Gravity Separation & Concentration Methods

Gravity Separation & Concentration Methods

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Gold losses by Churn Drills on Wet Alluvial (4 replies)

Marshal Meru
6 years ago
Marshal Meru 6 years ago

The performance of Churn Drills on wet alluvial merits some discussion as I agree that Churn Drills are EXCELLENT on DRY-ISH ground (terraces, small creeks, paleoplacers etc) but USELESS on WET ground such as floodplains or below water tables.

Possibly the largest systematic test on the performance of Churn Drills in the world was conducted in the Zaamar Goldfield by Soviet and Mongolian drillers.

More drilling confirmed similar discrepancies existed on a 50km stretch of the Tuul, justifying the investment in 5 large Siberian-built bucket-line dredges.

Without checking, the 5 large IZTM bucket-line dredges would not have been launched, halving Mongolia’s gold production in its painful cash-starved transition from a command economy to a market economy.

I believe 20 or more large placer dredge projects exist in the world, but written off as uneconomic long ago, due to poor false results from churn drills.

If alluvials are waterlogged, the correction factor for Churn Drill logs is often X3.

Claims elsewhere say gold production of large dredges with on-board sluices/jigs often tally with predictions derived from Churn Drills. But gold losses by wet Churn Drills means that a decent dredge and recovery system should produce far more than 100% of the churn-drilled gold resource.

Are there any large data sets for Churn Drill logs in wet alluvials available in public domain elsewhere in the world?
6 years ago
(unknown) 6 years ago

How did the expected total gold resource (as determined from churn drilling results) compare with the actual gold recovery added to the known gold losses (as determined from the Knelson or sluicing).

Are you saying that gold production has been three times greater than expected (on the basis of churn drill results)? Or gold production would have been three times greater than expected if 100% gold recovery had been achieved (on the basis of churn results)? Or are you simply comparing grades as obtained from lotok + blowing + micro-balance when considering churn drill verses bucket drill for IDENTICAL locations (which is difficult to fully establish - though good estimates can be determined)?

6 years ago
(unknown) 6 years ago

Curious at what was the size screen analysis of the recovered gold, in comparison of the churn drill results and the bucket drill and the actual recovery from the dredges.

6 years ago
(unknown) 6 years ago

In those days the difference in gold recovery from wet alluvials was well-known and well-documented by the Soviet (Russian) geologists, but us western geologists were largely in the dark. By the time Golden Tiger Corp (affiliate of Java Gold Inc CDNX) wanted to drill there were no Soviet Bucket Drills left in Mongolia, but lots of Soviet Churn Drills. I think the Mongolian drilling contractor for GTC was MAK, who were not exactly straightforward as some of the hard-rock drilling they did later for Mongolian Goldfield Corp (aka Tyhee) was allegedly salted with placer gold. Java Gold were told by MAK that in order to bring the new Churn drill results into line with the older Bucket drill results, a correction factor of at least 40% would be required. Naturally I have doubts on the MAK drilling results in view of their later track record. Anyway, the independent study by Competent Person rejected the 40% correction factor as unrealistic and untenable, so undermining Java Gold's press releases and in the end Java Gold ditched its Churn Drill results and submitted the much higher Bucket drill resource calculations to the IFC for funding. IFC rejected the funding application, and Java Gold later collapsed due to cash starvation. The splitting by each large Bucket Drill established block by a Churn Drill line seemed very logical at the time (I would have done the same) to create better defined resources, but unfortunately created a lower total resource based on alternating bucketchurnbucketchurn lines,

instead of the soviet bucket<<>>bucket<<>>bucket lines.

Without "some boosting of churn grades" followed by 40% correction factor the effect was a saw tooth.

I agree that these were not identical lines, so the jury will forever be out on this.

However, the SOVIET Churn Drills were each checked by a SOVIET Bucket Drill, by drilling very close to each other, not for splitting blocks. Therefore the SOVIET-SOVIET 'competition' is has jury and executioner, being supervised by a single team.

Zander Barcalow
6 years ago
Zander Barcalow 6 years ago

Whenever people suggest applying factors to assay numbers they are implicitly admitting they have sampling problems. When you use holes drilled by different machines, each producing its distinctive diameter, you can demonstrate the size of the problem by plotting frequency of results on log-normal cumulative graph paper. I suggest the total weights of gold recovered be used, rather than cubic measure.

The insuperable problem with alluvial gold deposits of dredge-able size is that the overall deposit is often of fairly consistent value but the internal variations of gold distribution are on such a scale that drilling does not reflect the true picture. When we plotted gold recoveries from successive clean-ups, a picture emerged of a multitude of 'fingers of high grade only a few of which the drilling detected. Overall recovery on a year to year basis was very consistent although the averages calculated from drill results fluctuated quite violently.

Based on years of familiarity with the deposits, the consultant routinely factored 'reserves' in his annual report to the directors for budget purposes. I have seen factors of 200% of drill-hole estimates used when the gold content indicated by drilling was lower than usual. Results usually came out very close. The reason for this success was that the gold content of any block that was dredged was fairly consistent from year to year. The drill-hole results suffered from a huge regression effect.

All this is easy once you have twenty years’ experience but it calls for guts at day one. We have no history of experience in applying geostatistics so would recommend avoiding its use. Far better to use statistics applied to particle sizes of the gold recovered. Overall, our drilling was about ten percent light. I ascribe this to the presence of a few flakes of coarse gold the frequency of which is impossible to estimate by drilling.

The outline of the deposit should be evident from drilling. Inside that outline, the best thing to do is juggle around with the statistics looking for fatal flaws. It's like playing the violin, some people get more out of it than others!

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