Spatial and Temporal Variations in Composition and P-T Conditions of Ore-Forming Fluids Along Structures Controlling Unconformity-Related Uranium Deposits in the Athatbasca Basin, Saskatchewan

Unconformity-related uranium (URU) deposits in the Athabasca Basin occur within certain segments of basement-rooted reverse faults near the basin-basement unconformity. It is widely accepted that these deposits formed through interaction between oxidized basinal fluids and reduced basement-derived fluids or reducing lithologies. However, the factors that control localization of orebodies within limited areas along the structures remain to be determined. This study tackles this problem by examining the spatial and temporal variation of composition and pressure-temperature conditions of ore-forming fluids along two major URU-controlling structures, the Patterson Lake corridor (PLC) in the southwestern margin of the basin and the P2 fault in McArthur River area in the eastern Athabasca Basin. The results provide new insights on the sources and paths of the ore-forming fluids, ore deposition mechanisms, and the role of structures in controlling the localization of mineralization. Fluid inclusion analyses indicate that the pre-Athabasca fluids have lower salinities and higher homogenization temperatures (average salinity of 9.0 wt.% NaCl equivalent and average Th of 147 ̊C) than syn-mineralization fluids (average salinity of 25.4 wt.% NaCl + CaCl2 and average Th of 128 ̊C). The overall similarities in fluid composition between the syn-mineralization fluids and fluid inclusions recorded within the basin, including elevated salinities, NaCl and CaCl2 as dominant solutes, and variable Ca/Na ratios from Ca-dominated to Na-dominated varieties, supports the general hypothesis that ore-forming fluids were derived from basinal brines. Microthermometric analyses on fluid inclusions from syn-mineralization drusy quartz veins indicate that fluids with similar Th, salinity, and Ca/Na values were present in mineralized zones and areas distal to mineralization: over ~31 km strike length and up to ~900 m vertical extent in the PLC and ~4.3 km strike length of the P2 fault. Coexistence of liquid-dominated, vapor-dominated, and vapor-only fluid inclusions in fluid inclusion assemblages indicate that fluid boiling, which is interpreted to be related to low fluid pressure caused by episodic fracturing, occurred in both mineralized and distal areas along the ore-hosting structures. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICPMS) of individual fluid inclusions indicate that U-rich fluids were present in both mineralized segments (0.4 to 213.9 ppm) and distal areas (0.3 to 144.2 ppm). Uraniumrich fluids were also detected by ICP-MS analysis of bulk fluid inclusions from both mineralized zones (0.4 to 1562 ppm) and distal areas (0.1 to 1.5 ppm), as well as by synchrotron X-ray fluorescence (SXRF). These results suggest that uraniferous fluids with similar thermal and compositional characteristics circulated along the entire structures and at different depths. The formation of a significant U mineralization at a given locality is therefore not determined by the presence or absence of U-rich fluids, but rather by the abundance or flow rate of uraniferous fluids and reducing fluids as well as their relative timing. The differences in fluid flow rate and timing of different fluids in different parts of a given structure may be related to factors such as heterogeneous permeabilities, variations in lithology and fault geometry that control fluid flow mechanisms.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Level thesis:degree_level

Doctoral -- first

Discipline thesis:degree_discipline

Geology

Grantor dc:publisher

Faculty of Graduate Studies and Research, University of Regina

Year dc:date.issued

2021

Author dc:creator

• Rabiei, Morteza

Advisors dc:contributor.advisor

• Chi, Guoxiang
• Potter, Eric

Committee members dc:contributor.committeemember

• Jia, Na
• Bethune, Kathryn
• Normand, Charles

Language dc:language.iso

en

DOI

https://doi.org/10.82465/5064

OAI identifier oai:identifier

oai:uregina.scholaris.ca:10294/14469