by Dan Oancea

The recent merger and acquisition frenzy which affects the industry clearly indicates that chances of finding a good mineral deposit are pretty slim in comparison with the certitude of acquiring one - if you cannot grow an apple tree why not buy some orchards on the market, business model.

At the same time risky exploration ventures are taking place in countries laden by corruption (the rotten apple), populist governments which could any time turn their back to the foreign investor (the sour fruit), or simply countries with great mineral potential but unfortunately encumbered by civil wars (the forbidden apple).

What is it left then? Some other companies have figured out the answer: historic mining districts could still provide a first class exploration ground with landholdings that have never been the subject of a modern approach.

It is there that the geologist and the mining engineer come in contact not only with a barren or mineralized rock but also with history. And history could be a good and wise friend if you understand it or a staunch adversary if you ignore it.

So many times the riches have been pulled out of the ground at a time when there was nobody to draw a map or build a cross-section through the deposit. Anyway some historical information can usually be located and assembling it constitutes the first step in understanding the deposit characteristics, possible strike and dip extensions or unmined resources.

Understanding old stopes’ locations, heights and configurations proves to be a more difficult task. Most of the time the old miners have mined out only the richest parts of the veins (the visible gold or silver mineralization) leaving in-situ a chaotic maze of valuable unmined resources.

I remember being guided by an old miner on an over two hundred years old adit when the ceiling practically disappeared in front of my eyes. The light provided by the carbide lamp proved unable to reach it, so I enlisted the help of a flashlight. The dome shaped underground void was so high that it could easily host a church. Many other galleries coming from nowhere also joined the void creating the impression of a hive.

It was a stockwork and disseminated gold mineralization that the old miners dug out leaving the smaller veins that cross-cut the structure to be mined by more recent small-scale miners. One of them was the old guy who accompanied me. Some 40 years ago, he helped his father work a small free gold vein on one of the low ceiling galleries. I politely declined his offer to get into that small hole in spite of the lure of the gold which presumably ornate the face some 30 meters away.

By the way, these old guys have taught me a ‘trick’: never whistle while looking for gold. Superstition? Of course, but starting that day nobody has ever seen me whistling while doing business underground. Subsequently, I started retrieving some nice native gold samples.

The company that I worked for managed to delineate some low-grade gold resources. Some years later another company explored the same deposit and produced a NI 43-101 compliant resource - pretty much the same as the old one. The authors of the technical report urged the company to estimate the volume of the old underground voids before doing anything else. A difficult but not impossible task – at least many of the old mining works were safely accessible. Most of the two thousand years old Roman galleries proved to be very safe and accessible too.

A few years later, another mountain, another historic mining area. The impressive gold vein system outcropped on the mountain ridge. It has been mined almost continuously for the last 2,000 years - the surface expression of ancient miners' work resembled a canyon, the site being visible from far away. Some of them have found their final resting place not far away on top of a slightly smaller hill; later on the hill became famous not only because of the discovery of another gold rich vein swarm but because of the occurrence of the world’s best samples of an extremely rare mineral – hessite, a silver telluride.

Our exploration project was (mis)guided by inaccurate archived maps and documents describing some of the old mining works. So many times our hopes of finding unmined intervals of the fabled gold vein (considered by many to be the largest/richest in Europe) have been crushed by the crude reality encountered in the underground: back-filled or void stopes.

Nowadays it seems that many exploration companies are still using the same old method of assessment: drive a gallery and hope for the best. An expensive drilling program is nothing else then a "hit or miss" operation: the drill hole actually samples a small area and no inference could be made about the size and configuration of the mining void, if detected. It can be considered that the drilling method has a limited ability in detecting mining voids. It just provides us some info (some yes or no type of answers) regarding the specific piercing point.

Managing such an exploration program could be frustrating not to mention time and money consuming. If you're not lucky there is only one available option left: abandon the exploration program. One thing for sure, it doesn't have to be such a gamble.

What else could be done to achieve more accurate data and get some guidance? It seems that the answer has been on engineers table for a while: do what they do when they're trying to build a house in a zone that sits on top of some uncharted old mining works; or learn from the exploration techniques employed in oil and coal mine exploration.

Let's roll back to the 1920's when the first ground penetrating radar test sounded the depth of a glacier in Austria. This application of the new technology was simply forgotten until the 1950's when a curious thing happened: the U.S. Air Force planes trying to land in Greenland systematically crashed because their radar systems 'misread' the altitude. Hard to believe but the radar was actually 'seeing' through the ice sheet. From that moment on developments of the new technology proved to be an important aid in subsurface imaging.

Another important technique in mining void detection is the seismic method with all its variations.

Most of the papers that I found on the Net recommend a combination of both methods; in some cases other electrical methods could also be employed as an aid (i.e. there is a different response signal generated by an intact mineralized vein in contrast with an air/water/back-filled void).

Microgravimetric surveys could also offer some rough data regarding the presence or absence of a mining void.

Any geophysical method detects a contrast between materials. That means that the geophysicist should be fully aware of what are you looking for and if possible provided with samples and guided in the underground to make him understand the 'materials' he's asked to identify.

Any combination of these geophysical methods could not totally replace the 'exploratory' drilling, but it would greatly reduce the need for it. Drilling should come on the second place and only after the void's irregular configuration is mapped and understood. Some focused drilling should take place to confirm and most important to check the survey's accuracy (i.e. to check the height and lateral extent of the old mining stopes).

Exploration, mine planning, panel delineation, safety and not on the last place budgets are going to greatly benefit from the application of these geophysical methods.

No need to get into technical details. You could get them from the web or by contacting a company which provides these kinds of services.

Here are a few links that I have found useful: