Welcome to Dreamland

Therese Dunphy

October 1, 2009

With the right geologic support, nightmarishly complex deposits can be transformed into an interesting and vibrant mine plan.

by Bill Langer

Almost every aggregate producer wishes for a nice, homogeneous deposit of rock or sand and gravel. If a bucket full of rock from one part of a deposit is exactly like a bucket of rock from any other part of the deposit, they are in dreamland.

In contrast, geologists like to solve complicated puzzles. My reaction to a deposit like the one described above is boring! And I am not referring to drill holes. Figures 1 through 3 demonstrate one example of what I consider to be a really great sand and gravel deposit; this one is along the San Andreas Fault in southern California.

Figure 1 shows an aerial photograph of the landscape that I have reassembled to make it appear the way it would have looked in the recent geologic past. Highlighted on that photo are two alluvial fans; Fan 1 was deposited first, and was then moved along the San Andreas Fault. Some time later, Fan 2 was deposited over Fan 1.

Alluvial fans commonly have boulders and coarse gravel near the apex (pointed portion) of the fan, and sand at the distal end (splayed out portion) of the deposit. Alluvial fans also tend to have sand at depth and coarser material near the land surface. So, as one can imagine, having one fan piled on top of another creates a very interesting challenge when trying to characterize the deposits and calculate reserves.

After the fans were happily settled in, the earth’s tectonic plates decided once again to move along the San Andreas Fault. As the plates slid past one another, they carried with them the alluvial fan deposits. That must have been quite an exciting time. After the dust settled, things ended up as shown on Figure 2.

To make things even more interesting, two new fans (Fans 3 and 4) were deposited adjacent to Fans 1 and 2 as shown in Figure 3. All four fans came from different source rock areas and, consequently, the properties of all four fans differ.

This, my friends, is an aggregate producer’s nightmare. But geologists commonly can solve the puzzle of complicated geologic deposits, just like was done in this example. The geologist can use that information to optimize test hole locations (red dots on the lower figure), interpret the drill logs, calculate reserves, and contribute to the mining plan, thus transforming what otherwise might be a nightmare into a visit to Dreamland.

Author’s note: This article was inspired by, simplifies, and summarizes the presentation ‘Multi-disciplinary Approach to Aggregate Resource Evaluation’ given at the Society for Mining, Metallurgy, and Exploration (SME) 2009 annual meeting by geologist Jeff Light, manager of geological services, Granite Construction Inc.


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