Pyrite trace element chemistry of gold deposits in the Red Lake greenstone belt, Northwestern Ontario, Canada

Abstract

The Red Lake greenstone belt located in northwestern Ontario, Canada, hosts a world class gold camp that contains the high grade Campbell-Red Lake, Cochenour-Willans, Howey, and Hasaga deposits, as well as several other gold deposits and gold occurrences. This thesis examined textures, trace element chemistry and sulfur isotope evolution of pyrite and its use as a tool to discriminate Red Lake gold deposits, as well as the potential to use of pyrite chemistry as vectoring and fertility assessment tools. Field and petrographic observations, along with results from scanning electron microscopy, LA-ICP-MS analyses and sulfur isotope composition of pyrite, were integrated to achieve the objectives of this study. Petrographic analyses of chemically etched pyrite grains reveal two main groups of pyrites associated with Red Lake deposits namely hydrothermal zoned pyrites (Py1a, Py1b and Py2) and recrystallized pyrites (Py3). At the Campbell-Red Lake (CRL) deposit, Py1a which is a mineral inclusion rich pyrite is enriched in element suite of Sb-Tl-Au-Ag-Te-W ± Hg with average δ34S values of approximately +6.6 ‰ and forms the core of zoned pyrites. It represent a stage of low temperature hydrothermal alteration. The second stage pyrite of pyrite growth at CRL is identified by the formation of arsenian, Au-rich Py1b crystals which represents metamorphic devolatization and mobilization of main stage mineralizing fluids with a narrow range of δ34S (+3‰ to +6.7‰). Py2, interpreted as the last stage of pyrite growth represents retrograde cooling of CRL deposit hydrothermal system resulting in the enrichment of Co, Ni and As and has δ34S value around +6‰ consistent with a metamorphic fluid source. Py3 at the CRL represents recrystallization of early pyrites under high temperature contact metamorphism from plutonic rocks with trace element content of Co, Ni and As around 1000ppm and average δ34S of +1.1‰ whereas Py3 at the Howey deposit with similar trace element contents to the CRL have δ34S of +2.5‰ consistent with association with fluids from intrusive rocks of the Howey diorite complex. In contrast to the CRL deposit, oscillatory zoned P1yb and Py2 are the early pyrite whereas mineral inclusion rich Py1a is the latest pyrite at the East Bay area deposits and gold occurrences. The paragenetic sequence of pyrite at the East Bay area and intrusion related deposits show evidence of coupled dissolution and reprecipitation of early pyrites to form gold-rich late stage Py1a. Sulfur isotope data from the East Bay area deposits suggests that early Py1b and Py2 formed from metamorphic fluid whereas Py1a have magmatic fluid input. The intrusion related Howey and Hasaga deposit have similar pyrite paragenesis and sulfur isotope characteristics compared to East Bay area deposit except the non occurrence of As-rich Py1b. The Sb/Tl, Sb/Bi and Bi/Te ratios of Py1a across several Red Lake deposits and occurrences provide potential fertility indicator tools. The Sb/Bi ratio of pyrite decreases with decreasing deposit size whereas Bi/Te increases. Gold, Te, and Mo concentrations of ore related Py1b is appears to be a good discriminator between the large CRL and the smaller systems in the Red Lake greenstone belt. Various elements in pyrite including Se, Te, Bi and Ni/Co ratio show systematic changes in concentration from the center of the CRL deposit to about 3km away approximately 1-2km more than the footprint of whole rock samples. Pyrite trace element composition from other Red Lake deposits effectively discriminates between deposit proximal or distal signature from a mineralized zone.