Fathom Geophysics Newsletter 25
Exploration News: Glacial sulfides point the way
SULFIDE grains survive glaciation-related sediment production and transport well enough that their geochemical signatures can be used in mineral exploration to trace the path back up-ice to likely source areas of possible ore mineralization.
Quebec-based researchers looking at the supracrustal gneissic Rachel-Laport Zone and the rift- and platform-related Labrador Trough areas of northern Quebec's Churchill Province examined the trace-element profiles of pyrite, chalcopyrite, and sulfarsenide grains extracted from glacial sediment samples. 
A thick cover of Quaternary sediments meant the Churchill Province's metals-bearing capacity was poorly known when it came to base and precious metals, the researchers said in their published paper.
"However, numerous showings have been identified and suggest a complex metallogenic setting," they said.
"The transitional geodynamic regime from a rift to a pro-foreland basin could be favorable to several types of ore deposits."
By looking at the different populations present of each sulfide grain type, the researchers concluded that (1) nearby metavolcanic and metasedimentary bedrock potentially hosted volcanogenic massive sulfide or sedimentary exhalative mineralization, and (2) nearby mafic and ultramafic intrusive rocks may host magmatic nickel-copper mineralization.
Sulfides survive in glacial drift
Sulfide-rich minerals are notorious for smoldering and burning up when exhumed during the mining process , and during typical weathering and erosion processes.
"This lack of preservation has led to the misconception that in all climates sulfide minerals are not preserved in surface sediments," the researchers said in their paper.
"However, burial rates are fast in glaciated terrains and till matrix can be relatively impermeable, which results in very limited chemical weathering," they said.
Similar findings of good glacial-environment survival have been documented for kimberlite indicator mineral grains, which degrade in non-glaciated regions. 
The researchers found that sulfide and sulfarsenide grains from their till and esker samples posessed little to no oxidation rind. What rind was present could be removed by treating grains with oxalic acid.
The use of glacial-drift analysis for mineral exploration reconnaissance purposes, called drift prospecting, has enjoyed a long history in Canada.  The country is said to contain more than half of the world's formerly glaciated land area.
Other jurisdictions employing the technique include Finland, Minnesota, and Ireland.   
Drift prospecting has been used in the chase for a variety of ore types in Canada, including gold and diamond deposits. 
Glaciation-influenced landscapes are home to a wide array of special features found nowhere else — such as eskers, moraines, and till mudboils. It means geoscientists working in these landscapes must be familiar with what exactly constitutes a scientifically rigorous sediment sampling campaign.  And they need to keep an eye on geomorphological research into the behavior of former ice sheets, which is continually advancing.  
The research project involved the collection of more than 600 till and esker samples from across the National Topographic System of Canada's 24G and 24B 1:250K map sheet areas. Each till sample involved the collection of 10 kilograms of material, and each esker sample weighed 15 kilograms.
First, the 250-1000 micrometer fraction of non-ferromagnetic heavy minerals was extracted from samples. Then the magnetic subset of that fraction was discarded and the desired sulfides and sulfarsenides were picked from the remaining diamagnetic subset.
Of all of the material collected from the field, 301 samples contained sulfide and sulfarsenide grains. On averge, one sample contained around 10 grains. A randomized selection process was used on samples containing hundreds of grains.
All up, the researchers study involved 1,831 grains. The grains were then prepared in polished epoxy sections and analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) to obtain time signal-based grain compositions. Semi-quantitative multi-element distribution maps were also obtained, to provide a picture of relative element abundances.
The researchers found that samples contained subpopulations of each sulfide/sulfarsenide grain type. Clues used to glean their possible origins included the internal structure of the grains (such as crystal zonation and exsolution features), the nature of inclusions within grains, and the abundances of trace elements. They also compared grain compositions with the known trace-element profiles of sulfides from magmatic and hydrothermal deposits.
"The combination of certain types of inclusions, element associations, and zonations may be indicative of ore processes," the researchers said.
"From an indicator mineral perspective, this is the first step to identify possible bedrock sources."
They found that the pyrite grain population included homogeneous and zoned crystals. Some pyrite was relatively enriched in platinum, palladium, tellurium and rhenium, which was typical of pyrite formed in magmatic environments. Other pyrite grains were relatively enriched in gold, bismuth, antimony, thallium, and mercury, which suggested formation in a hydrothermal environment. Some of these grains also contained an association of inclusions that supported the hydrothermal scenario (namely, sphalerite and molybdenite inclusions).
Most of chalcopyrite grains appeared homogeneous in multi-element maps, but time-signal results revealed that some grains hosted inclusions, or exsolution features, or both. The researchers said that chalcopyrite plus pentlandite suggested a magmatic origin, while chalcopyrite plus sphalerite, galena, or scheelite suggested formation in a hydrothermal setting.
Most sulfarsenide grains (arsenopyrite and lollingite) were found to be homogeneous in both multi-element maps and time-signal results. They contained varying abundances of cobalt, nickel, antimony, tellurium, gold, mercury, lead, and bismuth. Some grains were particularly enriched in these element relative to levels found in pyrite and chalcopyrite grains. However, because too little prior research had been done to establish links between the formation settings and compositions of sulfarsenides, the researchers were unable to definitively ascribe the origin of these grains. The elevated antimony, tellurium, gold, and bismth seemed consistent with a hydrothermal genesis, though, they said.
"Previous studies have shown the link between the occurrence of sulfide minerals in till samples and the proximity of underlying mineralization," the researchers said.
Their study had identified the presence of at least two distinct mineralization types — one related to magmatism, the other related to hydrothermal activity.
The combination of trace element chemistry, bedrock geology, and glacial geology, made it possible to delineate regional vectors related to each mineralization type.
"Overall, the integration of sulfide trace element data with sample distribution suggests a regional potential for magmatic mineralization along the Labrador Trough, as indicated by the wide dispersal [train] of chalcopyrite and pyrite grains of magmatic origin," the researchers said.
"In addition, we have identified a northwest- to north-trending pattern in the western part of the study area that is defined by the alignment of samples containing chalcopyrite and pyrite of hydrothermal origin and sulfarsenide minerals," they said.
"This vector strongly suggests the presence of local hydrothermal mineralization in the western part of the map sheet 24B or the southwestern part of the map sheet 24G. Moreover, this vector suggests a remarkably long glacial dispersal of about 100 kilometers from a source in the Rachel-Laporte Zone and highlights the long-distance transport that is characteristic of an ice stream."
Where to from here
The researchers were unable to locate very specific source areas of the magmatic sulfide-mineral grains due to their ubiquitousness and their wide distribution geographically among sites sampled.
But the task of finding out more precise proximities for the sources of the various sulfide grain types could be done in further work by sampling more densely and by ascertaining absolute sulfide grain abundances, they said.
They added that whenever possible with this kind of study, the geochemistry of sulfide mineral grains should be looked at in conjunction with information from the analysis of other minerals from the samples, such as other sulfides (sphalerite, galena and pentlandite), chromite, and pyroxene.
And blind tests could be done in well-studied areas to assess the effectiveness of this approach before being routinely used in an exploration program, they said.
 C.J. Duran, H. Dube-Loubert, P. Page, S.-J. Barnes, M. Roy, D. Savard, B.J. Cave, J.-P. Arguin and E.T. Mansur (Jan 2019) "Applications of trace element chemistry of pyrite and chalcopyrite in glacial sediments to mineral exploration targeting: Example from the Churchill Province, northern Quebec, Canada", Journal of Geochemical Exploration, 196, 105-130.
 S. Jung (2012) "Sulphide self-heating: Moisture content and sulfur formation", McGill University, Master of Engineering thesis, 104 pages.
 M.B. McClenaghan (2005) "Indicator mineral methods in mineral exploration", Geochemistry: Exploration, Environment, Analysis, 5, 233-245.
 W.W. Shilts (1993) "Geological Survey of Canada's contributions to understanding the composition of glacial sediments", Canadian Journal of Earth Sciences, 30, 333-353.
 P. Sarala (Ed.) (2015) "Novel technologies for greenfield exploration", Geological Survey of Finland, Special Paper 57, 197 pages.
 D. Elsenheimer (2012) "Geochemical soil survey in Archean granite-greenstone terrane, International Falls area, Koochiching County, Minnesota", Minnesota Department of Natural Resources, Open-File Report, Project 385, 115 pages.
 M. Dempster, M. Cooper, P. Dunlop and A. Scheib (2016) "Using soil geochemistry to investigate gold and base metal distribution and dispersal in the glaciated north of Ireland", In: M.E. Yound (Ed.) "Unearthed: Impacts of the Tellus surveys of the north of Ireland", Royal Irish Academy, Dublin, 89-99.
 Geological Survey of Canada (2017) "Till sampling and analytical protocols for GEM projects: From field to archive", Open File 6850 (revised), 75 pages.
 V.M. Levson, A. Plouffe, T. Ferbey and J.D. Bond (2014) "Refined ice-flow directions of the Cordilleran Ice Sheet and implications for mineral exploration", Geological Society of America Annual Meeting presentation.
 R.A. Klassen (2001) "A Quaternary geological perspective on geochemical exploration in glaciated terrain", In: M.B. McClenaghan, P.T. Bobrowsky, G.E.M. Hall and S.J. Cook (Eds) "Drift exploration in glaciated terrain", Geological Society, London, Special Publications, 185, 1-17.
About Fathom Geophysics
In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.
Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.
We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.