Abstract
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 Fäboliden 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 Fäboliden 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-löllingite and stibnite, and commonly found in fractures and as inclusions in the arsenopyrite-löllingite 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 Fäboliden deposit, oxygen, hydrogen, and sulfur isotope studies were undertaken. δ18O from quartz in veins shows a narrow range of + 10.6 to + 13.1‰. δD from biotite ranges between − 120 and − 67‰, with most data between − 95 and − 67‰. δ34S in arsenopyrite and pyrrhotite ranges from − 0.9 and + 3.6‰ and from − 1.5 and + 1.9‰, 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 18O-enriched magmatic fluid possibly interacted with the auriferous fluid, causing precipitation of Au and the formation of the Fäboliden hypozonal orogenic gold deposit.
Highlights
Gold has been one of the most sought-after commodities for thousands of years
It has been suggested that orogenic gold deposits cannot form at metamorphic conditions beyond midamphibolite facies due to the inhibited fluid flow that occurs at temperatures above approximately 600–650 °C, and gold deposits hosted by rocks subjected to higher metamorphic conditions are assumed to either have formed pre-peak metamorphism and subsequently overprinted by peak metamorphic conditions (Tomkins and Grundy 2009), or to have formed under retrograde PT conditions (e.g., Kolb et al 2015)
Metamorphic fluids have been implicated in forming many other gold deposits (Kerrich et al 2000; Goldfarb et al 2005; Pitcairn et al 2006, 2010; Large et al 2011; Phillips and Powell 2010; Tomkins 2010, 2013; Gaboury 2013; Groves and Santosh 2016; Goldfarb et al 2017), but orogenic gold deposits have been suggested to have formed from magmatichydrothermal fluids (Burrows et al 1986; Pattrick et al 1988; Burrows and Spooner 1989; de Ronde et al 2000; Xue et al 2013), from deep-convection of meteoric water (Nesbitt et al 1989; Hagemann et al 1994; Jenkin et al 1994), from mantle-derived CO2-rich fluids (Cameron 1988, 1989; Colvine 1989), or from fluids expelled during subduction of oceanic crust (Kerrich and Wyman 1990; Goldfarb et al 1991a; Jia et al 2003)
Summary
Gold has been one of the most sought-after commodities for thousands of years. The global historical production is estimated to be approximately 180,000 tons and about one-third of that gold comes from orogenic gold deposits (Frimmel 2008). It has been suggested that orogenic gold deposits cannot form at metamorphic conditions beyond midamphibolite facies due to the inhibited fluid flow that occurs at temperatures above approximately 600–650 °C, and gold deposits hosted by rocks subjected to higher metamorphic conditions are assumed to either have formed pre-peak metamorphism and subsequently overprinted by peak metamorphic conditions (Tomkins and Grundy 2009), or to have formed under retrograde PT conditions (e.g., Kolb et al 2015). This has led to a debate on whether all orogenic gold deposits in amphibolite and granulite facies (hypozonal group) are metamorphically overprinted greenschist deposits
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