So how do they sample rocks for gold and silver?
Gold, silver and platinum are categorized as precious metals. This means they are highly valuable with a high demand. This translates into a high monetary value for relatively small amount of the metal (Gold/Platinum ~ $1500/ounce, Silver ~$25/ounce). In addition to the high monetary valu, nature only supplies precious metals in small amounts (usually) which results in some difficulty in sampling at all stages.
Of particular problem to precious metals is called the nugget effect. This is where the tiny amount of precious metal may be rich in one part of a rock, and completely absence in another – it is never evenly distributed, Nature doesn’t like making things easy. Overcoming the nugget effect typically means larger sample sizes or targetted samples based on indicator minerals.
Indicator minerals are a mineral associated with the precious metal but much more common in the rocks. Typical indicator minerals are referred to as sulfides and include Pyrite (FeS2), arsenopyrite (FeAsS), Pyrrhotite (Fe1-xS), Chalcopyrite (CuFeS2) amongst others. These are minerals which are of no to little economic value in small quantities however the do indicate the potential presence of a more valuable precious metal. A very useful property any iron sulfides (contain iron (Fe)) is that they rust, leaving a nice rusty stain to see.
All samples are then assayed for the precious metal. The sample is pulverized into a powder, mixed to distribute any precious metal evenly, then divided down to a small manageable size for assaying. A 1 kg sample will end up with only a 30g or 50g sample size. Without proper mixing, the nugget effect can be greatly enhanced leading to either very negative results to over reporting of the precious metal content.
various techniques are used for assaying specific minerals, I wont go into that here. The end result is a report on the amount of precious metal in your sample of rock from which you can extrapolate potential value to proceed or move your exploration elsewhere.
Stage one sampling
Stage one sampling occurs at the outcrop level, these are bedrock (not boulders) exposures accessible with no to minimal excavation.
A quick way to see if a rock isn’t just a large boulder is to place your foot on it and hit it (not your foot) with a hammer. If it vibrates its likely a boulder, if not likely outcrop.
Looking for a nice rusty rock, with the proper indicator mineral, hammer off a chunk a little larger than your fist. Do the same thing in another section of the rock, rinse repeat. The more samples you take, the more representative it is. The more samples will also narrow down where the precious metal is depending on the amount, nature and ratios of indicator minerals which result in a positive gold showing when assayed.
Stage two sampling
Stage two sampling, in theory, provides a more reliable indication of precious metal value. Channel sampling uses a rock saw to cut two deep cuts forming a channel approximately 2 inches apart and 4-5 inches deep. The channel is cut perpendicular to any structure (across the vein) if present. using chisels and hammers, the channel rock between the two cuts is then removed/broken from the rock. Length of samples are determined by rock type, mineralization and localized constraints to precious metal mineralization.
Since these samples tend to be larger , and are cut from solid rock, the resulting sample tends to be “fresh” allowing for an easier indicator metal determination and lessen potential contamination from other rocks above them.
Stage three sampling
The most widely reported form of rock sampling comes from drill core. A diamond drill is use to produce a drill core of varying size. This cylindrical peice of rock from deeper in a deposit tends to be untouched by surface action and uncontaminated from materials above.
Drill core is usually drilled at an angle through the deposit so length of a mineralized “zone” in core does not correspond to the actual width of the deposit.
Core is examined for indicator minerals, mineralized zones, geological structures and whatever else can be learned from raw untouched rock. For sampling, the core is split, cut with a rock saw, in half – half stays with the core for future reference (43-101 compliance rules dictate this must happen). The other half is bagged and sent toan assay lab for processing like the other samples.