Successful mining starts with strong investigative skills and sound interpretation skills. “We are using data to help create a picture of what’s underground, long before we ever sink a mine shaft and cut a drift (underground tunnel),” said George Moellering, exploration manager at Doe Run’s Southeast Missouri Mining and Milling (SEMO) Division. “It’s a lot like putting together a puzzle without knowing in advance what the end image looks like.
“We collect data from core samples1 and historical geological information known about an area, and then begin interpreting that data to provide us with a picture of what we believe is taking place underground. Based on the data, we estimate tonnage and the ore grade2. If it appears the grade and tonnage provide a reasonable economic return, we begin developing the area for mining.”
Determining Economic Value
Before a new ore body is determined to have sufficient economic value to mine, several teams evaluate criteria, including ore grade, the distance from existing mined areas to the ore body, elevation and costs to reach the ore body.
As a company, Doe Run maintains a core library dating back to approximately 1949 when predecessor St. Joseph Lead Company explored what is today known as the Viburnum Trend. The library contains samples from more than 10,000 surface drill holes. The successful exploration and subsequent mining of this area of southeast Missouri has earned the Viburnum Trend a global reputation as the second-largest lead mining district in the world.
In 2012, the company began its review process for completing mining in this area of the Brushy Creek Mine. As part of their protocol, Doe Run reviews all past surface and underground prospecting data (both log books and core samples) and checks in with work crews to discuss any significant underground mineral observations to ensure that valuable mineralization will not be stranded and unattainable. During this process, the mine geology group noted copper mineralization had been observed in sections of the mine floor during the mining crew’s final pass through the area.
The review processes identified the core log entries from 1981, which supported the initial rationale for mining the area.
Doe Run’s Core Room
Measuring Core Samples
Examining Mineral Make-up
“Our mine geologists reviewed the core samples that indicated six ore runs were originally identified in the area of the Brushy Creek Mine,” said Moellering. “Five of those ore runs represented the upper zone of the Bonneterre Formation, which we had been mining for decades. The sixth ore run on the core sample represented a new zone on a previously unmined level of the mine, some 100 feet below our mined areas, near the Lamotte sandstone.”
This sixth run, indicated in the core sample, was ignored over the years because a series of surface exploration drilling did not suggest strong mineralization. Furthermore, once mining in the upper portion of the Bonneterre Formation took place, surface drilling to the lower ore zone was not possible and underground jackhammer drilling methods were not capable of reaching the lower ore zone.
Separately, these two puzzle pieces seemed insignificant, but when placed together in context, they revealed something completely different.
Based on these two findings, the historic core logs and newly observed mineralization, the mine team began underground exploration with a diamond drill in August 2012. And over the next several months, crews drilled 48 holes that indicated mineralization existed from the floor of the last mining pass at 122 feet elevation all the way to 35 feet below sea level. In some places, the mineralization was 150 feet thick. Most surprising, from a geological standpoint, was that the highest grade ore was located at the point of contact between the bottom of the Bonneterre Formation and the Lamotte sandstone.
As a result of the exploration, 11 separate areas (known as ore blocks) were identified as prospects. The final tonnage for the combined blocks was estimated at more than 430,000 tons of lead, zinc and copper ore with an ore grade about three times the mineralization traditionally found in the area.
It took development crews approximately eight months to develop this unique mining zone in order for mine crews to gain access to the ore.
“Getting to this ore body created some challenges,” said Randy Hanning, mine operations manager. “This is the first ore body next to the Lamotte sandstone, which was a new situation for us. The sandstone is pretty porous as opposed to our traditional mine horizons that are in dolomite, a much harder and more stable host rock.” To compensate, crews made modifications to the mine design in order to respond to the mine conditions and ore grade.
Addressing ventilation requirements at that depth also required a different approach with the mine crews drilling a 125-by-6-foot opening through the roof of the new area to the floor of the previous area to improve ventilation. Expedited advancements by the development crew to the lower zone allowed production from the area to begin in June of 2014. The extra effort has paid off with high mineralization of galena (lead), sphalerite (zinc) and chalcopyrite (copper).
Doe Run mined nearly 100,000 tons of ore from the Deep Ore Zone from June 2014 to December 2015.
“Unlike our extraction in the Bonneterre zone, where we have long understood the pattern and elevation of the ore, the Deep Ore Zone mineralization at Brushy Creek is located at a greater depth – nearly sea level – and the mineral content from one mining advance to the next is less predictable,” explained Chris Hogan, Brushy Creek Mine geologist. “So far, we have been successful in recovering nearly 100,000 tons of this resource between June 2014 and the end of 2015, but we still have only a portion of the puzzle put together for this area, so we believe we have a long way yet to go. This is what makes exploration so rewarding, and is what extends the life of our mines.”
(1) Cylinder mineral samples, generally 1 to 5 inches in diameter, drilled out of an area to determine the geologic and chemical makeup.
(2) Ore grade is based on the concentration of minerals found in the ore or host rock.