Rupert advances South African project — Delineation drilling seen as prelude to bulk sampling

Under the supervision of MPH Consulting, Rupert Resources (RUP-V) has launched a drill program on the past-producing Crown kimberlite pipe in South Africa.

Eight holes have been drilled to date, with the ninth in progress. The objective of the program is to determine the pipe’s dimensions and composition at depth.

The kimberlite pipe was mined prior to 1932 but has never before been drilled.

Rupert has the right to acquire up to a 75% interest in the sub-surface portion of the mine, and up to a half-interest in a tailings reprocessing operation. In return, the Vancouver-based junior must make cash payments and expenditures totalling US$15.8 million, of which US$5.6 million can be paid from project profits.

The mine, which lies 200 km southwest of Johannesburg and 25 km northwest of the city of Kroonstad, operated intermittently from 1902 to 1931. During that time, 701,346 carats were produced from the treatment of nearly 4.5 million tonnes of kimberlite, which equates to a recovered grade of 15.7 carats per 100 tonnes. De Beers Consolidated Mines acquired the inactive mine in 1939, only to dispose of it early last year.

The main kimberlite pipe was mined out by open-pit and underground chambering methods to a depth of 165 metres and partially mined to a 240-metre depth, with the lowest development level sitting at 330 metres below surface. The mine is currently flooded and the main shaft has collapsed near surface.

A second, smaller satellite pipe, 30 metres west of the main pipe, was discovered in 1921. This structure, which is not exposed at surface, has been tested by holes 4 and 6 in Rupert’s drill program. Hole 5 was also designed to intersect the pipe, but apparently missed.

Rupert has tested the main pipe to a vertical depth of more than 800 metres below surface. The pipe consists of tuffisitic kimberlite breccia and hypabyssal kimberlite, with varying degrees of purity depending on the abundance of country rock fragments.

Drilling indicates that, below a vertical depth of 600 metres, hypabyssal kimberlite dominates, with fewer country rock xenoliths and much higher diamond counts.

Tim Wilkes, managing director of MPH Consulting’s office in Botswana, presented an update of the project to a Vancouver audience in early June. He described the tufficitic kimberlite breccia as a “dirty-type of kimberlite” characterized by abundant country rock fragments of shale and Ventersdorp basalt (40-75%) set in a kimberlite matrix. It has a lower grade than the hypabyssal kimberlite as a result of the country rock dilution. Wilkes believes much of the historically mined material, which he estimates had an in situ grade of about 30 carats per 100 tonnes, was composed of the tuffisitic kimberlite breccia.

The hypabyssal kimberlite is characterized by lesser amounts of country rock fragments and the presence of fresh olivines. Based on drilling to date, Wilkes is confident one-third of the pipe is composed of the higher-grade hypabyssal kimberlite.

The eighth hole of the current program intersected 569.7 metres of kimberlite in the main pipe, beginning at a downhole depth of 324.2 metres.

The hole was drilled 50 metres south of, and parallel to, hole 7 in an east-westerly direction and at an inclination of minus 60. Microdiamond results are pending.

Hole 9 is being drilled back toward hole 8 in a west-easterly direction. It is expected to reach the main pipe at about 330-350 metres downhole.

Hole 7 was drilled back toward hole 6 at an angle of minus 55 to a completed depth of 652.9 metres. It intersected the main pipe from a downhole depth of 317.6-645 metres. This hole was the first hole to have encountered hypabyssal kimberlite above the deepest level of the old mine workings, at 328.8 metres downhole or 250 metres below surface.

A collection of 21 samples weighing a total of 514.99 kg and representing 273 metres of varying degrees of kimberlite purity, yielded a 0.829-carat parcel of stones comprising 143 macrodiamonds (i.e. greater than 0.5 mm in one dimension) and 400 micros. Eighty percent of the diamonds are described as “white transparent.”

Hole 6 was drilled to a downhole depth of 869.3 metres in a west-easterly direction at an angle of minus 55. The hole passed through the satellite pipe at a downhole depth of 267-376 metres prior to intersecting the main pipe from 547.3 to 832.5 metres.

A 108.56-kg sample of the satellite pipe representing a 40-metre core length of tuffisitic kimberlite breccia, returned 5 macros and 50 micros. A composite 402.55-kg sample representing 213 metres of the main kimberlite pipe yielded 47 macros and 333 micros.

Previously reported holes 1 and 3 also tested the main pipe. These holes were drilled back towards each other in a north-southerly orientation.

To date, the 1.7 tonnes of kimberlite collected from the four holes drilled in the main pipe (holes 1, 3, 6 and 7) have yielded 395 macros and 1,311 micros, for a total weight of 1.61 carats. Those diamonds greater than 1 mm in at least one dimension account for 61.1% of the total weight, or 0.984 carat. The five largest stones recovered to date range in weight from 0.0597 to 0.0944 carat.

In a technical summary of the project in 1997, consulting geologist Barbara Henderson wrote: “Diamond production from the Crown mine is stated to have consisted of good quality stones with a high proportion of inferior quality stones. Most diamonds recovered were less than 1 carat, though numerous good quality stones of larger size were recovered, including fancy yellow and blue-white diamonds. The largest documented diamond recovered from the Crown mine weighed 86.5 carats.”

Former owner De Beers estimated that, together, the main and satellite pipes containe a resource of 20.3 million tonnes to a depth of 440 metres, at a predicted average grade of 32 carats per 100 tonnes. Based on drilling to date, Wilkes has constructed a model of the two pipes with a resource estimate totalling 28 million tonnes at an average grade of 35.6 carats per 100 tonnes.

As part of its evaluation of the Crown mine, Rupert is directing and monitoring the processing of tailings through an on-site 200- tonne-per-hour pan plant. Wilkes says the company has recovered almost 5,000 carats to date from the tailings at an average grade of 7 carats per 100 tonnes. The recovered diamonds range in value from US$48 to $55 per carat.

Wilkes adds that, as a rule of thumb, a value for the in situ pipe diamonds can be predicted by doubling the tailings valuation. He stresses, however, that it is necessary to take an underground bulk sample in order to recover more than 5,000 carats for a definitive valuation.

Rupert has hired Steffen, Robertson & Kirsten, a firm of consulting engineers, to examine the costs of dewatering and refurbishing the old underground shaft, the objective being to provide access for bulk-sampling development work and further drilling. The firm is also preparing a scoping study.