News Releases:"true width" of core assay
Found this article on the internet, hope it helps
understand these press releases from mining companies.
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Diamond drilling using large machines is the preferred method of delineating mineralization in Canada where access is a problem. Most drill machines are designed to break down into pieces small enough to be lifted by helicopter which allows the drills to be moved rapidly from site to site. Diamond drill costs range from $20 per foot for easily accessible targets to $100 per foot for remote areas.
Diamond drills produce core which ranges from approximately one to two inches in diameter. Drill core is usually split in half by either a core splitter which produces samples of uneven size or by a diamond saw which produces halves of equal size. Half of the core is saved for geological examination and half is bagged and sent to the laboratory for analysis. Drill core is usually analyzed by assay methods.
A press release which contains drill core assays should always quote the length of the mineral intersection as a "true width" or a "drilled length". The difference in these two measurements is extremely important when attempting to assess the economic significance of the press release. A "true width" takes into account the angle the drill hole cut the mineralization and corrects for drill lengths which have traveled oblique to the direction of the mineralized bed or vein. A "drilled length" is the measured length of core that contains mineralization. This length can bear no relationship at all with the "true width" of mineralization and must be treated with caution.
Reserves
Advanced drilling projects are designed to define the average grade of mineralization and the amount of economic ore which could be profitably mined. These ore reserves are usually broken down into three main categories: Proven or Measured Ore, Probable or Indicated Ore, and Possible or Inferred Ore. The definitions for the above categories are given in detail in the Canadian Securities Exchange Policy No. 2A, which is the guideline used by the VSE. These definitions should be referred to when examining corporate information regarding tonnage and grade of an ore body.
It is probably sufficient to say here that "Proven Ore" must be drilled or sampled in three dimensions and have a solid assay database to support the quoted grades. "Probable Ore" can include data from projections over reasonable distances and "Possible Ore" is a flat-out arm wave based on a few drill holes and geological projections.
Should press releases not indicate in which category quoted reserves lie, always assume that the mineralization is "Possible" rather than "Proven" or "Probable".
ANALYSIS
Sample Preparation
The analysis of geological samples must start with the reduction and homogenization of the sample into a form which can be easily handled by the analytical personnel. This initial step is called sample preparation and is the source of many of the errors in a chemical analysis.
Soil and stream sediment samples are usually sieved so that particles larger than fine sand are removed. The fine particles are mixed and a portion is removed for chemical analysis.
Rock samples are treated in a multi-step procedure. Rocks, cuttings, or core are first crushed to about pea-size in a jaw crusher, then passed through a secondary crusher to reduce the size further - usually 1/10 inch. This crushed sample is mixed, split in a riffle splitter and reduced to about one-half pound or 250 grams. This 250 grams is placed in a pulverizer where it is reduced further to -150 mesh.
This reduction process for rocks is relatively trouble-free when testing for base metals, but can be fraught with problems when precious metals are involved. Certain types of pulverizers can smear large gold particles causing a loss of gold onto the pulverizer plates and of course a decrease in gold in the sample. The pulverizer can then spread gold to the following samples creating false anomalies.
Large gold particles can create the "nugget effect" in samples. When nuggets are present, repeat or check analysis of the same sample will not reproduce the initial gold concentration, which in turn leads to uncertainty in the complete sampling and analytical procedure. Measurements of this "nugget effect" can be made by performing a metallic gold analysis, which consists of removing the native metals by screening through a fine sieve. The coarse and fine fractions are then assayed separately. Knowing the weights of each fraction allows the calculation of the total amount of gold in the original sample.
Assay Techniques
Assay procedures differ from geochemical analytical techniques in that they more accurately represent the mass of the sample being analyzed. In general, assay techniques use a significantly larger weight in the determination. Assaying for base metals uses a combination of wet chemistry and instrumental techniques. Wet chemistry utilizes a physical measurement, either the color of a solution, the weight or volume of a reagent, or the conductivity of a solution after a specific reaction. Assaying of mineral concentrates or geological samples containing a high concentration of a specific element are relatively accurate and precise. They are the preferred techniques to determine element concentrations in ore samples.
Precious metals in rock or soil should be analyzed using the fire assay technique. The advantage of fire assaying is that a large sample can be used. One assay ton, the usual amount of sample used for fire assaying, weighs approximately 30 grams. This large size for test samples reduces the "nugget effect" and results in better analytical precision.
The usual fire assay technique for gold and silver transfers a one assay ton portion of the sample into a crucible where it is mixed with a variety of chemicals. This mixture is fused at a high temperature. During this fusion, beads of metallic lead are released into the molten mixture. The lead particles scavenge the precious metals and sink to the bottom of the crucible due to differences in density between lead and the siliceous component of the sample, known as slag.
When the fusion is complete, the molten mixture is poured into a mold and left to solidify. After cooling, the slag is removed from the lead and the lead button is transferred into a small crucible known as a cupel and placed back into a furnace. The lead is absorbed by the cupel, thus leaving a bead of the precious metals at the bottom of the cupel. This technique is the source of the Alchemist's claim of "turning lead into gold".
The gold and silver can be measured by weighing the bead on a balance, dissolving the silver in nitric acid, and then weighing the bead again to determine the mass of the undissolved gold. Silver is calculated by difference. Another method of finishing the determination is to dissolve the entire bead in an acid mixture known as aqua regia and measure gold and silver by atomic absorption. Other forms of measurement include neutron activation analysis and flameless atomic absorption.
An experienced fire assayer can recover nearly all precious metals from almost all matrices. In the past there have been claims that a proprietary technique must be used to recover gold. These claims must be treated with the highest degree of suspicion. In my experience, there has never been a sample containing gold where the gold could not be detected using the fire assay technique.