More Topics in the Placer Gold Category: https://bigrivergold.com/category/placer-gold-and-field-prospecting/
1. What an Assay Can and Cannot Prove
An assay is a laboratory test of a specific sample. It can tell you how much gold was measured in that submitted material, but it cannot prove that the whole mining claim is valuable. This is the most important point for a claim buyer. One rich rock chip, one pan concentrate, or one bag of gravel may come from a good spot, a bad spot, a salted sample, or even somewhere outside the claim. USGS analytical work has long used methods such as fire assay and atomic absorption for determining gold in geologic materials, but even a good laboratory result only applies to the sample actually tested. It does not automatically define the grade, size, continuity, depth, or mineability of the deposit. For buying a claim, the assay should answer a narrow question: “What is in this documented sample from this exact location?” It should not be treated as a guarantee. A buyer should document where the material came from, take photos, record GPS coordinates, label the bag, and keep the sample under the buyer’s control. Do not rely on a seller’s mystery sample. If the seller hands you a bag and says it came from the claim, that is not strong evidence. Better evidence is a sample you collect yourself from inside the claim boundary, with the location, material type, weight, and collection method recorded. USGS geochemical work commonly separates sample type, preparation, detection limits, and analytical method because those details affect how results should be interpreted.
2. What to Ask the Lab to Test
For a gold claim, ask the lab for a gold assay first. For rock samples, fire assay is a standard method for precious metals, and USGS lists a lead-fusion fire-assay method for gold, palladium, and platinum in geologic samples. For placer material, the lab may recommend a different preparation depending on whether you submit raw gravel, black sand concentrate, crushed rock, or pan concentrate. Ask the lab what sample size it needs, whether the sample should be crushed, pulverized, split, or submitted whole, and how results will be reported. Gold may be reported as ppm, grams per tonne, ounces per ton, or another unit. Make sure you understand the unit before comparing results. I would not stop at gold-only testing if you are seriously thinking of buying the claim. Ask for a multi-element geochemical panel or “metals panel” along with the gold assay. This is what people often mean when they say “heavy metals assay,” although the exact element list depends on the lab. The panel may include silver, copper, lead, zinc, arsenic, antimony, mercury, molybdenum, iron, manganese, nickel, cobalt, chromium, and other elements. Those results can help identify pathfinder patterns and possible environmental concerns. EPA describes abandoned mine drainage as acidic water rich in heavy metals that can form when sulfide-bearing mine materials react with air and water, so metals such as arsenic, lead, mercury, and others are not just academic details. They can matter for handling, water, tailings, and reclamation questions.
3. How to Use the Results Before Buying
Use the assay as one piece of evidence, not the whole decision. A good result from one sample should lead to more checking, not immediate purchase. If a rock chip is high in gold, ask whether that mineralization continues along a vein or is only a small pod. If a placer concentrate is high in gold, ask how much original material produced the concentrate. A cup of black sand with gold is not the same as a cubic yard of gravel with a known recovery value. A result from hand-picked quartz can be useful, but it may overstate the average grade. A result from random rock may understate a narrow vein. The sample type controls the meaning of the number. The best buyer practice is to take more than one sample. Take a duplicate sample from the same spot, a second sample nearby, and at least one background sample from less-promising material. For placer claims, test measured volumes from different bars, benches, inside bends, false bedrock cracks, and exposed bedrock traps. For lode claims, sample across the vein or structure rather than only grabbing the best-looking rock. Keep the lab report, sample notes, photos, GPS points, and map together. If gold is present but the multi-element panel shows high arsenic, lead, mercury, or other metals, that does not automatically kill the claim, but it does mean you should be more careful about water, concentrates, dust, tailings, and any plan to disturb old mine waste. EPA keeps technical resources for abandoned mine lands because mine waste and contamination issues can be site-specific and serious.
Related Reading
- The Complete Guide to Gold Prospecting Clues: Minerals, Alteration, Veins, and Host Rocks
- Why Gold Forms, Moves, and Concentrates
- How to Read Streams, Benches, Dry Creeks, Desert Washes, Marine Terraces, Dredge Tailings, and Old Placer GroundHere are
References
[1] U.S. Geological Survey. “Determination of Gold in Geologic Materials by Solvent Extraction and Atomic Absorption.”
[2] U.S. Geological Survey. “Gold, Palladium, and Platinum by Lead Fusion Fire Assay Method.”
[3] U.S. Geological Survey. “Sample Preparation and Analysis.”
[4] U.S. Environmental Protection Agency. “Abandoned Mine Drainage.”
[5] U.S. Environmental Protection Agency. “Abandoned Mine Lands: Technical Resources.”