Precursor and ambient rock paleothermometry to assess the thermicity of burial dolomitization in the southern Cantabrian Zone (northern Spain)

International audiencePaleozoic rocks in the Cantabrian Zone, and the Variscan foreland fold-and-thrust belt on the Iberian Peninsula have been affected by a sequence of diagenetic to epizonal thermal events. Late- to Post-Variscan hot fluid circulation caused a large-scale burial dolomitization and ore mineralization, mostly in Cambrian and a Lower to Middle Carboniferous carbonate sucessions. The goal of this study is to analyze and compare the temperatures experienced by the carbonate precursor rocks, as well as the under- and over-lying siliciclastic ambient rocks to gain a better understanding of the thermicity of dolomitization. These temperatures are evaluated based on published paleothermal datasets combined with new data obtained from Rock–Eval pyrolysis and vitrinite reflectance analysis of Carboniferous rocks rich in organic matter. The overall results indicate that reworking of detrital sediments in synorogenic ambient siliciclastics results in an anomalously high thermal maturity recorded by bulk rock techniques such as illite crystallinity and Rock–Eval pyrolysis. In situ VR-derived paleotemperatures recorded by ambient siliciclastic rocks appear to be higher compared to CAI-derived temperatures for carbonate precursor rocks. This variation in thermal maturity is likely related to the analytical techniques used to obtain CAI and VR data, and the empirical equations applied to calculate corresponding paleotemperatures. Conodont fragments were not as sensitive compared to vitrinite, and the color alteration process could have suffered from hydrothermal alteration. A secondary cause might be a different response to mechanical deformation between siliciclastic and carbonate units during the Variscan and post-Variscan geodynamic evolution of the study area. Rigid precursor carbonate units experienced fluid circulation mainly along distinct and spaced fracture zones, creating fracture-related dolomite geobodies and ore mineralization. Soft ambient siliciclastic rocks experienced more diffuse fluid circulation and heat dissipation. The different paleothermometry datasets compiled for the study area indicate that the fluids circulating during Late- to Post-Variscan times, with associated fracture-related dolomitization and ore mineralization in carbonate precursors, are hydrothermal. The highest paleotemperatures were recorded in ambient and precursor rocks in the highly tectonized northern part of the study area, where several thrusts and faults allowed intense fluid circulation. Positive temperature anomalies within the precursor carbonates correlate well with the occurrence of dolomite geobodies and ore mineral deposits. Such anomalies could thus be used as an exploration tool for hydrothermal dolomite bodies in analog sub-surface settings

Mixing of magmatic-hydrothermal and metamorphic fluids and the origin of peribatholitic Sn vein-type deposits in Rwanda

The fluid sources of granite-related Sn-quartz vein deposits are commonly obscured by fluid mixing or fluid-rock interactions. As a result, fluid inclusions, minerals and isotopes in these veins indicate an intermediate composition between magmatic and metamorphic, but the degree of mixing between these endmembers is currently unquantified. This study presents a novel quantitative approach to assess the degree of mixing between magmatic-hydrothermal and external metamorphic fluids in the formation of peribatholitic Sn-quartz veins. In particular, fluid mixing in the Sn-mineralized Rwamagana-Musha-Ntunga pegmatite-quartz vein field in East Rwanda has been evaluated by the following four methods: quartz stable isotopes, muscovite geochemistry, fluid inclusion microthermometry and LA-ICP-MS, and geochemical modelling. The quartz stable isotope data (δ18O: +13.1 to +15.8 ‰ V-SMOW; δD: −27.6 to –59.7 ‰ V-SMOW) cannot uniquely differentiate between a metamorphic fluid origin or an initial magmatic hydrothermal fluid origin with subsequent metamorphic fluid mixing or host-rock interaction. However, granitophile element concentrations in magmatic muscovite from pegmatites and hydrothermal muscovite from associated Sn-quartz veins are equally high, indicating a close genetic link (Rb: 530 – 8740 ppm, Li: 110 – 1990 ppm, Sn: 87 – 810 ppm, Cs: 62 – 420 ppm). Primary H2O-CO2-N2-NaCl medium saline magmatic fluid inclusions in quartz of pegmatites (∼12.7 wt% NaCleq) and H2O-CO2-(N2)-NaCl low saline fluid inclusions in barren metamorphic quartz veins (∼4.9 wt% NaCleq) were analyzed by LA-ICP-MS. These results show an enrichment in Li, Rb, Sn and Cs for the magmatic fluid, while the metamorphic fluid is characterized by low granitophile element concentrations and high Sr and Ba contents. The expected Rb-Cs and Rb-Sn signature of the Sn-quartz vein muscovite was modelled using the measured fluid endmember compositions, confirming mixing between magmatic and metamorphic fluids in the formation of the veins. The quantification suggests that the hydrothermal Sn-quartz vein fluid contains 5 – 80 % of an external metamorphic fluid component

Geological setting and timing of the cassiterite vein type mineralization of the Kalima area (Maniema, Democratic Republic of Congo)

The Central African Mesoproterozoic Karagwe-Ankole belt in the Great Lakes area (DRCongo, Rwanda, Burundi, Uganda and Tanzania) forms a metallogenic province that hosts a variety of granite-related mineralization, which contains cassiterite, columbite-tantalite, wolframite/ferberite, spodumene and beryl. The Kalima area in the Maniema province of the DRCongo forms one of the most important areas for cassiterite mineralization in the eastern part of the DRCongo, even after many decades of exploitation. The mineralization dominantly consists of quartz veins that are hosted in Mesoproterozoic metasediments at the contact with granitic rocks of the Kalima granite (Avuanga and Yubuli) or directly crosscutting these granitic rocks (Atondo). Only limited – and mainly unmineralized pegmatites – have been described in the Lutshurukuru area. Mineralized quartz veins – and some granite bodies – intruded following the regional tectonic foliation or existing fracture zones, confirming the late-to post-tectonic origin of the fertile granite system. The emplacement of the quartz veins resulted in an alteration of the metasedimentary and granitic host-rocks, mainly resulting in muscovitization, tourmalinization and silicification. Cassiterite itself formed relatively late during vein formation and is associated with muscovite in fractures in or along the margins of the quartz veins. 40Ar-39Ar age dating of muscovite of an unmineralized pegmatite from the Lutshurukuru area gave an excellent plateau age of 1024 ± 5.5 Ma, while the muscovite associated with mineralization gave plateau ages of 986 ± 5.3 Ma for the Atondo deposit and 992.4 ± 5.4 Ma for the Yubuli deposit. The rather large spread in ages between the supposed parental granite/pegmatite and quartz veins is interpreted to reflect different magmatic events in the evolution of a composite granite system, starting at ∼1020 Ma and ending with mineralized quartz vein formation at ∼990 Ma. The latter age corresponds with the U–Pb age reported for columbite-tantalite in the area (993 ± 1 Ma at Kamisuku), which could be interpreted as the primary formation age of a new generation of mineralized pegmatites in the Kalima area, or as the resetting age of the U–Pb system during the ∼990 Ma mineralizing event. Muscovite of a mineralized greisen sample of Avuanga gave a plateau age with relaxed constraints of 1010.3 ± 5.9 Ma, which has been interpreted as a partially resetting of muscovite formed at ∼1020 Ma age, during the ∼990 Ma event

Precursor and ambient rock paleothermometry to assess the thermicity of burial dolomitization in the southern Cantabrian Zone (northern Spain)

© 2017, Springer-Verlag GmbH Germany. Paleozoic rocks in the Cantabrian Zone, and the Variscan foreland fold-and-thrust belt on the Iberian Peninsula have been affected by a sequence of diagenetic to epizonal thermal events. Late- to Post-Variscan hot fluid circulation caused a large-scale burial dolomitization and ore mineralization, mostly in Cambrian and a Lower to Middle Carboniferous carbonate sucessions. The goal of this study is to analyze and compare the temperatures experienced by the carbonate precursor rocks, as well as the under- and over-lying siliciclastic ambient rocks to gain a better understanding of the thermicity of dolomitization. These temperatures are evaluated based on published paleothermal datasets combined with new data obtained from Rock–Eval pyrolysis and vitrinite reflectance analysis of Carboniferous rocks rich in organic matter. The overall results indicate that reworking of detrital sediments in synorogenic ambient siliciclastics results in an anomalously high thermal maturity recorded by bulk rock techniques such as illite crystallinity and Rock–Eval pyrolysis. In situ VR-derived paleotemperatures recorded by ambient siliciclastic rocks appear to be higher compared to CAI-derived temperatures for carbonate precursor rocks. This variation in thermal maturity is likely related to the analytical techniques used to obtain CAI and VR data, and the empirical equations applied to calculate corresponding paleotemperatures. Conodont fragments were not as sensitive compared to vitrinite, and the color alteration process could have suffered from hydrothermal alteration. A secondary cause might be a different response to mechanical deformation between siliciclastic and carbonate units during the Variscan and post-Variscan geodynamic evolution of the study area. Rigid precursor carbonate units experienced fluid circulation mainly along distinct and spaced fracture zones, creating fracture-related dolomite geobodies and ore mineralization. Soft ambient siliciclastic rocks experienced more diffuse fluid circulation and heat dissipation. The different paleothermometry datasets compiled for the study area indicate that the fluids circulating during Late- to Post-Variscan times, with associated fracture-related dolomitization and ore mineralization in carbonate precursors, are hydrothermal. The highest paleotemperatures were recorded in ambient and precursor rocks in the highly tectonized northern part of the study area, where several thrusts and faults allowed intense fluid circulation. Positive temperature anomalies within the precursor carbonates correlate well with the occurrence of dolomite geobodies and ore mineral deposits. Such anomalies could thus be used as an exploration tool for hydrothermal dolomite bodies in analog sub-surface settings.status: publishe

Geological setting and timing of the world-class Sn, Nb-Ta and Li mineralization of Manono-Kitotolo (Katanga, Democratic Republic of Congo)

The Central African Mesoproterozoic Kibara belt in Katanga (DRCongo) forms a metallogenic province that hosts a variety of granite-related mineralization, rich in cassiterite, columbite–tantalite, wolframite/ferberite, spodumene and beryl. This mineralization is mainly present in pegmatites and quartz veins that are thought to be associated with the youngest granite generation in the Kibara belt (i.e., so-called “E-group” granite generation). Manono-Kitotolo is one of the world's largest Sn, Nb–Ta and Li mineralized pegmatites, with a large resource of spodumene, columbite–tantalite and cassiterite still remaining. Mineralized pegmatites have intruded along the foliation in the Mesoproterozoic metasedimentary rocks and dolerites of the Manono-Kitotolo area. The pegmatites have been emplaced very late during the climax of the Kibaran orogeny, probably during the transition from orogenic collapse to extensional tectonics, based on their structural position. The emplacement of the pegmatites resulted in an intense alteration of the doleritic and metasedimentary host-rocks, resulting in muscovitization, tourmalinization and silicification. A mineralogical and geochemical zonation typical for granitic pegmatites has been identified that was affected by metasomatic/hydrothermal alteration, mainly albitization and greisenization. The latter alteration is associated with the main phase of cassiterite mineralization. A first stage with Nb–Ta, Li and minor Sn, however, already formed pre-alteration, directly associated with pegmatite crystallization. 40Ar–39Ar muscovite dating of unaltered pegmatites resulted in ages of 938.8 ± 5.1 Ma and 934.0 ± 5.9 Ma. These ages are overlapping with the U–Pb age of the Nb–Ta mineralization (940 ± 5.1 Ma), also giving new temporal constraints on the fertile E-group granite generation in the KIB. The 40Ar–39Ar muscovite age of a mineralized greisen is 923.3 ± 8.3 Ma, which is younger but still partly within error of the 40Ar–39Ar ages of the unaltered pegmatites. Based on the partial overlap in age, but also on field and paragenetic relationships and the stable isotope composition of the fluids, this greisenization and associated cassiterite mineralization can still be linked to the pegmatite crystallization

Petrographic, fluid inclusion and isotopic study of the Dikulushi Cu–Ag deposit, Katanga (D.R.C.): implications for exploration

The Dikulushi Cu–Ag vein-type deposit is located on the Kundelungu Plateau, in the southeastern part of the Democratic Republic of Congo (D.R.C.). The Kundelungu Plateau is situated to the north of the Lufilian Arc that hosts the world-class stratiform Cu–Co deposits of the Central African Copperbelt. A combined petrographic, fluid inclusion and stable isotope study revealed that the mineralisation at Dikulushi developed during two spatially and temporally distinct mineralising episodes. An early Cu– Pb–Zn–Fe mineralisation took place during the Lufilian Orogeny in a zone of crosscutting EW- and NE-oriented faults and consists of a sequence of sulphides that precipitated from moderate-temperature, saline H2O– NaCl–CaCl2-rich fluids. These fluids interacted extensively with the country rocks. Sulphur was probably derived from thermochemical reduction of Neoproterozoic seawater sulphate. Undeformed, post-orogenic Cu–Ag mineralisation remobilised the upper part of the Cu–Pb–Zn–Fe mineralisation in an oxidising environment along reactivated and newly formed NE-oriented faults in the eastern part of the deposit. This mineralisation is dominated by massive Agrich chalcocite that precipitated from low-temperature H2O–NaCl–KCl fluids, generated by mixing of moderateand low-saline fluids. The same evolution in mineralisation assemblages and types of mineralising fluids is observed in three other Cu deposits on the Kundelungu Plateau. Therefore, the recognition of two distinct types of (veintype)mineralisation in the study area has a profound impact on the exploration in the Kundelungu Plateau region. The identification of a Cu–Ag type mineralisation at the surface could imply the presence of a Cu–Pb–Zn–Fe mineralisation at depth

Timing and geological setting of the world-class Sn, Nb-Ta and Li mineralization of Manono-Kitotolo (Katanga, Democratic Republic of Congo): current state of knowledge

The Central African Mesoproterozoic Kibara belt forms a metallogenic province that hosts a variety of granite-related mineralization rich in cassiterite, columbite-tantalite, wolframite/ ferberite, spodumene and beryl. Between 997 +/- 8 Ma and 966 +/- 21 Ma, granites intruded, associated with pegmatites and quartz veins, locally carrying Sn-Nb-Ta mineralization. Different highly evolved pegmatites intruded in the Manono-Kitotolo area. The emplacement of the pegmatite veins resulted in an intense alteration of the doleritic and metasedimentary host-rocks. After emplacement, the pegmatites developed a regular zonation during cooling that is affected by later metasomatic/hydrothermal alteration. Nb-Ta mineralization formed prior to metasomatic alteration and has been dated at 940 +/- 5.1 Ma, which is significantly younger than the age of pegmatites that have been dated at 973 13 Ma in Katanga. This is interpreted as resetting during (metasomatic) post-crystallization processes. The major part of cassiterite mineralization formed during metasomatic alteration, i.e. during the muscovitization. Muscovite associated with the cassiterite mineralization, in so-called "poches de greisen" has been dated at 923 +/- 8.3 Ma. This age is too young to reflect crystallisation during cooling of the granites and is interpreted as resetting during a Late Neoproterozoic event