Of the crystallizing magma chamber at voiseys bay

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: c magma is rich in volatiles and rare elements • such a melt is very fluid and invades cracks and results in large crystals Pegmatites l ook like hydrothermal veins but they are injections of granitic melt and are mined for uranium, thorium, beryllium, lithium, tantalum, … II. Deposits associated with metamorphism • Many important ore deposits are produced by metamorphism of country rocks adjacent to an intrusion (especially reactive carbonate-rich rocks which act as a chemical trap for metals) • These are referred to as “skarns” • sphalerite (zinc) • chalcopyrite (copper) • scheelite (tungsten) 9 Skarns: Contact Metamorphism Skarns are characterized by abundant calcic minerals such as garnet an pyroxene III. Hydrothermal ore deposits • Associated with igneous activity - 1) veins • Among the best known and most important ore deposits are generated from hot water • Many originate from hot, metal-rich fluids expelled from crystallizing intrusions • Fluids move along fractures, cool, and precipitate the metallic ions to produce vein deposits Vein deposits are concentrated in areas of intense faulting 10 • Hydrothermal fluids in open fractures precipitate vein minerals as they cool • Quartz and calcite are among the most common vein minerals Where do hydrothermal fluids come from? • Some originate when water dissolved in magma is released as the magma crystallizes • Some are formed from rainwater that circulates deep in the crust, close to hot intrusions • In the oceans, such as near midocean ridges, hot seawater is circulated into the ocean crust These fluids are corrosive and can leach metals from the rocks Dissolved metals precipitate from the fluids as they rise and cool III. Hydrothermal ore deposits Associated with igneous activity - 2) disseminated • In addition to veins, metals can also occur as disseminated deposits in which the metals are distributed throughout the rock body, rather than concentrated in veins (low grade, large volume) • Most of the world’s copper comes from hydrothermal veins and disseminations derived from large porphyritic intrusions (these are referred to as porphyry copper deposits) 11 The “ root zones” of volcanoes CCGK Bingham Canyon, Utah Morenci mine, Arizona GSC World Minerals Project Global Distribution of Porphyry Deposits 12 III. Hydrothermal ore deposits • Associated with igneous activity - 3) VMS • Volcanogenic Massive Sulphide (VMS) deposits are lenses of sulphide minerals that accumulate on the seafloor from hydrothermal vents • Heated seawater, rich in dissolved metals vent from the seafloor as black smokers Hydrothermal Deposits at Black Smokers The same process seen today was responsible for many VMS deposits (as old as 3 billion years) Lenses of Cu, Fe, Zn and Pb sulphides accumulate on the seafloor and are buried by younger lavas 13 III. Hydrothermal ore deposits • Associated with sediments • Massive sulphide deposits also form from seafloor hydrotherma...
View Full Document

This note was uploaded on 11/10/2012 for the course SCIENCE 1, 2, 3 taught by Professor All during the Spring '12 term at University of Ottawa.

Ask a homework question - tutors are online