Resource estimate considerations (cont.) In a series of posts from July 17-22, Bloomfield provides us with a number of comparative notes regarding the resource estimates provided by GH Giroux (2006) and Peter George and voices his concerns that “the 2012 estimate will find a lot more indicated resource largely because of easy parameters” employed in the resource calculations. But is this explanation too simple?
The near-surface open pit calculations provided by Giroux pertain only to a certain type of ore that exists in the higher benches (elevations) of Cow Mtn. Giroux wrote: “Within the area covered in this resource estimate, only the quartz-type veins are of economic value.”
Hall (1999) wrote: The Cariboo Gold Quartz Mining Company Limited explored for pyrite-type ore during the 10 year period prior to closure of the No. 1 mine in 1959 but most of it was carried out below Jack of Clubs Lake. Exploration for replacement ore within the Baker section of stratigraphy and above the 1500 level was minimal.
The Giroux study concentrated almost exclusively on the zones above the higher 1200 level (4200-4700 ft. elevations) and employs stringent resource calculations that pertain to the quartz-type vein ore in those zones.
The 2012 resource estimate is modeled to fit both quartz-type vein ore and higher grade pyrite replacement ore in the lower 3550-4550 ft. elevations and in new zones drilled in 2011 well outside the Rainbow-Sanders-Pinkerton zones.
Whereas Giroux used 25 X 25 X 15 ft. blocks, as “Giroux reasoned that the gold mineralization was confined to relatively narrow zones” in quartz veins, George’s 25 X 25 X25 ft. block model must now incorporate the predominant pyrite replacement ore mineralization into the geological model, where “individual stopes show pancake-shapes in plan rather than rod-shapes, with marked plunge continuity, more typical of the Island Mountain and Mosquito Creek Gold mines”. (Hall, 1999). And the less-stringent search ellipse in the George model also finds validation in the fact that the Island Mountain mineral distribution showed that “Quartz-type ore with an average grade of 0.35 ounces of gold per ton (12.0 g/t) and pyrite-type (“replacement") ore with an average grade of 0.67 ounces of gold per ton (23.0 g/t) were mined. Pyrite-type ore was higher quality ore accounting for about 40% of tonnage mined and about 60% of the gold produced.” (Pickett, Dec. 2000). We must also consider that the Giroux calculations for narrow quartz vein mineralization were constrained by higher cut-off grades (gold was $272/oz in Dec 2000) in mineralized zones that graded 0.89-2.47 gr/tonne, the lowest grades within the bench estimates for the 2012 resource estimate, and failed to allow any concessions for the fact that that the ore that was mined from these quartz-type vein systems nevertheless yielded an average .039 oz/ton throughout the historic underground mine life history, well above grades necessary for an open pit model.