Trace element content variation within sulfides of the Fäboliden gold deposit

The Fäboliden gold deposit is an ore body in northern Sweden’s Bothnian Basin and has been the subject of studies and test mining since the early 2000s when the Gold Line, an area of anomalously high-Au glacial till in Northern Sweden, became a center of economic interest. The deposit is a hypozonal orogenic gold deposit that displays many characteristic features of ore bodies of this type, including the presence of compound sulfide grains composed of a core of löllingite surrounded by a rim of arsenopyrite, and an abundance of pyrrhotite throughout the deposit and surrounding alteration zone. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to first map the distribution of trace elements in several of the arsenpyrite-löllingite grains, then to perform spot analyses of pyrrhotite grains in samples from across the distal and proximal alteration zones of the deposit. The maps of the trace elements in the compound löllingite-arsenopyrite grains show that: As and Au are found in greater abundance in the löllingite than in the arsenopyrite; Cu, Fe, S, and Ti are found in greater abundance in the arsenopyrite than in the löllingite; and Ag, Au, Bi, La, Mn, Pb, Sr, Ti and Tl are found in zones of secondary enrichment along fractures within the grains. Uranium and V may also be enriched along fractures, although this isn’t clear on all of the maps. Gold is an element of particular interest, because its distribution throughout the sulfide phases can be used to constrain the mechanism and relative timing of mineralization within the deposit. The presence of invisible gold within the löllingite, but not within the arsenopyrite, shows that gold is not incorporated into arsenopyrite at high temperatures and will instead be concentrated in the löllingite core of the composite grain and at the margin between the two sulfide phases as the löllingite is altered to arsenopyrite. Previous research on the Fäboliden gold deposit, including geothermometric analysis of vein-hosted graphite within fluid inclusions and garnet-biotite geothermometry as well as structural evidence provided by regional deformation, indicate that mineralization occurred post-peak metamorphism. The textures seen are therefore not conclusive of mineralization occurring syn- or pre-peak metamorphism, as has previously been proposed based on research of orogenic gold deposits in Western Australia. The spot analysis of pyrrhotite samples from across the deposit shows a distinct decrease in Ni and Co content in the proximal alteration zone, suggesting uptake of these elements by other minerals such as the löllingite and arsenopyrite. Multiple analyses performed on single grains show local variation, but are insufficient to establish the presence or determine the character of growth zonation within pyrrhotite