) values of +6.5 to +9.1. The provenance is dominated by local syndepositional arc-related igneous rocks with older detritus possibly from Laurentia, again based on the age distribution of the detrital zircons. The Central Qilian and Hualong terranes show strong affinities with the North Wulan terrane and together constituted a single coherent Qilian block prior to their involvement in the Qilian–North Qaidam orogen. The Qilian block was probably once part of the Tarim craton and had a strong linkage to South Tarim, which drifted from North Tarim during the breakup of Columbia in the early Mesoproterozoic. We suggest that, from the late Mesoproterozoic to early Neoproterozoic, the South Tarim–Qilian formed an active continental margin located close to Laurentia during the assembly of Rodinia. The final collision occurred in the early Neoproterozoic with the formation of a significant continent that included the reunified Tarim-Qilian as well as Qaidam-Kunlun and Qinling terranes, Alxa block, Kyrgyz-Chinese Tianshan, and Yili block.

Petrología y geoquímica de los Esquistos del Silgará en el area de Silos, Macizo de Santander, Colombia

Los Esquistos del Silgará consisten en metapelitas, metasemipelitas, rocas cuarzofeldespáticas, rocas carbonatadas puras y rocas calcosilicatadas. El contenido y patrón de REE y las relaciones Zr/Sc, Th/Sc y los altos contenidos de Hf y Zr, apuntan a una fuente de composición félsica de la corteza superior con alto grado de reciclaje. Los Esquistos del Silgará fueron metamorfizados en condiciones de pico indicado por la paragénesis Bt + Grt + St + Qz + Ky + Pl a condiciones de T = 670 – 770° C y P = 7.8 – 11.3 Kbar. La litología y las condiciones P-T del metamorfismo barroviense de los Esquistos del Silgará de la franja de Silos son muy similares a las del área de Mutiscua.

Fluid inclusion and stable isotope (O, H, C) constraints on the genesis of the Pedra Branca gold deposit, Troia Massif, Borborema Province, NE Brazil: An example of hypozonal orogenic gold mineralization

At the Archean-Paleoproterozoic Troia Massif, NE Brazil, two major Paleoproterozoic greenstone belts are documented (Algodões and Serra das Pipocas). They share similar lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and West Africa craton). Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. In the mineralized area (the Pedra Branca gold deposit) the main gold mineralization is found in association with quartz veins, calc–silicate alteration (e.g. diopside, K-feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Pyrrhotite is commonly found as inclusions in pyrite, and late magnetite replaces pyrite, suggesting progressive oxidation of the ore–forming fluid. Free-milling gold commonly precipitates at the late stages of alteration, in close association with magnetite and tellurides. The main tellurides found are hessite, altaite, sylvanite, calaverite and tellurobismuthite. Two fluid inclusion assemblages were identified from mineralized quartz veins. Assemblage 1 is characterized by pseudo-secondary trails that show the coexistence of CO2–rich and low salinity (0–8 wt% NaCl equiv.) CO2–H2O–NaCl and H2O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). Assemblage 2 is represented by late secondary low–temperature (Th < 200 °C) H2O–NaCl inclusions, probably unrelated to gold mineralization. The δ18O, δD and δ13C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) and fluid inclusions are compatible with a magmatic–hydrothermal ore-forming fluid that underwent fluid–rock interaction with the greenstone sequence. The calculated fluid δ18O values range from +8.3 to +11.0‰ (n = 59), fluid δD from −98 to –32‰ (n = 24) and δ13C values of calcite from −6.35 to −9.40‰ (n = 3). Oxygen isotope thermometry for quartz–magnetite pairs yields temperatures from 467 to 526 °C (n = 7, average 503 °C). This probably represents the temperature of gold deposition, because gold is generally associated with magnetite in the ore mineral assemblage. The estimated PT conditions of the gold mineralization range from 467 °C and 1.93 kbar (7 km) to 526 °C and 3.48 kbar (13 km), repectively, and they were obtained from the intersection of isochores with the temperature range derived from the quartz–magnetite isotope geothermometer. This PT range is akin to hypozonal orogenic gold deposits. The association of gold with magnetite and tellurides implies an ore–forming fluid derived from oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion-related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields).

Paleozoic tectonothermal event in Mt. Dongbatu, Dunhuang terrane, southernmost Central Asian Orogenic Belt (CAOB): Implications for petrogenesis and geological evolution

In this contribution, systematic petrographical, whole-rock geochemical, geochronological and zircon Lu-Hf isotopic investigations were conducted on Paleozoic (ca. 410–400 Ma) metamorphic-magmatic rocks from Mt. Dongbatu in the central Dunhuang terrane. The metamorphic rocks in the Shangkouzi area are of meta-calcsilicate rocks, and granitoid rocks occurring within these rocks are geochemically of trondhjemite. Whereas, metamorphic rocks in the Gangou area are metabasaltic rocks. LA-ICP MS zircon U-Pb dating results reveal that these metamorphic-magmatic rocks recorded ca. 1.6 Ga magmatic event and metamorphic events of ca. 410–400 Ma and ca. 370 Ma. Field occurrence, whole-rock geochemistry, mineral composition and zircon Hf isotopes suggest that the 1.6 Ga mantle-derived basaltic magma, together with iron-rich and Ca-rich components and mica-quartz schist of Paleoproterozoic supracrustal rocks probably contributed to protolith of meta-calcsilicate rocks in the Shangkouzi area, and granitoid rocks within meta-calcsilicate rocks were possibly generated by partial melting of calcsilicate rocks.Combined with previously published data, it is suggested that the protoliths of Paleozoic high-grade metamafic rocks in the Dunhuang terrane are predominantly composed at least of basaltic rocks produced by magmatic events of 2.5–1.8 Ga, ca. 1.8 Ga and ca. 1.6 Ga. The Paleozoic granitoid rocks in the Dunhuang terrane might have been derived from different magma sources as partial melting of subducted plate, or thickened lower continental crust, or pre-existing basaltic rocks produced by 1.6 Ga and 1.8 Ga magmatic activities, and calcsilicate rocks genetically associated basaltic magmatism at ca. 1.6 Ga. Considering regional tectonic evolution, it can be concluded that the Dunhuang terrane was probably in a post-collisional extension setting at ca. 1.6 Ga, producing certain amount of basaltic and calcsilicate rocks. From the beginning of Silurian (ca. 440 Ma), the Dunhuang terrane was involved in the orogenic processes related to the closure of the southern Paleo-Asian Ocean, becoming the active continental margin of the southernmost CAOB.

Geology and metamorphism of the neoproterozoic Mangabal Complex: An example of Ni[sbnd]Cu-PGE mineralized intrusion in the Goiás Magmatic Arc, central Brazil

Inserted in the context of the Goiás Magmatic Arc, the Mangabal Complex represents a partially metamorphosed mafic-ultramafic intrusion, which is part of a cluster of neoproterozoic layered bodies formed during the Brasiliano Orogeny in central Brazil. The Mangabal Complex is composed of two exposures, the northern and the southern members, which exhibit the similar igneous mineralogy, crystallization sequence and were affected by the same metamorphic conditions. The stratigraphy of the Mangabal Complex can be divided into three main zones: i. the Lower Mafic Zone (LMZ), composed of adcumulatic norite and, forming the basal portion of the southern member; it is composed of adcumulatic norite; ii. the Ultramafic Zone (UZ), characterized by dunite and harzburgite cumulates, and iii. the Upper Mafic Zone (UMZ), consisting predominantly of norite, with restricted interlayeres of feldspathic dunite-orthocumulate. Petrographic data reveal a crystallization sequence for the Mangabal Complex formed by Olivine + Cr-Spinel > Olivine + Orthopyroxene > Orthopyroxene + Plagioclase > Clinopyroxene. In the olivine cumulus, the forsterita (Fo) content (Fo 78–85) suggest a moderately primitive parental magma. The NiCu-PGE mineralization occurs as disseminated sulfides, located in the UMZ, UZ and LMZ cumulate rocks, or forming massive to net-textured bodies in the metamorphic rocks of the UMZ. The main sulphide assemblage is composed of pyrrhotite + pentlandite + chalcopyrite ± pyrite, with minor pyrite + millerite. Disseminated sulphides are present in igneous rocks as well as in the metamorphic rocks that still partially preserve their original primary texture. This fact, together with the occurrence of abundant olivine and chromium-bearing spinel cumulates, suggests a fertile magma for the Mangabal Complex. However, the highest S, Ni, Cu, and PGE contents correlate positively with and are related to amphibolite and calc-silicate rocks associated with shear zones, indicating that metamorphism and deformation processes were critical in promoting the remobilization of sulphides and their deposition along shear zones. The primary mineral assemblage was largely replaced due to high-pressure upper-amphibolite facies metamorphism. The paragenesis representing metamorphic peak conditions is composed of kyanite + orthoamphibole + hornblende, which reveals pressures of approximately 8.5 kbar and a temperature up to 750 °C. The metamorphic mineral assemblage, together with the proximity to the São Luis de Montes Belos shear zone, suggests that kyanite crystallization in the Mangabal Complex and similar layered intrusions worldwide might be linked to the development of major crustal-scale structures.

The Tonian Embu Complex in the Ribeira Belt (Brazil): revision, depositional age and setting in Rodinia and West Gondwana

The Embu Complex in the Neoproterozoic Ribeira Belt of Brazil provides a record of sedimentation associated with the Rodinia supercontinent and its subsequent deformation and metamorphism during assembly of West Gondwana. It is composed of a succession of variably migmatized pelitic schists and paragneisses, as well as minor calcsilicate rocks with quartzite intercalations, and orthogneiss. Detrital zircon U-Pb (LA-ICP-MS and SHRIMP) determinations for key samples of the Embu Complex in its type-area (West and Southwest of São Paulo City), and samples from neighboring units (Votuverava and São Roque groups) indicate distinct provenance histories and in combination with other data, discrete times of sediment accumulation. Detrital zircons from the Embu Complex are characterized by ages in the range 1500–1000 Ma and 2000–1700 Ma, with a very minor number of older ages. The youngest detrital grain is dated at 974 ± 12 Ma. Metamorphic zircon overgrowths range from 850 to 570 Ma with age peaks at ca. 820–790 and 650–570 Ma. Thus, the depositional age of the Embu Complex is constrained to 970–850 Ma. The detrital zircons of the Votuverava and São Roque groups are dominated by ages in the range 2200–2000 Ma, along with minor Paleoproterozoic and Archean ages. The youngest detrital grains are ∼1400 Ma (Votuverava Group) and ∼1750 Ma (São Roque Group). Metamafic rocks from the Votuverava Group have crystallization ages of ca. 1300 Ma, with a higher metamorphic grade amphibolite showing a range of discordant ages with upper and lower intercepts of 1300 Ma and ∼800 Ma, respectively. These results demonstrate that the Embu Complex was probably accreted to the Apiaí Terrane during a Tonian (820–790 Ma) orogenic event, prior to final development of the Ediacaran Ribeira belt arc-related magmatism and a transcurrent shear zone system. The main source area for detrital zircons of the Embu Complex is likely to be the Grenville-Sveconorwegian-Sunsás orogen or time-equivalent regions rather than the central portions of the neighboring cratons. The Embu Complex geological context is consistent with accumulation on the margin of the Rodinia supercontinent.

Carbonate-related metallic and non-metallic mineralization within and proximal to granites (Fichtelgebirge Pluton, Germany): “Mantle-crust marker mineralization”

Metacarbonate rocks (marble, calcsilicate rock, skarn) hosting strata- and structurebound Fe-, As-Sb, Bi-, Ni-Co, Cu-Pb-Zn, U-, W- ore minerals as well as talc, clay minerals, barite and fluorite are widespread in the country rocks south of the collisional calc-alkaline felsic to intermediate intrusive rocks of the Fichtelgebirge Pluton, Germany. At its western rim, thrustbound and vein-type mineral assemblages with Au-As-Sb and F-Ba minerals associated with carbonate gangue minerals developed. Only recently, a calcite-hosted Sb mineralization was encountered in a deep-seated lineamentary fault zone cutting through metaultrabasic rocks along the northern edge of the granites. The same structure zone forms the loci of calcite-bearing U episyenite situated within this pluton. A composite geological, mineralogical and chemical (major and trace elements, REE, C- and O isotopes) study has been conducted to distinguish the heat and element source in subcrustal or deep-seated crustal areas and in the largely exposed granite complex. Only Sn-W skarn deposits are genetically related to the highly fractionated granitic members of the Variscan pluton. Pegmatite skarn has a strong subcrustal component as to the heat source and the provenance of rare elements and a moderate crustal one as far as the silicates are concerned. Deep-seated fault zones were active over a rather long period of time and acted as conduits venting magmas and hydrothermal fluids from the waning stages of the Variscan deformation through the Neogene. Calcsilicate and carbonate mineral assemblages are an efficacious tool to constrain the physico-chemical regime in this mineral province, covering the temperature range from 745 °C down to 53 °C in a medium to low pressure regime at strongly varying redox conditions. The major and trace elements, the REE variation, the Ce and Eu anomalies as well as carbon and oxygen isotopes of the various mineral assemblages enable us to identify the fluid sources and depict the element concentration processes, e.g., mixing of fluids, connate fluid and meteoric fluid interaction. From the economic point of view, the mineralizing system is most prospective for rare element deposits, talc, kaolinite-group minerals, iron, and uranium. Accumulations of fluorite and barite are subeconomic in mineral assemblages inside as well as outside the granites, while base metals and precious metals are only of mineralogical interest.

Campaign-style U-Pb titanite petrochronology: Along-strike variations in timing of metamorphism in the Himalayan metamorphic core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core (HMC), in order to document and investigate the causes of along-strike variations in its Oligocene–Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya. This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ∼30 Ma to 12 Ma, recording (re-)crystallization between 625 °C and 815 °C. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen. Disparities in the duration and timing of metamorphism within the HMC are best explained by along-strike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes. These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger, thicker crystalline core in the eastern Himalaya.

Zircon ages and geochemistry of amphibolitic rocks from the Paleoproterozoic Erdaowa Group in the Khondalite Belt, North China Craton and their tectonic implications

A currently popular model regards the Khondalite Belt in the North China Craton as a typical continent–continent collisional belt along which the Yinshan and Ordos blocks amalgamated to form the Western Block at ∼1.95 Ga. Although this model is fully supported by the geological data of high-grade metamorphic rocks from the belt, it remains unknown whether it is consistent with the low- to medium-grade units in the belt. One of such low- to medium-grade units from the Khondalite Belt is the Paleoproterozoic Erdaowa Group that consists of low-grade schists, quartzites, amphibolitic rocks, calc-silicate rocks, marbles and minor meta-conglomerates, of which amphibolitic rocks can provide important insights into understanding the tectonic setting and timing of collision of the Yinshan and Ordos blocks along the Khondalite Belt. The Erdaowa amphibolitic rocks can be divided into two groups of which Group1 is assigned to tholeiitic basalts, whereas Group 2 belongs to basaltic andesites, but they both experienced the fractional crystallization of the primary magma that was derived from the partial melting of the lithospheric mantle with minor crustal contamination. Such partial melting of the lithospheric mantle most likely occurred in a subduction zone as evidenced by their REEs and trace elements characterized by depletions in high field strength element (HFSE), Ti, P, Nb and Ta, suggesting that their igneous protoliths have affinities to continental marginal arcs. Metamorphic zircons from two amphibolitic rock samples yield Pb/Pb ages of 1941 + 14 Ma and 1910 + 9 Ma, similar to those metamorphic ages obtained for high-grade pelitic granulites from the Khondalite Belt. This confirms that the Yinshan-Ordos collision occurred at 1.95–1.90 Ga, and provides direct evidence for the involvement of low- and medium-grade rocks during the collision.

Petrofabric of forsterite marbles and related rocks from a low-pressure metamorphic terrain (Almadén de la Plata massif, Ossa-Morena Zone, SW Spain) and its kinematic interpretation

A fabric study of forsterite marble and calc-silicate rock paragenetic minerals is presented for samples collected in the Almadén de la Plata massif of SW Spain, that experienced Variscan low-pressure metamorphism related to the tectonic evolution of the suture between the Ossa-Morena and South-Portuguese zones of the Iberian Massif. Two tectonically stacked metamorphic units contain marble, metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist (structuraly upper unit) and the upper amphibolite/granulite facies (structuraly lower unit). Different reactional paths lead to formation of forsterite marbles in both units. Dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn-metamorphic ductile deformation under temperatures between 500 and 700 °C. Dolomite formed (harder) porphyroclast systems and boudinaged bands, whereas weaker calcite underwent dynamic recrystallization. In both cases deformation was facilitated by activation of various intracrystalline slip systems, complemented by other deformation mechanisms. The fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral/stretching lineation, including a systematic asymmetry suggestive of non-coaxial deformation components. Foliations and lineations are interpreted as solid-state flow planes and directions bearing a kinematic significance. In particular, regional sinistral shear at mid-deep crustal realms coeval with low-pressure metamorphism explains the imbrication of metamorphic units and their exhumation, concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the Ossa-Morena/South-Portuguese suture between the late Devonian (∼360 Ma) and the latest Carboniferous (∼300 Ma).

Evolution of metamorphic fluid recorded in granulite facies metacarbonate rocks from the middle segment of the Mogok metamorphic belt in central Myanmar

AbstractThe Mogok metamorphic belt of Palaeogene age, which records subduction‐ and collision‐related events between the Indian and Eurasian plates, lies along the western margin of the Shan plateau in central Myanmar and continues northwards to the eastern Himalayan syntaxis. Reaction textures of clinohumite‐ and scapolite‐bearing assemblages in Mogok granulite facies metacarbonate rocks provide insights into the drastic change in fluid composition during exhumation of the collision zone. Characteristic high‐grade assemblages of marble and calcsilicate rock are clinohumite+forsterite+spinel+phlogopite+pargasite/edenite+calcite+dolomite, and scapolite+diopside+anorthite+quartz+calcite respectively. Calculated petrogenetic grids in CaO–MgO–Al2O3–SiO2–H2O–CO2 and subsets of this system were employed to deduce the pressure–temperature–fluid evolution of the clinohumite‐ and scapolite‐bearing assemblages. These assemblages suggest higher temperature (&gt;780–810°C) and [=CO2/(CO2+H2O) &gt;0.17–0.60] values in the metamorphic fluid for the peak granulite facies stage, assuming a pressure of 0.8 GPa. Calcite grains commonly show exsolution textures with dolomite particles, and their reintegrated compositions yield temperatures of 720–880°C. Retrograde reactions are mainly characterized by a reaction zone consisting of a dolomite layer and a symplectitic aggregate of tremolite and dolomite grown between clinohumite and calcite in marble, and a replacement texture of scapolite by clinozoisite in calcsilicate rock. These textures indicate that the retrograde reactions developed under lower temperature (&lt;620°C) and (&lt;0.08–0.16) conditions, assuming a pressure of 0.5 GPa. The metacarbonate rocks share metamorphic temperatures similar to the Mogok paragneiss at the peak granulite facies stage. The values of the metacarbonate rock at peak metamorphic stage are, however, distinctly higher than those previously deduced from carbonate mineral‐free paragneiss. Primary clinohumite, phlogopite and pargasite/edenite in marble have F‐rich compositions, and scapolite in calcsilicate rock contains Cl, suggesting a contrast in the halogen compositions of the metamorphic fluids between these two lithologies. The metamorphic fluid compositions were probably buffered within each lithology, and the effective migration of metamorphic fluid, which would have extensively changed the fluid compositions, did not occur during the prograde granulite facies stage throughout the Mogok metamorphic belt. The lower conditions of the Mogok metacarbonate rocks during the retrograde stage distinctly contrast with higher conditions recorded in metacarbonate rocks from other metamorphic belts of granulite facies. The characteristic low conditions were probably due to far‐ranging infiltration of H2O‐dominant fluid throughout the middle segment of the Mogok metamorphic belt under low‐amphibolite facies conditions during the exhumation and hydration stage.

The Rio Doce Group revisited: An Ediacaran arc-related volcano-sedimentary basin, Araçuaí orogen (SE Brazil)

The Araçuaí orogen developed between the São Francisco and Congo cratons, during the Western Gondwana amalgamation in Late Neoproterozoic time. Located in the eastern Araçuaí orogen, the Ediacaran Rio Doce magmatic arc includes a large amount of plutonic rocks (the G1 supersuite, ca. 630-580 Ma) spatially associated with a supracrustal succession, the Rio Doce Group (RDG). This group, poorly studied until recently, encompasses four formations (from base to top): Palmital do Sul, Tumiritinga, São Tomé, and João Pinto. The Palmital do Sul and Tumiritinga formations mostly include pelitic schists to paragneisses, interbedded with metavolcanic and meta-volcaniclastic rocks, representing proximal deposits in relation to the volcanic arc front. Pyroclastic deposits represented by lapilli-rich meta-tuffs with scattered volcanic bombs denote explosive volcanism of dacitic to rhyolitic composition. Zircon U-Pb data provide magmatic crystallization ages of 595 ± 13 Ma and ca. 584 ± 5 Ma for the volcanic protoliths. The São Tomé Formation mainly includes psammitic to pelitic meta-turbidites with lenses of calcsilicate rocks, indicating relatively distal marine deposits with a maximum depositional age of ca. 594 Ma and a striking arc-related lithochemical signature. The João Pinto Formation mostly includes quartz sandstones and micaceous to feldspathic sandstones, with a maximum sedimentation age of ca. 620 Ma, representing fluvial to continental shelf environments related to the late stages of the Rio Doce arc development. The Rio Doce Group can be correlated with other orogenic supracrustal successions (i.e., parts of the Andrelândia Group) found in the region covered by the Rio Doce arc, connecting the Araçuaí and Ribeira orogens.