2.2 Site characteristics 2.2.1 Geology The magnesite deposits in the vicinity of Hochfilzen are bound to Lower Paleozoic rock units, that were historically grouped into “Western Greywacke Zone” of The Eastern Alps. Recent interpretations place them into Staufen-Höllengebirge Nappe of Tirolic-Noric Nappe System, whereof the host rocks of the mineralization are within the Wildseeloder Complex, the Hochhörndler Complex and the Glemmtal Complex (Heinisch et al. 2015; Geosphere Austria 2024) At the mine site the Wildseeloder Complex is represented by massive dolomites in footwall position “Spielbergdolomit” and porphyritic rocks (originated from acidic volcanic ash) termed “Porphyroid”. Finely laminated, organic-rich black dolomites, Porphyroid and massive grey dolomites represent the Hochhörndler Complex. The hanging wall is formed by schists of “Wildschönauer Schiefer” of the Glemmtal Complex. Locally two layers of Porphyroid are known; especially the one directly at the contact with Spielbergdolomit is intensely sheared to a fault gauge. Its thickness reaches 60 m in uppermost outcrops, with depth thickness is diminishing, concomitantly the rock quality is deteriorating. It is the biggest structural element at Bürgl site. Spielbergdolomit and massive dolomites of the Hochhörndler Complex are hosting the metasomatic and thus shape-wise highly irregular magnesite lenses. They are of sparry type, generally dip concordantly with the host rock at an angle of 30 to 70° towards south. They reach a thickness up to 80 m and extend for hundreds of meters in strike and dip direction. At Bürgl, four magnesite ore bodies of significant volume are known. Three of the main lenses are in footwall position to the sheared Porphyroid layer, one is in hangingwall position, both with direct contacts. The ore bodies are beside primary and secondary dolomites, practically monomineralic, fine-grained (grain size below 5 mm) and of intense coloring (brown, grey, red, black). Rarely, magnesite shows relict bedding and normally behaves isotropic with a distinct little porosity and UCS up to 180 MPa. 2.2.2 Backfill Regulative framework does not exclude mining methods with caving or remaining open voids in particular; nevertheless, permitting process is beneficial in avoiding that. For that reason, the mining company has decided to exclude such methods from planning efforts. Huge dumps from the mining period starting 1957 until 1971 are remaining at Bürgl site. From economic as well as material properties it is the best medium available for backfill. As products have evolved over the years CaO cut-offs are less stringent nowadays, meaning magnesite that had to be dumped in the past is usable today. It seems feasible that in the coarse fraction magnesite could be sorted out using NIR technology and just the reject as well as fines used as backfill. This potential could mitigate some of the financial burden related to the backfill operation.
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