Chapter 83: Karst-Hosted Bauxite Deposit
"Chief Engineer Ding, the CSAMT survey conducted by the Geological Survey Institute—will you be the one presenting the results to everyone?" Li Tang looked at Ding Shaoying.
"Sure," Ding Shaoying replied. Although he specialized in geology, he was familiar enough with electromagnetic methods.
He directed his technicians to lay out a series of hand-drawn CSAMT profile charts on the ground.
Controlled Source Audio-Frequency Magnetotelluric (CSAMT) sounding involves injecting current into the ground and receiving signals at a distance. Different strata and rock types have different apparent resistivities, and the signals they return vary accordingly.
By adjusting the transmission frequency, different depths could be probed, allowing the inversion of subsurface geological conditions.
It was like performing a CT scan at a hospital—peeling back the surface to see what's underneath.
"According to your instructions, we conducted CSAMT surveys in the 200 to 600-meter depth range within the project area."
He pointed at the colorful cross-sections spread across the ground. "As you can see, the strata are stable. There are hardly any major structures in this region."
He gestured toward the gravity survey map they had just discussed. "I honestly don't understand what kind of subsurface rock body is causing that gravity anomaly."
Li Tang crouched down and studied the charts closely. Because different depths represented different stratigraphy, the apparent resistivity changed in a visually intuitive way, which they had color-coded.
Apparent resistivity ranged from high to low using deep red, red, yellow, light green, green, and other gradients.
Since the subsurface here consisted mostly of sedimentary strata, the bands appeared as near-horizontal color stripes—rainbow-like.
Some charts even showed rounded color patches, reminiscent of Van Gogh's Starry Night.
"What's this?" Li Tang pointed at a long red band.
"Limestone," Ding Shaoying replied after checking the data. "Its apparent resistivity exceeds 3,000 ohm-meters."
"And this green layer beneath it?"
"Shale—apparent resistivity between 300 and 400."
"And the red layer below that?"
"Dolomite—around 2,500."
"Mm."
Li Tang stood up, resting his chin in his hand, pondering for a long time. Suddenly, he said with doubt, "Have you all noticed? The contact between the limestone and the shale is smooth—almost a straight inclined plane. But the boundary between the shale and the underlying dolomite is wavy. These spots…"
He pointed to several suspicious areas on the profile. "Don't they look like trenches or submarine canyons? Isn't that strange?"
"When local subsurface structures are developed, you get features like this," one of the CSAMT technicians explained.
"Okay. I understand now."
Li Tang turned without expression and picked up the gravity anomaly contour map. He pointed at one of the high-anomaly areas. "Let's place the first drill hole here."
"We're still drilling under these conditions?" Ding Shaoying stepped forward again to object.
"Why not drill?" Li Tang showed no hesitation and was unshaken in his stance. "The project has come this far. Of course we need to verify through drilling!"
"We need a basis for drilling!"
"These survey results are the basis!"
"No one would base drilling decisions on just these geophysical results!"
"Right here!" Li Tang's finger swept across the green zone on the CSAMT profile and declared firmly, "This is the bauxite layer!"
"Oh?" The previously silent Bi Junjie now took interest. He smiled and asked, "And how do you know for sure that this is a bauxite layer?"
"Both the gravity and electromagnetic survey data point to one thing: a karst-hosted bauxite deposit!"
"Karst-hosted bauxite?"
"The bauxite deposit here has an overall layer-like appearance."
With Li Tang's guidance, Bi Junjie crouched down again to study the rainbow-like charts spread on the ground. He clicked his tongue in amazement. "You know, looking at the anomaly shapes, they really do resemble karst-hosted bauxite! And on a large scale too!"
Ding Shaoying was stunned. He reexamined the charts and then shook his head. "But karst-hosted bauxite doesn't occur below 200 meters!"
"What's karst-hosted bauxite?" several students began whispering among themselves.
This real-life encounter with a geological phenomenon instantly sparked their thirst for knowledge.
Zhang Zhongyu was glad to see their enthusiasm and began explaining: "Karst depressions are a type of karst landform. They usually have a funnel or V-shaped cross-section. Limestone and dolomite are dissolved by surface water over time, forming these bowl-shaped features. Some are just a few square meters in size, while others can stretch across dozens of square kilometers."
He briefly introduced this common geological feature and then continued, "As for karst-hosted bauxite deposits, they form when bauxite layers originally created through surface weathering and leaching are carried by water into karst depressions and re-accumulate there. Generally, bauxite forms near the surface, and karst-hosted bauxite is no exception—either exposed or covered by thin Quaternary deposits."
Seeing the students' eagerness to learn, Bi Junjie chimed in cheerfully to share more. "Since 1955, our country has discovered 15 large bauxite deposits. Every one of them was found within 150 meters of the surface. Shallow deposits are easy to find, which is why no new large bauxite discoveries have been made for many years."
"Ohhh—" The students murmured in understanding.
No wonder the Geological Survey Institute staff had been speaking ill of Li Tang behind his back. Some students even overheard Ding Shaoying saying Li Tang was still in his rebellious phase.
It turned out Li Tang was trying to find bauxite below 200 meters—a move that clearly went against conventional knowledge.
As the saying goes, ignore your elders' advice, and you'll suffer immediately. People were used to following the wisdom of their seniors.
If you didn't, you were labeled an outlier.
Seeing the students so focused, Bi Junjie continued—perhaps showing off a bit. "Which country has the largest bauxite reserves in the world? Guinea. That country is built on mineral resources and is considered a geological wonder—yet it's still poor and underdeveloped. Some of our Chinese companies are working on bauxite projects there. I had the chance to go and provide technical support. The bauxite there is lateritic and extremely rich. You can see vast fields of red bauxite stretching out to the horizon—just dig it up, and it's money in your hands!"
"Can we go there and mine too?" a student murmured, already dreaming of riches.
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