Gillen, David (2010) A study of IOCG-related hydrothermal fluids in the Wernecke Mountains, Yukon Territory, Canada. PhD thesis, James Cook University.

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Abstract

An extensive study of fluid inclusions from the Wernecke Mountains, Canada, has revealed a number of important findings related to the nature and source of iron oxide – copper - gold (IOCG) –related hydrothermal fluids in this region. This study is the first to characterise fluid compositions in detail in the Wernecke region, and uses this information to illuminate aspects of IOCG formation including fluid source and evolution that were previously inaccessible using traditional methods.

Direct analysis of fluid inclusions was performed using a range of advanced microanalytical techniques including microthermometry, laser Raman, proton induced Xray emission (PIXE), laser induced inductively coupled plasma mass spectrometry (LAICPMS), and extended Ar-Ar methodology, in order to obtain a wide range of compositional information. Comparison of results from fluids with different timing, and in particular from IOCG prospects at different levels in the stratigraphy of the Wernecke Supergroup, was key to gaining insight into the evolution of circulating fluids.

Microanalytical results characterised the hydrothermal fluids overall as moderate to high temperature (110-400°C), moderate to high salinity (15 to 45 wt% NaCl equiv.), Na and Ca rich fluids (6 to 10 wt% Na and up to 9 wt% Ca) with moderate to low K (typically 1 to 3 wt%) and Fe (mostly below 0.5 wt%). Overall ore metal content is low, however the measurement of Cu in these fluids for the first time, confirms that Cu was transported by hydrothermal fluids within the WSG. Importantly, the measured Cu levels (ranging mostly from 5 to 500ppm) are low compared with the Cu contents of ore forming fluids in many economic IOCG deposits. The overall characteristics of the studied fluids have more in common with moderate salinity, moderate temperature, calcic sedimentary-derived basinal brines, as disparate to high temperature, hypersaline, K- and Fe-rich magma-derived fluids. Analysis of halogens and noble gases further support a dominantly formation water origin for the bulk of fluids, but for the first time also identified a minor basement-derived (possibly magmatic) component. The majority of fluid populations were shown to be consistent with evolution from a common parent brine, and distinct differences in fluid salinities were attributed to variable dissolution of halite horizons.

Fluids were shown to evolve through fluid-rock interaction via circulation upwards through the WSG, based on differences in fluid chemistry at different stratigraphic levels. Fluid evolution and wall-rock alteration were also shown to be co-dependent, based on measured correlations between elements lost from fluids and gained in wall-rocks. The alteration was driven by changing temperature (cooling) with depth, evolving fluid composition, and local changes in redox conditions. Ore metals were derived mostly via leaching from wall-rock during alteration of WSG rocks, and geochemistry results suggest minor mafic intrusions were likely the chief donor of ore metals given the metal-poor nature of WSG sedimentary rocks.

The findings of this study strongly favour a dominant sedimentary formation water origin for IOCG-forming fluids in the WSG, and clearly place the Wernecke IOCG district into the non-magmatic class of IOCGs. The opportunity to study an end-member non-magmatic IOCG system has provided insights into the importance of non-magmatic processes in IOCG formation. Important factors for ore formation in this (and potentially other nonmagmatic systems) include the enhancement of salinity through evaporite dissolution, and scavenging of ore metals from wall-rocks via extensive sodic metasomatism, with metal enrichment depending further on the metal content of host-rocks. The major factor limiting ore deposit formation in the Wernecke Mountains was the lack of a significant ore metal budget. This can be attributed to the lack of a hypersaline metal-rich magmatic fluid pulse (the key source of metals for many IOCG deposits), or the lack of metal-rich host rocks. Either of which could have significant boosted the abundance of ore metals in the system.

Item ID:	11620
Item Type:	Thesis (PhD)
Keywords:	hydrothermal fluids, fluid inclusions, Wernecke mineralogy, mineral formation, mineralization, ore formation
Date Deposited:	08 Jul 2010 02:50
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040306 Mineralogy and Crystallography @ 100%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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