Das PGE-führende Ni-Cu-Co Vorkommen im Haidbachgraben bei Mittersill

Research output: ThesisMaster's ThesisResearch

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Abstract

The Haidbach valley south of Mittersill is host of a small Cu-Ni orebody in chlorite-epidote schist and chlorite-epidote-albite gneiss. It is located in the Habach Group, which forms part of the Subpennine nappe system that is exposed in the Tauern Window. The group comprises a complex metamorphosed sequence of Pre-Permian magmatic and sedimentary rocks. Since the 17th century the deposit had been intermittently explored until 1939. The estimated reserves are 10.000 to 15.000 t including 150 t of each Ni and Cu. Anomalous concentrations of platinum-group elements (PGE) initiated a detailed microscopical and geochemical investigation of the underground mine. Sulfide mineralization occurs as layers or lenses oriented parallel to the foliation, and as disseminations. Major ore minerals comprise pyrite, pyrrhotite, chalcopyrite and pentlandite. Massive ore carries between 0.3-2.0 wt.% Ni, 0.2 and 5.4 wt.% Cu and 200 – 3000 ppm Co. Precious metal concentrations reach up to 2.5 ppm Pd, 1 ppm Pt, 23 ppm Ag, 0.3 ppm Re and 0.4 ppm Au. In addition some of the minor metals reveal elevated concentrations, most notably As, Se and Te. Elevated concentrations of Cr are noteworthy, reflected by the chemistry of chlorite and the occasional presence of chromian spinel. Reflected light and electron microscopy confirm the presence of a large number of rare-metal-bearing minerals that usually occur included in the major sulphides. They comprise Ni-Fe-Co sulfarsenides (gersdorffite, cobaltite, arsenopyrite), Pd-melonite (Ni,Pd)(Te,Bi)2, merenskyite Pd(Te,Bi)2, sperrylite PtAs2, hexastibiopanickelite (Pd,Ni)(Sb,Te), sudburyite Pd(Sb,Te), testibiopalladite PdSb(Sb,Te), hessite Ag2Te, empressite AgTe, kotulskite PdTe, molybdenite, sphalerite, irarsite IrAsS, rheniite ReS2, Re-Pb-sulfide, and electrum AuAg. These minerals reach up to 80 µm (Ni-Co-Fe sulfarsenides) or 30 µm (platinum-group minerals in their longest dimension) in grain size. The abundance of discrete euhedral small grains of Re sulfides included in Fe-Cu sulfides is highly unusual. These minerals are not associated with any of the PGE-bearing phases. The Haidbach mineralization is interpreted as a stratiform sulphide accumulation in basic to intermediate magmatic rocks having geochemical affinities to MORB-type basalt. The host rock has a similar chemistry with an increased concentration of Na2O+K2O. Comparison with literature data shows that the mineralogy of the PGE-mineralization is quite similar to other well known Ni-Cu-sulphide deposits such as Noril'sk, Russia, or the Lusatian Block, Germany. The analyses lead to the theory that the rocks in the project area represent a magmatic fractionation sequence. The ore is depleted in alkalis due to alteration processes. The genesis within an ophiolite complex or in a back-arc area is possible. Primary textures are masked by strong hydrothermal and metamorphic overprint, as well as polyphase deformation. The mineralization has a magmatic origin in the sense that a sulfide melt was segregated from a silicate magma because of S-saturation and the density difference. This is proven by melt droplets, which form via a MSS/ISS intermediary state from a sulfide melt. Hydrothermal alteration leads to a wider distribution of minerals, which is why some occur within the silicate zones.

Details

Translated title of the contributionThe PGE-bearing Ni-Cu-Co mineralization in the Haidbachgraben near Mittersill
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
Award date1 Jul 2016
Publication statusPublished - 2016