EXPLORATION ’93 — New focus on Colorado’s Sloan pipe

Considering the current level of interest in diamond exploration, it was only a matter of time before the hopes and dreams of explorationists once again settled on the Sloan diatreme, 135 km north of Denver, Colo.

This well-known project has been revived by Royalstar Resources (VSE), which recently announced plans to carry out a 45-tonne bulk sample, followed by a larger 2,730-tonne evaluation. The company is finding, however, that its target carries the stigma of being a textbook example of an uneconomic diamond deposit.

The 7.8-hectare diatreme is a Devonian kimberlite with eclogitic-type diamonds. This and other targets in the region were explored in the 1980s by a joint venture involving Lac Minerals and Superior Oil.

The program involved heavy mineral stream-sampling, extremely low-frequency electromagnetic surveys and excavator trenching.

Bulk sampling was then carried out, involving 52 times 50-tonne surface pits, spread more or less evenly over the pipe area. The total 2,668 tonnes were processed through a diamond mill and 23,033 stones totaling 313.12 carats were recovered. The nominal grade, averaged over all pits, was 11.74 carats per 100 tonnes.

Two contiguous pipes were recognized in the diatreme; Sloan 1 and Sloan 2. According to Royalstar, seven different types of kimberlite are now recognized in the total pipe. Since one of the facies sampled returned grades well above the average of all pits, Royalstar believes selective mining of the most favorable lithological facies is possible.

The program will not just determine the grade of certain facies. The Sloan pipe’s reputation of being uneconomic also relates to the size distribution and quality of its diamonds.

Previous operators of the project found impressive microdiamond abundances from drill holes. However, during bulk sampling, it was found the grades of the large diamonds, as well as their sizes, were not supported by these populations.

Royalstar is of the view that because one of the facies is in economic range, the Sloan diatreme represents an encouraging target for further evaluation. The junior predicts more larger stones will be recovered by the larger-sized bulk samples.

The junior has signed an agreement whereby Dia Met Minerals (TSE) will, at its own expense, evaluate and process an initial 45-tonne surface sample from the Sloan diatreme. Dia Met’s Fort Collins diamond processing plant is about 1.5 km from the Sloan target.

Royalstar will fund the cost of the 2,730-tonne evaluation, by paying a rental fee to Dia Met for the use of its plant. After completion of this phase, Dia Met will have the right to obtain a 51% interest by reimbursing Royalstar 76% of the costs of the 2,730-tonne underground sample. Results from the first phase of test work should be available by mid-1994.