Rare Earth Assay Results: What They Tell You and What They Don’t

Posted by admin at 2:20 PM on Aug 3, 2009


By Clint Cox

There are a LOT of companies coming out with rare earth assay results now. REEs have become a hot topic, and many companies are now throwing their assay results into the ring — hoping to become the next contender in the rare earth sector.

Assay results are often what companies use to project their legitimacy in the marketplace, and therefore it is critical to understand what these tests are really telling us. According to the US Bureau of Mines (as provided by EduMine here) “assay” is defined as following:

Definition: assay
To analyze the proportions of metals in an ore; to test an ore or mineral for composition, purity, weight, or other properties of commercial interest.

So what do assay results tell us?
1. How much rare earth is in that particular sample. Usually expressed in parts per million (ppm), assay results show the composition of a given sample of ore (or potential ore). This gives a sense, in general terms, of what elements are found in the sample. It is often converted to a percentage as well. They often don’t test for all of the REEs or show “trace” for the amounts.

2. The comparative percentage of one element to another. This can show if the rare earth content of the sample skews towards the light rare earths (LREEs) or the heavies (HREEs). Also, certain values of elements other than the REEs can provide clues as to the environment in which the REEs are found.


What rare earths found within?

It is also important to know what assay results DO NOT tell us:
1. What is the rare earth mineral (or minerals)? Sometimes it is assumed that you may have a particular mineral when you have a given set of assay results — I have made this assumption in the past myself. It is true that certain minerals may have a typical distribution of rare earths, but there may be other rare earth minerals involved. A good example of this would be Thor Lake, which can have a number of rare earth-bearing minerals in a given sample. Other testing methods must be used to ensure that the rare earth minerals are properly identified. After all, it is critical to correctly identify the mineral that hosts the rare earths.

2. Is the assayed sample representative? Beware of the infamous “grab sample”. Many companies take special care to take samples only from what they believe to be the prospective ore body. However, it is often difficult to resist assaying the fantastic grab sample – perhaps that one sample found 40 meters up the cliff face that has that giant perfect crystal of bigdollarite! Just make sure that the results that you are looking at come from samples that are taken from areas that are representative of the potential ore body.

3. Metallurgy. You can receive fabulous assay results from complex mineralogy. However, it may not ever be economic to get the REEs out the minerals. It takes lots of time, effort, and money to properly determine a process to create a saleable concentrate of rare earths. Many companies are taking the proper steps to establish the processing needed to pull out the REEs, but beware of the assumption that the REEs can be easily pulled out of the ground. If a company has $500 rock in the ground, but it takes you $2500 to pull out the rare earths – that project may not be feasible.