Gold Ore - Finding the Motherlode - Part One
Finding gold ore takes an experienced eye and a little bit of luck. Even with all of the technology we have today, there isn't nothing that can positively tell you where gold ore is. Through time prospectors, geologists and geophysical engineers have conducted reasearch on the genesis of gold ores but not one has been able to say, here is where all the gold is. Todays prospecting methods include anything from breaking rocks with a hammer to panning to doing Mag VLF surveys and so on. Over the years some methods have proven to be better than others but one thing is for sure, you wont find any gold unless you put your feet on the ground.
It seems like everywhere you look now, Gold is on Tv, the internet and social media. Television series like Gold Rush, Bering sea gold, Alaskan gold, Ice cold gold and so on are glorifying the art of placer mining and prospecting. Making it look much easier than it is and only tending to the 1% of the art. More often than not prospectors come up empty handed, with little more than a few bruises and a sore back, they don't have all of the high tech equipment that these companies do.
So what does a prospector do to locate his motherlode? Well whether it be copper, gold, silver or tungsten there are a few things you can keep in mind while prospecting.
Gold - Without getting into an overly techinical discussion, Lode gold deposits form in a wide variety of geological settings. Lode gold is typically high grade hosted in quartz veins which are thin by nature. These veins are usually formed in association with fault activity giving the opportunity for open space filling to occur, allowing for minerals like gold to be precipitated into a solution, carried and deposited. Depending on the process and chemicals involved gold ores do not always present Native gold as the primary source. Often times gold is associated with sulphides or tellurides. In short , sulphides and tellurides are other minerals which contain gold content. Arsenopyrite is an example of a sulphide mineral which can contain economical amounts of gold. Calaverite although uncommon is a gold telluride mineral, mixing silver and gold together to form another mineral. These minerals are just a few of which occur in conjunction with gold. Typically gold is found in association with Basalt, turbidites, volcanic sequences and in laterites which is a sedimentary rock. A secondary form of gold deposition is found in association with granite intrusions, this type of deposit, hence the name is Intrusion related gold. Porphyry dikes are also a source of gold deposition but they are heavily reliant on gold already being present in magmas, rather than circulation of fluids during geological processes. To really achieve a better understanding of each of these processes I recommend researching them independently or searching the blog for articles associated to each.
As we generalize along here, we look into another aspect of finding gold ores. If you do locate a geological structure which is favorable for gold deposition, your next step is to conduct geochemical testing. Which involves taking rock samples or soil samples to quantify the amount of gold present in the rock. Typically gold will occur below a detection limit of 5ppb, however low anomalies in the 20+ ppb range are worth looking into, depending on the rock type. If the rock type would be classified as a sedimentary rock, there is a possiblity that your anomalies will be a little higher, given that these rocks have been reworked several times by geological processes. Saying this doesn't write off the possibility of having a gold bearing system within that sedimentary rock either, especially if it is in association with faulting or geological contact zones.
As you can probably tell so far, the geology of gold ores can be complicated. Disseminated gold deposits can be entirely invisible to the naked eye, either dissolved into the parent rock or in what appears to be milky white quartz. A Prime example of this is the Devil's pike brook gold in New Brunswick. Devil's pike brook was located by a provincial geologist who seen a hematized boulder along a brook. (Hematized is referring to a reddish color caused by iron content, in ancient times hematite refers to "Like Blood"). Upon seeing the boulder it was sampled and returned an unbelievable 1300 grams of gold, per ton of rock, which we generally refer to as G/T. This gold ore was a prime example as to how difficult it is to detect valuable gold deposits with the naked eye. This would have been considered invisible gold, however below I've added a nice picture of some coarse grained gold in a quartz matrix that you might enjoy.
Happy Holidays to all and I'll have part 2 written up in the next few days.
JMR
