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Assays and Geochemical Analysis

A chemical analysis that determines the proportion of metallic (Cu, Pb Zn, Au, Ag, etc.) or non- metallic (Fl, S, P, etc.) elements in a sample is commonly referred to as an assay. A wide variety of geological materials can be chemically analyzed: these include water, vegetation, soil, sediment and rock.

Assay labs can provide you with single and multi-element analyses by a variety of methods. Rock and soil samples are crushed, powdered, fused or digested in acid and then analysed using any one of several analytical methods/instruments. The most common methods are: AA: atomic absorption spectrometry; FA: fire assay; ICP: Inductively Coupled Plasma - Atomic Emission Spectrometer; ICP-MS: Inductively Coupled Plasma - Mass Spectrometer; and INAA: Induced Neutron Activation Analysis.

Submitting Samples for Assay: A Checklist

Have you...

  • Made sure the sample is clearly labeled or secured in a clearly labeled sample bag? (use waterproof ink and place a card with sample number into the bag)
  • Packaged the samples securely?
  • Specified the analyses that you require, and given any special instructions?
  • Given the lab a contact name, phone number and address?
  • Specified if you want the pulps (left-over powdered sample) returned to you. This may be done at your cost.
  • Specified how you want to receive the assay results (mail, fax, e-mail, diskette)?

Avoid contaminating your samples at this stage. (Use clean sample bags and avoid handling samples if you are wearing a gold ring.)

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What Elements Do I Get Determined?

Several factors (aside from availability of funds) should determine what elements are to be determined. Obviously, the elements that are selected should include those for which you are prospecting. But don't necessarily restrict yourself to these. Some minerals (and thus elements) commonly occur together in some geological situations: zinc, lead, copper, gold and silver in Buchans (VMS-type) ore. Clearly it would be useful to analyse for all of these elements, when searching for such VMS deposits. Barite is another important mineral in the VMS deposits, so Ba should also be requested when prospecting in such an environment.

There are elements, called 'pathfinder elements', which, although not economic in themselves, can be used in indicating the potential of certain rocks to contain ore minerals. Enriched antimony and/or arsenic values can be used to indicate that an alteration zone has gold potential. High lead and silver may indicate the presence of certain types of gold deposits. In many cases, the presence of these more reactive, chemically mobile elements associated with gold (which is stable or unreactive) present a larger, more readily identifiable prospecting target. A high content of fluorine may indicate that tin mineralization is present.

Most analytical laboratories offer either single element determinations or packages that include a number of elements. The latter normally do not include gold, which must be analyzed separately at an extra cost. The standard assays are limited, in that they can only determine elemental concentrations up to a certain level. If there is a large amount of an economic element in the sample (e.g., over 1 percent copper), then a separate ore grade assay has to be requested.

Sample preparation (crushing, grinding, sieving, drying etc) is an additional cost, as is sample digestion. The difference in price between the various packages is not large, but there is no need to spend money on elements that are irrelevant. However, the packages are usually a better buy than getting a series of different elements done at different times. Most labs will be quite happy to discuss what would be the best value for money.

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Local Assay and Assay Preparation Laboratories

Local Service Companies Database

There are several other analytical labs elsewhere in Canada: e.g., Acme, Chemex, Barringer, Bondar-Klegg, XRAL, etc. All have websites that give addresses and costs of various analytical packages.

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Chemical Abbreviations

Assay sheets normally use abbreviations for the chemical elements. Some of the abbreviations may not be so obvious when you first encounter them. Most of these are derived from the Latin (given in italics)

  • Au - gold (aurum)
  • Fe - iron (ferrum)
  • Pb - lead (plumbum)
  • Cu - copper (from the Latin for Cyprus: cuprum)
  • Ag - silver (argentum)
  • Sb - antimony (from the Latin stibium, which means mark. The antimony mineral stibnite was once used as mascara by the Greeks. How's that for trivia.)

Others are simply abbreviated forms of the term:

  • As - arsenic
  • Mn - manganese
  • Zn - zinc
  • Co - cobalt
  • Ni - nickel (did you know that the word comes from the German "kupfernickel", which means the "devil's copper").
  • Mo - molybdenum

Many of the following elements are included in extended assay packages:

Al (aluminum), Ca (calcium), K (potassium), Mg (magnesium), Na (sodium), P (phosphorus), Ti (titanium), Rb (rubidium), Ba (barium), Ce (cerium), La (lanthanum), W (tungsten), Bi (bismuth), Cr (chromium), Hg (mercury), Sr (strontium), V (vanadium), Be (beryllium), Cd (cadmium).

There are a number of periodic table websites which you can check out on line. Try the Los Alamos Laboratory opens new window website as a start.

Abbreviations for concentrations

  • ppm (parts per million); a measure of concentration of an element in a rock, soil, water, etc. 1 ppm equals 1 gram per tonne
  • 10,000 ppm equals 1 percent

ppb

  • parts per billion; a measure of concentration of an element, particularly precious metals such as gold. 1000 ppb equals 1 gram per tonne.
  • 34,285 ppb equals 1 ounce/short ton

g/t

  • gram per ton
  • equals about .029 ounces per short ton or 1000 ppb

%

  • percent (the same as parts per hundred)

Dwt

  • pennyweight: you may encounter this term in some old reports. Its rarely, if ever, used any more. A pennyweight equals about 0.05 ounces per ton

oz

  • troy ounce

kg

  • kilogram
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Anomalous Chemical Values

An anomaly is simply a departure from the normal or from what is expected.

The concentrations of elements in your assayed sample may be what is normally expected from that type of rock. Or it may be anomalous - anomalously high or anomalously low.

In the case of most of the economically important metals and non-metals, you'll be most interested in the anomalously high values. The anomalously low concentrations of some elements can be meaningful, too. For example, they may indicate the presence of hydrothermal alteration in rocks that formed close to an area of economic mineralization.

Different rock types are characterized by different concentrations of chemical elements. Here are some examples of what is considered to be normal concentrations for different types of rock. Note that these are broad generalities only, and that there can a wide range in concentrations within varieties of a single rock type that may not be economically significant. For example, an assay of 2000 ppm nickel may seem anomalously high, but it is only background if the rock assayed is an olivine-bearing ultramafic rock.

Keep in mind that depending on which analytical method you choose, the detection limit may be greater than the average composition for any one element.

Table of Average Abundances for Select Rock Types

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Definitions

Detection Limit: The smallest concentration or amount of a component of interest that can be detected by a single measurement with a stated level of confidence.

Fire Assay: Highly precise and accurate method for the total determination of Au and other precious metals in (ore grade) samples.

Whole Rock Analysis: Total determination of major element concentrations typically in rock samples. Elements are expressed as common oxides for each element (i.e. SiO2, Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, TiO2). The analyses also includes measuring the concentration of all volatile phases by loss on ignition (LOI). These volatile phases could include water, sulphur and carbon dioxide.

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