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KATX - Importance of Volcanic-associated massive sulphide (VMS)- An Overview

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 halcopyrite,
sphalerite, and galena as the principal economic minerals. Barite and cherty silica are common gangue accessory minerals.



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 and compositional 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.

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.

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 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. 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.

Volcanogenic Massive Sulfide Deposit Density


View of the United Verde mine (upper left), the Edith and Audrey shafts of the United Verde Extension mine (foreground), and the abandoned Little Daisy Hotel, which was a dormitory for the miners (upper right), Jerome, Arizona. (U.S. Geological Survey photograph taken by Dan Mosier in 2002)

Abstract

A mineral-deposit density model for volcanogenic massive sulfide deposits was constructed from 38 well-explored control areas from around the world. Control areas contain at least one exposed volcanogenic massive sulfide deposit. The control areas used in this study contain 150 kuroko, 14 Urals, and 25 Cyprus massive sulfide subtypes of volcanogenic massive sulfide deposits. For each control area, extent of permissive rock, number of exposed volcanogenic massive sulfide deposits, map scale, deposit age, and deposit density were determined. The frequency distribution of deposit densities in these 38 control areas provides probabilistic estimates of the number of deposits for tracts that are permissive for volcanogenic massive sulfide deposits—90 percent of the control areas have densities of 100 or more deposits per 100,000 square kilometers, 50 percent of the control areas have densities of 700 or more deposits per 100,000 square kilometers, and 10 percent of the control areas have densities of 3,700 or more deposits per 100,000 square kilometers. Both map scale and the size of the control area are shown to be predictors of deposit density. Probabilistic estimates of the number of volcanogenic massive sulfide deposits can be made by conditioning the estimates on sizes of permissive area.

The model constructed for this study provides a powerful tool for estimating the number of undiscovered volcanogenic massive sulfide deposits when conducting resource assessments. The value of these deposit densities is due to the consistency of these models with the grade and tonnage and the descriptive models. Mineral-deposit
density models combined with grade and tonnage models allow reasonable estimates of the number, size, and grades
of volcanogenic massive sulfide deposits to be made.

For Complete Report Please Visit:
http://pubs.usgs.gov/sir/2007/5082/sir2007-5082.pdf