WHAT DOES ROCK CHIP SAMPLING TELL YOU?

Tips for the ASX punters by Wayne McCrae

Put simply, rock chip sampling involves a geologist or similar professional walking across a mining tenement, looking for outcrops of rocks. (An outcrop is when rocks protrude from the ground or are visible at ground level, or in a trench called a costean in mining terms, which is excavated to a depth of 1m-2m and a meter wide by an excavator). The geologist may also chip exposed rocks from an underground mine.

So, what does it actually mean?

In most cases, it means that some of the samples collected during rock chip sampling are sent to a laboratory for assaying (measuring the metal content). In 99 out of 100 cases, only the rock chip samples that show visible mineralization, or those that the geologist suspects might carry some grade, are forwarded for assay. (Assaying is very expensive.) That’s all it means. In most cases, a company will report only very high mineral grades. The best-looking samples are selected for assaying, especially those that show visible mineralization.

Rock Chip Sampling has zero reflection on the possibility of there being any commercially viable minerals that would warrant a mining operation. In most cases, a geologist may chip 500 rocks, take them back to their vehicle, inspect them, and discard any that look bland. Typically, only a handful are selected. That’s the reality of rock chip sampling.

Geophysical Survey
Now, having said the above, a geophysical survey is where a grid pattern is set up over the tenement area, and the soil is sampled to around 6 inches deep within the grid. The grid pattern can vary in size, from 50m to 500m, depending on the size of the mining tenement. Today, the geologist carries a backpack full of electronics and recording equipment, along with a small hand pick (a hammer with a pointed end). The backpack contains an XRF handheld analyzer (X-Ray Fluorescent) that immediately analyzes the mineralization and records the exact position using the GPS attached to the XRF analyzer in the backpack. It records the exact location within centimeters.

At the end of the survey, the geologist plugs the information from the backpack GPS and analyzer data recorder into their computer, and the entire survey is recorded with exact positions, along with the assays of the minerals that have been analyzed. The grid and mineral grades recorded by the XRF may provide information on the mineralized structure and the direction of possible zones of mineralization for drilling.

Did you know?

g/t = grams per tonne
ppm = parts per million
They both mean the same thing.
50 g/t Au = 50 ppm Au
10,000 g/t or 10,000 ppm = 1%

So if a company reports an assay of 6,500 ppm Cu, it means the grade is 0.65% Cu.
If the grade is 3% Cu, it means the grade of copper is actually 30,000 ppm Cu, or 30,000 g/t Cu. Take your pick. Most companies use g/t for the grade of precious metals (Gold, Silver, Platinum) and ppm for base metals. You can use either one in reality.