Detection of alteration at the Millennium uranium deposit in the Athabasca Basin: a comparison of data from two airborne electromagnetic systems with ground resistivity data

Abstract

The Millennium uranium deposit is located within the Athabasca Basin of northern Saskatchewan. The basement rocks, comprised primarily of paleo‐Proterozoic gneisses, are electrically resistive. However, the deposit is associated with highly conductive graphitic metasediments that are intercalated with the gneisses. An unconformity separates the basement rocks from the overlying, horizontally stratified, Proterozoic sandstones of the Athabasca Group (which are also highly resistive). The strike extents of the graphitic metasedimenary packages are extensive and therefore electromagnetic (EM) survey techniques are successful at identifying these zones but do not identify the specific locations where they are enriched in uranium. Through drilling it has been noted that hydrothermal processes associated with mineralization has altered the rocks in the vicinity of the deposits, which should in theory result in a resistivity low. A significant resistivity low has been mapped coincident with the Millennium deposit using ground resistivity survey techniques. However, a comparison of the airborne EM and ground resistivity results reveals that the two data sets have imaged different features. The resistive‐limit (on‐time) windows of the MEGATEM data show conductive features corresponding to lakes located to the west and south of the deposit. The late‐time windows show a feature to the east of the deposit, interpreted as being associated with the east‐dipping graphitic basement conductors (similar to that observed in historical ground EM data collected in this area). The early‐time TEMPEST windows (delay times less than 0.2 ms) show a broad resistivity low located at approximately the same location as where the alteration has been identified through drilling. Modelling the data is not easy but a response that decays prior to 0.3 ms is consistent with 500 Ωm material in the sandstone, a resistivity value close to the lower limit with respect to the hydrothermally altered Athabasca group sediments in this area. The MEGATEM system does not see a conductive zone over the alteration as clearly but the high signal‐to‐noise ratio in the late‐time MEGATEM data means that the conductive material at a greater depth is more coherently imaged.

A comparison of airborne electromagnetic data with ground resistivity data over the Midwest deposit in the Athabasca basin

“…The work with TEMPEST at the Millennium deposit (Smith et al . ) concluded that this might be possible; the work described in this paper was to see if these conclusions also applied at Midwest.…”

“…AEM methods have been tested at the Millennium deposit and show an anomalous response that could be interpreted as due to alteration (Smith et al . ).…”

Surficial geochemical surveys over concealed uranium ore of the Phoenix and Millennium deposits in the Athabasca Basin, Saskatchewan

“…The Marker Fault is rooted in graphite-rich pelitic gneisses. The Paleoproterozoic basement rocks in the area belong to the transition zone between the Wollaston and Mudjatik Domains of the Trans Hudson orogeny (Smith et al, 2010). The rocks of the Mudjatik Domain are similar to those of the Wollaston Domain, but the former is dominated by granitic gneisses.…”

“…750 m below the surface along the Marker fault in the basement. Modified afterSmith et al (2010) andWood et al (2012).…”

Fast approximate 2D inversion of airborne TEM data: Born approximation and empirical approach

“…The principal applications are mining exploration (Smith et al, 2010;Wolfgram and Golden, 2001), subsurface hydrology (Danielsen et al, 2003;Baldridge et al, 2007;Auken et al, 2009;Siemon et al, 2009;Kirkegaard et al, 2011) and on-shore hydrocarbon exploration (Huang and Rudd, 2008;Pfaffhuber et al, 2009). AEM data are often interpreted using apparent conductivity transforms (Wolfgram and Karlik, 1995;Macnae et al, 1998), equivalent plate inversion (Keating and Crossley, 1990;Tartaras et al, 2000;Zhdanov et al, 2002), diffusion velocity analysis (Eaton and Hohmann, 1987), Zohdy's method (Zohdy, 1989;Sattel, 2005) or layered earth inversion (Huang and Palacky, 1991;Chen and Raiche, 1998;Farquharson et al, 1999;Christensen, 2002;Viezzoli et al, 2008;Vallee and Smith, 2009;Guillemoteau et al, 2011).…”