"Am I right i thinking that the cutting of drilling assays is to eliminate the nugget influence from the drilling results ? "
No. It is a Noranda practice. You have to understand that Noranda ran one of the biggest gold companies in North America, but it was NOT a gold mining company. They were a copper mining company who recovered gold in their process from sulfide ore of the Abitibi. Which should be world famous for one thing. Sulfide mines that contain lots of told. The Horne was to put it mildly on of the world's best gold mine. It made far more per ounce than the Hollinger Mine which was the grade leader - for its size - in the world for quite a while. But Noranda knew nothing about running primary gold mines. They indirectly owned a few like Kerr Addison, but they did no technology transfer. No common staff.
And believe it or not, in assessing ore, the Dome people were led far astray by simple stuff like sampling towers. Sampling towers were the invention of a bunch of US custom milling people in the 1880's who wanted a reliable way to pay people on custom ores that they would trust, i.e. the client, but would allow them a healthy margin of gold in the mill after losses. That the client would never wise up to. Sampling towers are statisically weak. Only an expert mathematician and geologist could figure that out. Few did. Dome never got the joke and relied on them to assess the grade of the Musselwhite. (Pickle Lake) I believe I had a hand in being such a vocal critic of this process around Timmins to all and sundry from the local baker at to the recording officer that it may have sparked Dome to try again and re sample the Musselwhite with underground structurally oriented channeling. Shades of 1870! They just about doubled their grade. Finally the drill holes lined up. They even offered me a job working at the project underground, but conflicts of another kind got in the way.
Noranda knew so little about assessing the grade of a gold mine that they had to ask an assayer in Haileybury I knew to solve the "grade problem" of their Hemlo mine. They could not establish its averages because the could not repeat their assays, or match the grade in core reliably. It was too variable. Contrary to what you may have heard about Hemlo's famous continuity etc.. So the assayer took the core, and said he would reprocess, it possibly destructively to come up with a definitive answer they could hanf their hat on. Schiarelli had given them a drop dead date and $300 million of expenditures had to come up green or the project reverted. Sweat was in blood at head office. What the assayer did was take every foot of exploration core intersection that the geologist told him thought would make ore, ground it up, put it in a big cyanide bath and recovered the gold therein by Merril Crowe. I believe at Corey Miller's plant in Timmins. Once the gold was weighed and the tailings assays in, the simply divided by the weight of the core, and told them the answer. Their grade was 0.29 OPT. The fease was positive and the mine construction was started. He never told them until after he had done it, that was going to be his technique. But he did say that he would stake his professional reputation on it being right.
If I had been the geo on the project I would have done it another way. I would have saved the sludges. All of them. And lovingly stored them in tin pails with reference to core photographs in a book and assays. All on a spread sheet with assay tags #, structural 3d pix wire frame and hole numbers. Whence came the inevitable management frittering, you simple smile and say there is a fool proof way to get a 1/200% cross section sample of the orebody. Bear in mind a single drill hole, 50 feet centers crosses a 30 foot intersection to create 6250 tons of ore. The drill hole of BQ weighs 1.81 lbs per foot. So 54 lbs = 6250 tons. The cuttings weigh 114 lbs. over the same interval - 16,000 ore to one sample, but so much more reliable than the core that it's funny. That is why cuttings are ALWAYS more reliable than core for assessment of true grade. They are always close to what you mine. You will hear of people telling you, oh no, the cuttings concentrate gold, the smear it, etc.. now stop for a minute. How can this be? The cuttings take up more gold than they do rock? How could this happen? The rock stays behind but the heavier gold rises in the water flow back to the drill?. uh huh.. well it does NOT happen. If anything you lose gold. So why do you often, very often get a wider intersection of gold in the cuttings assays than in the core. People will point to the imprecision of the cuttings sample point. But they should think again. If it were true that cuttings allowed one to include barren rock then the grade of the "edges" of a sludge sample assay should be lower all the time. But if properly controlled, they are not. What happens is the cutting get wall rock gold that the core samples miss, by virtue of simply the law of averages. The annulus of a diamond drill hole core or the kerf in sawyer's terms, is 2 times the volume of the core, so it stands to reason it gets 2 times the gold and then some. Then a simple thing happens. Mixing. The gold that does exist in the cuttings is not randomly distributed around the core, anymore. It is also not subject to diamond core saw losses of being smashed out of "nickels" of core when splitting, or simply falling out of shears into the box or on the floor. Those nickels of core ( slices of core in thin sections) split at the direction of shear emplaced gold and thus the loss rate is higher. Given that a highly nickeled core is argillitic or carbonaceous and not terrifically gold bearing, there is usually some leeway here but it is a factor. All the gold gets into the cutting split in sludge sampling and is faithfully recorded over the whole core length. It is a much more reliable form of sampling. Now we must deal with the dreaded air core, or RC drill and why it always gets a lower sample than the diamond drill. Why does that happen? well it is a whole other question. It grinds the core right up and spits it back to surface by air pressure. It has terrific volume, but always underestimates the gold. I will leave it to you as to why.
So how many people do cuttings today? Not many at all. The geos just don't know enough. No experience. You would need about 30 years of exploration gold drilling and mining to see that. Few people have that.
Oh. nugget effect? Nugget effect drives grade up, not down. So cut backs are the opposite. What they try to do is reduce the effect of anomalously high samples that are 4th standard deviation. rare. Is this fair? Well no. Texts point out that it is NEVER justified to do cut backs until a reliable milling method of known recovery has worked the ore which has been shown to have a known dilution over the drilled area. Then factors can be developed. The Salmita and many other mines would NEVER have been mined with any sort of basic one ounce cut back rule. Only every tenth assay at the Salmita made ore. And it made ore at 10 to 30 times the average grade! So what do you do then? In fact it maybe more intelligent to just drill enough and do arithmetic averaging. It is instructive to read texts on kriging. Kriging degrades on sufficient sample density to arithmetic averaging. Note the word degrades. What does it do with insufficient sample density? Guess. No I don't mean you guess. Kriging guesses. What are guesses worth. Well that question is answered by the mathematical game of risk and uncertainty analyses. This tells you how large your holes must be for so many tons of an orebody so wide, and how large your assays have to be, for such and such a confidence. I would say offhand you can mine an orebody that is 8 feet wide, with drill holes 65 feet apart across the plunge, 130 feet apart down plunge, with N core, using 60 gram samples on the sludges and 90 gram samples on the core, fire assay with careful Ballings calcs on each assay until you know the metallurgy. 2.5 foot runs on core. Then you can go standard formula assay, probably four formulas for the orebody. High sulfide, lots of quartz, chlorite zones, high carb, etc.. I can assay ten feet apart on vein. Small volume chips not large, they are more accurate than large. If you do that you can get to 0.01 OPT of real smelter return on fine gold distributed ore. On nuggety stuff, from 15 to 30%. Most nugget effect is poor sample extraction, ratios and assaying.
"The often complex, erratic, and localized nature of gold is a common feature of many vein-style gold deposits. This style of mineralization is often referred to as being nuggety or possessing a high-nugget effect. As a result of these complexities resource estimation is difficult and in general, only Exploration Results can be provided or an Inferred Mineral Resource estimated from surface drilling data alone. Underground development, further drilling, and probably bulk sampling will be required to delineate Indicated and Measured Resources. Tonnages can generally be estimated from diamond drill and development information with a reasonable degree of confidence. Grade is much more difficult to define with confidence because it is commonly highly erratic and discontinuous in nature. The dependency of higher confidence Resource categories on development information may create a Catch 22 situation, with funding for such development often depending on the prior definition of at least Indicated Resources. There are no easy solutions to these challenges posed by high-nugget effect deposits, and it is important when classifying and reporting not to downplay the uncertainties often associated with Mineral Resource and Ore Reserve estimates for such deposits. However, in common with all deposit types, if the principles that underpin the estimation, classification, and reporting procedures are borne in mind and common sense applied, most issues can be satisfactorily resolved. This paper discusses the classification and reporting of Mineral Resources for high-nugget effect gold vein deposits within the framework of the JORC Code (JORC, 1999)."
S. C. Dominy, S. C. DOMINY, M. A. NOPPE, and A. E. ANNELS
Errors and Uncertainty in Mineral Resource and Ore Reserve Estimation: The Importance of Getting it Right
"The Occurrence of High-grade Gold Pockets in Quartz Reefs at the Gwynfynydd Mine, Wales, United Kingdom: A Geological Explanation for the Nugget Effect
IAN M. PLATTEN
GGI Consulting, Hertfordshire, England, United Kingdom
SIMON C. DOMINY
Economic Geology Research Unit, School of Earth Sciences James Cook University, Townsville, Queensland, Australia
Gold-bearing quartz reefs commonly show extremely erratic and unpredictable grade variation, although gross geological continuity may be good. This type of variation is often described as being nuggety or having a high nugget effect and can be measured quantitatively using the semi-variogram. Understanding of geological features such as reef texture and structure will provide improved models for the interpretation of assay data, drill core descriptions, etc. In this paper, a case study from the quartz-hosted gold occurrences in the Gwynfynydd mine, United Kingdom, is described. The work provides a starting point for models of lateral variation and demonstrates the types of structural and textural features that may be sought to give clues to the prediction of gold distribution in similar deposits.
Gold deposition at Gwynfynydd is primarily associated with the reef footwall. The host veins form a discrete group of structures that may have been emplaced early in the sequence of quartz veins forming the reef. This potentially exposes them to a maximum number of dissection events during repeat dilations of the lode. Gold was deposited in the local porosity at extreme concentrations, equivalent to kg/t grades, when the host fissure had become filled with a porous framework of crystalline quartz and sulfides. Distribution of porosity varied laterally along the vein as the result of the formation of clump-like growths of minerals from the vein walls. The growth style is relevant to the distribution of gold although it may not be relevant to the occurrence of gold in a particular vein. The preservation of pores with some connectivity late in the vein fill sequence may be important in permitting continued but slow fluid flows. This facilitates effective fluid reaction with wall rock-derived methane, thus changing the relative rates of gold and quartz deposition in favor of gold.
The textural studies explain the first-order control of nugget distribution at Gwynfynydd but do not allow for prediction and optimization of the resource estimation process. This is principally because of the very low geological continuity and poor predictability of the high-grade pockets. Nonetheless, the work does provide a clear geological explanation for the erratic nature of the gold. It indicates textures and structures that can be used to determine geological continuity of the gold-bearing elements within the gross reef envelope.
"
"Sample Size and Meaningful Gold Analysis
This note gives some numerical scenarios to illustrate the severity of nugget effects. Please refer to the two graphs in the Field Geologists Manual, 2nd Edition 1982, p97-98. These are reproduced from the USGS Professional Paper 625-C by Clifton et al, 1969, "Sample size and meaningful gold analysis".
1. Assume -80# samples (-177 microns).
Clifton et als' sampling statistics considered a binomial distribution for particulate gold. To achieve ±50% precision 95% of the time (ie 20 particles of flake gold of -80 mesh required) on a 50g sample taken for analysis (eg by fire assay), the results will only be reliable at and above 3 ppm (3000 ppb). So much for this method of sampling and analysis!
One can only get away with this, and have reproducible results, if the gold flakes within the -80# sediment are very much finer than 80#, as they mostly are. For example if the gold was only 30 micron sized (80# is 177 microns), then 50 g would be a reliable analytical sample at and above 16 ppb (graph page 98).
If visible gold can be panned, however, by definition nugget effects can be guaranteed some of the time, as such gold is relatively coarse.
Crustal abundance for gold is approximately 2 ppb. A 50g sample running 2 ppb (±50%) consistently, implies that the gold is as 8 micron spheres, or 15 micron flakes (graph, p98). This may be a barren, background result, indicating 20 grains of this size, or >20 grains of a smaller size — none of which is pannable or visible. But suppose that within this spectrum of grain sizes we have one flake of coarser, 125 micron gold per kilogram which would report as perhaps 5 flakes of "VFG" in a typical pan concentrate. This might be sourced from a small, distal auriferous quartz vein.
Referring to the graph on p97, this flake would weigh approximately 3.5 micrograms. On a unit basis, this equals 0.0035 micrograms per kilogram, or 3.5 parts per billion. Added to the 2 ppb median, the result should be 5.5 ppb. However, a 50g sub-sample taken for aqua regia digest would include this 125 micron flake one time in twenty, reporting "72 ppb" as a strong spot anomaly.
Follow up of such spot anomalies consumes time and money.
NB. One should look at spatially clustered gold anomalous areas rather than following up isolated extreme highs, provided sampling density is adequate. The anomalous site should also make geological sense — does it drain structures or formations/intrusions of potential interest? (check float lithologies in field log).
Reanalysis of 50g of the same sample would produce a value of only 2 ppb, which should alert one to the problem. Sieving to -200# or even -150# would have eliminated the random flake and given a reproducible 2 ppb result. In the 50g of -200# (75 microns) case, results above approximately 250 ppb would always be reliable (graph, p98). The same aqua regia digest could also be used to determine arsenic, bismuth and chalcophile elements.
BCL (or BLEG) sampling attempts to solve this problem by taking a large, more representative sample, say 2 kg, for analysis. Such a sample can be treated with an alkaline cyanide leach (the principle of CIP and CIL extraction) at room temperature, whereas fire assay or hot aqua regia digest on such a large sample would be difficult. In the above example, the one 3.5 microgram flake in a 2 kg sample would add 1.75 ppb to the median value.
Assume 2 kg of -40# BLEG.
In practice there are many variations, taking sample weights from several hundred grams to up to 10 kg, with sieve sizes ranging from fine to quite coarse, of the order of 10 or even 5 mesh. Some practitioners add Magnafloc to coagulate suspended clays, while analytical variations include using a static leach, where the sample is not agitated continuously. This would promote considerable scope for analytical error.
Methods of concentration of gold from the cyanide solution for final presentation to flameless AAS or ICP or ICP-MS include precipitation with zinc dust, coprecititation with tellurium on reduction, adsorbtion onto activated charcoal, or solvent extraction. Silver, palladium and copper can be determined from the same cyanide leach, although in my experience correlation of copper with conventional acid leach copper is not good.
Assuming 2 kg of -40# sample, a 4 ppb result would be reliable (± 50%) if all the gold within the -40# (-475 microns) fraction was present as 20 grains of 65 micron gold, or over 20 grains of finer gold. ( see graph in Field Geologists Manual, p 98). 200# is 75 microns = 0.075mm.
But, if we had two flakes of 0.22mm gold within this 2 kg sample, reference to graph p 97 shows this would add 16 ppb to the sample to make it 4 + 16 = 20 ppb instead of 4 ppb.
The answer is to eliminate random, coarser gold by fine sieving but still taking a reasonable bulk for BCL analysis. There is, however, a practical trade off in how long it takes to sieve a lot of fine sample, particularly from coarse soils or sediments, or high energy streams."
"Abstract:
High-nugget effect gold-quartz reefs are one of the most challenging styles of mineralization to evaluate and exploit. The estimation of mineral resources, and subsequent ore reserves, forms a critical base for exploitation. The understanding and application of grade and geological continuity issues during estimation is of importance. Most reporting codes make specific references to grade and geological continuity issues. Within the high nugget environment, confidence in the tonnage estimate is variable depending upon geological continuity, though it is usually higher than the confidence in the grade estimate. Estimation of grade is problematic as a result of the erratic and low continuity of gold values and gold-bearing domains. Detailed geological interpretation supported by 3D modelling to produce either 'conceptual' or 'visual' continuity models is critical. The inherent risk associated with the estimation of mineral resources and ore reserves and subsequent classification can be reduced by a strong understanding of geological and grade continuity.The level of investment required to acquire this information is generally substantial, and will almost certainly involve underground development supported by close-spaced diamond drilling and bulk sampling/trial mining."
Grade and geological continuity in high-nugget effect gold–quartz reefs: implications for resource estimation and reporting
Authors: Dominy, S.C.; Platten, I.M.; Raine, M.D.
Source: Applied Earth Science : IMM Transactions section B, Volume 112, Number 3, December 2003 , pp. 239-259(21)
***************************************** |
