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.

From http://earthsci.org/mineral/mindep/d...