The Gold AnomalyIn early-stage gold exploration there is often talk of gold ‘anomalies’. In simple terms this is where samples have been taken and the amount of gold found in them is higher than what is normal for that area. This is good news because it means there is a better chance that a gold deposit occurs in this area.
Defining value equals an anomaly can be difficult however because it is a relative term - there is no one value above which would be called anomalous. Anomalous gold values be very low, much less than economic grade but still significant if in the context of stream or soil sampling. The significance of any gold anomaly depends intrinsically on context. This context includes the spatial extent of the anomalous samples; the material the sample was taken from; the size of the sample, and the background gold level for that area. Two examples where gold anomalies exist but are not of any significance due to their context. Example 1: Contrasting background gold levels of adjacent rock types The background level of gold in many rocks can be zero, or it can be up to 4 parts per billion (0.004 grams gold per tonne of material). This is well below the economic grade. Figure 1.1 shows a 12 square kilometre soil sampling grid. Most samples have 0 ppb gold, except for a 3 square kilometre zone which has 4 ppb gold. Technically this 3 square kilometre is anomalous in gold. If the spatial extent represents an underlying gold deposit with higher grade then a major discovery could be at hand. However, in this case there is no use getting excited if see the underling geology shown in Figure 1.2. The 4 ppb Au results correlate exactly with black siltstone, a rock type known to typically have a 4 ppb Au level. The remaining area with no gold is a rhyolite, a rock known to be have gold level less than 1 ppb. |
|
Example 2:
Isolated highly anomalous stream sample vs low level anomalous but consistent stream sample values In Figure 2.1 below a single stream-sediment sample returns a value of 8 ppm gold (8 grams gold per tonne of material). This is a very high value and any deposit mined with this grade would in most cases be extremely profitable. However, the sample taken was only 50 grams of sediment, which is relatively small for a sample of this type. The smaller the sample the more likely a small gold grain can 'spike' the gold value. If you take a larger sample this is less likely to happen. In this can the sample happened to catch a small grain of gold within it to give the high result of 8 g/t Au. This is not representative of the stream sediment of this area in general. A second sample taken in the same place may return zero gold. This one-time 8 ppm Au sample is therefore not significant outside the context of a larger program with some supporting samples, or some repeat sampling that is able to replicate such a high result. A single-point or small sample number program may return high value results, but will be difficult to interpret in terms of spatial consistency and gold source. This makes it hard to draw conclusions about where to do follow up work. Contrastingly, a stream sediment program such as that in Figure 2.2 which returns values much lower than the earlier 8 g/t Au value would in fact be more significant if they show a clear spatial consistency that indicates a common source (see Stream Sediment Sampling for an illustration). The takeaway is that when talking gold anomalies, context is everything. When reading company releases we must realise all is not what it may seem at first glance. |