Thanx pw and enginerd for your valuations.

I found a few more details which I want to share.

First, in wikipedia I found the s.g. of felsic rock, which is a mixture of FELdspar and SIliCa. Feldspar has 2.5, silica has 2.65 (unfortunately they German wikipedia says amorphous silica has 2.2). So if we assume 2.5, we are on the conservative side.

Then I was confused on the factors between grams and ounces. The factor 34.2 applied by pw may be the factor of 31.1 grams per troy ounce, combined with a recovery factor. Using a higher factor takes into account that the gold recovery is not 100% but perhaps 90 %. Is that a good guess?

dr