Week 4 Geology Flashcards
What are Hydrothermal deposits
A hydrothermal ore deposit is one in which the ore minerals were precipitated from aqueous high-temperature fluid solutions.
Most hydrothermal fluids at high temperature and pressure, and have a density intermediate between that of liquid water and that of water vapour at surface conditions.
What is the basic concept of a hydrothermal system
Structural features like faults, and fractures represent the pathway of fluid migration.
Energy source to drive fluid migration to thermal (e.g. magma emplacement) or mechanical (e.g. deformation event).
Chemical driving force of ore precipitation - result of changing solubility of the ore mineral arrow changing temperature, pressure or fluid composition.
explain (Cu) saturation in fluids.
Cu in fluid is high as there is no pre-existing mineral phase to collect Cu.
Cu in fluid is low as there is a pre - existing mineral phase (Cu-sulphide?) to collect Cu.
Summary part 1
Hydrothermal deposits that have a close spatial and temporal association to magmatic activity.
Fluids can derive from depth (e.g. magmatic or metamorphic) or from earth surfaces (e.g. meteoric water).
Fluids main role is to concentrate element (leaching), and to trigger ore precipitation (e.g. change of elements solubility).
Where are most Porphyry formations found
Almost all porphyry deposits from within volcanic arcs, both in continental arcs the Cordillera of Western North and South America. And in intra- oceanic island arcs “Southwest Pacific” arcs of Papua New Guinea, Solomon Islands.
They from at the same time as active subduction, or immediately after cessation of subduction (post subduction) in the arc.
What happens with the Hydrated Oceanic lithosphere in tectonic environment of Porphyry formation.
Subduction of hydrated oceanic lithospheres recycles volatiles (H20, S, CL) and alkalis into the asthenosphere mantle wedge, and progressively increases the mantle oxidation sate.
Metasomatised mantle - includes mantle-driven chalcophile and siderophile metals to the upper plate lithosphere.
