ORE DEP (ORE-BEARING FLUIDS) Flashcards
Factors affecting ore fluid movement
1 Precepts of Geohydrology
2 Permeability
3 Porosity
4. Density
5. Viscosity
6. Presence of Fractures
7. pressure Gradients
8. Time
9. Rules of Igneous Petrology
Important modes of movements for Ore formation from Magmatic fluids
1 Filter Pressing
2 Late Liquid Gravitative Accumulation
3. Magmatic Injection
A partly melted magma, when exposed to external stress, can be squeezed away the melt fraction from the crystalline mush like a toothpaste
Filter Pressing
Sinking of high density globules due to immiscibility and density variation within and from parent fluid after some differentiation
Late Liquid Gravitative Accumulation
Deposts which are products of 6
Podiform Chromites
Residual liquid is squeezed into the surroindung country rocks
Magmatic Injection
Deposit formed from Magmatic Injectin
Magmatic Injection deposits
Deposit example of 8
Manetite-Apatite Dike
What causes magma to move up
1Expansion of Gases and decrease in Sp.G
2Tectonic stresses squeezing the magma or fraction of magma into the overlying or adjacent rock
3 Movement through stoping
The process by which country rock is broken up and removed by the upward movement of magma
Stoping
Ratio of pore volume and total volume of rocks
Porosity
Measure of the interconnectedness of pores which gives a rock the capacity to transmit a fluid through it across a pressure gradient
Permeability
Permeability which is intrinsic to the rock
Primary
Permeability which is superimposed on the rocks
Secondary
This forms when contact metamorphism and ore emplacement occur sequentially in a limestone section around a coolint pluton
Skarn
What are the processes which may increase permeability in rocks
Exposure to Heat
Movement or pressure of fluids
Tectonic processes
These are very important in epigenetic deposit which are emplaced during a tectonic activity
Structural Cntrol
A type of structural control formed due to phreatomagmaytic phreatic and hydrothermal process
Breccia
At shallow depths, which type of permeability is more significant to ore transport and deposition
Secondary
Type of shallow deposit
Epithermal Au
Why do permeability and porosity decrease with depth?
Because Lithostatic Pressure increases with depth
Deep fluid flow depends on what type or permeability
Primary
Until how much depth is secondary permeability significant
<3kms
The spontaneous movement of molecular or ionic particels down concentration gradients that cause one substance to become uniformly intermingle with another
Major mechanism of mass transport in deep environment of restriceted mechanical fluid flow
Diffusion
Sample deposit where diffusion plays part
Orogenic Au Deposits
Factors which affetcs ore deposition
ORE DEPOSITION CONTROLS
1) Physical effects - Gravity settling, Settling due to agitation
2) Cooling of Temp
3) Pressure Changes
?Retrograde Boiling (due to depressuring)
?Throttling (Depressurization w/ heat loss or adiabatic decompression)
4) Chemical Changes
?Reaction of ore fluid w/ wall rocks
?Mixing of Different ore fluids
Deposition of Magmatic Segregation Deposits
?Direct effects of igneous melt differentiation
?Magmatic Sedimentation - settling and accumulation of crystallizing mienrals in high-T mafic Magmas (Bushveld Cr-Pt, Magn-Ilme)
?Direct crystallization of magma chamber walls or floors (Stillwater PGE)
?Separation of mgamatic liquids and their solidifcations
?Consolidation of an igneous rock w/ an entrained accessory economic mineral
?Crstallization of melt
Depostional textures of hydrothermal fluid deposits varies with
?Nature of fluids
?Physical and chemical character of the host rock
?Mode of emplacement
What are the data which can be extracted from depositional texture of hydrothermal fluid deposits?
1) Temporal relationship of sucessive miernal assemblages in a rock
2) Overall environment of formation
3) Manner of deposition
Types of Deposition of HFs
1) Exsolution
2) Replacement
3) In-Filling
(ManyOre deposits shows Evidences of both replacement and open-space filling)
Compositional or Structual adjustments as the fluids cool down
Particularly formation of two separate solid phase from a high-temp solid solution
Exsolution or inversion
Exsolution pairs imply that 2 minerals are in
Isomorphic Solid Solution at higher temp
Differentiate Exsolution from replacement as seen in the samples
Exsolution shows depletion at the junction of intersecting lamellae while Replcement shows greater concentrations at intersections
Exsolution Pairs
?Pyrrho-Pentl
?Chalcopy-Bornite
?Spha-Chalcopy
?Magnet-Ilmen
Inversion Pairs
?Argentite-Acanthite (173 deg C)
?Enargite-Luzonite (275-300 degC)
Exsolution Textures
?Lamellar
?Emulsoidal
?Myrmekitic
The process of simultaneous capillary solution and deposition by which a new mineral of partly or wholly differing chemical compostion may grow in the body of an old minral aggregates (diffusion)
Replacement (Metasomatism)
Rules of Replacement
1) Sulfides (Arsenides,Sulfosalts) + Tellurides - any Gangue, Rock and Ore Minerals
2) Gangue - Only Gangue and Rock Minerals, rarely Oxides
3) Oxides - Only Gangue and Rock rarely sulfide and tellurides
Criterial for replcement textures
?Pseudomorph - replacement mineral or mineral aggregates retaining the EXTERNAL Form of the origg mineral or mineral aggregates
?Widening of a fracture filling to an irregular mass where a fracture crosses a chemically reactive mineral grain or rock
?Islands (Isolated) unreplace host minerals or wall rock
?Concave surfaces into the host (Cusp and Caries)
Most common feature in Shallow zones (<3Kms below) where Brittle Rocks yield breaking rather than plastic flow
Open Space Filling
Examples of Open Spaces which can be filled
1) Veins
2) Factures fills
3) Vugs or Open Cavities
4) Breccias
The distribution in time or the sequene of mienrals or element depositon which can be determined using Ore Textures
Paragenesis
any regular patten in the distribution of mienrals or elements in space
Zoning (Take note a bit dfferent to zoning in minerals)
