Timing of magmatic-hydrothermal activity in the Variscan Orogenic Belt: LA-ICP-MS U–Pb geochronology of skarn-related garnet from the Schwarzenberg District, Erzgebirge

Here, we present in situ U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) ages of andradite-grossular garnet from four magmatic-hydrothermal polymetallic skarn prospects in the Schwarzenberg District, Erzgebirge (Germany), located in the internal zone of the Variscan Orogenic Belt. Within the geochronological framework of igneous rocks and hydrothermal mineralization in the Erzgebirge, the obtained garnet ages define three distinct episodes of Variscan skarn formation: (I) early late-collisional mineralization (338–331 Ma) recording the onset of magmatic-hydrothermal fluid flow shortly after the peak metamorphic event, (II) late-collisional mineralization (~ 327–310 Ma) related to the emplacement of large peraluminous granites following large-scale extension caused by orogenic collapse and (III) post-collisional mineralization (~ 310–295 Ma) contemporaneous with widespread volcanism associated with Permian crustal reorganization. Our results demonstrate that the formation of skarns in the Schwarzenberg District occurred episodically in all sub-stages of the Variscan orogenic cycle over a time range of at least 40 Ma. This observation is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal zones of the Variscan Orogenic Belt in central and western Europe. In analogy to the time–space relationship of major porphyry-Cu belts in South America, the congruent magmatic-hydrothermal evolution in the internal zones and the distinctly later (by ~ 30 Ma) occurrence of magmatic-hydrothermal ore deposits in the external zones of the Variscan Orogenic Belt may be interpreted as a function of their tectonic position relative to the Variscan collisional front.

Eksplorasi Geokimia untuk Menentukan Daerah Prospek Mineralisasi Emas Tipe Urat Epitermal: Studi Kasus di Daerah Tompaso, Kabupaten Minahasa Selatan, Sulawesi Utara

ABSTRAK Sulawesi Utara termasuk daerah Tompaso, Kabupaten Minahasa Selatan merupakan jalur magmatik yang potensial menghasilkan endapan bijih hidrotermal. Penelitian ini bertujuan menentukan daerah prospek mineralisasi emas khususnya tipe urat epitermal berdasarkan eksplorasi geokimia meliputi geokimia batuan dan sedimen sungai, khususnya metode BLEG (Bulk Leach Extractable Gold). Metode penelitian meliputi pemetaan geologi, alterasi dan jalur urat, percontoan (bijih/batuan dan sedimen sungai), dan analisis geokimia. Sampel bijih dianalisis dengan metode FA/AAS untuk emas dan metode AAS untuk unsur lain, sedangkan analisis sampel sedimen sungai dilakukan dengan metode cyanide leach dan AAS. Hasil penelitian menunjukkan batuan induk mineralisasi berupa lava andesit dan intrusi diorit. Batuan ini mengalami alterasi silica-clay, argilik, dan propilitik. Perpaduan antara pemetaan geologi, zona alterasi, dan jalur urat dengan eksplorasigeokimia batuan dapat menentukan daerah prospek mineralisasi yaitu Prospek Asam dan Polangkok. Pada Prospek Asam, sampel bijih memiliki kandungan emas sampai 0,03 ppm dengan anomali emas pada sampel BLEG menunjukkan nilai threshold 13,52 ppb Au. Pada Prospek Polangkok ditemukan 2 jalur urat (P1 dan P2) berarah baratlaut-tenggara dengan lebar sampai 5 m. Urat P1 memiliki kadar Au mencapai 0,31 ppm dan pada urat P2 mencapai 0,16 ppm Au. Mineralisasi pada Prospek Polangkok didukung oleh anomali Ag pada sampel BLEG dengan nilai threshold 67,18 ppb. Kedua daerah prospek tersebut direkomendasikan untuk eksplorasi lanjut (follow-up exploration). ABSTRACT North Sulawesi, including the Tompaso region, South Minahasa district, is a part of the magmatic belts hosting potential hydrothermal ore deposits. This study is aimed to determine the prospect area for epithermal vein-type gold mineralization, based on geochemical exploration including rock and BLEG (Bulk Leach Extractable Gold) stream sediment geochemistry. Mapping of geology, alteration and vein direction, sampling (ore/rock and stream sediment), and geochemical analysis were performed. Gold in rock samples was analyzed by FA/AAS, and other elements were detected by AAS, while BLEG samples were analyzed using the cyanide leach and AAS methods. The study area is occupied by andesitic lava and diorite, which are suffered by silica-clay, argillic and propylitic alteration. An integration of geological mapping, alteration zones and vein direction with lithogeochemical survey enables to determine the prospect areas, which consist of Asam and Polangkok prospects. At the Asam prospect, the ore sample contains gold up to 0.03 ppm, where the gold anomaly in BLEG samples show a threshold of 13.52 ppb Au. At the Polangkok prospect, two NW-SE trending veins (P1 and P2 Veins) were discovered with a width of up to 5 m. Vein P1 and P2 contains of up to 0.31 and 0.16 ppm Au, respectively. Mineralization at the Polangkok prospect coincides with Ag anomaly of BLEG samples with a threshold of 67.18 ppb. The two prospect areas are recommended for follow-up exploration.

Constraining the time-span of magmatic-hydrothermal activity in the Variscan Orogenic Belt – U-Pb geochronology of skarn-related garnet from the Schwarzenberg district, Erzgebirge, Germany

<p>Europe´s major Sn and W resources are hosted by magmatic-hydrothermal ore deposits of the Variscan Belt: e.g. in Cornwall, the Erzgebirge, the Iberian Massif, and the French Massif Central. In the Erzgebirge, several major skarn bodies are located in the Schwarzenberg district (12 x 15 km). Although recent geochronological data relates (skarn) ore-formation to late- and post-orogenic magmatic-hydrothermal activity, details on the nature and duration of mineralization events remain insufficiently understood.</p><p>In this study we present innovative in-situ LA-ICP-MS U-Pb geochronology of garnet from several skarn prospects in the Schwarzenberg district, which is complemented with available geochronological data on intrusions and mineralization in order to constrain the timing of skarn formation within the Variscan orogenic cycle.</p><p>Eighteen garnet dates range from 338.2 ± 2.5 to 294 ± 8.3 Ma. Associated errors are in the range of 2.5 to 8.4 Ma, but generally tend to be <7 Ma. The oldest ages (338-331 Ma, stage I) are related to metasomatic garnets of the Globenstein skarn (n=5) – a skarn that is exceptionally enriched in W compared to the other skarn prospects in the same district. Conversely, the other skarns (Antonsthal, Breitenbrunn, Hämmerlein) are younger and range from 327 to 313 Ma (stage II) and 304 to 294 Ma (stage III), respectively. Stage I and II garnets lie within the range of available zircon ages of major intrusive bodies in this area (Aue-Schwarzenberg granite suite: 334-322 Ma; Eibenstock granite: 326-311 Ma). The third stage, in contrast, does not overlap with the age of any known granite intrusions in the Schwarzenberg district. However, it coincides with widespread early Permian volcanic rocks, which presumably have intrusive roots that are not yet exposed in the Erzgebirge region.</p><p>The distribution of garnet ages implies that skarn formation occurred episodically during the ~45 Ma life-time of the Variscan orogen, with the onset of magmatic-hydrothermal activity occurring significantly earlier than previously assumed – at 338 Ma, immediately after the peak of regional metamorphism. Tin and W deposits (skarn, greisen and vein-type) seem to have formed episodically over the entire 45 Ma orogenic cycle of the Erzgebirge – this is consistent with the age range of available geochronological data related to magmatic-hydrothermal ore deposits from other internal parts of the European Variscan Belt.</p>

Petrologic Reconstruction of the Tieshan Magma Plumbing System: Implications for the Genesis of Magmatic-Hydrothermal Ore Deposits within Originally Water-Poor Magmatic Systems

Most genetic models for magmatic-hydrothermal ore deposits are based on the prerequisite that the parental magmas associated with mineralization are enriched in water (> ∼4 wt %). However, it has been recognized that a number of magmatic-hydrothermal ore deposits also formed within tectono-magmatic settings that produce initially water-poor magmas such as Climax-type porphyry deposits. Here, we present a detailed reconstruction of the Tieshan magma plumbing system related to skarn-porphyry Cu–Fe–Au mineralization in the Edong district, in which primitive magmas typically show water-poor features. Applications of multiple thermodynamic calibrations on various magmatic units from the Tieshan and Tonglushan deposits provide a wealth of information regarding the structure and evolution of the transcrustal magmatic system. Petrographic observations and clinopyroxene-liquid thermobarometry calculations indicate that the Tieshan magmatic-hydrothermal system was fed by a deep crustal magma reservoir. An accurate picture of the evolution of H2O within the magma plumbing system is presented using the plagioclase-liquid hygrometer in combination with the amphibole hygrometer. Three critical stages during the evolution of water within the plumbing system have been recognized, associated with H2O contents of 0·8–1·7 wt %, 2·1–2·8 wt % and 3·2–4·6 wt %, respectively. The first enrichment of water in the magmas can be attributed to the separation and transfer of evolved melts from the deep magma reservoir to the shallow crust. Continuous cooling and solidification of the shallow magma body gave rise to the second enrichment of H2O in residual melts, leading to magmas that were fertile for the formation of ore deposits. The detailed chemical evolution of the magma plumbing system was investigated using mineral trace element compositions in combination with the partition coefficients predicted by the lattice strain model. The earliest equilibrium melts are characterized by high Sr contents (the average = 658 ± 64 ppm), suggesting that high Sr/Y signatures were likely derived from their magma sources or fractionation at deeper levels in initially water-poor environments. Variations of some particular geochemical fingerprints in equilibrium melts such as, Dy/Dy* and Eu/Eu*, also provide fundamental information on the evolution of the magma plumbing system. Our study confirms the critical role of a deep crustal magma reservoir on the formation of magmatic-hydrothermal ore deposits. The fertility of magmas with respect to ore deposit formation was enhanced by the extraction and transfer of evolved magmas from the deep reservoir to shallower levels, particularly due to the enrichment of magmatic water contents. In addition, the presence of a deep magma reservoir also sustains the incremental growth of shallow magma chambers, which provide ore-forming fluids.

U-Pb dating of skarn garnets from Bulgarian deposits

<p>Calcic garnets from grossular-andradite (grandite) series have proven their ability to record the conditions and timing of their formation processes. Typically these minerals occur in skarn systems, together with other calc-silicates (diopside, epidote) and commonly host economic Cu, Zn-Pb-Ag, Au, Sn, W or Mo mineralization. Based on the U-content in the garnet structure, we used in-situ LA-ICP-MS U-Pb geochronology to determine the age record in more than 15 skarn deposits from different tectonic zones in Bulgaria. The data is partly complemented with ID-TIMS dating. The mineralogical, geochemical and petrological characteristics of the materials were described additionally. Both contact and infiltration skarns were studied.</p><p>The obtained data revealed that the garnet composition in terms of major elements does not affect the precision of age determination. Both andradite and grossular members yield age data with very high accuracy. The dating results, however, depend on the geochemical signature of the garnets and especially on the U-content and U/Pb ratio. Our data shows that skarn samples from the vicinities of magmatic bodies or along contacts of causative pegmatite veins usually have increased U-incorporation from several to more than 70 ppm, as suggested by their proximal position to the source. The contact skarn garnets formed by intrusion of silicate melts (or pegmatites) onto carbonate-rich hosts mostly produce precise ages, which are in good agreement with the geochronological zircon data about the magmatism in the studied regions (e.g. Central Pirin, Teshevo, Plana, Gutsal, Rila-West Rhodope, Sv. Nikola etc. plutons). The infiltration skarns, though, generally reveal ages with low accuracy and significant errors, mainly due to U-content below 1 ppm. The reason for the low U-concentration and U/Pb ratio is either connected with a primary U-deficit and its depletion in the garnet-precipitating fluids with time and space but might be also related to garnet retrograde hydrothermal alteration.</p><p>The time span of the Bulgarian skarn garnets is closely connected with the causative magmatic bodies. The studied skarns reveal Paleogene (~30-42 Ma - Central Pirin and Teshevo plutons and pegmatites from Rila-West Rhodope batholith; Djurkovo, Murzian and Zvezdel Pb-Zn deposits; ~ 58 Ma - skarns from Western Rila Mts., ~ 68 Ma – Babyak Mo-Ag-Au-W-Bi-Cu-Pb-Zn deposit), Cretaceous (~ 76 Ma- Gutsal pluton, 81 Ma - scheelite bearing skarns from the Plana pluton, 86 Ma – Iglika skarn deposit) and Paleozoic (~ 303 Ma – Martinovo Fe-skarn deposit) ages. Given the occurrence of Ca-garnet in contact rocks and hydrothermal ore deposits, our results highlight the potential of grandite as a powerful U-Pb geochronometer for dating magmatism and skarn-related mineralizations.</p><p><em>Acknowledgements.</em> The study is partly supported by the DNTS 02/15 bilateral project between Bulgaria and the Russian Federation, financed by the Bulgarian National Science Fund.</p>

Tracing metal sources for the giant McArthur River Zn-Pb deposit (Australia) using lead isotopes

Giant hydrothermal ore deposits form where fluids carrying massive amounts of metals scavenged from source rocks or magmas encounter conditions favorable for their localized deposition. However, in most cases, the ultimate origin of metals remains highly disputed. Here, we show for the first time that two metal sources have provided, in comparable amounts, the 8 Mt of lead of the giant McArthur River zinc-lead deposit (McArthur Basin, Northern Territory, Australia). By using high-resolution secondary ion mass spectrometry (SIMS) analysis of lead isotopes in galena, we demonstrate that the two metal sources were repeatedly involved in the metal deposition in the different ore lenses ca. 1640 Ma. Modeling of lead isotope fractionation between mantle and crustal reservoirs implicates felsic rocks of the crystalline basement and the derived sedimentary rocks in the basin as the main lead sources that were leached by the ore-forming fluids. This sheds light on the crucial importance of metal tracing as a prerequisite to constrain large-scale ore-forming systems, and calls for a paradigm shift in the way hydrothermal systems form giant ore deposits: leaching of metals from several sources may be key in accounting for their huge metal tonnage.