This is intended to serve as an introduction for the preparation of the geological mapping at Hochfilzen site for MultiMiner project. The areas to be mapped are defined and a brief geological overview is presented. The study area is placed in a modern regional geological framework and the rocks most relevant for the mapping work are presented.
As a base for a more in-depth familiarization, the explanations for sheet 122 Kitzbühl [@Heinisch2015] and especially the rich literature list there should be mentioned. A geological overview of the magnesite deposits near Hochfilzen is given by e.g. @Vavtar1976.
Magnesite mineralizations are currently known in the Spielberg dolomite group and in the dolomite-radiolarite complex (see below). The known deposits, such as Weißenstein and Bürgl, are located in the Hochhörndler complex [@Heinisch2015]. Regarding the magnesite deposits in the study area, it should be mentioned that there is a discussion about their age and their genesis going back to the 19th century with significant differences of opinion. The inferred age of mineralization ranges from middle Silurian over Variscan to Alpidic and the spectrum of genesis models include primary-sedimentary, synsedimentary to early diagenetic to metasomatic. Also, there are different ideas about the origin of magnesium intake [@tollman1977geologie].\
See also @sec-MgsGenetix on this topic.
## Location of the study area
Geographically, the study area is located about five to seven kilometers southwest of Hochfilzen -- see @fig-AOI_Overview. It is mostly located in the province of Tyrol and a small part is located in the province of Salzburg.
{#fig-AOI_Overview}
## Map sheets
The areas to be mapped concern sheets 122 Kitzbühl [@Heinisch2003] and 123 Zell am See [@Heinisch1995]. Explanatory notes [@Heinisch2015] are available for the former map sheet. The boundary of the two map sheets in the study area is the, here approximately north-south oriented, Fieberbrunner Ache. The majority of the areas to be investigated are located on sheet 122.\.
Mentionable "sheet edge faults" between these sheets are not known in the study area and are not of major importance here because no mapping area overlaps the sheets. Important lithologies have different numbers on the map sheets, but are named roughly the same.
## Questions
The basic concern of the mapping work is the magnesite mineralization. Accordingly, the focus is on geological parameters that can be assumed to be relevant as evidence for magnesite mineralization processes. Important geological features include contacts to the adjacent rocks, differences in grain sizes, faults, \dots\
It must be kept in mind that in the course of the MultiMiner project, the study area will be surveyed using spaceborne and drone-based Earth Observation (EO) methods, and the results of the mapping work will be linked to the results of these methods. In this respect, the field work should also take into account what a satellite or drone "sees" and thus, for example, not only nice outcrops should be examined, but also weathered rock.
At this point a few words about the practical field work and its preparation:
* Online, the regions and areas are shown with different map material at <https://ardigeos.geologie.ac.at/mumimap>.
* The mapping areas can be obtained as GeoPackage through the link <https://rhea.geologie.ac.at/index.php/s/rUqqImMlXdzGAUl>.
* The two map sheets can be obtained through the Tethys Research Data Repository, of Geosphere Austria, through the following links as PDFs and as GeoPackages:
* GK 123 Zell am See: <https://doi.tethys.at/10.24341/tethys.54>
* The Geological Maps of the Republic of Austria 1:50,000 can be integrated into GIS programs as raster layers via the following WMS/WMTS link: <https://gisgba.geologie.ac.at/arcgis/services/image/AT_GBA_GK50/ImageServer/WMSServer?request=GetCapabilities>
* WGS84, EPSG:4326, is to be used as the coordinate system for marking field points. This is the same coordinate system used by GPS devices.
* The positions of points recorded by GPS must be checked with a GIS program and satellite images (e.g. the orthophoto layer of basemap.at) and corrected if necessary prior to further use.
* Structural measurements are to be reported in (dipDirection, dip)-value pairs -- as they are provided by structure compasses after Clar -- and not in (strike, dip)-value pairs as it is still common in the Anglo-American area.
* The use of 10\% of hydrochloric acid to distinguish magnesite from dolomite has not proved successful. This can be deceptive, as aragonite may also be present in part. Macroscopic identification of carbonates is more appropriate.
* Sample labeling should not be done on the sample with a permanent or paint marker, rather the sample should be placed in a bag and the bag should be labeled. The reason is a possible spectral analysis on the sample in the laboratory and colors added by markers could make this analysis difficult or impossible.
* For field work, the period June-July seems to be ideal, because here mostly a stable weather situation (especially July) prevails and the days are long. In August, precipitation is traditionally expected. In September, however, favorable weather conditions could arise again, although the days are already shorter at this time.
# Regions and areas
The areas to be mapped are located in a roughly west-east oriented strip south of Fieberbrunn and Hochfilzen with the active open pit Weißenstein roughly in the middle -- see @fig-mappingAreasOverview and @fig-mappingAreasOverview_GK. For (preliminary) structuring, the study area is divided into three regions and associated areas. These are now briefly described here -- from east to west and with decreasing priority.
{#fig-mappingAreasOverview}
{#fig-mappingAreasOverview_GK}
## East Region
Lies to the east of the active Weißenstein open pit and is the only one of the regions on map sheet 123. It is bounded by the former Bürgl open pit, to the west, and Inschlagalpe underground mine, to the east. Two varieties of magnesite can be identified in this region: One fine-grained and one coarse-grained.
### Area 1
East of the former Bürgl open pit resp. the eastern side of the Spielberggraben. This area should be easily accessible: Access via Burgeralm and parking there and on. After consultation with the landowners, it may also be possible to use the alpine path to the Kleberkopf saddle.\
For parts of this area there is a detailed description and mapping by @Riedler2010.
### Area 2
The area around the former mine site Inschlagalpe. Here, a well exposed area to the west of the former mine site is of great interest. (South)East of this, the terrain is quite steep and forested -- however mapping information exists from when the mine was in operation. A relatively coarse-grained magnesite with zero porosity is found here.\
Access via the Schwarzleotal with parking at the show mine Leogang. Possibly, the private road behind the gate can be used after consultation with the landowner.
### Area 3
This is still vaguely defined. Here, laser scans should be consulted in order to identify potential outcrops. If none of them could be identified the southern border of the Spielberg dolomite should be followed.\
Access via the Schwarzleotal with parking possibility at the Leogang show mine. Possibly, the private road behind the barrier can be used after consultation with the landowner.
## Central Region
This region is on the eastern edge of map sheet 122. It is the area to the west and north around the Weißenstein open pit.
This is a hiking area including the Blumenweg track and some via ferrata near the ridge in the north. Of great interest here is an outcrop of black dolomite and especially its contacts with the neighbouring rock. Furthermore, this is an area where the Glemmtal complex and the Hochhörndler complex are exposed and therefore more metabasites could be found here.
This region is intended for model verification of spaceborne and drone-based Earth Observation (EO). The EO shall first provide indications of possible magnesite mineralization, which will subsequently be searched for.\
This region is characterized by the Pletzerbach and its tributaries, the Lengfilzenbach and Grubalm with Sulztalbach.
### Area 6
In this area Magnesite is known as rolled pieces in the bedload of the Pletzerbach creek. Their origin in the form of outcrops is not known and should be clarified if possible. In the north of the Pletzergraben mighty Spielberg dolomites are exposed. Especially their boundaries to the south are to be localized and worked on. The area is part of the Pletzergraben and should be mapped on both sides of the Pletzenbach.\
This area has the lowest priority and should be mapped depending on possibilities.
# Geological framework
On the map sheets mentioned above, the study area is located in the Wildseeloder unit $\subset$ Grauwackenzone $\subset$ Upper Austroalpine. Small-scale contacts with the Glemmtal unit -- also graywacke zone -- and the Northern Calcareous Alps $\subset$ Upper Austroalpine occur.\
This traditional nomenclature of geological units no longer corresponds to the current state of knowledge. According to modern tectonic nomenclature, the area under investigation is part of the Staufen-Höllengebirge nappe, which is assigned to the Tyrolian-Noric nappe system [@Schmid2004; @Heinisch2015; @Huet2019].\
An overview tectonic map of the Eastern Alps is given in @fig-TectUnitsofEasternAlps_afterSchmid and @fig-EAlpsTectUnits shows a schematic diagram of the major tectonic units of the Eastern Alps and the incorporation of the Tyrolian-Noric nappe system into these units. The palaeogeographic position of the Austroalpine from Cambrian to Devonian is outlined in @fig-PagGeo__AA__RockyAT.\
The lithostratigraphic division of Paleozoic rocks within the Staufen-Höllengebirge nappe [@Huet2019; @Huet2022] was based on the explanations of GK 122 Kitzbühl [@Heinisch2015]. Four complexes are distinguished, which correspond to Variscan tectonic units. From the footwall to the hanging wall, these are the following lithodemic units: Uttendorf complex, Glemmtal complex, Hochhörndler complex and Wildseeloder complex. These are unconformably overlain by Permomesozoic lithostratigraphic units.
The metamorphic history of the Staufen-Höllengebirge nappe is poorly studied. In general, it is assumed that there is an increase in the degree of metamorphism from north to south [e.g. @SchlaegelBlaut1990; @Heinisch2015; @Rantitsch2009]. In general maximum greenschist facies pressure-temperature conditions can be assumed for the study area: Approximately in the range of \SIrange{350}{400}{\celsius} and pressures greater than \SI{3}{\kilo\bar} and less than \SIrange{4.5}{8}{\kilo\bar} [@SchlaegelBlaut1990; and references therein].
In detail, however, it seems difficult to assign the deformations and metamorphism to the Variscan or the Eoalpid event [@Huet2019].\
Based on data from illite crystallinity, the degree of graphitization, the presence of chloritoid and the Conodont Alteration Index (CAI), metamorphism in the (lower) greenschist facies is assumed to have occurred during the (Cretaceous) Eoalpidian event. \isotope[40]{Ar}/\isotope[39]{Ar} ages, from white mica fine fractions, in the \SIrange{115}{95}{\Ma} range indicate cooling of the rocks of the Staufen-Höllengebirge nappe and their exhumation during this event [@Schuster_etal_2004; @Heinisch2015; and references therein]. This metamorphism was again weakly overprinted by advective heat transport and circulating fluids during the course of the Alpine orogeny, in the Oligocene to Miocene [@Rantitsch2009].
Variscan metamorphism is not documented in the study area; however, weak (lowermost greenschist facies) prealpine deformation and metamorphism is assumed [@Heinisch2015].
This is consistent with \isotope[40]{Ar}/\isotope[39]{Ar} dating [@Panwitz2006] of detrital muscovites indicating Neoproterozoic ages (in the range \SIrange{600}{800}{\Ma}). The closure temperature of muscovite in the \ce{Ar}/\ce{Ar} system, \SI{390 \pm 50}{\celsius} [@Schaen2020], has thus not been (significantly) exceeded since that time.
## Note {.unnumbered .unlisted}
Regarding the terms "Greywacke Zone" and "Northern Calcareous Alps", it should be mentioned that these historical terms date from a time before today's understanding of the tectonic nappe structure of the Alps [@Schuster2015]. One can probably understand these terms as geological units - in the sense that they refer to rocks characteristic for them. However, they are not tectonic or lithostratigraphic units in the strict sense [@Huet2019].\
The term graywacke zone describes a geographic unit [@Schuster2015] which represents an east-west oriented strip of Paleozoic rocks. This is several hundreds of kilometers long, extends roughly from Schwaz in Tyrol to Lower Austria at the margin of the Vienna Basin, and has a maximum width of about \SI{25}{\km} [@Heinisch2015]. Based on the spatial allocation, the Western and Eastern Grauwackenzone can be distinguished.\
Thus, the term graywacke zone - and the accompanying subgroups such as northern, southern, western and eastern - should be seen as an informal term without stratigraphic and tectonic implications.
![Tectonic map of the eastern Alps and the northern foreland, nomenclature after @Schmid2004, Modified after @Schuster2022. Numbers reference to tectonic units in @fig-EAlpsTectUnits.\
![Schematic diagram of the major tectonic units of the Eastern Alps, after @Schuster2022. Highlighted is the Tyrolian-Noric nappe system in which the mapping area is located.\
- Left: Degree of metamorphism during the Eoalpine (Cretaceous) and Alpine (Cenozoic) events and the time of peak metamorphism;\
- Middle: Major lithologic content of tectonic units, red numbers indicate time of emplacement in Alpine orogen wedge, numbers in colored boxes reference tectonic units on map in @fig-TectUnitsofEasternAlps_afterSchmid;\.
![Palaeogeographic position of the Austroalpine (red ellipse) from Cambrian to Devonian. Pieces of crust split off at the northern edge of Gondwana and drift north towards Laurentia or Baltica. From an initial position near the South Pole it drifts to a position near the equator with changed climatic conditions. From [@schuster2013rocky]](Figs/AA_PalGeoEvolution.pdf){#fig-PagGeo__AA__RockyAT}
# Rock types and lithological units {#sec-lithoContents}
Lithologically particularly relevant for the investigations here is the Wildseeloder complex, which is made up of Middle Ordovician metaignimbrite (Blasseneck porphyroid), Upper Devonian siliciclastics and various Silurian to Upper Devonian carbonate rocks (including the Spielberg dolomite group) [@Heinisch2015; @Huet2019].\
Rocks of the Glemmtal complex and the Hochhörndler complex can occur primarily on the southern edges of the areas to be examined. The Glemmtal complex comprises mostly Early Paleozoic to Lower Carboniferous mudstone, siltstone, sandstone and conglomerate with a turbiditic origin with subordinate inclusions of Ordovician and Devonian metabasites and also locally occurrences of Blasseneck porphyroid and carbonate rocks [@Huet2019]. The Hochhörndler complex consists of a siliciclastic matrix with isolated carbonate and magmatic elements from the Wildseeloder complex and the Glemmtal complex and it probably represents a sequence with olistoliths and/or a tectonic zone [@Huet2019]. Hence, rock types of the neighboring complexes are present in the Hochhörndler complex as reworked components.
In the map sheets, only the Wildseeloder and Glemmtal complexes are distinguished. These represent different facies with their specific lithologies. The Hochhörndler complex is implicitly to be understood as a "transition zone" -- in which the lithologies of the other two complexes are mixed.\
For practical fieldwork, the rocks can be assigned to the complexes as follows (oral comm. Benjamin Huet):
Wildseeloder Complex
: Thick carbonates and metaignimbrite, almost no basites, almost no pelites
Hochhörndler Complex
: Isolated, small-scale bodies of basites, carbonates, porphyroids in a matrix of siliciclastics such as slates, quartzites, \dots\
In the geological maps, the Hochhörndler Complex can be recognized by the fact that units of the Wildseeloder Complex lie next to units of the Glemmtal Complex without faults in between.
Glemmtal Complex
: Basites in carbonates and siliciclastics
Important lithologies are briefly described here and are summarized in stratigraphic divisions in @fig-lithoStrat_StaufenHoellen_Heinisch and @fig-LithStratModel_StaufenHoellen_Huet.
## Blasseneck porphyroid
In the study area, this porphyroid can be found, for example, at the Wildseeloder with a succession of up to \SI{600}{\m} thickness [@Heinisch2015]. It has been correlated with the Blasseneck porphyroid, named after the Styrian type locality, on the basis of lithological criteria and its stratigraphic location under the Llandovery [^Llandovery] limestones [@Hubmann2014].\
This porphyroid is interpreted as a subaerial pumice-rich deposit, resulting from a pyroclastic density current that was washed into shallow sea basins. The volcanism is considered to be \ce{SiO2}-rich, alkaline rhyolitic to rhyolitic. Hence, the chemistry suggests an extensional regime based on the alkaline rhyolitic character. Therefore, it could have been rift-related magmatism, which led to a partial melting of the continental crust due to an increased heat flow [@Heinisch1981; @Heinisch2015].\
More recent investigations which include U-Pb dating from zircons and studies on zircon systematics by @Blatt2013 show ages in the range of \SIrange{471}{461}{\Ma} and also suggest an interpretation of continental rifting. The paleogeographical position is assumed to be the northern edge of Gondwana.
[^Llandovery]: Llandovery: Lower Silurian stratigraphic series: rounded \SIrange{444}{433}{\Ma }; named after the town of Llandovery in Wales
<!-- The rock is now metamorphic and is thus addressed as metaignimbrite.-->
## Spielberg dolomite group
This group forms the host rock of some deposits such as siderite, baryte, fahlore and - here of particular importance - magnesite.\
These dolomites represent the most important summits in the study area and form a coherent mountain range from the Kitzbühlerhorn via the Wildseeloder, the Spielbergerhorn to the Inschlagalpe. The type locality of these dolomites, the Spielbergerhorn [@Hubmann2014], is one of the study area boundaries (Region 2, @fig-mappingAreasOverview and @fig-mappingAreasOverview_GK).\
This rock group is interpreted as a Silurian-Devonian carbonate platform [@Blatt2013] - which is now dolomitized. An age classification based on macrofossil remains, albeit sparse and weakly metamorphic, of corals, crinoids, and occasional conodonts indicates a Lower to Upper Devonian age [@Heinisch2015; and references there].\
The typical formation of a carbonate platform with reef complexes and a lagoon can be inferred from the rock and the fossil remains. The transition to the mainland can be seen from the inputs of clayey-sandy clastics, influences of which can be found in the upper part. The different facies manifest themselves in different forms in the dolomite rock. According to @Heinisch2015, a distinction can be made between:
"Massenfazies" -- massive dolomite
: Occasional coral relics indicate that this massive dolomite represents the reef facies of the carbonate platform
"Bankfazies" -- banked dolomite
: The transition from the "Massenfazies" to the "Bankfazies" is differse and thus the bank thickness varies from two meters (rarely) to decimeters (more often).\
This facies or this dolomite outweighs the other dolomites of this group in terms of frequency of occurrence. Due to the thickness and extent of these rocks, an extensive (Devonian) carbonate platform is assumed.\
Sedimentary structures are occasionally preserved as laminites seen as remnants of algal mats from the stillwater area of a lagoon facies. Furthermore, crinoid remains can occasionally be seen, which could represent a transition from reef debris facies to reef areas.
"Flaserdolomit-Fazies" -- red "Flaserdolomit"
: In the facies transition to the "Bankfazies", thinnly banked colored zones occur and layers of argillaceous slate up to centimeters thick can be observed. The red color of the "Flaserdolomit" can be seen as an aeolian input of desert dust.
"Dolomit-Sandstein-Folge"
: Banked dolomites alternate with quartz sandstone banks and clay slates. Quartz clasts have also been found within the dolomites. Together with the "Flaserdolomit", this sequence is interpreted as a terrigenous sedimentary input from an adjacent continent. In terms of facies, these two dolomites could represent the transition from the lagoon facies to the beach area.
Overall, the Spielberg dolomite group represents a shallow water facies close to the coast on the northern edge of Gondwana [e.g. @Blatt2013; @Heinisch2015; and references there]. A sketch of such a possible deposition space is given in @fig-rimmedCarbonatplattform platform.
![Sketch of a carbonate platform as the possible environment from which the Spielberg dolomite group emerged. Different areas or facies can be identified and dolomites from this group are assigned to them. From [@Boggs2009]](Figs/RimmedPlatform__Boggs__2009__Fig__9_39){width=90% #fig-rimmedCarbonatplattform}
### Südfazies {#sec-suedfaz}
A term that appears in the (older) literature is the so-called "Südfazies", which goes back to @Mavridis1969. He divides the carbonate rocks, in the area between Wörgl in the west and Bischofshofen in the east [@Haditsch1970], into two facies for the first time: the "northern facies" and the "southern facies". These two facies were describes as both lithologically and stratigraphically distinct and separated by a somewhat west-east oriented fault. @Mostler1970 also saw a "dolomite barrier" between the northern Spielberg dolomite and the Südfazies. The term Spielberg dolomite was introduced for the northern facies and its stratigraphic range was defined as Emsian to Eifelian (upper Lower Devonian to lower Middle Devonian). The southern facies has been defined as an association of distinct dolomites (stratigraphically ranging from footwall to hanging wall): black dolomite, light gray dolomite and red "Flaserdolomit", and light to dark gray coarse sparry dolomite. The stratigraphic range of these dolomites extends from the Younger Silurian (Ludlow) to the Upper Devonian [@Mavridis1969].\
In the classification of the current map sheets [@Heinisch1995; @Heinisch2003] and the explanations [@Heinisch2015] the term "Südfazies" is no longer in use. Rather, parts of the "southern facies" were included in the Spielberg dolomite group and the black dolomite mentioned was assigned to the Dolomit-Kieselschiefer complex. The term "Südfazies" is also not found in the current (revised) stratigraphic table [@Hubmann2014], but is mentioned there as a synonym for some lithologies.\
Consequently, the term "Südfazies" should also be avoided when working on the MultiMiner project. However, what can be relevant for the work here is the distinction of the different dolomites as reef and basin sediments (\ce{SiO2}-bearing dolomites, siliceous slate layers, \dots).
In association with the Spielberg dolomite, alternating layers of slate occur occasionally. In the hanging wall of the platform dolomite, a smooth transition into these very fine clastic sediments can be observed. Due to the macroflora residues contained in this slate, its age can be narrowed down to the Upper Devonian or younger [@Heinisch2015].
## Radiolarite
Associated with the banked and massive dolomite sequences of the Spielberg dolomite group, alternating sequences of black, banked dolomite, radiolarite (lydite), black slate and gray clay slate can occasionally occur. These occurrences can be assigned stratigraphically at the base of the carbonate platform, but also represent décollements and are found along faults or as small rock bodies [@Heinisch2015]. Chronostratigraphically, this alternation sequence is assumed to belong to the middle to younger Silurian -- middle Wenlock to lower Ludlow -- and its origin is considered to be a deep marine environment with euxinic conditions [@Hubmann2014].\
The black dolomite mentioned above should be particularly emphasized here, as it is the (main) carrier rock of the magnesite mineralization at the Weißenstein.
## Metabasites of the Glemmtal complex
The metabasites are marked in the maps as metabasalt, metatuff, metatuffite, and gabbroic and dioritic dykes. This suggests some diversity in terms of volcanic genesis. Compared to recent volcanic provinces, analogies to pillow and sheet lava stacks, different variants of basaltic pyroclastics, epiclastic rearrangements and subaquatic eruptions with different production rates can be identified. These volcanics are variously intruded by gabbroic rocks. Here, sills and vertical dykes as well as stock-like intrusions occur [@Heinisch2015].\
To sum it up, these metabasites can be interpreted as a product of multiphase basic volcanism with subvolcanic intrusives. Detailed geochemical investigations by @SchlaegelBlaut1990 were able to rule out connections between volcanism and active plate boundaries -- both oceanic ridges and subduction zones. Rather, they suggest a basic intraplate volcanism, which represents volcanic high zones (seamounts, island volcanoes) in a shallow marginal sea.\
In summary, these metabasites are interpreted by @Heinisch2015 as mostly shallow marine seamounts. Biostratigraphically, this event can be assigned to the Lower Devonian based on conodonts. Furthermore, the Glemmtal complex is described as basin facies.\
Geochronological dating of these rocks using the U/Pb and Sm/Nd methods yielded ages in the range of \SIrange{492}{454}{\Ma} [@Heinisch2015 and references there]. This covers a broad period from the late Cambrian to the middle Upper Ordovician and is in contradiction to the conodont stratigraphic classification given above. According to @Heinisch2015, this contradiction must be considered open.
**Note on geological vocabulary**\
Regarding the subvolcanic intrusives mentioned above, it should be noted that in the literature they are partly summarized as basaltic-gabbroid material and also as diabase (schist). The former is supposed to express the transitional character of the fabric of subvolcanics. Subvolcanites are intrusive rocks formed by cooling and crystallization below the surface of the earth -- but at shallower depths and usually with smaller volumes than typical plutonites. Their fine-grainedness is often not as pronounced as that of the volcanic rocks and they represent transition from the volcanic rocks to the plutonic rocks. These relatively coarse-grained basaltic rocks, often with an intergranular to ophitic texture [@vinx2015gesteinsbestimmung], are called dolerites or micrograbbros (grain size over three millimeters). The term diabase, which is often used synonymously, should be avoided due to its ambiguity [@LeMaitre2002].
![Lithostratigraphic table of the Staufen-Höllengebirge nappe, from [@Heinisch2015]. Left: Wildseeloder complex with dolomites of the Spielberg dolomite group and the Dolomit-Kieselschiefer complex; Right: Glemmtal complex with basites; Center: Hochhörndler complex; Blasseneck Porphyroid is found in all units. Variscan tectonics and a Permomesozoic covering are also shown](Figs/Heinisch__2015__Erlaeuterungen__LithoStrat_Fig_7_p45.pdf){#fig-lithoStrat_StaufenHoellen_Heinisch}
![Modern lithostratigraphic model of the Staufen-Höllengebirge nappe, modified after [@Huet2019; @Huet2022]; Green: proposed new denotations, for the units according to @Heinisch2015 (see @fig-lithoStrat_StaufenHoellen_Heinisch), following a modern lithostratigraphic resp. lithodemic nomenclature [@NACSN2005]. Detailed lithostratigraphy of each complex in @fig-litStrat_Wildseeloder-Cmplx, @fig-litStrat_Glemmtal-Cmplx, @fig-litStrat_Hochhoerndler-Cmplx](Figs/Huet_StaufenHoellengebirge_Nappe/Huet__StaufenHoellengebirge_Nappe__NewTerminology.pdf){#fig-LithStratModel_StaufenHoellen_Huet}
![Lithostratigraphy of the Wildseeloder complex, modified after [@Huet2019; @Huet2022]](Figs/Huet_StaufenHoellengebirge_Nappe/Huet__Wildseeloder_Cmplx.pdf){#fig-litStrat_Wildseeloder-Cmplx}
![Lithostratigraphy Glemmtal complex, modified after [@Huet2019; @Huet2022]](Figs/Huet_StaufenHoellengebirge_Nappe/Huet__Glemmtal_Cmplx.pdf){#fig-litStrat_Glemmtal-Cmplx}
![Lithostratigraphy of the Hochhörndler complex, modified after [@Huet2019; @Huet2022]](Figs/Huet_StaufenHoellengebirge_Nappe/Huet__Hochhoerdnler_Cmplx.pdf){#fig-litStrat_Hochhoerndler-Cmplx}
# Magnesite deposits in the Hochfilzen area, Eastern Alps: Genetic considerations {#sec-MgsGenetix}
Worldwide, most economic magnesite deposits are either of the cryptocrystalline Kraubath type or the sparry Veitsch type. Magnesite formed in sedimentary lacustrine environments only plays a subordinate role [@Pohl2020].
Cryptocrystalline magnesite of the Kraubath type (named after an occurrence in Styria, Austria) occurs as white and very fine-grained masses (bone magnesite) in veins, stockworks and massive bodies within (meta)ultramafic rocks. It is the product of low- to moderate-temperature reactions between \ce{Mg}-rich rocks and \ce{CO2}-rich aqueous fluids of hypogene or supergene origin [e.g. @Pohl1990]. Sparry magnesite of the Veitsch type (named after an abandoned mine in Styria, Austria) forms irregular to strata-bound bodies that are preferentially hosted in carbonate-dominated sequences. Deposits of this magnesite type are massive, commonly coarse-grained and often show a sparry to pinolitic texture. Marine clastic to carbonate shelf sediments are the most common precursor rocks which are transformed by \ce{Mg}-rich fluids in a process known as \ce{Mg}-metasomatism. Metasomatic replacement textures are widely documented in these deposits, but the timing, the nature of the fluids, the source of magnesium and the exact pressure, temperature and chemical (pTX) conditions of magnesite formation are a matter of debate since more than 150 years.\
Historically, two schools evolved that either propagated epigenetic (metasomatic-hydrothermal) or syngenetic (sedimentary to early diagenetic) models -- for a review see @Deelman2020.
Most magnesite deposits in the Eastern Alps are of the sparry Veitsch type and occur in carbonate-dominated Paleozoic formations within the tectonic highest Austroalpine geological, i.e., the Veitsch-Silbersberg nappe system, Tirolian-Noric nappe system (geographically “Greywacke Zone”), or the Graz Paleozoic nappe complex. The magnesite deposits and their country rocks commonly record low-grade to rarely medium grade pre-Alpine and Alpine regional metamorphism.
The main deposits currently mined by RHI Magnesita in Austria are located at Breitenau (Styria), in the Hochfilzen area (Tyrol) and at Radenthein (Carinthia). Smaller active mining operations of other companies are located at Oberdorf an der Laming and Hohentauern in Styria. The total annual Austrian magnesite production in 2022 was about \SI{844000}{\tonne} [@MayerJauck2023].
Magnesite deposits in the Hochfilzen area are situated in the Tirolian-Noric nappe system (geographically “Northern greywacke zone”) and include several active and abandoned mines at Weißenstein, Bürgl, and Inschlagalpe. These mines stretch along a ca. \SI{6}{\km} long corridor that is associated to the tectonically complex Hochhörndler complex (see @sec-lithoContents). The carbonate host rocks are mainly dolostones and limestones of Silurian to Early Devonian age. Previous studies distinguished a northern reef-dominated dolomitic facies (Early to Middle Devonian Spielberg Dolomite) and a southern facies ("Südfazies") of Silurian fossiliferous limestones to Early Devonian dolostones [@Mavridis1971] -- see also @sec-suedfaz. According to @Mostler1970, the magnesite deposits are restricted to the southern facies.\
It is to be noted that the magnesite deposits in the western “Greywacke Zone” formed in different lithologies and in carbonate sequences of different stratigraphic age [@Mostler1973]. Hence, they are not strictly stratiform but strata-bound, at best. The magnesite deposits have been affected by low-grade greenschist facies metamorphism as indicated by chloritoid, pyrophyllite and paragonite in the surrounding metapelites [@Morteani1989; @Morteani1990].
The spatial vicinity of the magnesite deposits in the western “Greywacke Zone” to tectonic nappe boundaries was pointed out in previous studies [@Mostler1973; @Morteani1989]. Variscan thrust planes were thought to be possible fluid pathways for the \ce{Mg}-rich mineralizing fluids in a metamorphic-metasomatic model. In this metamorphic-metasomatic model the replacement of dolomite by magnesite is controlled by inversion of the temperature gradient due to tectonic overthrusting and influx of \ce{Mg^2+}-rich fluids from the overthrust unit [@Morteani1989; @Morteani1990]. The rare earth element (REE) contents and patterns of magnesite compared to those of the meta-sedimentary carbonate host rocks support this interpretation [@Morteani1982].
Magnesite from the Weißenstein quarry is macrocrystalline but it lacks the cm-sized sparry magnesite texture that is typical for many other deposits. Banding is rarely preserved in finer grained magnesite. It has been interpreted as relict of an older sedimentary fabric whereby indicating an initial stage of sedimentary magnesite formation prior to subsequent metamorphic recrystallization [@Vavtar1976; @Schulz1989].
Magnesite was also reported from the Permian Gröden Formation (redbed sediments), which unconformably overlie the Early Paleozoic strata hosting the sparry magnesite deposits in Hochfilzen area. These Permian magnesites occur as nodules, in discrete layers within mudstones as well as intergranular cement [@Spoetl1994]. Magnesite in these redbed sediments formed diagenetically in a playa-lake system and the magnesium was derived from weathering of Devonian dolostones and associated magnesite deposits [@Spoetl1994]. Magnesite was also documented in the Permian basal breccia -- Brunnsink breccia -- [@Siegl1953; @Angel1955]. This breccia contains magnesite-bearing clasts, which were interpreted as sedimentary components and provided a key argument for supporting the pre-Permian (Variscan) age of the magnesite formation in the Alps [@Angel1955]. However, detailed textural observations indicate that magnesite formation in this breccia is in fact post-depositional [@Siegl1964; @Mostler1970].\
It remains an interesting question if and how the processes forming the sparry magnesite deposits in the Hochfilzen area and those in the Permian formations are genetically linked.
Some progress in our understanding of the fluids involved in the formation of sparry magnesite deposits comes from crush-leach analyses [@Prochaska2001]. \ce{Na}/\ce{Br} vs. \ce{Cl}/\ce{Br} plots generated by this method reveal that sparry magnesites plot at the end of the seawater evaporation trend indicating that the saline fluids (“bittern brines”) derived from strongly evaporated seawater [@Prochaska2016]. The few conventional fluid inclusion studies done on magnesite deposits in the Eastern Alps confirm that the mineralizing fluids are saline aqueous brines belonging to the \ce{H2O}-\ce{NaCl} system [@AzimZadeh2011]. The temperatures obtained from microthermometry seem to reflect the metamorphic grade of the deposits (i.e., the fluid inclusions have undergone at least partial metamorphic re-equilibration).
Establishing a correct genetic model for sparry magnesite deposits in the Eastern Alps strongly depends on availability of reliable age data. So far only a very limited number of age data is available. The most precise age was determined in the Breitenau deposit where magnesite formation was dated at \SI{229.3 \pm 2.4}{\Ma} with the \ce{Sm}-\ce{Nd} method [@HenjesKunst2014]. This Middle to Upper Triassic age invalidates models of orogenic (Variscan, Alpine) magnesite formation as repeatedly proposed in the historic metasomatic models. Instead, the formation of sparry magnesite deposits must be seen in the larger geodynamic context of lithospheric extension with enhanced crustal heat flow due to the break-up of Pangea and formation of the Neotethys.
It must be stressed that neither modern fluid inclusion studies nor age data are yet available for the Hochfilzen deposits. Both would be required to establish a sound genetic model that could be the basis of future exploration and help in interpretation of airborne data.
# Rock descriptions
To give an impression of the main rock types in the study area, the descriptions given by @Heinisch2015 where extracted and translated. Further detailed rock descriptions are given, for example, by @Vavtar1976 and @SchlaegelBlaut1990.\
## Blasseneck porphyroid
Lithology numbers (53 | 49) in sheet (122 | 123)
> "In the field, the light-colored, rather coarse-banked to massive protrude rocks can be mapped well. A pastel green to yellowish color is characteristic. The porphyry structure is perfectly preserved or also heavily overprinted depending on the degree of alteration and deformation. The macroscopically recognizable, mm-sized porphyroclasts are recognized in the thin section as quartz and alkali feldspar. Idiomorphic forms (high quartz), some with magmatic corrosion bays, are common. Alkaline feldspar generally shows perthite texture, with individual domains being sericitized to varying degrees. Plagioclase occurs only subordinately and is always strongly altered. The matrix is composed of a fine felt of quartz, albite, sericite and possibly chlorite. Idiomorphic zircons demonstrate the primarily magmatic character. [...]
>
> The variable content of xenolith fragments compared to phenocrystals can be used for a first field differentiation (of the lithotypes). Almost crystal-free, dense porphyroid layers, yellow-green in appearance, can also be distinguished. These, as well as thinner layers, are usually heavily foliated and have conspicuously shiny silvery discontinuities, some of which appear as soapy seperation surfaces.
>
> In the ideal case, however, the rocks crumble in a coarse-blocky and gneiss-like manner and then form detrital material for local moraines (Wildseeloder). They can be traced to the foothills of the alps as the main drift. [...]
>
> Especially on the Wildseeloder, volcanological details can still be recognized in the field today."
## Spielberg dolomite group
Lithology numbers (44-47 | 42-44) in sheet (122 | 123)
>"These are dolomite rocks of different characteristics in their facies with a thickness of up to 600 m. This creates a typical weathered dolomite landscape, which is initially rounded off by glaciers, tends to form debris and only supports sparse vegetation. The karstification that occurs throughout leads to corresponding dry vegetation. [...]
>
> The rocks are greyish when weathered, but mostly rust-brown in colour, and are occasionally covered with map lichen. Freshly broken, they appear pure white to pale pink, occasionally also light grey. Primary sedimentary structures are extremely rare due to diagenetic recrystallization and metamorphic overprinting.
>
> Mineralogically, it consits predominantly of dolomite with admixtures of ankerite or iron-rich dolomites. This causes the weathering color, which is often rusty brown. Calcite rarely occurs. The grain size ranges from \SI{0.1}{\mm} to \SI{1}{\mm}. The rocks can therefore be described as dolomite marble. Gritted decay and sanding are common secondary effects within this grain size."
>
> __Dolomit-Sandstein-Folge__ `#44`: "Well bedded dolomite alternates in layers with quartz sandstone beds and slates. Microscopic quartz clasts can also be found within the dolomite beds."
>
> __Banked Dolomite__ (`#45`): Shows "[...] bank thicknesses from \SI{2}{\m} to decimeter bank thickness. The latter case is the more common. [...] In favorable cases there are relict sedimentary structures preserved in the form of laminites [...]. In other cases, fragments of crinoid calyxes and crinoid stalk segments are found [...]".
>
> __Massive Dolomite__ (`#46`): This dolomite "[...] segregates in coarse blocky. Karst vents are common, as is striation. Irregular fracturing is common. Occasional relics of coral [...]" may occur.
>
> __Red Flaserdolomit, Flasermarmor with violet slate__ (`#47`): These "[...] stand out because of their intense play of colors (white-violet), their cm-thick banks and their mostly wavy, flaky structure. [... ] The content of clayey material varies greatly, there can even be cm-thick slate interlayers. [...] Occasionally, the violet slates are also secondarily bleached, giving the rock assemblage a green-grey colour."
**Note**: According to @Heinisch1995, the massive dolomite can be a carrier of magnesite mineralization -- e.g. deposits Bürgelkopf and Inschlagalpe -- and is therefore of particular importance.
## Slate combined with Spielberg dolomite
Lithology numbers (43 | 41) in sheet (122 | 123)
>"Gray argillaceous slate occasionally occurs in alternating layers with Spielberg dolomite (43) (to the west of "Platte" (mountain) in the direction of Pletzergraben, Malernalm near Kitzbühel). However, they also develop in the hangingwall with a sedimentary transition from the platform dolomites. A corresponding profile can be found on the Wildseeloder, to the south of the
Griessenbodenalm. Here it can be shown that these are the youngest preserved sedimentary sequences of the Wildseeloder unit below the Variscan angular unconformity."
## Dolomite-radiolarite complex
Lithology numbers (49 | 46) in sheet (122 | 123)
>"Alternating sequences of dolomites, radiolarite (lydite), black shale and gray slate, which are summarized as the Dolomit-Kieselschiefer complex (49). [...]
>They often function [...] as décollement and can be found along faults (e.g. Lämmerbichlalm) or in rock slices (e.g. Lachtalbach, Römerweg-Barmleiten, Brunnalm, Jufenkamm).\
>The common characteristic of the alternation is the black colour, due to the high proportion of organic carbon. Banked up to decimeters, dark, sugar-grained dolomite alternates in layers in the cm to dm range with lydites and black shales. Gray slates are also intercalated. In view of the perculiar colouration, the usually thin sequence forms a well mappable guiding horizon. Maximum thicknesses of up to 80 m are known. However, given the tendency for internal small folding, thrust tectonics and décollement, stratigraphically undisturbed profiles are very rare. This is due to the high contrast in consistency to the often neighboring banked and massive dolomite sequences."
### Metabasalt, massive or with a pillow structure
Lithology numbers (55 | 50) in sheet (122 | 123)
>"Metabasalts (55) can be found in the entire Glemmtal unit, with primary structures and fabrics (pillows, bubbles, glass edges) commonly preserved. Connected metamorphic lava sequences reach a maximum thickness of 300 m. [...]
>
>The pillows show sizes of maximum 1.5 x 1 m and minimum 0.3 x 0.1 m. A clear variation of the pillow size from the footwall to the hanging wall was not detectable in any of the sequences, however, multiple shifts between layered and pillow lavas can be observed. Due to the concave top, convex bottom and adaptation to the predefined morphology at the base of well-preserved pillows stratigraphically direction in some places can be shown. This agrees with the results from the biostratigraphically datable profiles. In other cases, the tectonic reshaping of the pillow structures made interpretation difficult. Occasionally relict glass fragments are preserved in the metabasalts. These appear dense, dark gray to black and are 1–3 cm across. The former glass is fully devitrified and transformed into a fine fringe of chlorite and ore pigment.
>
>Two types of metabasalt can be distinguished macroscopically. Both occur as metamorphic pillow lavas and layered lavas. Dark gray to dark green, irregularly breaking rocks are partly developed as "Diabasmandelstein" ("amygdaloidal diabase"); they have numerous filled bladder shaped cavities averaging around 0.3 cm in diameter. Light grey, very hard, splintery and sharp-edged types, on the other hand, occur in the lower Saalachtal (sheet 123 Zell am See). The latter show a lower proportion of bubbles.[...]\
>The thin section investigations show pyroxene and plagioclase as well as pseudomorphoses after clinopyroxene, orthopyroxene, olivine and plagioclase as phenocrystals. Plagioclase, hornblende, chlorite, epidote, sericite, calcite, leucoxene, stilpnomelane, clinozoisite and quartz can be observed in the matrix. The content of phenocrysts and their grain size vary greatly, with phenocryst-rich and aphanitic textures present. The phenocrysts are a maximum of 7 mm long. They are often completely replaced by secondary minerals."
### Metatuff (pyroclastic volcanic rock)
Lithology numbers (59 | 55) in sheet (122 | 123)
>"Vulcaniclastic rocks can occur both in association with the metabasalts and in layers in the metasediments. These are volcanic slate, which often still contain identifiable components. For example, the primarily pyroclastic nature of the components can be demonstrated, e.g. as lapilli or scoria. In other cases the volcanic fragments are clearly epiclastic.
>The lion's share of pyroclastics is found in the fine fraction (coarse ash to fine ash tuff). These occur today as dull green to blue-green appearing volcanic slates. They make up the main part of the outcrop area, for example at the end of the Glemmtal valley or along the Pass-Thurn-Straße south of Jochberg. As a rule, they show clear foliation; parts that occasionally appear more massive represent former coarse ash tuffs and are easily confused with layered lava flows. Microscopically, the mineral composition is poorly resolvable. It is a fine felt of chlorite, epidote, albite and occasionally calcite. Fine pigmentation by opaque minerals is the rule. Phenocrystal relics are almost always metamorphically altered."
>"Metamorphic gabbroid rocks (57) occur unconformably as dykes or stock-like masses and concordantly as sills throughout the Glemmtal unit (thicknesses often in the meter range, rarely more than 10 m up to a maximum thickness of about 100 m). In the front part of Glemmtal a close association of gabbroid sills and metamorphic basaltic lavas is characteristic [...]. In the remaining areas, metamorphic basalts and metagabbros are found less frequently together. Concordant and discordant dykes or stocky gabbroic intrusive rocks were mapped within the metamorphic volcaniclastic rocks or within the siliciclastic metasediments[...] Some dykes are only dm thick, in which case they can easily be mistaken for greywacke beds.
>
>[...] The macroscopic and microscopic appearance of the metamorphic gabbroid intrusives is very different. In outcrop, these rocks appear massive, dark green to grey. Grain size and structure vary from uniform (coarse, medium to fine-grained) to porphyry-grained with maximum 1.5 cm long pyroxene crystals. A wide range of variations in the content of pyroxene and feldspar is noticeable even in the hand sample. The microscopically determined mineral composition consists mainly of clinopyroxene, plagioclase, hornblende, epidote/clinozoisite, leukoxene and stilpnomelane. Alkali feldspar, apatite, titanite and zircon could be observed as accessories. The most common are subophitic texture (rocks with predominantly pyroxene) and intersertal texture (rocks with predominantly plagioclase, pyroxene then as gusset filling, [...])."
### Dioritic dykes
Lithology numbers (58 | 52) in sheet (122 | 123)
>"The term dioritic dykes (58) summarizes variants with a tendency towards intermediate chemistry. These metadiorites are especially significant as sills. [...]
>
>In hand sample, the rocks appear massive, mostly fine-grained to dense. The rock color shows a medium grey. Occasionally, small feldspar-slats are identified on the fresh fractured rock surface. As a result, they are usually confused with greywacke beds. However, the leather-brown weathering color and occasional pyrite leads to a correct assignment in the field, which often had to be confirmed by thin sections.
>
>The microscopically determined mineral composition consists mainly of plagioclase, hornblende, sericite, chlorite, pyroxene and epidote. Quartz and opaque minerals are subordinate in appearance. Apatite and zircon can be observed on accessories. The main components of the rock are hypidiomorphic, twinned, zonar plagioclase strips (about 1 mm long). They form a bulky irregular intersertal or intergranular structure. The interstices are filled with opaque minerals, sericite and chlorite; in addition, small hypidiomorphic hornblendes and isolated pyroxenes occur. Quartz is also part of the gusset fillings. It is not clear to decide whether it is primarily igneous quartz or metamorphic recrystallisation.
>
>Due to the proximity to gabbros, the rocks are interpreted as product of differentiation."
## Magnesite
Magnesite is not shown as a distinctive rock unit on the geological maps. Due to its major importance for this mapping project, the description of the magnesite occurring in the investigation area is taken from [@Vavtar1976]:
>"Magnesite occurs in a wide variety of color variations. There are all possible transitions from [...] red-colored to orange to yellow to dark brown and gray-colored magnesite. Three more distinctive types can be distinguished:
>
> 1. Yellow to grey-yellow magnesite with pyrite, goethite and lepidocrocite.\
> 2. Red [...] magnesite with hematite and minor pyrite.\
> 3. Gray to black magnesite with pyrite. [...]
>
> The magnesites differ not only in color but also in hardness. Bulky, relatively hard as well as porous, softer types coexist.\
>Various factors are responsible for the different appearance of the magnesite:
>
>_Colouring pigment_: Sericite layers rich in hematite or the finest hematite-scales color the magnesite red. Locally, the hematite can be enriched to nodules parallel to the lamination. Pyrite-rich layers of sericite cause the magnesite to have a dark gray color. [...]
>
>_Weathering_: The magnesite is colored brown by weathering of the pyrite via lepidocrocite to goethite.
>
>_Recent magnesium mobilization_: Yellow magnesite can often be observed along joints, which partly extends into the bedding from such fine cracks and gives the magnesite a "cloudy" appearance. This is a more recent magnesium mobilization since the layered change in grain size that is otherwise often observed is absent and these magnesites are completely pigment-free even where they penetrate pigmented magnesite or extend parallel into layers.
>
>_Grain size change_: The rhythmic change in grain size can also cause grey-yellow layering of the magnesite. [...] In the rhythmic change in grain size, however, no polar or geopetal sructure in the form of vertical grain sorting can be determined."
*Note*: According to the current state of knowledge, the black coloring is due to a high proportion of organic carbon.
