A typical VMS deposit consists of a concordant lens of massive sulfides underlain by a discordant pipe-like stringer zone. The sulfide lenses are formed by hot fluids discharging into the seafloor. The stringer zone and the discharge zone immediately above it, contain much of the chalcopyrite ore. In the more distal parts of the pipe and higher up in the stratigraphy, pyrite, sphalerite and galena dominate. The Cu-Zn-Pb sulfides grade outwards from a Cu-rich core to a Zn-Pb-rich margin. Gangue minerals associated with the ore include barite, quartz, chlorite and carbonate. A siliceous cap rock generally overlies the deposit.

Host rocks to the deposit are characterized by chemical and mineralogical zonation. Extensive hydrothermal alteration is typical of the proximal and distal parts of the deposit, which is enveloped by silicified, sericitized and chloritized alteration zones. The chlorite-rich zone is proximal and the sericite-rich zone is generally more distal to the orebody. Such well defined alteration halo provides an excellent exploration target for VMS deposits.

PETROGRAPHIC REPORTS will include microscopic examination of thin sections with transmitted and reflected lights, detailed description of mineralogy and texture, classification of rocks where possible, visually estimated modal proportion of minerals, and description of primary and secondary minerals and their paragenetic sequence. In mineralized samples, the genesis of ore will be estimated. All reports will include 2 or more photomicrographs of each thin section taken by a digital Leica cooled camera, a cd with file(s) and 2 hard copies of the report.

MAGMATIC SULFIDES

Magmatic sulfide deposits (Ni-Cu-Co) are commonly hosted by mafic and ultramafic igneous rocks. The sulfide ores (often enriched in platinum group elements) are the result of the separation of an immiscible sulfide melt from the sulfur-saturated silicate melt. As the dense sulfide droplets separate out and settle through the less dense silictae melt they besome enriched in copper, cobalt and platinum group elements - all of which have a srong affinity for the sulfides with respect to the silicates.  Chalcopyrite, pentlandite and pyrrhotite are the major minerals in magmatic sulfide deposits. Some of the most famous deposits include Sudbury (Ontario, Canada), Noril'sk (Russia), Kambalda (W. Australia), Merensky Reef, Bushveld Complex (South Africa) and Voisey's Bay (Newfoundland, Canada).

The photomicrographs below show some typical ore textures and some host rocks to the ores.

(x-axis of photos: 1.6mm)

• Biotite rims olivine in troctolite Biotite rims olivine in troctolite
• Fragmented pentlandite rimmed by pyrrhotite Fragmented pentlandite rimmed by pyrrhotite
• Green hercynite replaces plagioclase Green hercynite replaces plagioclase
• Olivine aggregates and plagioclase in troctolite Olivine aggregates and plagioclase in troctolite
• Pentlandite included in pyrrhotite Pentlandite included in pyrrhotite
• Pentlandite is intergrown with magnetite Pentlandite is intergrown with magnetite
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