Kevin L. Shelton - Geological Sciences, University of Missouri-Columbia

Deciphering the complex fluid history of a greenstone-hosted gold deposit: Fluid inclusion and stable isotope studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Kevin L. Shelton, Todd A. McMenamy
Department of Geological Sciences, University of Missouri, Columbia, MO 65211

Edmond H. P. van Hees
Department of Geology, Wayne State University, Detroit, MI 48202

and Hendrik Falck
C. S. Lord Northern Geoscience Centre, P.O. Box 1320, Yellowknife, NWT,
Canada X1A 2L9

Abstract

Mesothermal, greenstone-hosted gold deposits are typically products of complex hydrothermal systems that have experienced multiple fluids at various times throughout their histories. Such complex fluid histories make it a challenge to look back through the numerous fluid overprints to deduce the chemistry of the ore-forming fluid. The Giant mine is an example of such an extremely complicated system because: (1) there are at least two episodes of gold ore introduction, including both refractory sulfide-hosted and free-milling, vein-hosted ores; (2) the presence of multiple regional gold-depositing events; (3) intrusion of granodiorite, resulting in decrepitation of many early fluid inclusions; (4) fluid overprinting associated with multiple generations of post-ore veins and vugs; and (5) post-ore deformation and recrystallization of ore veins, especially along faults. At least three main stages of mineralization have been recognized in the Giant mine, each deposited by a chemically and isotopically distinct fluid, identified using fluid inclusions and stable isotopes.

Early quartz ± carbonate veins related to gold-ore were deposited from H₂O-CO₂-NaCl fluids with T_h values of 180-360°C and salinities of 4 to 9 wt. % equiv. NaCl. The d¹⁸O 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 (d¹⁸O_water = 4.4-9.8‰) that equilibrated with large volumes of metasedimentary and metavolcanic rock during greenschist metamorphism. The d¹⁸O values of vein quartz (8.6-11.4‰) outside the ore-related alteration zone indicate deposition from fluids with d¹⁸O_water values of 2.8 to 5.6‰. Evidence of sporadic fluid unmixing indicates gold was deposited at temperatures near 350°C and pressures of 1-2 kbars.

Post-ore carbonate veins associated with minor Pb-Zn-Sb-Ag mineralization were deposited from highly saline NaCl-CaCl₂ brines with T_m values of -32 to -36°C and T_h values of 72-273°C. The d¹⁸O values of these carbonates (20.6-26.1‰) indicate that their parent brines (d¹⁸O_water = 9-14‰) also equilibrated with large volumes of metasedimentary and metavolcanic rocks. Subsequent dissolution of these carbonate veins created abundant vuggy porosity.

Late, primarily vug-filling dolomite ± stibnite, overgrown by scalenohedral calcite, were deposited from dilute fluids (<6.0 wt. % equiv. NaCl) with T_h values of 95-115°C. The d¹⁸O values of dolomite (17.4 toward 13.4‰) and latest calcite (10.9‰) indicate deposition from progressively less evolved meteoric waters with decreasing d¹⁸O values from 1.1 toward -6.9‰.

By combining stable isotope and fluid inclusion techniques, it has been possible to see through the complexity of various fluid overprints to deduce the chemistry of the early ore fluids and to determine the mechanisms of gold ore deposition for the Giant ore bodies. Although the chemistries of CO₂-H₂O-NaCl ore fluids are grossly similar, the two types of gold mineralization in the Giant mine were deposited from isotopically distinct fluids via unique processes. Reaction of mineralizing fluids with metavolcanic wall rock Fe²⁺ resulted in deposition of refractory, sulfide-hosted gold mineralization. Fluid unmixing in high-grade veins, with loss of H₂S to the vapor phase, resulted in deposition of free-milling gold-quartz vein ores.

Great Slave Lake