Calculating Tonnage
June 01, 2011
This is a very simple task, but one without which you cannot guestimate metal content. Precious metals are measured frequently as ‘grams per tonne’. Tonnage is a function of three dimensions plus density. If the ore body were conveniently fitted into a right-angled box of three dimensions, all we’d need to know would be its length, width, depth, and specific density. Specific density is a ratio relative to that of water. One cubic metre of water (at 4 degrees Celsius) weighs one tonne. Toolbox calculators on grahamanalytics use 2.5 as a fixed specific density (SD) for ore-bearing rock. This is close enough for our back-of-envelope estimates, but would not do for formal resource calculations.
To illustrate, if your ore body were 10 metres by 20 metres by 5 metres, using the 2.5 figure as SD, the tonnage of ore would be 2,500 tonnes.
Similarly, if your ore body were 1300 metres by 600 metres by 300 metres (SD = 2.5), you’d have 585 million tonnes. If that ore body sported a grade, say, of 1.31 grams of gold per tonne (g/t), you’d have 24,638,722 ounces of gold. Dream about that for a moment.
Unfortunately, ore bodies don’t form regular geometric shapes. They shift, turn, bend, and fold. So, what do you do? What I do is pick a shape into which the ore body very roughly fits, then start eyeballing where it doesn’t fit, trimming sections (and percentages) here and there accordingly. For example, a ridge approximates a triangular prism. If the ridge doesn’t hold its height from end to end, I need to trim.
Next, I plot drill holes on the map, using Google Earth. Resource estimates cannot speak to dimensions outside of drill hole locations. Extrapolation does not extend past exploration. It only works inside the drilled extent of the site. Working from the length, width, and depth of collected drill holes, inside the geometric shape, more trimming may be necessary. This really is an eyeballing exercise, but without the modeling software, it’s better than nothing. It’s certainly better than blindly accepting what someone else says.
I hope this adds value for you.
Kevin Graham