How to develop a hard rock gold deposit

An open pit gold mine at Val D'or, Quebec.  The refinery is in the background.

Once a prospector has found a viable gold deposit where a mine is going to be built the job is turned over to a firm that specializes in turning the prospect into a working mine.  It is because of this one of the great associations of the mining business got its name – the Prospectors and Developers Association of Canada (PDAC).

To create a working mine many different things have to come together, and the mine developer is like the conductor of a symphony orchestra.  One of the very first things that happen is deciding the form the mine is going to take open pit or underground.  Both types of mine have their strongpoints and sometimes the two schemes are worked together as at the Kidd Creek Mine in Timmins, Ontario.  Even to a layman it’s obvious that an open pit mine is more economical to operate then an underground mine because it requires much less equipment.  It can also use large equipment that can’t be used underground.

The Oryx Mine in South Africa - An example of an underground gold mine.
Photo by Babakathy

Most countries have environmental laws that require getting all kinds of environmental permits before starting construction of a mine.  This also falls under the purview of the developer although the actual permitting process is under the direction of a lawyer.  Another important document in the permitting process is the Mine Closure Plan that is usually prepared by an engineer or landscape architect that describes what will happen to the real estate once the mine closes for good.

Depending on the size of the mine determines how much physical work will be required to bring the mine up to production.  This can be a major construction project in itself involving all kinds of construction equipment let along mining equipment.

This is why the Mexicans say, “It takes a silver mine to open a gold mine,? 

How to sample a lode gold deposit

_Pieces of drill cores
Photo by Constantino Figini 

Once you have discovered a promising outcrop of lode gold the next step is to sample the deposit.  This can be done in several different ways, but sometimes the old tried and true methods are the best.  The simplest way is to chisel a channel right across the outcrop of ore in which you are interested.  This is a lot of hard work but is well worth the effort.  The chiseling is done with a one inch (25 mm) stone chisel and a four pound hammer (2 kg), a short handled striking hammer works best for this.  The chips are saved in a cloth sample bag for later assaying.

Another situation you may come across is several outcrops of potential ore around your site that is in this case too much to sample the outcrops singly in a first sampling program on the site.  In this case you can go around the site and simply chip off a small sample from each outcrop, then have it assayed.  This isn’t the best method for sampling channeling is, but at least it will give some idea of what you have discovered.  Be sure you keep the samples random, and don’t favor what looks like hi-grade ore because this will skew the results of your sampling.

If you want to be sure about the accuracy of your samples send them to three different assayers then average the readings so you will come up with an average reading instead of a single assay.  This has been found to be more accurate.

After the preliminary sampling has been done, and your site has been proved to hold gold it is time to go onto more sampling to find out how much gold your site really holds, and if it is large enough to develop a mine.  It can take a lot of money to develop a hard rock gold mine, sometimes more then a billion dollars.  You should also keep in mind that only about one in a hundred gold discoveries are eventually developed into a paying gold mine.  They call it lode gold because it takes a load of money to develop it into a mine.

Drilling the orebody is the accepted method of proving the size of a gold deposit that is commercially viable.  This is done using a diamond core drill that gives you a solid core sample to examine.  Core drilling rigs come in several sizes with the smallest ones a drilling rig that can be back-packed.  There are several companies that make these drills that are capable of going at least 15 meters into solid rock.  Even larger rigs are available capable of drilling more then 1,200 feet into bedrock.

A back-pack rig is within the scope of something a prospector can carry into the bush with him, but larger rigs are best handled by specialized drilling companies.  The samples these take are cylinders or stone bearing the ore.  The first thing that happens when a core is retrieved is to split it in half lengthwise so the prospector retains half of the core with the remainder being shipped off to an assayer.  A series of core drill cores are used to delimit the actual size of an orebody by moving the drill rig further to the initial hole until the last hole bored is beyond the last hole bearing payable ore.  Many of these drilling programs encompass drilling kilometers of core drill holes.

Worldwide Gold Deposit Maps

Gold Deposits found in Mesozoic Terrane

Gold deposits found in Cenozoic Terrane

Alluvial gold deposits

This collection of maps is pretty much self explanatory and they do depict where gold can be found throughout the world.  The maps themselves are in Public Domain.

This depicts the gold deposits that are found in Precambrian Terrane

This depicts gold deposits found in Paleozoic Terrane

The Difference between an Ore Deposit and a Mineral Deposit

A mineral deposit is the showing of any type of mineral whether it has value or is just of scientific interest. An ore deposit is a mineral deposit that has an economic value as well as a scientific value.

The copper mineral malachite
Photo by Rob Lavinsky

There are times when you can actually have a very large mineral deposit worth millions of dollars, but because of its location or the cost of production to mineral deposit is not classified as ore. A good example of this is a group of prospectors found a copper deposit in northern Canada that after performing a considerable amount of drilling it was determined that this copper deposit held $100 million worth of copper. Because of its isolated location and the amount of money that would've been required to bring this into production was actually more than the deposit was worth.

This example drives home the difference between ore deposits and mineral deposits to the extent that the prospectors of all eventually called this the $100,000,000. nuisance.