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massive sulphides

Volcanogenic Massive Sulphide Deposits

adapted into HTML from notes
from The Prospecting School on the Web
by Dave Watkins, Falconbridge Copper Corp
and Bruce Downing, Geological Consultant

and

Prospecting and Evaluating  Ni-Cu-Co Type Magmatic Massive Sulphides

Adapted to HTML from the magazine Newfoundland Mining
for The Prospecting School on the Web


VMS Deposits

Volcanogenic Massive Sulphide Deopsits

classification
composition
alteration
mineral and metal zoning distribution
genetic model
references

adapted into HTML from notes by Dave Watkins, Falconbridge Copper Corp
for The Prospecting School on the Web

 Volcanic-associated massive sulphide (VMS) deposits occur throughout the world and throughout the geological time column in virtually every tectonic domain that has submarine volcanic rocks as an important constituent. VMS deposits are major sources of Cu and Zn and contain significant quantities of Au, Ag, Pb, Se, Cd, Bi, Sn as well as minor amounts of other metals.

As a group, VMS deposits consist of massive accumulations of sulphide minerals (more than 60% sulphide minerals) which occur in lens-like or tabular bodies parallel to the volcanic stratigraphy or bedding.                                                                                                    

They are usually underlain by a footwall stockwork of vein and stringer sulphide mineralization and hydrothermal alteration (Figure 1). They may occur in any rock type, but the predominant hosts are volcanic rocks and fine-grained, clay-rich sediments. The deposits consist of ubiquitous iron sulphide (pyrite, pyrrhotite) with chalcopyrite, sphalerite, and galena as the principal economic minerals. Barite and cherty silica are common gangue accessory minerals.

Volcanogenic Massive Sulphides Figure 1( NOTE :LARGE .jpg  64K)

Classification

 VMS deposits are classified with respect to host rock type and on the basis of ore composition. The host rock classification is a useful field system as it can be relates to the geological environment which can be determined from geologic maps. The major groups are:

 1.felsic volcanic hosted - 50% of deposits - eg. Buttle Lake (Westmin - Vancouver Island, B.C.), Noranda

2.mafic volcinic hosted - 30% of deposits - eg. Anyox

3.mixed volcanic/sedimentary association - 20% of deposits - eg. Windy Craggy, Tatshenshini Area, B.C.

Compositionally, VMS deposits form two broad groups:

 1.Cu-Zn - eg. Noranda, Windy Craggy, Britannia (Britannia Beach, B.C.)

2.Zn-Pb-Cu - eg. Buttle Lake                                                                            

 Economically significant quantities of Au and Ag may occur in all the above lithological andcompositional groups. There is only a poor correlation between the ore composition types and host rock type.

 Another massive sulphide category - Pb, Zn deposits -forms in a sedimentary environment

 VMS deposits tend to occur in districts. Up to two dozen deposits, might be clustered in an area of a few tens, of square kilometres. Known VMS districts are good hunting grounds for new discoveries.

Deposits within a specific district tend to have similar metal ratios and a fairly narrow range in composition. In any given district, deposits will tend to range in size from less than one million tonnes to several tens of millions of tonnes, with most deposits at the small end of the range and only a few large deposits.                                                                                                          

Alteration

 Petrologically and chemically distinctive alteration zones produced by the reaction of ore forming fluid with wall rocks underlie, and in some instances also overlie VMS deposits. The alteration zones may greatly increase target size for exploration because they extend beyond the deposit boundaries and may be several times larger than: the deposit itself. They fall into three main groups:

 1.pipes beneath deposits

a) Cu-Zn deposits: vertically extensive conical shaped stringer zones with black colored chlorite or talc rich core enveloped by a sericite - quartz halo; Na2O, CaO and sometimes SiO2 are depleted from the core of the zone; K2O may be enriched on the fringe. (Figure 1) eg. Millenbach Mine, Noranda, Quebec

b) carbonate rich volcanic and sedimentary rocks: sericite + quartz + siderite; not zoned - eg. Misttabi Mine, Sturgeon Lake, Ontario                                                                           

c) Zn-Pb-Cu deposits: zonation is opposite to Cu-Zn deposits with sericite + quartz core surrounded by chloritic outer fringe eg. Buttle Lake, B.C.

 2.semi-conformable alteration zones - regionally extensive semi-conformable zones at depth below deposits possibly representing a geothermal aquifer; characterized by Fe, Mg enrichment, Na depletion; variable silicification and quartz + epitote alteration (Figure 1) eg. Anderson Lake Mine, Snow Lake, Manitoba

 3.hanging wall alteration (Figure 1) - occurs in some deposits as a mineralogically defined zone of diffuse clay minerals + sericite + dolomite in relatively unmetamorphosed rock to epidote + silica + (sericite) in low grade metamorphic areas ea. Kuroko deposits, Hokuroko District, Japan.

Mineral and Metal Zoning

The distribution of metalss and sulphide types is commonly zoned on the scale of an individual lens and in clusters of lenses.

 Cu is usually high relative to Zn + Pb in the core of the pipe and in the spine of the massive sulphides.The ratio of Zn + Pb to Cu increases around the outside of the pipe and towards the upper part and margins of the massive zone.                                                                      

 Au and Ag usually are highest in the fringe areas. Barite also tends to occur at fringes.  Proportions of Zn, Pb and Ba also tend to increase in lenses peripheral to the center of the deposit, both laterally and vertically (up-strastigraphy).

 Pyrrhotite + magnetite may occur in the core zone with pyrite usually becoming dominant at the fringes.

Distribution

 VMS deposits tend to cluster in districts (or camps) and locally within districts. The average massive sulphide camp in Canada has about 9 deposits, but ranges from four (Manitowadge) to 21 (Noranda), However, an individual deposit may consist of a number of closely associated, discrete lenses ranging from several thousand to several million tons in size (ea. Millenbach Mine was 16 geologically discrete ore lenses). The largest deposits in this group may be in excess of 100 million tons (ea. Kidd Creek, Bathurst No. 12).

 Within a camp, deposits may occur laterally at a discrete time - stratigraphic interval. However, they may also be vertically stacked through several thousand feet of volcanic stratigraphy.

 VMS deposits are spatially associated with structural features and rock types that are reflective of the geological environment of deposition. Common relationships include:

 - synvolcanic faults and scarps that focus, channel, or trap hydrothermal fluids  

- dyke swarms, diatremes, ring structures and other features indicative of proximity to volcanic centres

- features associated with rapid subsidence or collapse (ea. calderas, grabens) felsic domes, breccia domes, etc. that occupy volcanic centers

- subvolcanic intrusions in the footwall sequence

Genetic Model

 VMS deposits are generally accepted to have formed at or near discharge vents of hydrothermal systems on the sea floor (Figure 1) . Moat models of the hydrothermal system accept a seawater convection cell driven by the heat of a cooling subvolcanic igneous body with metals being leached from surrounding rocks through which the hydrothermal fluids circulate.

Discharge is focused along fault or fracture systems. Sedimentary structures in the massive component of the deposits may result from mechanical reworking and downslope transportation of sulphide ores after initial deposition. Underlying alteration and stringer mineralization result from the interaction of hot discharging fluids with the footwall rocks.                                                        

 "Black smokers" are modern day analogues to fossil VMS deposits. They have been observed over the past several years forming in deep submarine trenches off the Pacific Coast of North America. A schematic representation of the growth of a modern mound-chimney sulphide deposit is provided in

References

Franklin, J.M.; Sangster, D.M.; Lydon, J.W.; 1981, Volcanic Associated Massive Sulphide Deposits; Economic Geology 75th Anniversary Volume; pp. 485-627.
 
 
 


Ni-Cu-Co Sulphide Deposits

Ni-Cu-Co Type Magmatic Massive Sulphides

Adapted to HTML from the magazine Newfoundland Mining
for The Prospecting School on the Web

 making sense of mining company press releases
the nature of the target
finding a deposit
geophysical surveys
gossans
rock sampling
three dimensional sampling
evaluating 3D assay results
grade and widthaffect of waste rock and mill recovery
affect of future prices
evaluation "rules of thumb"

 
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.


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