Kevin L. Shelton Zoned saddle dolomite crystals Research
 

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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 Th 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 (d18Owater = 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 d18Owater 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 Tm values of -32 to -36°C and Th values of 72-273°C. The d18O values of these carbonates (20.6-26.1‰) indicate that their parent brines (d18Owater = 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 Th 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

Great Slave Lake

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Ed van Hees

Ed van Hees

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Geological Map of the Yellowknife greenstone belt, NWT, Canada


Pillow Basalts at the
Giant mine

Pillow Basalts

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Giant gold mine
sodium depletion

Giant gold mine sodium depletion

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Native Gold in Quartz

Gold-bearing quartz

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Gold-Bearing Quartz

Native gold in quartz

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last revised: spring 2007
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