Concentration of gold during retrograde metamorphism of Archean banded iron formations, Slave Province, Canada

Abstract

Gold-bearing iron formations are widely distributed within extensive metasedimentary terranes of the Archean Slave Province, situated in the northwestern Canadian Precambrian Shield. Mineralized iron formations occur within thick turbidite sequences overprinted by a protracted history of deformation, metamorphism, and plutonism. Economically significant gold prospects are specifically sited at structural culminations characterized by polyphase folding. Based on garnet–biotite geothermometry on the stable prograde metamorphic assemblage of enveloping metapelites, peak metamorphic conditions are approximated to be 570 °C and 4 kbar (1 kbar = 100 MPa). Diagnostic prograde mineralogy reveals that two facies of silicate iron formation are represented at the five gold occurrences investigated: (1) amphibolitic iron formation (AIF), characterized by quartz + grunerite + hornblende + pyrrhotite ± garnet ± graphite + ilmenite, and (2) pelitic iron formation (PIF), consisting of quartz + biotite + garnet + ilmenite ± grunerite ± hornblende. Textures reveal that grunerite crystallization preceded hornblende and garnet. Within AIF, banded pyrrhotite is in textural equilibrium with prograde metamorphic minerals. Retrograde hornblende, garnet, zoisite, apatite, carbonate, ferroactinolite, and gold-bearing sulphide minerals replace the prograde mineral assemblages on the margins of quartz veins that intensify at AIF fold hinges.It is hypothesized that the iron-formation-hosted gold deposits of the Slave Province are a result of multistage processes. Gold concentrated at high background levels within pyrrhotite-bearing AIF was remobilized during fluid migration into brittle AIF fold hinges in subsequent metamorphic and deformational events. Metamorphic fluid, ponded in fractured AIF hinge domains, caused retrogressive replacement, quartz veining, and gold-bearing sulphide precipitation during waning temperature. Although the mineralized hinge zones commonly display evidence of late chloritization, this alteration did not further affect gold distribution. The gold precipitated with destabilization of thio complexes due to sulphidation prior to low-temperature hydrothermal activity.