L9 - Magmatic Framework for porphyry, skarn & epithermal deposits

What are the 5 main fluid reservoirs in the crust?

1. Connate (sedimentary basin)

2. Meteoric (surficial)

3. Magmatic hydrothermal

4. Sea water

5. Metamorphic

What fluid reservior is responsible for skarn, porphyry, and epithermal deposits?

Magmatic hydrothermal

Define metasomatism

A process where the chemical composition of a rock is altered in a solid state through the introduction or removal of chemical components by hydrothermal and other reactive fluids

What elements are water soluble?

LILE (Large Ion Lithophile Elements)

What is a LILE?

Large Ion Lithophile Elements (LILE) are cations with large ionic radii that preferentially partition into fluids or melts.

How does a convergent margin lead to the genesis of magmatic-hydrothermal mineral deposits?

1. Subducting slab dehydrates (vis metamorphism), producing fluids

2. Fluids become enriched in incompatible, water-soluble elements

3. Fluids rise and metasomatize (changes the composition) of the overlying asthenospheric mantle wedge

When fluids interact with the mantle/host rock, is the country rock reduced or oxidized?

Oxidized

Are metasomatized rocks oxidized or reduced?

oxidized

What minerals are being altered to dehydrate the oceanic crust during subduction?

Amphiboles (hbl, ziosite, chlorotoid etc) due to lower stability of hydrous minerals

Amphibolite → eclogite metamorphic facies transition

Why does magma stalling/ underplating/MASH occur?

The basaltic magmas from the mantle wedge are more dense than most continental rocks

How does the underplated basaltic magma start to move through the continental crust? What process is occuring?

Basaltic magmas start to crystallize, differentiate, and interact with the lower continental crustal rocks.

• MASH process begins

What does MASH stand for?

• M = melting

• A = assimilation

• S = storage

• H = homogenization

How do these basaltic magmas generate intermediate to felsic magma?

• M = melting (partial melting of crustal rocks in the path of ascent)

• A = assimilation (incorporates incompatible elements into hydrous magma and becomes enriched)

• S = storage (incompatible elements and water are concentrated and stored in remaining magma)

• H = homogenization (as melt becomes homogeneous with surrounding crustal rock, the magma will continue to ascend as it becomes more bouyant/felsic)

What fluid reservoir is responsible for Orogenic gold deposits?

Metamorphic fluid reservoir

Where do fuilds com from in VMS deposits?

ocean (circulation)

What type of fluid reservoir is responsible for Epithermal and carlin type deposits

Meteoric

What fluid reservior is resposible for a SEDEX, MVT, and Sedimentary Cu deposit?

Connate (sedimentary basin) fluid source

What structure might magma form to ascend through the upper continental crust?

dikes

At what depth might a ascending dike pool to form a batholithic magma chamber?

At a depth of 5-10 km

Where do I-type magmas form?

either in MASH zone or at the base of crust

Where do S-type magmas form?

zone of partial melting within the lower crust

What structures form when magma begins to accumulate within the crust?

• horizontal lenses/sills

What tectonic regime do S-type magmas form?

collisional margins/orogens

• Partial melting of pelitic protoliths occurs during crustal thickening in the deep crust

Do S-type melts tend to be enriched in Chalcophile, or Lithophile elements? Why?

S-type granit melts tend to be enriched in lithophile elements more then chalcophile

• Depleted in Chalcophile elements because they do not partition efficiently into silicate melt (leave behind sulphides during partial melting)

• Enriched in lithophile elements since they inherit enriched crustal nature

Rank the following magmatic systems from least enriched (in incompatible E) to most enriched:

1. pegmatite

2. basaltic magma

3. residual melt of basaltic magma (co-genetic)

basaltic magma → residual melt of basaltic magma → pegmatite

What do miarolitic cavities indicate?

evidence of fluid in magma

How does pressure dropping (from magma rising) affect water solubility in melt?

pressure drop = reduced water solubility in melt

• water exsolves from melt while magma rises

What is 1st boiling?

1st boiling - exsolution of separate fluid phase due to depressurization

• Saturation of silicate melt with respect to its volatile components

What is 2nd boiling?

fluid exsolution from a melt that reaches fluid saturation due to mineral crystallization

• lower mol% H2O than the residual melt

• need crystallization to reach H2O saturation at upper crustal depths

Free volatile phase generation during cooling depends on what 3 factors?

1. Initial volatile content (H2O, CO2, Cl-, S, etc)

2. Confining Pressure

3. Degree of crystallization

As more anhydrous minerals crystallize, how does the pressure of H2O saturation react?

as more anhydrous minerals crystallize, pressure needed to saturate the melt decreases

• higher fluid content = more pressure to prevent fluid saturation

• lower fluid content = less pressure to prevent fluid saturation

Where do volatile phases partition into?

Volatile phases partition into co-existing fluid rather than magma

What elements can be carried in highly saline (Cl-) fluids at high Temp?

Cu, Au, Zn, Fe

Based on this graph, which elements are incompatible in the fluids? How do you know?

Elements larger than D=1 are incompatible with the melt and incorporated into the fluid

How does an Ore deposit form with an ore fluid?

• If ore fluids (enriched in ore elements) escape the magma chamber, they are in chemical and thermal disequilibrium with the surrounding rock

• Physicochemical disequilibrium between hydrothermal fluids and surrounding rock results in an exchange of heat and chemical components between the ore fluid and the rock

• ppt of metal

Why might chlorinity of a fluid change the metal-carrying capacity?

Based on charge balance

• Cl- or H3O+ ions can carry metals with -/+ charge

• higher [Cl-] = higher [Cu +] - and other ore elements

If the melt is oxidized, will Cu stay in the melt or be incorporated into forming crystal?

• Oxidized magma (Loss of electron) = S- incorporated into crystal (loss of e-) → traps Cu in crystal. BAD

• Reduced magma (gain of electron) = S- remains in melt (gains e-) → traps Cu in melt. GOOD

What is the relative tonnage and grade of a porphyry deposit?

Porphyry deposits have a very high tonnage and a relatively low grade deposit

porphyry deposits are the world’s primary source of which ore elements?

• Cu

• Mo

• Sn (tin) -W (tungsten)

• + Au

What type of magmatic structure are porphyry deposits, and what is their composition?

associated with:

• shallow crustal intrusions

• intermediate to felsic composition (52-77wt% SiO2)

What depth do Porphyry deposits typically appear?

2-4 km depth, but as deep as 8km

how do porphyry deposits typically form? What type of fluid do they use?

• Formation involves the exsolution of metalliferous and S-rich SALINE (NaCl) hydrothermal fluids from a silicate melt

What does co-genetic mean?

Separate batches of molten rock that share a common parental source or are derived from one another through magmatic evolution

• typically identified by trace elements

What are the typical host rocks for porphyry deposits?

typically intermediate -felsic PORPHYRITIC intrusive rocks (commonly co-genetic), volcanic rocks (sometimes co-genetic) and lesser sedimentary rocks

Define hypogene

Formed from beneath

• Mineralization that formed directly from magma upwelling hydrothermal solutions derived from magma, metamorphic rocks, pore fluids, meteoric water, seawater.

• primary mineralization

Define supergene

Formed from above

• Mineralization that formed from a pre-existing hypogene deposit via interaction with surface waters

• secondary mineralization

How much wt% H2O is in a typical andesitic melt?

3-6wt% H2O

How are subtypes of porphyry deposits defined?

Defined by the metals that are essential to the economics of the deposits

• ordered from most wt% to least

examples, grade, tonnage, and value of different deposit furthur explained in slide