Secondary Gold Precipitation Involving Immiscible Methane-Nitrogen-Carbon Dioxide and Aqueous Fluids, Lupin Gold Deposit, Nunavut

Jacob J Hanley1, Steven Crowell1 and Daniel Kontak2, (1)Saint Mary's University, Halifax, NS, Canada, (2)Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian University, Sudbury, ON, Canada

Contact First Author: Jacob J Hanley; jacob.hanley@smu.ca

Previously Published Material: The results of this study were recently presented as a poster contribution at the NWT Geoscience Forum in November of 2014

Abstract ID#: 36629

 

English Abstract:
Fluid inclusions in gold-rich, late-stage “ladder” quartz veins in sulfidic BIF of the Lupin Deposit, Nunavut, preserve CH4-N2-CO2-H2O fluids responsible for secondary gold transport and deposition.

Three inclusion types are identified in the mineralized vein quartz: (i) gas-rich inclusions containing single phase (at room T) CH4-N2-CO2 homogenizing to liquid between -113º and -19oC (but exhibiting similar Th in inclusion assemblages), consistent with a wide inter-assemblage range in CH4:N2:CO2 ratios and/or variable Ptrapping; (ii) liquid-rich inclusions containing a vapour bubble and a saline aqueous fluid that exhibit clathrate melting (Tmclath) between -0.3º and 1.8oC (2s range; mean of 0.6 oC; n=64), corresponding to salinities of 15.9-13.6 wt.% eq. NaCl, and homogenize (ThL-V>L) between 254º and 308oC; and (iii) mixed CH4-N2-CO2-H2O inclusions that decrepitate on heating above ~300oC rather than homogenize and which occur in groups with type (i) and (ii) inclusions. Petrographic and microthermometric characteristics of type (iii) trails are consistent with entrapment of only locally immiscible (coeval) CH4-N2-CO2-rich and aqueous-rich fluid “end-members”. These end-member fluids were derived from separate sources and later interacted at the point of type iii inclusion entrapment. Entrapment conditions for end-member fluids are estimated between 1-3 kbar and 350-550oC, with localized immiscibility occurring at <2 kbar and 400oC, well below conditions of peak metamorphism and late pluton emplacement associated with primary gold deposition in the Lupin BIF.

The results of this study introduce possible mechanisms for localized gold enrichment at Lupin and may explain the lack of gold enrichment associated with quartz veining in shallow parts of the deposit where immiscible fluids entering structures from depth had already precipitated gold. Importantly, cumulative gold deposition at Lupin was protracted and continued to much lower conditions than previously thought, requiring structural ponding of residual Au-bearing metamorphic fluids prior to secondary gold deposition.

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