especially like these paragraphs:

let's consider the nature of the target.

In Labrador, the Voisey's Bay deposit serves as a model for explorationists. The massive sulphide "ovoid" is the focus of attention; its surface plan is roughly circular and about 300 metres in diameter and it extends to a maximum depth of more than 100 m.

Explorationists, then, are looking for a body of rock about the size of the "Rocks Area" in Sydney and the height of a 10 storey building, which may be deeply buried under gravel and/or rock!

Finding such a target is not a trivial problem and is complicated by the fact that the geological processes that formed the very rich Voisey's Bay ovoid have also formed many smaller deposits and occurrences, most of which will be too small or too low grade to ever be mined. It is not always easy or even possible to tell at first glance if the mineralization you have encountered in outcrop is or is not the tip of a very large iceberg. The simple fact of a new discovery or an exciting drill hole does not guarantee that a mineable deposit has been found.

It is important to realize that magnetic and electromagnetic anomalies can result from geological features other than orebodies. An airborne anomaly in an area of favourable geology is a good target for further exploration but it may or may not be caused by a sulphide body - ground exploration is required to confirm this.

Grade and Width

Consider an intersection over some significant width reported to grade 2% Ni and 1% Cu. The amount of Ni present in a ton of this rock is easily calculated. One tonne contains 1000 kilograms (kg) and at 2% Ni, would contain 1000 x.02 =20 kg of Ni. Similarly, it contains 1000 x .01 = 10 kg of Cu. 1996 Ni and Cu prices were about $3.85/lb. and $1.30/lb., respectively ($US) (we use non-metric price unit measures because metal prices are typically quoted in $US/lb), so the value of a tonne of rock at these grades is (20*$3.85*2.204) + (10*$1.30*2.204) = approximately $200.

This is quite valuable rock. For comparison, a tonne of rock from the Buchans mines, some of the richest deposits of their kind in the world, at 1996 prices, would have been worth around $270, a tonne of ore from the Daniel's Harbour Zinc Mine around $85, and a tonne of ore from the Hope Brook gold mine around $60.   

A few brief rules of thumb for evaluating the significance of new massive sulphide discovery might be as follows:

massive sulphide deposits are small, difficult to find targets. For every large, high grade deposit, there are likely to be large numbers of small, lower grade deposits. Massive sulphides generally have higher grades than disseminated sulphides;

Airborne geophysical anomalies may indicate the presence of sulphides and are excellent targets for further exploration; but they may also result from other geological features; grab samples with interesting assays clearly indicate the presence of mineralization on the ground, but provide little indication of the potential size or overall grade of the occurrence; channel samples provide a good systematic sampling of mineralization at outcrop scale and an indication of the width of the mineralization as well diamond drill holes normally provide the first three-dimensional picture of an occurrence. assays of drill core provide vital grade and width information, both of which must be considered in evaluating the significance of a result. Grades provide good information on the relative value of the mineralized zone but are not necessarily the grades that will eventually be mined; estimates of the value of an orebody must include some estimate of the metal prices in the future.