Discovery Under Cover
One of the greatest challenges in mineral exploration is determining the location of economic mineral deposits. In areas with extensive glaciation and/or weathering this difficulty is compounded by the depth of the material covering the bedrock and can be hampered by vegetation cover, water bodies, snow and glaciers. Due to the lack of outcrops and visible mineralization, this exploration challenge is often referred to as "blind mineralization". Using "blind" as a description for under-cover deposits, however, leads to the common misconception that undiscovered mineral deposits are encapsulated and nonreactive with no surface expression.
Buried mineral deposits are subject to "weathering", beneath the surface, via water-bedrock interactions and microbial actions in the form of precipitation infiltration, ground water advection (flow due to pressure, gravity or remnant glacial tectonic pressure), interfacing with the water table and microbial-enhanced bedrock-dissolution. On the Earth's surface, weathering processes are fairly well studied and understood. Subsurface rock interactions are not as well known, but it is has been shown that water and microbial actions do occur at the bedrock interface, as well as deep underground, and these processes contribute the cycles at the Earth's surface through gas-bubble transport from microbial action or degassing as well as capillary action (upward fluid transport).
Three examples of deposits found under cover include the discovery of the Twin Creeks Gold Mine in Nevada, USA; "modern" exploration at the Keno Hill Silver Camp in Yukon Territory, Canada; and the extension of the Cadia gold mineralization leading to the location of the Ridgeway Gold Deposit in New South Wales, Australia.
The Twin Creeks Gold Mine was discovered in 1984 following a sagebrush biogeochemical survey to determine the mineral potential of the area around the recently discovered Vista Deposit. The mineralization at Twin Creeks was considered "blind" and was covered with between 12 and 200 metres of overburden consisting of alluvial material (re-deposited material in a non-marine setting). Although sagebrush roots average between 1 to 4 metres, the results showed a distinct pattern overlying the buried mineral deposit which lead to its discovery.
"Modern" exploration at the Keno Hill Silver Camp was conducted in the early 1960's resulting in the birth of "overburden drilling". A skid-mounted rotary-percussion drill, modified for use as a bedrock-sampling tool, was pulled through the bush over parts of the Camp covered by glacial overburden. By 1988 they had drilled 594 kilometres from 15,000 holes to an average depth of 37 metres. Sampling the overburden resulted in the location of new veins as well as anomalies. They were also able to trace the extent of known veins and define the limits of the open-pit mines. There were hiccups with this method as, in one instance, sampling within a gulley down-slope from an old mine resulted in the interpretation of a split from the main vein. It was later found that galena-pyargyrite rich float had migrated down-slope from the mine, over resistant subcrop, and into the gully.
The Ridgeway Gold Deposit was one of several porphyry deposits discovered near Cadia in 1996. The deposit is located ~500 metres beneath the surface under 20 to 80 metres of Miocene cover and approximately 450 metres into the Ordovician host rock. In this case, structural interpretation, magnetic data and geochemical/alteration vectors based on three-dimensional patterns discovered at Cadia, lead to the site selection of the deep-drilled discovery hole: 145 metres from 598 metres at 4.30 g/t Au and 1.20 % Cu; and 84 metres from 821 metres at 7.40 g/t Au and 1.27 % Cu.
More advancements are being pursued for discovering "blind mineralization" as fewer targets are being generated from surface techniques. Based on greater understanding of the biological and chemical processes beneath the Earth's surface and working from the methods of previous discoveries, exploration programs can be developed to find deposits under deep cover:
Using appropriate statistical techniques on regional stream and lake sediment data to determine prospective areas of the chosen commodity and/or pathfinder elements.
Biogeochemical surveys and/or overburden drilling to locate local anomalies and geochemical/alteration halos.
Geophysical surveys and structural mapping to assist with targeting as well as determining depth and angles of drill holes.
About the Author:
Diana has over 20 years of experience working in the mineral exploration industry searching for diamonds and metals in a range of roles: from heavy minerals lab technician to till sampler, rig geologist, project manager and business owner. She has a Bachelor of Science in General Biology and, following a Master of Science degree in Earth Sciences for diamond indicator mineral geochemistry, Diana has conducted field work in Canada (BC, NWT, YT and ON) as well as in Greenland. She has also been involved, remotely through a BC-based office, on mineral exploration projects located in South America, Africa, Eurasia, and the Middle East. Diana finished a Ph.D. at UNBC in 2017 researching geochemical multivariate statistical analysis techniques for use in interpreting biogeochemical data for mineral exploration. Currently, Diana is the owner of Takom Exploration Ltd., a small geological and environmental consulting firm focused on metal exploration in BC and the Yukon.