Uh-oh: Silver solder in drill bits can contaminate core — Silver Spruce and ODM solve a ‘silver bonanza’ puzzle

Silver Spruce Resources (SSE-V, SSEBF-O) has learned through experience in Mexico that silver solder is routinely used in diamond drill bits and that destruction of these bits, especially if water circulation is lost and the drill cuttings are not flushed from the core barrel, can seriously contaminate the drill core.

In areas where high-grade epithermal silver mineralization is expected, the resulting high silver assays can easily be mistaken for and reported as “bonanza-grade” drill intersections.

This happened to the company last fall in drilling on the Centauro precious metals project in northern Mexico’s Chihuahua state.

The problem was discovered when check-sample analysis of the anomalous drill-core sections returned highly variable silver values.

Luckily, the company also intersected genuine gold-silver mineralization in other drill holes, but the episode has indicated that companies must be vigilant to avoid such contamination.

Subsequent enquiries have determined that many geologists and diamond-drilling contractors are unaware that silver is used in diamond drill bits.

Here’s a little more background about what happened with Silver Spruce.

On Oct. 7, 2008, Silver Spruce issued a news release headlined “Bonanza silver grades intersected — Centauro, Mexico gold/silver project” that highlighted an exceptional silver value of 1,140 grams silver per tonne (or 33 oz. per ton) over 1.5 metres in drill hole CEN-08-4 (hole 4).

It was further reported that two check assays of 697 and 1,140 grams silver had been obtained from a Canadian laboratory and that the high silver values were consistent with epithermal vein mineralization.

However, the company cautioned that additional check assaying of the immediately overlying and underlying sections in hole 4 was being undertaken at two other labs for three reasons: the silver was not accompanied by gold as expected from known mineralization in the area; the core was badly broken with over 50% loss; and the only similar high silver value obtained from the 14-hole program was from the last sample of hole 1, which was terminated due to poor ground conditions.

On Dec. 9, Silver Spruce issued another release, reporting that the additional check sampling in hole 4 had given erratic results, and that drill-bit fragments had been noted in the core box.

The company further advised that the crushed-core lab rejects of the three problematic samples from hole 4, or the remaining boxed core where rejects were not available, had been sent to Overburden Drilling Management (ODM) in Nepean, Ont.

Rejects from the suspect sample at the bottom of hole 1, and from a sample in hole 10 which had yielded combined gold and silver values, were also submitted.

ODM processed the samples by micro-panning to check for natural or unnatural silver or gold-bearing particles, then further investigated the particles, drill core and crusher rejects by scanning electron microscope (SEM) analysis.

This work established that the erratic high silver values in holes 1 and 4 are related to silver solder fragments in sludge sections in the drill core.

These sludge sections consist of a clayey paste that has hardened into lumps of the same diameter as the intact core.

Their presence indicates that drilling continued after water flow to the bit was blocked, a procedure that would quickly “burn” (shred) the soft “brass” matrix of the bit in which the cutting agents — synthetic diamonds and tungsten carbide powder — are set.

Various alloys are used for this type of brass, depending on the desired hardness and toughness. SEM analysis showed that the particles in the Silver Spruce samples consisted of a silver-copper alloy commonly called silver solder.

Interestingly, the silver and copper had combined with sulphur, possibly dissolved from anhydrite by the drill water, to produce the secondary silver and copper sulphide minerals acanthite and chalcocite, which were distinguishable from their natural counterparts only by the included tungsten carbide powder and a few adhering diamonds.

Ironically, the sample from the section in hole 10 that had returned significant values in both silver and gold (104 grams silver per tonne and 2.49 grams gold over 3.1 metres) was found to contain about 5,000 grains of natural acanthite and 1,000 grains of native gold.

Together, these results showed that any genuine silver anomalies occurring in other core samples from the drilling program could be distinguished from drill bit contamination by the presence of gold and absence of copper.

The highly erratic silver values, including three analyses ranging from 2 to 4,040 parts per million in one of the samples in hole 4, evidently reflect varying proportions of contaminated sludge and uncontaminated rock in the core samples that were submitted for analysis.

Contamination of this magnitude is a serious problem for silver exploration programs, especially those targeting narrow, high-grade veins.

Therefore any drill sludge that accumulates in the core barrel should be removed and either discarded or analyzed separately from the intact portion of the core.

For the full news releases, please see www.silverspruceresources.com.

— Peter Dimmell is vice-president of exploration of Silver Spruce Resources and a former president of the Prospectors and Developers Association of Canada. He can be reached at pdimmell@silverspruceresources.com.

Stu Averill is president of mineralogy and exploration consulting firm Overburden Drilling Management, www.odm.ca. He is at stuaverill@storm.ca or tel. 613-226-1771.