Resource blocks, Stope coverage and CleopatraThe bulk sample of the E585 XC (crosscut) was 2,167 tonnes. That sample was from planned mining sections (stopes) and waste or support areas. Pretivm is says that 1,451 tonnes (67%) of the sample should have come from the planned stopes. Therefore, 716 tonnes (33%) of the sample came from areas that were not planned for mining. Presumably, those non-stope areas would be lower grade, but not devoid of minerals.
The 1451 tonnes from stope areas was supposed to have 4.6 grams/tonne gold and 6.8 grams/tonne silver. Using 31.1 grams/ounce, that converts into an expectation of 215 ounces of gold and 317 ounces of silver. The actual bulk sample recoveries were 281 ounces gold and 531 ounces silver. That looks impressive, but we do not know how much of the gold and silver came from the non-stope areas. A bulk sample does not give that degree of granularity.
If one looks at both the feasibility study stope outline and the presentation slide on Bulk Sample Within Resource Blocks (see below), it appears that the stope area sample has block grades in the ranges of 5-15 and 15-30 grams/tonne gold. It is somewhat surprising that Pretivm is saying that only 4.6 grams were expected when their presentation shows 5-30 grams.
For the whole bulk sample of 10,000 tonnes, 4,000 ounces of gold is expected. The 585E XC sample is 22% of the total tonnes, but only 13% of total expected gold ounces. The remaining 87% of expected gold (3,719 ounces) must come from the remaining 78% of the bulk sample. Over the whole bulk sample, the gold average must be 12.4 grams/tonne in order to make the 4000 ounces.
Some people dismiss these concerns by saying that 585E is the lowest grade crosscut. That does not appear to be the case to me. Below, I have developed a table based upon the two presentation diagrams mentioned above. I counted the sample rounds for each crosscut and determined which of them were in stope areas. As well, I eyeballed the percentage of the crosscut that is in each grade range. You will see that 585E XC has 24% of “in stope” rounds and about 70% of block grades >5 g/t. It would appear to me that E555 XC has both a smaller number of rounds and lower grades.
If Pretivm is to get close to 4000 ounces for the total bulk sample, it will depend very much on the high grade associated with 615E XC and 615 E-L. Those are the areas associated with the Cleopatra vein and generally higher grades. Those are likely to be the last reported areas.
So much depends upon Cleopatra. Is this bulk sample really representative? What would happen if Cleopatra was not included?.