Uncovering Gold Discoveries by Till Sampling

Covered by 30 meter thick glacial sediment deposits in its namesake Rainy River greenstone belt of northern Ontario, the gold zones on this property might never have been discovered if not for modern till geochemistry.

With the help of reverse circulation (RC) drilling and till sampling techniques, gold anomalies were first detected on the property by the Ontario Geographical Survey (OGS) in the late 1980s, followed by a series of major and junior resource companies. A 20 hole RC drilling and till sampling program in 1994 at Rainy River identified the largest and strongest gold-grain anomaly encountered in till in Canada to that date and led to the discovery of the ODM/17 Zone, still today considered the largest gold zone on the property.

Later, these same techniques allowed Rainy River Resources’ exploration team to identify glacially dispersed subtrains of gold grains and to trace these back to their bedrock source. After its C$310 million acquisition of Rainy River Resources in 2013, New Gold worked to bring the Rainy River property into commercial production in 2017.

Today, the Rainy River gold mine hosts proven and probable reserves of 2.5 million ounces of gold and 7.2 million ounces of silver. In addition to measured and indicated resources of 2 million ounces of gold and 5.1 million ounces of silver.

What is till sampling?

Transported and deposited by active glaciers, till is a surface mixture of sediment and crushed bedrock that can contain varying size of material from fine clay particles all the way up to large boulders. Much of the till overburden in Canada’s glaciated shield terrains has only been exposed to weathering processes over the past 8,000 to 12,000 years, or fairly recently in the geological time scale. Hence, the composition of the till is typically highly indicative of the primary composition of the bedrock material in the region.

Till overburden can contain glacially liberated gold and gold-bearing grains scraped from bedrock and dispersed in a train of material as short as a few meters to many kilometers from the original bedrock source, or the “head of the train.”

Glacial till sampling is a tried-and-true mineral exploration technique based on glacial erosion and the down-ice transportation of primary bedrock mineralization. This relatively new method has led to several critical gold discoveries in Canada in the past few decades.

As noted by the Geological Survey of Canada, the North American country “is a mineral-resource-rich glaciated landscape where till geochemistry and indicator mineral methods are important exploration tools.” After Rainy River, another prime example of a significant gold discovery brought about by gold-in-till sampling is Norther Superior’s TPK gold-silver-copper project in Ontario, considered to host one of the largest gold grain-in-till dispersal aprons in North America.

Depending on the location and thickness of the glacial till as well as the objectives of the exploration program, sampling tools and techniques can range from simple handheld shovels and small portable drills to excavators and RC drills. Gold discoveries on the till-covered terrain of the Rainy River property were made via RC drilling and digging with a backhoe in thicker till and digging by hand in shallower till cover.

Till samples are collected in clean plastic bags to be shipped to labs for gold grain counts and geochemical analysis. Geochemical analysis involves the detection of anomalous concentrations of indicator minerals consistent with the targeted deposit type. The elements are also known as pathfinder minerals because they leave a path for exploration teams to locate mineral deposits. In the case of till sampling, the pathfinder minerals are dispersed gold grains.

“Through till sampling, the concentration of gold grains, as well as their shape and size, can indicate the presence of a nearby gold deposit in glaciated shield terrains,” Rory Ritchie, Vice President of Exploration of Prosper Gold (TSXV:PGX) told the Investing News Network. “Typically, the higher the concentration of gold grains and the larger the till anomaly, the higher the potential for a gold deposit of potential economic significance. The shape or morphology of the gold grains provide insight as to how nearby the source may be.”

Three types of gold grains: pristine, modified and reshaped

The three types of dispersed gold grains — pristine, modified and reshaped — are categorized based on their relationship between grain wear and distance of transport. During glacial activity, the shape of gold grains is changed as they are liberated and transported from the bedrock source. The further the gold grains are transported from the primary source, the more worn down they become, modifying their shape.

Reshaped gold grains show the most wear and tear, indicating they have been transported across a long distance (1 to 10 kilometers). Pristine gold grains have retained their original shape and tecture as they have only travelled a short distance (100 meters). Modified gold grains fall somewhere in the middle. Till samples dominated by modified and pristine gold grains indicate a nearby bedrock source.

Till sampling and gold grain counting together with conventional geochemical analysis led directly to the discovery of the Rainy River gold deposit. When the first gold-grain anomalies were identified in the late 1980s by the OGS, the Rainy River district was considered a “New Frontier” because the geology and mineral potential of the bedrock was still unknown to geologists.

Exploration work in later years would soon identify several subtrains within the overall gold-grain dispersal train averaging greater than 100 gold grains per sample, indicating the presence of undiscovered gold zones. Between 2005 to 2010, drilling by Rainy River Resources further defined the gold-grain subtrains and finally locate their bedrock sources in six gold zones.

On the trail for new gold discoveries with till sampling

Richfield Ventures Corp., led by Peter Bernier, President & CEO of Prosper Gold, discovered the Blackwater-Davidson deposit which was acquired by NewGold in 2011 and the later sold to Artemis Gold in 2020. The property currently has proven and probable reserves of approximately 8.2 million gold ounces and 60.8 million silver ounces. A till sampling case study was completed over the Blackwater-Davidson deposit after the resource had been largely delineated, and the results showed that till sampling would have more than likely led to its discovery, had the project not already been drilled by historical operators.

The Prosper Gold team is on the trail of another significant gold-in-till discovery at the Golden Sidewalk Project in Ontario. The gold exploration project lies within the Birch-Uchi greenstone belt, approximately 65 kilometers east of the Red Lake underground gold mine, which Goldcorp sold to Evolution Mining in 2019 for C$475 million. The Red Lake camp has produced over 30 million high-grade ounces, with the majority of those ounces coming from the Red Lake – Campbell mine (> 27 million ounces).

Prosper Gold has defined several regional targets at Golden Sidewalk, most notably the Golden Corridor, characterized as a highly prospective, greater than 5 kilometer trend of coincident favorable magnetic and resistivity lineaments supported by pristine gold-in-till anomaly covering 3.3 by 0.5 kilometers.

Till sampling results from Prosper’s Q4 2020 exploration season included a sample containing as much as 882 pristine gold grains with a calculated assay figure of 110 g/t gold. The size, shape and concentration of the gold grains in the large till anomaly suggest strong potential for a gold deposit in the nearby vicinity. The company will commence an inaugural drilling program at the Golden Corridor in May of 2021 .

Takeaway

Success stories such as Rainy River, TPK and Blackwater demonstrate that gold-in-till sampling represents an effective exploration strategy for significant gold discoveries in Canada’s deep glacial overburden. The examples also offer investors insights into how to evaluate the investment potential of gold exploration projects in Canada’s glaciated shield terrains. Experienced geological teams turning up large concentrations of pristine gold-in-till results offer a stronger path to discovery.

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