source (1.)adapted into HTML from notes from The
Prospecting School on the Web by Dave Watkins, Falconbridge Copper Corp
and Bruce Downing, Geological Consultant
(2.)adapted to HTML from the magazine Newfoundland Mining for The
Prospecting School on the Web
1. VMS Volcanogenic Massive Sulphide Deposits
and Ni-Cu-Co Type Magmatic Massive Sulphide Deposits
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 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.
Schematic diagram of the modern TAG sulphide deposit on the Mid-Atlantic
Ridge. This represents a classic cross-section of a VMS deposit, with
concordant semi-massive to massive sulphide lens underlain by a discordant
stockwork vein system and associated alteration halo, or "pipe". From
Hannington et al. (1998)
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
1.Cu-Zn - eg. Noranda, Windy Craggy, Britannia (Britannia
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
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.
The development and maturation of a generic subseafloor hydrothermal
system involves three stages.
(A) The relatively deep emplacement of a subvolcanic intrusion below
a rift/caldera and the establishment of a shallow circulating,
low-temperature seawater convection system. This results in shallow
subseafloor alteration and associated formation of hydrothermal
(B) Higher level intrusion of subvolcanic magmas and resultant
generation of a deep-seated subseafloor seawater convection system in
which net gains and losses of elements are dictated by subhorizontal
(C) Development of a mature, large-scale hydrothermal system in
which subhorizontal isotherms control the formation of semiconformable
hydrothermal alteration assemblages. The high-temperature reaction
zone next to the cooling intrusion is periodically breached due to
seismic activity or dyke emplacement, allowing focused upflow of
metal-rich fluids to the seafloor and formation of VMS deposits. From
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
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 - 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
Graphic representation of the lithological classification for VMS deposits
by Barrie and Hannington (1999), with the addition of a "high
sulphidation" type to the bimodal felsic group.
Average and median sizes for each type for all Canadian deposits, along
with average grade, are shown
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.
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
Common relationships include:
- synvolcanic faults and scarps that focus, channel, or trap
- 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
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
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
"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.
There are three principal tectonic environments in which VMS deposits
form, each representing a stage in the formation of the Earth's crust.
(A) Early Earth evolution was dominated by mantle plume activity,
during which numerous incipient rift events formed basins
characterized by early ocean crust in the form of primitive basalts
and/or komatiites, followed by siliciclastic infill and associated
Fe-formation and mafic-ultramafic sills. In the Phanerozoic, similar
types of incipient rifts formed during transpressional, back-arc
rifting (Windy Craggy).
(B) The formation of ocean basins was associated with the
development of ocean spreading centers along which mafic-dominated VMS
deposits formed. The development of subduction zones resulted in
oceanic arc formation with associated extensional domains in which
bimodal mafic, bimodal felsic, and mafic-dominated VMS deposits
(C) The formation of mature arc and ocean-continent subduction
fronts resulted in successor arc and continental volcanic arc
assemblages that host most of the felsic-dominated and bimodal
siliciclastic deposits. Thin black arrows represent direction of
extension and thick, pale arrows represent mantle movement.
The three most crucial factors for the formation of large and super-large
magmatic sulfide deposits are:
(1) a large volume of mantle-derived mafic-ultramafic magmas that
participated in the formation of the deposits;
(2) fractional crystallization and crustal contamination,
particularly the input of sulfur from crustal rocks, resulting in sulfide
immiscibility and segregation;
(3) the timing of sulfide concentration in the intrusion.
The super-large magmatic Ni-Cu sulfide deposits around the world have been
found in small mafic-ultramafic intrusions, except for the Sudbury
deposit. Studies in the past decade indicated that the intrusions hosting
large and super-large magmatic sulfide deposits occur in magma conduits,
such as those in China, including Jinchuan (Gansu), Yangliuping (Sichuan),
Kalatongke (Xinjiang), and Hongqiling (Jilin).
Magma conduits as open magma systems provide a perfect environment for
extensive concentration of immiscible sulfide melts, which have been found
to occur along deep regional faults.
The origin of many mantle-derived magmas is closely associated with mantle
plumes, intracontinental rifts, or post-collisional extension.
Although it has been confirmed that sulfide immiscibility results from
crustal contamination, grades of sulfide ores are also related to the
nature of the parental magmas, the ratio between silicate magma and
immiscible sulfide melt, the reaction between the sulfide melts and newly
injected silicate magmas, and fractionation of the sulfide melt.
The field relationships of the ore-bearing intrusion and the sulfide ore
body are controlled by the geological features of the wall rocks.
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
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
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
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.
The search for a buried sulphide deposit often begins with an airborne
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
"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.
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.
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
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
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
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
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.