Abstract

Mesothermal, greenstone-hosted gold deposits are typically products of complex hydrothermal systems that involved multiple fluids at various times throughout their histories. The Giant mine is an example of such an extremely complicated system, including (1) multiple, local and regional gold-depositing events; (2) at least two styles of gold ore introduction in the mine area, including both refractory, sulfide-hosted and free-milling, vein-hosted ores, whose relative timing is enigmatic; (3) fluid overprinting associated with deposition of multiple generations of postore vein- and vug-filling minerals; and (4) postore deformation and recrystallization of ore veins, especially along faults.

Three main stages of quartz-carbonate mineralization are recognized in the Giant mine. Stage I encompasses deposition of dominant refractory, sulfide-hosted ores and subordinate free-milling, vein-hosted gold ores. Refractory, metavolcanic rock-hosted orebodies are connected to metasedimentary rocks east of the mine by an east-dipping alteration zone characterized by a depletion in Na and enrichments in K, Ag, As, S, and Sb. Quartz veins within the wall-rock alteration zone have δ18O values that decrease systematically from 14.7 per mil in deeper metasedimentary rocks toward 11.6 per mil in shallower metabasalts in the mine. This decrease is interpreted to indicate that 18O-enriched ore fluids originated in deeper, metasedimentary rocks and reacted extensively with wall rocks along the entire extent of their flow paths before depositing dominantly refractory gold ores within more 16O-enriched, shallower, Ti-rich tholeiitic metabasalts. Quartz ± carbonate veins related to these gold ores were deposited from H2O-CO2-NaCl fluids with Th values of 180° to 360°C and salinities of 4 to 9 wt percent NaCl equiv. Evidence of sporadic fluid unmixing indicates that gold was deposited at temperatures near 350°C and pressures of 1 to 2 kbars.

The δ18O values (SMOW) of vein quartz (11.6–14.7‰) and calcite (8.6–14.1‰) within the ore-related wall-rock alteration zone indicate deposition from fluids (δ18Owater = 4.4–9.8‰) that equilibrated with metasedimentary and metavolcanic rocks during greenschist metamorphism. The δ18O values (8.6–11.4‰) of vein quartz that locally contains free gold, hosted in metavolcanic rocks outside of the ore-related alteration zone, indicate deposition from fluids with lower δ8Owater values of 2.8 to 5.6 per mil. The difference in ranges of δ18Owater values may indicate that multiple events were responsible for gold-bearing quartz veining, or alternatively, that fluids depositing veins within and outside of the alteration zone represented distinct fluid reservoirs that evolved through reaction with isotopically distinct rocks in their source regions (i.e., 18O-enriched metasedimentary vs. 16O-enriched metavolcanic rocks).

Postore (stage II) carbonate veins associated with minor Pb-Zn-Sb-Ag mineralization were deposited from highly saline NaCl-CaCl2 brines with Tm values of –32° to –36°C and Th values of 72° to 273°C. The δ18O values of these carbonates (20.6–26.1‰) indicate that their parent brines (δ18Owater = 9–14‰) also equilibrated with metasedimentary and metavolcanic rocks. Subsequent dissolution of these carbonate veins created abundant vuggy porosity.

Late (stage III), primarily vug-filling dolomite ± stibnite, overgrown by scalenohedral calcite, was deposited from dilute fluids (<6.0 wt % NaCl equiv) with Th values of 95° to 115°C. The δ18O values of dolomite (17.4 toward 13.4‰) and latest calcite (10.9‰) indicate deposition from progressively less evolved meteoric waters with decreasing δ18O values from 1.1 toward –6.9 per mil.

Refractory ores in the main alteration zone and gold-bearing quartz veins contained therein formed from chemically similar CO2-H2O-NaCl ore fluids whose oxygen isotope compositions are consistent with a metasedimentary source. These two styles of gold mineralization appear to be part of the same mineralizing event in which the style of mineralization was dictated by the mechanism of ore deposition. Refractory, sulfide-hosted gold mineralization resulted from reaction of mineralizing fluids with metavolcanic wall-rock Fe2+. Free-milling, gold-bearing quartz vein ores were deposited as a result of fluid unmixing, with loss of H2S to the vapor phase.

Gold-bearing quartz veins outside of the main wall-rock alteration zone in the Giant mine were also deposited by unmixing of similar H2O-CO2-NaCl fluids. However, these fluids were isotopically distinct from those within the alteration zone and may not be part of the refractory, sulfide-hosted gold-depositing event. They may instead represent a separate mineralizing event whose fluids and ore-forming constituents were derived solely from within metavolcanic rocks.

This overprinting of one event on the other may have been a necessary condition for the development of world-class gold deposits in the Yellowknife district. The important roles of both metavolcanic and metasedimentary source rocks may explain why some smaller greenstone belts, with limited volumes of metavolcanic rocks, can host substantial economic gold mineralization and has important implications for regional resource evaluation and exploration for gold deposits in greenstone belts.

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