The article delves into the exploration potential of hydrothermal alteration zones surrounding stratiform Pb-Zn-Ag-Ba deposits in the Graz Paleozoic of Austria. It presents comprehensive geochemical and mineralogical investigations of these zones, highlighting the presence of Fe-rich carbonates, chlorite, and REE- and HFSE-bearing minerals. The study aims to use these findings to develop a new exploration model, which could significantly enhance the efficiency of mineral exploration in similar deposits. The research focuses on the Schönberg Formation, known for its rich mineralization, and includes detailed analyses of ore horizons and their associated alteration zones. The article offers valuable insights into the mineralogical composition and geochemical signatures of these zones, which could serve as proxies for ore deposits. The ongoing investigations aim to refine these proxies and integrate them into a revised exploration model, promising advancements in the understanding and exploration of SEDEX deposits.
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Abstract
In the Graz Paleozoic in the Eastern Alps (Austria), stratiform Pb-Zn-Ag-Ba deposits have been mined in the past. They are classified as sedimentary-exhalative (SEDEX) deposits and are hosted by polyphase deformed greenschist facies metasediments and volcanics of the Schönberg Formation (Schöckel nappe; Drauzug-Gurktal nappe system), which is upper Silurian to Lower Devonian in age. The stratiform ore horizons are dominated by galena, sphalerite, barite, pyrite, pyrrhotite, and magnetite and are accompanied by chalcopyrite, arsenopyrite, fahlore, pyrargyrite, tetradymite, cobaltite, ullmannite, breithauptite, electrum, and others. The project MRI_SEDEXPOT investigates these deposits to evaluate their exploration potential by focusing on hydrothermal alteration zones occurring in the footwall and hanging wall of the ore. Such alteration zones are globally used as exploration tools not only for SEDEX deposits. The investigated alteration zones show distinct mineralogical and geochemical characteristics. They contain Fe rich carbonates and chlorite, K‑ and Ba-feldspars, Ba bearing white mica, fluorapatite, REE and HFSE minerals. Disseminated sulphides are widespread, and albitization is typical below sulphide rich horizons. Geochemical profiles correlate well with the observed mineralogy. Ongoing investigations of these alteration zones should lead into defining proximity indicators to the ore and evaluation of the exploration potential of the stratiform Pb-Zn-Ba-Ag deposits of the Graz Paleozoic.
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1 Introduction
Sedimentary-exhalative (= SEDEX) deposits include some of the largest base metal occurrences on the global scale and contain more than 50% of the world’s resources for Zn and Pb [1]. They are further important for Cu, Ag, other minor elements (e.g. In, Ge, Ga, Cd and Ni), and especially critical raw materials like barite, Co, Sb [2]. Important deposits of this type are found in Australia (Broken Hill, Mount Isa), Canada (Sullivan), USA (Red Dog/Alaska), India (Rampura Agucha), South Africa (Gamsberg), and Germany (Rammelsberg, Meggen) with resources of up to several 100 Mt [1, 3]. SEDEX ores are hosted by marine sedimentary rocks, in especially carbonaceous shale, siltstone, and black shale [4] and are sometimes associated with volcanic rocks. A typical feature that is broadly used for the exploration of SEDEX deposits is a zone of hydrothermal alteration (typically silicification, carbonatization, chloritization) around the ore bodies [5‐7].
In the course of the MRI-project (“Initiative GBA-Forschungspartnerschaften Mineralrohstoffe”) MRI_SEDEXPOT, the SEDEX deposits of the Graz Paleozoic in Styria/Austria (Fig. 1) are investigated with respect to their exploration potential. The focus is on the hydrothermal alteration zones, which occur in the footwall and hanging wall of the ore horizons. Geochemical and mineralogical investigations are performed to characterize the alteration zones and to get information on the alteration processes during deposition and later metamorphic overprints. The data on the spatial expansion and composition of hydrothermal alteration zones will be used to calibrate a new exploration model for SEDEX deposits. Finally, they should be used as exploration parameters and lead into the establishment of a vector to the ore.
Fig. 1
Tectonic map of the Graz Paleozoic in Styria with the locations of the stratiform Pb-Zn-Ag-Ba deposits. Map according to ADB 500 of GeoSphere Austria 1.3.2023
The SEDEX deposits of the Graz Paleozoic were mined for about 680 years until 1927. The main mining sites were Arzberg, Haufenreith, and Kaltenberg-Burgstall east of the Mur river and Deutschfeistritz, Rabenstein, and Großstübing west of it, covering an area of about 35 × 20 km. During this study, ore horizons and accompanied alteration zones from the underground mines Kaltenberg-Burgstall, Haufenreith, Arzberg, Guggenbach, Peggau-Taschen, and Arzwaldgraben were investigated with petrographical, mineral chemical, and geochemical methods.
2 Geological Setting and Stratiform Ore
The Graz Paleozoic nappe stack is part of the Drauzug-Gurktal nappe system of the Austroalpine Unit in the Eastern Alps. According to the literature, the Graz Paleozoic is divided into a lower, an intermediate and an upper nappe group [8, 9]; Fig. 1) and in several lithostratigraphic units [10, 11] respectively. The stratiform Pb-Zn-Ag-Ba deposits, which are classified as sedimentary-exhalative (SEDEX) deposits, occur exclusively in the upper Silurian to Lower Devonian Schönberg Formation of the Schöckel nappe, which consists of polyphase deformed greenschist facies metasediments, metavolcanics, and metavolcanoclastics, mostly represented by black shale, carbonaceous rocks, and greenschist [12]. Greenschist facies metamorphic overprints occurred during Permian and Cretaceous times [13]. In several mining sites, more than one ore horizon occurs, but their regional correlation is questionable. In general, they are dominated by galena, sphalerite, barite, pyrite, pyrrhotite, magnetite, ilmenite, and rutile and are accompanied by chalcopyrite, arsenopyrite, fahlore, pyrargyrite, tetradymite, cobaltite, ullmannite, breithauptite, electrum, and others [12, 14].
3 Results
In the following description, the country rocks of the stratiform Pb-Zn-Ag-Ba deposits are referred to as host rock. Therein highly mineralized stratiform horizons of sulphide or barite ore occur. In the surrounding of the ore, altered host rock with disseminated sulphide minerals form an alteration zone. The investigations focus on the alteration zones in the vicinity of the ore horizons. Investigated samples include hand specimens and several drill cores from different localities. Whole rock geochemistry has been applied on 25 samples from a section of the H1 (Haufenreith) drill core, which was drilled during an exploration program in the 1970–80s [12].
3.1 Petrography
The investigated sulphide and barite ore horizons reach thicknesses of several cm up to about a few metres (Fig. 2). In some areas, both, the sulphide and barite ore, are found. For example, in the locality Arzberg, an upper ore horizon is dominated by sulphide minerals (Fig. 2a, b), whereas the lower one is rich in barite (Fig. 2c). The following descriptions only cover the sulphide ore, which has varying contents of different sulphides: Fe-rich sulphides as pyrite or pyrrhotite predominate, with galena, sphalerite, magnetite, titanite, ilmenite, and rutile as common constituents (Fig. 3). Additionally, chalcopyrite, cobaltite, arsenopyrite, gersdorffite, ullmannite, and other rare Ni sulphides are present as minor sulphide phases. The contact to the host rocks is not sharp but diffuse. In distances of up to a few tens of metres to the sulphide ore, the host rock is diffusively altered and sulphides occur in an alteration zone as either layered, disseminated, or within concordant vein structures. The extent of these zones is hardly visible in the field, but detectable by petrographical and geochemical methods. Apart from the sulphides, the alteration zones consist mainly of carbonate minerals, chlorite, mica, quartz, and feldspars. Minor constitutes are rare earth element (REE) and high field strength element (HFSE) rich minerals.
Fig. 2
a Typical sulphide ore in the abandoned mine Arzberg. b Folded sulphide ore in the abandoned mine Arzberg. c Barite ore in the abandoned mine Arzberg, width of image is 0.5 m. d Two sulphide ore horizons in an abandoned mine of Kaltenberg-Burgstall
a Sulphide ore hand specimen showing galena (Ga) and pyrite (Py), associated with ilmenite (Ilm) and rutile (Rt) (Arzberg mine). b Sulphide ore hand specimen showing galena (Ga) and pyrite (Py), associated with magnetite (Mag) (Arzberg mine). c Optical micrographs of a sulphide ore sample showing galena and pyrite under reflected light, plane polarizers and d transmitted light, plane polarizers. Ga galena, Py pyrite, Ilm lmenite, Rt rutile, Mag magnetite
In this section, the mineralogy of the altered host rock is described (Fig. 4). Carbonates are enriched in Fe in the vicinity of sulphide minerals. The corresponding minerals are siderite with up to 48 wt% Fe or from the dolomite-ankerite series with concentrations ranging between 4–25 wt% Fe and 2–11 wt% Mg. Furthermore, siderite and ankerite are enriched in Mn compared to calcite and dolomite, reaching 8.5 wt% Mn. Chlorite has elevated FeO concentrations (up to 44 wt% FeO) within the alteration zone, compared to host rocks (up to 35 wt%). Up to 7.5 wt% BaO are found in white mica. Additionally, Na‑, K‑, and Ba-feldspars are bound to the altered host rock. Albite is the most abundant, but orthoclase, celsian as well as the Ba‑K feldspar hyalophane are present in concordant layers within the mineralization.
Fig. 4
BSE images showing a galena filling cracks of pyrite in the sulphide ore horizon (sample of G7 Guggenbach drill core); b alteration mineralogy in the direct vicinity to the sulphide ore horizon (sample of H1 Haufenreith drill core); c concordant Ba-feldspar layer adjacent to a sulphide ore horizon (sample of H1 Haufenreith drill core); d typical mineralogy in the alteration zone of Kaltenberg-Burgstall. Ab albite, Ank ankerite, Ccp chalcopyrite, Chl chlorite, Cls celsian, Eux-(Y) euxenite-(Y), F‑Ap fluorapatite, Ga galena, Hya hyalophane, Mnz monazite, Or orthoclase, Ph phengite, Po pyrrhotite, Qtz ; quartz, Rt rutile, Sph sphalerite
Disseminated REE minerals are observed throughout the alteration zone mainly as monazite and fluorocarbonates, which both are enriched in Ce. Xenotime and rare allanite-(Ce) are also present. The microprobe analysis shows that monazite contains a median of 1.01 wt% ThO2 and 0.03 wt% UO2, whereas PbO is below the detection limit, hence inhibiting age dating. Within the alteration zone, minerals containing high field strength elements (HFSE) encompassing Ti, Zr, Nb, Th, and U are commonly observed. (Urano‑)Thorite is the most frequent Th/U phase, but experienced Pb loss thus preventing age dating. Additional rare Th/U minerals are uraninite and brannerite. Furthermore, rutile and ilmenite as well as zircon are common constituents. In the calcite-rich metatuffite and carbonate phyllite in the footwall of the Pb-Zn-Ag sulphide ore, titanite and ilmenite are present, whereas, in the ore horizon and the ankerite-rich carbonate phyllite in the hanging wall, only rutile occurs. The REE and HFSE bearing phases occur predominantly interspersed in mica-dominated mm-scale layers, but rarely thin monazite veins and rutile occurring as chain-like successions are observed. Both are associated with fluorapatite. Fluorapatite is either dispersed mainly in mica-rich layers or forms layer-like aggregations. Niobium-rich minerals are exclusively found in the Kaltenberg-Burgstall district, where columbite-(Fe) and euxenite-(Y) are observed in marble adjacent to an ore horizon.
3.3 Geochemistry
The geochemical data are visualized in Fig. 5. The Pb-Zn-Ag sulphide ore is obvious by a distinct positive anomaly of Zn and Pb concentrations within the carbonate phyllite host rock. Total sulphur corresponds to this Zn and Pb peak but has several additional peaks. The Na2O distribution has two main zones of elevated concentration, which are either below the Zn and Pb anomaly or between two total sulphur peaks. High concentrations of P2O5, TiO2, MnO, and Ce correlate with Zn, Pb, or total sulphur, whereas Ce shows higher concentrations above the Zn and Pb maximum. Most HFSE and REE are similarly distributed as Ce. Besides, Cu, Ni, Co, and Ba are enriched in the footwall of the ore horizon. TiO2 peaks in the Pb-Zn-Ag sulphide ore and displays slightly higher concentrations below than above the ore.
Fig. 5
Whole rock geochemical data profiles of the H1 drill core section, showing the most relevant geochemical signatures
In order to use the knowledge of mineralogy as a proximity indicator for mineralization, whole rock geochemical data are useful. The data show that the observed minerals and their modal quantity correlate well with the respective element distribution in the whole rock geochemical data.
The Pb and Zn peaks correlate with the Pb-Zn-Ag ore horizon of the H1 drill core. The same is true for total sulphur, whose additional peaks reflect Fe sulphide rich horizons. The elevated Na2O concentrations indicate albitization zones developed below the ore horizon and below the Fe sulphide rich horizons. Albitization occurs in carbonate phyllite and metatuffite equally. Its intensity does not seem to be affected by lithological changes. The increase of MnO in the proximity of the ore horizon and high total sulphur concentrations respectively can be attributed to increasing Mn concentrations in carbonate minerals. The TiO2 peak in the Pb-Zn-Ag sulphide ore corresponds to an enrichment of Ti phases in the ore, compared to the surrounding rocks. Ce, representing monazite, shows a distinct peak in the ore horizon and another one above, which is correlated with elevated TiO2 and P2O5. Below the ore horizon monazite is absent, corresponding to lower Ce concentrations. Furthermore, P2O5, representing fluorapatite, shows peaks at the ore horizon together with Ce. Generally, it is higher in the hanging wall compared to the footwall.
To explain the observed distribution of mineral associations and element zonations with processes related to ore deposition and later re-crystallisation/re-mobilisation during metamorphic overprints is the challenge of ongoing investigations. Therefore, additional investigations and analyses, also for other occurrences in the Graz Paleozoic, are in progress. They hopefully result in the definition of suitable proxies as vectors to the ore. The concluding step will be the implementation of these proxies in an exploration model and a revised understanding of the genesis of the investigated stratiform Pb-Zn-Ag-Ba deposits. In a continuing project, the exploration model will be used and adapted for the relatively similar deposit Meiselding in Carinthia (Austria).
5 Conclusion
The stratiform Pb-Zn-Ag-Ba deposits of the Graz Paleozoic are restricted to the upper Silurian to Lower Devonian Schönberg Formation of the Schöckel nappe. The ore horizons show a distinct mineralogical composition with variable modal contents of galena, sphalerite, barite, pyrite, pyrrhotite, magnetite, chalcopyrite, arsenopyrite, fahlore, pyrargyrite, tetradymite, cobaltite, ullmannite, breithauptite, electrum, and others. In the footwall and hanging wall of the ore horizons, alteration zones occur. They are composed of Fe rich carbonates and chlorite, K‑ and Ba-feldspars, Ba bearing white mica, fluorapatite, REE and HFSE minerals with disseminated sulphides as in the ore horizons. The modal mineralogical composition and the mineral chemical data correlate well with whole rock geochemical data measured on the H1 drill core.
Acknowledgements
The authors are very grateful to the whole project team: Christian Auer, Christian Benold, Bernhard Grasemann, Bernhard Hubmann, Kurt Krenn, Duncan Large, Holger Paulick and Heinz Reitner. Leopold Weber is greatly thanked for providing his big support and expertise. The Federal Ministry for Education, Science and Research of the Republic of Austria is thanked for funding the project by the “Initiative GBA-Forschungspartnerschaften Mineralrohstoffe—MRI”.
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