Character and Evolution of Ore Mineralisation in the Te-Rich Enåsen Au-Cu Deposit, Central Sweden

Abstract: The Enåsen gold deposit is located in Gävleborg county in central Sweden. Mining operations at Enåsen took place from 1984 to 1991 with Au as the main target for exploitation. The deposit has been interpreted as a metamorphosed Palaeoproterozoic analogue to near-recent epithermal Au mineralisations of a high sulphidation type. Its present mineralogy, textural-structural features, and morphology have been suggested to be the result of a combination of later deformation and regional Svecokarelian metamorphism at upper amphibolite to granulite facies conditions of the original epithermal deposit and it’s hydrothermally altered host rock. The main ore body now consists of a mineralised sillimanite quartzite gneiss. The aim of the project was to characterise the ore mineralogy, petrography and its paragenesis, evaluate the potential of Enåsen in terms of critical metals, and to test a hypothesis of partial ore melting.Among the most frequent ore minerals in the deposit are pyrite, chalcopyrite, pyrrhotite, bornite and tetrahedrite-tennantite, with variable but less abundant sulphides including covellite, digenite, mawsonite, stannite, arsenopyrite, cobaltite, galena, marcasite, sphalerite and pentlandite. Additionally, native gold, Se-bearing tellurobismuthite, hessite, tsumoite, pilsenite, rickardite, vulcanite, altaite, molybdenite, frohbergite, montbrayite, tellurantimony, löllingite and tellurbismuthantimony. While not an ore mineral here, rutile occurs abundantly. The ore mineral assemblages have seemingly at least partially melted. This is evidenced by failed quenching textures in the form of abundant multi-scale symplectites, potential sharp dihedral angles, localised concentrations of low melting point chalcophile elements (LMCE) + Au and Ag and arrays of multiphase sulphide/sulphosalt ± gold inclusions, as well as available mineral stability data considering that the ore assemblages have been subjected to upper amphibolite/granulite facies conditions followed by ductile and brittle deformation stages. Some ore mineral relationships have been described. Further studies would be required for a full paragenesis. The potential of Enåsen type deposits in terms of critical or near critical metals and semi-metals is likely to be as biproduct extraction in a mining operation aimed at gold. The most relevant element is likely to be bismuth, followed by tellurium and antimony.