Fluid and metal sources in the Faboliden hypozonal orogenic gold deposit, Sweden

To model the formation of orogenic gold deposits, in a global perspective, it is important to understand the ore-forming conditions not only for deposits hosted in greenschist facies rocks but also in amphibolite facies. The Paleoproterozoic Faboliden deposit in northern Sweden belongs to the globally rare hypozonal group of orogenic gold deposits and, as such, constitutes a key addition to the understanding of amphibolite facies orogenic gold deposits. The Faboliden deposit is characterized by auriferous arsenopyrite-rich quartz veins, hosted by amphibolite facies supracrustal rocks and controlled by a roughly N-striking shear zone. Gold is closely associated with arsenopyrite-lollingite and stibnite, and commonly found in fractures and as inclusions in the arsenopyrite-lollingite grains. The timing of mineralization is estimated from geothermometric data and field relations at c. 1.8 Ga. In order to constrain the origin of gold-bearing fluids in the Faboliden deposit, oxygen, hydrogen, and sulfur isotope studies were undertaken. delta O-18 from quartz in veins shows a narrow range of + 10.6 to + 13.1 parts per thousand. delta D from biotite ranges between - 120 and - 67 parts per thousand, with most data between - 95 and - 67 parts per thousand. delta S-34 in arsenopyrite and pyrrhotite ranges from - 0.9 and + 3.6 parts per thousand and from - 1.5 and + 1.9 parts per thousand, respectively. These stable isotope data, interpreted in the context of the regional and local geology and the estimated timing of mineralization, suggest that the sulfur- and gold-bearing fluid was generated from deep-crustal sedimentary rocks during decompressional uplift, late in the orogenic evolution of the area. At the site of gold ore formation, an O-18-enriched magmatic fluid possibly interacted with the auriferous fluid, causing precipitation of Au and the formation of the Faboliden hypozonal orogenic gold deposit.