Ni-Cu-Co Type Magmatic Massive Sulphides

Making sense of mining company press releases

The latest press releases from mineral exploration companies active in Labrador have just come out.

Company A says that it has completed its airborne geophysical surveys and has "several interesting magnetic and electromagnetic anomalies that require further testing".

Company B reports five grab samples from outcrops on their Labrador claims containing "up to 2% Ni, 0.5% Cu and 0.12% Co".

Company C reports that their latest drill hole has intersected 2 metres of massive sulphide grading 3% Ni, 1% Cu and 0.15% Co while

Company D describes a drill intersection of 30 metres of 'disseminated sulphides'

What do you make of these? How do these descriptions relate to the possibility that any of these companies has found a potentially viable ore deposit?

Mineral exploration is a complex activity and there are many variables that affect the potential viability of any discovery. It is important, when attempting to evaluate the significance of any particular announcement, to be aware of some of the fundamentals.

First, 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.

How does one go about finding one of these deposits?

Orebodies such as the Voisey's Bay deposit consist mainly of sulphide minerals in which the Ni, Cu and Co are chemically bound to sulphur.

The main Ni- and Co-bearing mineral is pentlandite [(Fe,Ni)9S2] while the main copper mineral is chalcopyrite [CuFeS2].

The deposits also typically contain a lot of pyrrhotite, an iron sulphide that is of no commercial value but which is an important factor in exploration as it is magnetic and can be detected by magnetometer surveys (see below). When a deposit consists almost entirely of sulphides, it is termed "massive sulphide".

When it consists of clots or patches of sulphides in the country rocks, it is termed "disseminated". The proportion of sulphide minerals, and therefore the metal grades, are generally higher in the former. However, it is not uncommon for disseminated sulphides to be sufficiently concentrated to achieve ore grades.

Geophysical Exploration

The search for a buried sulphide deposit often begins with an airborne geophysical survey.

Some sulphide minerals are magnetic (i.e. pyrrhotite) and therefore may cause a measurable perturbation in the earth's magnetic field in the vicinity of the deposit; others are good electrical conductors, and can measurably affect the strength and orientation of electromagnetic fields in their vicinity. In conducting an airborne survey, the aircraft systematically crisscrosses the property while sensitive instruments on board measure the earth's magnetic field and the intensity of electromagnetic fields.

"Anomalies", or unexpected perturbations in these fields, are recorded and compared with responses theoretically expected from sulphide bodies.

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.

Gossans

When prospectors or geologists first start to look for a sulphide body, they pay special attention to "gossans". A gossan is simply a patch of rusty rock in which iron-bearing sulphide minerals have been oxidized by the action of air and water.

Gossans are easily recognized on the ground (and even from the air) by their characteristic rusty brown to reddish colour. Because gossans have been highly leached, some or all of the metals may have been removed or locally concentrated by the weathering and the metal content of gossan samples may not be representative of the fresh rock underneath.

Rock Sampling

When prospectors or geologists find a mineralized outcrop, they will usually start their evaluation by taking a few "grab samples". As the name implies, these are randomly selected samples hammered off (grabbed from) the outcrop. These samples are then assayed.

An "assay" is simply a chemical analysis that determines the amounts of easily extractable metals in the sample; assays of the base metals (e.g. Ni, Cu. Zn, Pb, Co) are usually expressed in weight percent; assays of precious metals (e.g. Au, Ag) are usually expressed in grams of metal per tonne of rock.

Assays of grab samples give a good indication of the grade of visibly mineralized samples. However, if they have been taken in such a way as to sample the highest grade material in preference to lower grade material, they may not be representative of the outcrop as a whole. They do not necessarily provide systematic information about the overall grade or size of the mineralized zone.

Three Dimensional Sampling

A more representative technique for sampling surface outcrops is taking a "channel sample", in which the outcrop is systematically sampled by a continuous cut with a diamond saw or hammer and chisel. A channel sample provides both a representative assay of the outcrop and a width across which the assay has been taken.

Observations at surface only provide two dimensional information about an occurrence.

Eventually, a prospect must be drilled to evaluate its three-dimensional extent and grade. The hole is drilled using a diamond bit and the core is recovered to provide a continuous sample of the rock encountered in the hole.

Usually, on a good surface prospect or geophysical target, a program of several drill holes is planned to test the mineralization at various depths and at various places along its lateral extent. The nature and intensity of any mineralization is clearly visible in the drill core and when mineralization is encountered, a portion of the core is sent out for assay.

Evaluating Three Dimensional Assay Results

How does one evaluate the assay numbers that come out of this work? It is important to realize that each assay consists of two important parts, the width and the grade.

The width gives the explorationist the best idea of the size of the deposit andby extrapolating widths of mineralized intersections from hole to hole vertically and horizontally, it is possible to calculate the volume of mineralized rock in the occurrence and hence its tonnage.

The grade is simply the proportion of rock that consists of payable material, i.e: recoverable metals of value such as Ni, Cu. Co. Because different ores contain different metals in different proportions, the layman can not always easily evaluate the significance of grades reported from drill holes from different areas. One way to do this is to estimate the value of a metric tonne of rock (for massive sulphides, this would be a cube of rock about 60 cm on a side) at the quoted grades and then compare it with values from other areas.

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.

The Affect of Waste Rock and Mill Recovery

It is important to realize that grades reported in exploration drill holes are not necessarily those that can or will eventually be mined. In order to recover the ore, it might eventually be necessary to include a certain amount of country rock in the mining operation. The inclusion of waste rock in the mined product is called dilution and will inevitably mean a lower grade for the mine overall than for the sulphide body alone. For various reasons, not all of the metal can be recovered from the mined ore during milling and some percentage will be lost in the tailings. The percentage of metal that can be recovered in the mill is termed the recovery, and incorporation of this in the mining plan will further reduce the overall cash value of the deposit.

The Affect of Future Prices

A final point to emphasize in connection with the grades of the deposit is that the eventual value, and therefore profitability, of the deposit will be determined, not by today's prices, but by those in effect when the mine is operating, which may be anywhere from 5 to 20 years hence. Metal prices fluctuate on both short term and long term cycles with shifting demand and the state of the world economy.

Although relatively stronger at present, most metals have just come through a period of very low prices, principally as a result of the economic recession in the western economies. The decline in the price of Ni was more precipitous than others, mainly as a result of flooding of world markets with cheap Ni from the former Soviet Union following its collapse. To continue with our original example, the tonne of 2% Ni - 1% Cu ore that is worth about $180 today was only worth about $90 in late 1993, when Cu and Ni prices bottomed out at about $0.75/lb. and $1.90/lb. respectively. Of course, if prices continue their present upward trend, that tonne of ore could be worth considerably more than its present value when it is mined.

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.