Thursday, November 18, 2010

ORE GEOLOGY4.Ores In Felsic Igneous Rocks

CLASSICAL MAGMATIC HYDL. DEPOSITS
(Porphyry Cu. Mo, Pegmatites, Vein-type ores, Skarns etc)

Origin relates to processes operating at the end-stage of felsic
magmatism

The effectiveness of these end-stage processes depends mainly on
(1)magma comp. (H2O % content, metal, S and Cl)
(2)Geologic environment including depth of magma emplacement
Hence, to understand the genesis of the above ones the first consideration
of end-stage processes of felsic magmatism is a must
Three basic processes, according the geological and geochemical
environmental factors (geologic-geotectonic settings, P, T and phase
assemblages) operate.
These are:
(a) ORTHOMAGMATIC: Melt ⇔Crustal equilibrium
Controlled by viscosity and density contrasts, include
(i) some important processes for generation of hydrous metal-bearing magmas form different sources rocks
(ii) magma emplacement and crystallisation

(b) HYDROTHERMAL equilibrium Crystal ⇔ fluid
The other end of the process continuum- involving aq. fluids and solid
phases
(wall rocks + hydrous minerals)- can be of many types. Broadly
involving fluids of
(i) magmatic origin and
(ii) other extraneous nature

(c) TRANSITIONAL:
Between the two extremes (orthomagmatic & hydrothermal processes)-
Upper limit:
arbitrarily set at the point of separation of a magmatic aqueous phase.
lower limit:
H2O saturated solidus of the magma

ORTHOMAGMATIC PROCESSES
A SOURCE ROCKS AND MAGMA GENERATION
IMORTANT STEPS IN THE CONC. OF ORE FORMING ELEMENTS
Partial melting of
(1) Mafic oceanic crust in the subduction
2) Mafic amphibolites of the lower continental crust (
high average conc. of base and precious metal → the melts get enriched in these elements)(3) Felsic metasediment rocks of the lower continental crust (the melts get
enriched in Sn, W, Be, Ta, Nb
). Prior to melting, H2O in the source rocks is
bound as (OH) in hydrous minerals (
at the great pressure in the lower
continental crust or on the subduction zone) – amphibole or biotite.
But
even in these rocks H2O (tot.) < 2 wt% → places a definite constrain on the
amount of H2O generated at a given depth (P) and T

From
the above the P-T projection the following inferences can be drawn:
(1) At a depth < 70km,
a non-porous mafic amphibolite begins to melt at 940-1040°c and
H2O content of the melt must exceed approx. 2.7%
(30% of the orig. rock can be melted for each % of H20 in the rock.)
(2) At a depth > 75 to 80km -
in the subduction zone -
amphibolite is not stable and melting can begin at a temperature of 660°C – However, at such low temperature for melting to occur ,
the melt must contain 27% H2O-
the amount of initial formed melt <5% for each % H2O in the origin rock
(3) Non-porous mus- bearing meta-sediment rocks begin to melt at a temperature of 670-720°C, at the ambient pressure of
the lower continental crust
→ the first formed melt must contain in excess of 8.4wt% H2O
→Hence only 10 −12 % of the rock can be melted for each % of H2O in the original rock.


Melting Relations: ORE GENETIC SIGNIFICANCE


(1)Partial melting of amphibolites or micaceous metasediment rocks (zero porosity), under high-P produces initial melts, with at least 2−7 wt% H2O, regardless of the amount of H2O originally bound in hyd. minerals

(2)The melts produced, range in composition from granitic to dioritic
(calc-alk.) - Representative of igneous rocks associated with   magmatic/hydrothermal deposits

(3)The amount of initial melt formed is directly proportional to the H2O
content of the initial rock
(geologically reasonable 1% H2O = wt% of the
melt =10−25%)

(a) Min. of 2.7 wt% H2O - Once the magma produced and emplaced in
shallow crustal levels
-will evolve a separated magmatic aq. phase (hydl.
fluid) upon cooling and crystallization

(b)It enhances the solubility of metal sulphide by an order of magnitudes,
compound to the same in anhydrous melts of same composition by the
reaction
2FeS(solid)+2H2O(melt)+SiO2(melt) = Fe2SiO4(melt)+ 2H2S(melt)
a hydrous magma with high S-capacity

(c) Leads to crystallization of Bt and /or Hbl up on cooling and
crystallization at a depth > 2 km
→ providing exploration guides to intrusives associated with hydrothermal ore forming process
Calc-alkaline melt comp. → As the magmatic /hydrothermal ore deposits
are associated with rocks of this compositional range-
the hydrothermal activity is associated with these compositional type of rocks- because hydrous minerals in the source rocks (mica or amph.) play a major role in the generation of hydrous calc. alkaline magmas
Amount of melt vs. %H2O of source rock (dependence)- provides a
mechanism for enrichment of partial melt with certain elements relative to their original conc.

(a) elements (e.g. Cu in FeS in amphibolites, minor
phase) which dissolve completely in the early formed melt: enrichment
factor is inversely proportional to XH2O in the rock
(Cu, en. fact. >5)
(b) On the other hand, elements which are present in major mineral ss., that co-exist with the early formed melt (e.g. Pb in K-feldspar)- enrichment
factors are dependent on the kdi values
amongst the co-existing phases;
Hence for Pb- little enrichment

PARTIAL MELTING → CRITICAL INITIAL MULTIPLICITY PROCESS- e.g. Cu

EMPLACEMENT OF MAGMA

Shallow depth of emplacement (generally <10 km,
commonly 1−2 km)
Geological evidence points towards a rather negligible role played by
gravitational settlingunlike mafic/ultramafic melts
Hydrous magmas of intermediate composition
are generally not in chemical equilibrium with more silicic and potassic wall rocks through which they pass or are emplaced (initial disequilibrium) - tendency to assimilate at marginal part enriching in SiO2, K and perhaps Sn– might be an important factor in porphyry Sn deposits, but not in other
magmatic hydrothermal systems

Critical factors:
• Nature of the source rocks
• Process of hydrous magma generation
Emplacement
• Separation (Evolution) of a separate H2O-rich volatile phase
     -controlled by H2O solubility in Silicate melts
      At P                 H2O Saturation             % crystallization
     2km                 2.7 to 3.0 wt% H2O         33%
     8 km                6.1 to 6.4 wt%H2O          69%
     18km               9 to 10 wt% H2O             80%


TRANSITIONAL PROCESSES
(MELT ⇔ FLUID EQUILIBRIUM. )
The H2O content (Wt %) of an uprising magma at any stage of crystallization (Wt) can be worked out by simple mass balance considerations as follows:
Wt (1- F)+ C(F) = Wto ⇒ Hence,
Wt (in wt%) = [Wto - C(F)] / (1- F)] x 100
where Wto is the wt. fraction of H2O in the completely liquid magma,
C is the wt. fraction of H2O after
a finite wt. fraction of the melt is crystallized (F)

The formation of a separate magmatic aq.(volatile) phase by retrograde
(2nd) boiling
marks the beginning of transitional processes (within the
condensed crystal - melt system = orthomagmatic regime)
Chemically, contrary to xl ⇔ melt equilibrium during the orthomagmatic
processes, transitional processes are dominated by melt⇔fluid (volatile) equilibrium

Physically, orthomagmatic processes are controlled by viscosity and
density contrasts between the melt and xl,
whereas transitional processes are dominated by volume changes accompanying the 2nd boiling reaction:
H2O saturated melt→ minerals + volatile phases

PHYSICAL TRANSITIONAL PROCESSES

• A body of hydrothermal magma emplaced in colder wall rocks (whether
shallow seated porphyry magma or deep-seated pegmatite) must lose heat
to the surroundings → crystallization proceed inward from the walls of the
magma chamber
. But due to low diffusivity of dissolved H2O in silicate
melts=H2O saturation starts at the margin
(H2O-satd. rind or carapace in a quasi-static condition.). The interior of magma body is isolated to transfer
matter (except H) =conditions essential for formation of pegmatites,
porphyry Cu/Mo systems and explosive volcanism
•As 2nd boiling operates inside the H2O saturated rind, the magma bodymust either expand or the int. press. must increase due the 2nd boiling
reaction,
which leads to volume expansion at all crustal levels (pressure)
•A first approx. the vol. expansion is directly proportional to the H2O
content of saturation
and inversely proportional to pressure. A body of
H2O saturated pegmatitic melt
at 2 kbar (6.4 wt% H2O)
will expand approx.11%  upon complete crystallization
whereas the same body saturated with H2O
at 5 kbar (10%H2O) will expand only 5%
→ explains why gem-bearing pegmatites (with void space, low-P) vs. non-gem bearing pegmatites (no void space, high-P) The total change in volume accompanying the 2nd boiling reaction (H2O satd. melt→ xls + aq. phase), ΔVr=f(P,T)
Due to
above volume expansion and as at shallow depths the wall rockshave high rigidity,
no plastic deformation is possible to accommodate strain
.

Hence, as cooling and crystallization proceed, pressure inside the
H2O-satd. carapace
must increase and theoretically this excess int. press.can reach several thousands of bars (≈ 3 to 4 kbar), whereas the tensile strength of the strongest wall rock is only few hundred bars
Moreover 2nd boiling leads to production of mechanical Energy
[(PΔVr )Ergs km-3 of magma) x10–23]
because vol. per unit mass of xl + vap >>vol.of an equal mass of H2O saturated melt
Volume expansion and the release of mech. energy leads to brittle
deformation and formation of fractures
– essential for localization of ores
in most magmatic hydl. systems, the fractures types are stock works, large
veins, larger caldera and collapse structure .

CHEMICAL (TRANSITIONAL) PROCESSES

Generation of the magmatic aq. phase is accompanied by partitioning of all elements in the system such that μi or fi of each species is same in the phases in equil.

Cl: The dissolved Cl (in the hyd. magma) gets strongly partitioned into the aq. phase because
(1) chloride minerals are not stable in magmas of calc-alk. comp.
(2) Cl forms Cl– complexes with H, Na, Ka, Ca, Mg and heavy
metals (Cu, Zn, Pb, An) in the aq. soln. at moderate P-T

F: F also forms stable F- complexes in mag. fluids, but high solubility of the element in silicate melts and high thermal stabilities of minerals such as fluorite, topaz, mica→ cause F to be partitioned largely into the condensed crystalline phases

S: S initially dissolved as HS− in the melt is strongly partitioned into the
aq.phase
, provided a sulfide mineral such as Po is not stable

CO2: CO2 is sparingly soluble in the felsic melts and gets strongly
partitioned into the aq. phase
and thus plays a minor chemical role at this
stage
Di values of the above volatile phases (between the aq.fluid and melt)
appear to be relatively T- independent and except for S are also P-
insensitive.
Because S originally occurs as HS– can exist both as H2S and
SO2 (in the aq.phase), DS= f(fH2O,fO2) due to the reaction
H2S +1.5O2 =SO2 +H2O
At a given fO2 - increasing fH2O (hence fH2) increases the H2S/SO2 in the aq.
phase- hence DS ( ΣSv / ΣSm ) decreases. On the contrary, increasing fO2 at
a given pressure increases the SO2 /H2O ratio; hence the DS (ΣSv / ΣSm)
increases → these phenomena, arising out of low solubility of SO2 in the
melt (compared to H2O)
– are of great significance to sulfide ore formation
– by providing mechanisms for generation of high conc. of both S and ore
metals (almost entirely as Cl–) in the magmatic aq.phase.
fO2: The fO2 in the magma prior to the onset of 2nd boiling is largely
determined by the Fe3+/Fe2+ ratio-dependent on the type of source rocks

I -type Magma:
The fO2 in felsic magmas, generated by partial melting of
metamorphosed igneous
rocks is generally higher than the FMQ buffer. Hence, fSO2/fH2S ratio (and mole ratio) of fluids in equilibrium
with these magmas are close to ≥1 (ranges 1 to 10)

S- type Magma:
fO2
in felsic magmas, generated by partial melting of
carbonaceous metasediments
is generally lower than that of FMQ
buffer and the fCO2/fCH4 ratio (and mole ratio) =1. Consequently, the
fSO2/ fH2S ratio << 0.01

Aq. fluids separated from the I-type melts tend to produce S-rich porphyry
Cu-Mo deposits
and
those separated from S- type melts produce S-poor Sn- W deposits.

Chloride
(1) Granitic melt :The major Cl– complexes in equil. with typical granitic
melt are NaCl, KCl ,HCl (NaCl +KCl ≈ 90%)
However, with xllization of phases like mus (containing K+ & OH-), the HCl and KCl contents decrease (HCl drastically decreases) and NaCl content
increases
to maintain the stoichiometry (NaCl / KCl)fl becomes greater than

Na /K ratio in the melt.

(2) In granodioritic melt: Cl– complexes with Ca (CaCl2) and especially Fe
(FeCl2 + FeCl3), MgCl2 is mino
r. The addition of CaCl2 and FeCl2 (+ FeCl3)
complexes does not affect the HCl content, for a total Cl- conc. or the
equality between (NaCl / KCl)aq and (Na/K)m, but ∑ NaCl + KCl is reduced
by 2 or more times the ∑Fe + Ca - Chloride can be complexed with Na, K,
Ca + Fe-more complex situation
(3) In granitic system, pptn. of K-mica causes NaCl / KCl)fl >1.
But in granodioritic system early xllization of Na-baring Hbl- the trend is in reverse direction than Granite irrespective of initial values (eg. NaCl / KCl << 1).
In presence of both Hbl and Bt, NaCl / KCl ≈ 1. - the high (KCl /lNaCl)fl phase assoc. with granodioritic melt (Hbl-bearing) provides an explanation for K-metasomatism in porphyry Cu systems. On the other hand, high conc. of
HCl in the aq. phase prior to appearance of hydrous minerals - occurrence
of topaz in Sn- greisen and pegmatites.
Also very high conc. of Fe in the aq. phase accounts for contact metasomatic skarn ores in carb. rocks and dominance of pyrite in porphyry Cu-Mo ores



ORE METALS
By analogy with Mn and Zn which are partitioned in favor of the aq.phase
by a factor of 2 (∑mCl-)2, it is expected that elements which are more chalcophile than Mn -- will be strongly partitioned into the aq. phase.
Therefore, aq. chloride conc. where fSO2 ≥ fH2S– metals will be strongly
partitioned

HYDROTHERMAL PROCESSES
With falling temp. or decrease in int- fluid press-transitional chem.
processes give way to those hydl. processes which are dominated by
crystal (mineral)- fluid equil.

The boundary between these two regimes is arbitrarily chosen as the H2O std. solidus of magma may be relatively sharp in some system ( sudden brittle fracturing in porphyry Cu dep) and gradational in some other . Eg. in pegmatites both regimes can co-exist in different parts of the system and communicate with each other through volatile phases → coexistence of two inter-communicating regimes is apparently essential for development of mineralogical zoning in pegmatites Rapid passage of magmatic aq. phase into hydl. regime in development of fracture system in porphyry Cu-Mo dep -→leads to conditions of gross disequilibrium between the hydl. soln. and cooler wall rocks. The extent of disequil. depends on the initial condition of equil. in magmatic system as
well as on the nature of the wall rocks and to the extent of P-T decrease

 Aq. chl. solutions from a high temperature magmatic source tend to be enriched in HCl and react with the feldspathic
wall rocks to produce Al-silicate (mainly andalusite and topaz) alteration
with or without Bt at high temperature and
muscovite (+sericitic+ phyllic ) altn. at low temperature.

Fluid that equilibrated initially with Hbl- bearing
magmas - KCl/NaCl ratio is more
-- when interact with non-carb. wall rocks
– fixation of K in Fel and Bt (K – alteration) by exchange of Na+ & Ca++
lowers the KCl/HCl ratio→enter into the stability field of mus

FLUIDS DERIVED FROM I- TYPE MAGMAS (high fO2)
 If during cooling the fluid interacts with Fe2+ -minerals then
→(SO2 +6FeO +H2O = H2S +3Fe2O3) → aH2S increases at the expense of aSO2
→resultant increase in aH2S causes pptn. of metal sulfide from metal -
chloride complexes of mostly Fe which in turn produces HCl due to the
reaction 4FeCl2 + 7H2S + H2SO4= 4FeS2 + 4H2O+ 8HCl

Production of HCl is further enhanced by pptn. of anhydrite:
CaCl2 +H2SO4= CaSO4 + 2HCl
(CaCl2 is either produced by earlier K⇔Ca reaction with wall rock plag or direct reaction with H2SO4 of Ca- bearing wall rock minerals)-- since the amount of HCl produced in the above reactions is
related (mSfl); the low temperature acid (HCl) alteration may be much
greater than that produced by the HCl in the original magmatic fluid

FLUIDS DERIVED FROM S-TYPE MAGMAS (low fO2)
May contain same amount of H2S as that in high fO2 fFLUIDS DERIVED FROM S-TYPE MAGMAS (low fO2)
May contain same amount of H2S as that in high fO2 fluids - but due to low
fO2 they contain less SO2; hence less ΣS.
Consequently they ppt. less sulfide (mainly Po) but largely oxides (cassiterite) upon cooling.
mCO2≈mCH4≈mH2S>> mSO2, as a result they are of very high mCH4/mSO2 ratio
and mCO2/mCH4 ratio remains const. during cooling and the fO2-T paths tend to lie near CO2/CH4=1 line
Corroborated by rich conc. of C- species (CO2+CH4) in fluid inclusion from
granite-related Sn deposits.
In feldspathic wall rocks the amount of HCl produced is controlled by
hydrolysis reactions involving silicate minerals – commonly yielding mus
or other aluminous minerals.
In carb. rocks, HCl content of the fluid is fixed at lower levels
as: CaCO3 +2HCl = CaCl2 +H2O+ CO2, consumption of HCl in turn leads to pptn. of sulfide minerals
(carb. replacement) by reactions such as: ZnCl2 + H2S=ZnS + 2HCl (skarns)
The above reactions ore commonly accompanied /preceded by other carb.
replacement reactions involving pptn. of Fe- bearing silicates (Gt, Px)
oxides ( Mt) -typical Skarn forming reactions with evolution of CO2 and
further inhibiting calc-silicate formation; such as CaMgSi2O6 + 2CO2
=CaMg(CO3)2 +2SiO2 → resulting in complete replacement of calcite by
dolomite and silica














 




 




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