Thursday, November 18, 2010

ORE GEOLOGY3.Fluid Inclusions Study

Fluid Inclusions : Microgeological Systems
 types:
1)primary:
trapped during xenolith growth
2)Secondary:
formed after the xenolith growth, along healed cracks
3)pseudo-secondary:
occur along healed cracks and fractures that terminate at the grain boundaries → cracks formed during xenolith growth. PRACTICALLY FURNISH THE SAME DATA as primary inclusions.
Mechanisms of trapping:
Any process, which interferes with the growth of a perfect xenolith, may cause trapping of primary inclusions.
(1) A period of rapid crystal growth forming a porous dendritic layer
succeeded by a slower growth controlled xllization, thus covering solid
impervious layers and trapping many inclusions.
(2) If the material (nutrients) is supplied to the growing xenolith faces by a mass flow of fluid, large inclusion may be trapped as a result of temporary
starvation of centre of faces relative to faster growing edges
(having
easier access to the fluid) →the excess fluid is trapped.
(3) If some xenolith-growing blocks grow faster than others
→the surface becomes rough with many angular re-entrants,
which are filled in by the fluids
followed by covering during later growth periods yielding negative xenolith cavities.

ANY INCLUSION WHICH IS TRAPPED ALONG THE PRIMARY GROWTH
ZONE IN A MINERAL GRAIN IS PRIMARY. ISOLATED INCLUSIONS
FORMING A RANDOM 3-D NETWORK IN UNDEFORMED HOST MINERAL
GRAIN IS ALSO CONSIDERED AS PRIMARY

Trail
terminology:
main distinction is
made between
trans-granular (secondary),
inter-granular and
intra-granular
(pseudo- secondary)



Assumptions
(i)Homogeneous entrapment - proof: inclusions showing apparently same phase ratios
(ii) No change in cavity volume after entrapment
(iii) Inclusions behaved as thermodynamically closed systems
(iv) The origin of the inclusion is known

PHASE CHANGES SINCE TRAPPING

(A) SHRINKAGE:
leads to formation of the vapour bubble.
As the volume co-efficient of thermal expansion for most minerals is one to three order of magnitude lower than that for H2O - during cooling the cavity
(mineral) shrinks much less than the fluid inside.

When pressure in the inclusion drops below the total vapour pressure (P < Pv) of the multicomponent fluid (Hence Vfl <Vincl)
→ a bubble nucleates and grows in size. Hence, by reversal of this natural process in the lab. (heating), temperature (Th) can be determined.
Types of: L + V → L, L + V → V,
CRITICAL HOMOGENIZATION: by fading of the L- V miniscus
(B) IMMISCIBILITY:
There is no dividing line between the separation of a vapour bubble by fluid shrinkage and separation by immiscibility.
dH2O(pure H2O) =.0006 at 100°c,
but if the temperature is around 370°C then dH2O ≈ 0.32 grm.cm-3 (500 times greater) most gases present (CO2, CH4etc.) will preferentially enter the vapour phase dV > dL and the gas bubble will sink in the liquid.
If Xco2 is high – then CO2 phase will split into LCO2 and VCO2 and they will homogenize at any temperature below
31.1°c (Tcrit,CO2) down –16°c ( depending on dCO2).
(C) CRYSTALLIZATION ON THE WALLS:
during cooling the host mineral, if supersaturated should xllize.
Then theoretically speaking, it should redisolve on heating-- But seldom seen in inclusions from ore deposits.
(D) DAUGHTER MINERALS:
Fluid could become saturated wrt some
dissolved electrolytes
(salts) and they nucleate as daughter minerals
(NaCl, KCl, CaSO4 etc). These daughter minerals dissolve (Ts,NaCl etc.)
during heating – contrary to captive phases.
(E) METASTABILITY:
Inclusions are very small system even from the atomic viewpoint.
An inclusion of 10μ size containing 30% solution of NaCl may have only 1011-1012 molecules of NaCl.
inclusion containing Similarly, another 10 ppm PbS of the same size would contain around 25x106 molecules of PbS –→ results in metastability i.e., failure to nucleate new but stable phases.
Noticeable metastability during freezing the inclusion (failure to nucleate ice, NaCl .2H2O, CO2 5.75 H2O
etc).

MICROTHERMOMETRY
Most commonly used non-destructive analytical technique involving careful
observation of phase changes (e.g., ice melting, homogenization, solid dissolution etc) as a function of temperature (-195°C to 700°C) in individual inclusions, carried out in microscopic heating- freezing stages.The data obtained can be used with the help of experimental data inpertinent systems to constrain the chemical (salinity, gross chemistry) and physical (density, P, T) parameters of the fluid.
The constraints are in general, semi-quantitative for comparing complex
multi- component natural fluids to simplified experimental systems.

The H2O system
Many inclusions have comp. approaching pure H2O.
Hence, phase equil. of the most important compound on Earth is the logical place to start.
In this one component system, the solid- liquid- vapor triple point (+0.01°C an 0.006 bar) is invariant (change in P or T will reduce the no. of phases). As
dLH2O > dice (at the triple point) and
there are high- P polymorphs of ice →several unique features of low- T phase equil. of H2O → potential confusion in interpreting melting data.

Three possible cases can occur:
P-T diagram for pure H2O,
showing boiling curve (L-V equilibrium.) and isochores of various densities.
The inset shows an expanded view of the area near the triple point and melting
paths of two model inclusions.



i)
Relatively low- density H2O inclusion containing a vapor bubble
will freeze→ ice + V (incl. A) which will follow ice + vapor curve during warming until it melts and vanishes at +0.01°C.
(ii)
In somewhat denser inclusion (B), the vapor bubble eliminated during by
expansion of H2O. On warming, the inclusion will begin to melt at temp. <
0°C → final disappearance of ice at +0.01°C, where the bubble should re-nucleate.
(iii)
Still denser inclusion (not shown above), without a vapor bubble at room
temp. → freeze to a mixture of (ice I + ice III) → begin melting at -21°C
and finish melting at temp. below 0°C.

THE INEVITABLE PRESENCE OF OTHER SPECIES IN NATURAL H2O
COMPLICATE THE ABOVE INTERPRETATION

H2O- NaCl system
Most common salt system. Apart from ice, two new solid phases form.
They are: halite and hydrohalite (NaCl.H2O).
For all compositions < 61.9 % NaCl, at temp. < -21.2°C,
an incl. will contain (ice + HH+ V).
Aq. incl. irrespective of their comp. generally must be frozen to –30° to –40°C →instantaneously frozen to a disequilibrium mixture of ice + HH, which may begin to melt upon heating at the metastable eutectic at –28°C.
Let's assume that incl. A has been frozen to an equilibrium mixture of ice HH+ V. Upon heating to –21.2°C, the first melt forms.
The temp. remain fixed until either all the ice or all the HH melts.
Since the bulk comp. is to the left of the eutectic, HH will disappear.
Correct observation of first melting is critical for determining the pertinent incl.fluid comp. Upon further warming of incl. A, the liquid Increases (lever rule) until the ice disappears at a temp. that fixes its comp. For comp. <23.2 wt % NaCl, the final ice melting temp. (Tm) of the ice can be converted to salinity using eqn. (1)
wt % NaCl = (-1.78Tm)-[0.042(Tm)^2]-[0.00057(Tm)^3]....................(1)
For inclusion A (Tm= -10°C), has a salinity of 13.9 wt % NaCl. Note, however,
the same Tm of –10°C can also be observed for bulk comp. 23.2 and 26.3 wt
% NaCl, where HH rather than ice is the final phase to melt, for which
salinity can be calculated using eqn. (2)
wt % NaCl = 26.271 + (0.181 Tm) + [0.002 (Tm)^2].............(2)

Ice- or HH- melting (?)

Ice has a RI lower than H2O,
HH
is strongly birefringent and has a higher RI
Incl. containing > 26.3 wt % NaCl must have a halite daughter xl stable at
room temp. The bulk salinity of such incl. is calculated from the halite
dissolution temp, Ts,NaCl  using eqn. (3)
Wt % NaCl = 26.242 + (0.4928 Ts) + [1.42 (Ts)2]- [0.223 (Ts)3] + [0.04129  (Ts)4]            +[0.006295 (Ts)5]- [0.001967 (Ts)6]+ [0.00011112 (Ts)7](3)
where Ts = Ts,NaCl/ 100
Homogenization temperature (Th)



Influence of salinity on the boiling curve and slope of the
isochores in the H2O- NaCl system

In the above figure, for a given press. the temp. diff. (Tt- Th) increases as
the density decreases
as the isochores (lines of const. density/molar 10volume) flatten out. (Tt − Th) is referred as pressure correction.
Density can be calculated from the freezing data, but in the absence of correct press. estimate, Th is considered as MIN. TEMP. OF ENTRAPMENT (OR
FORMATION)
, unless boiling condition is established.

Since boiling in hydrothermal systems is essentially a near-isothermal decompression (or near-isobaric cooling) process,
inclusions showing both liquid- as well as vapor-state homogenization is indicative of entrapment from a boiling fluid


A MODEL BOILING FLUID ENTRAPMENT

Estimate of fluid density based on Th is a prerequisite for correct
estimation of Tt.
In real inclusions solutes such as NaCl, KCl, MgCl2 and
CaCl2 further complicate the P-T estimation. In the above figure, inclusion
A and B through trapped at the same press (1kb) and homogenizing at the same temperature. (Th=140°c) require grossly different pressure correction
as evident from the difference in T t(A) & T t(B)
Dissolution of daughter minerals
The daughter minerals (halite, sylvite) dissolve at different rates while
heating the inclusion (KCl faster).
For a saturated NaCl–H2O system, the
final solution temperature of halite (TS,NaCl) is directly proportional to the wt% NaCl in the solution.
When more than one daughter phase is present
(I)Correct identification of the phase and
(2) Knowledge of the solubility of each in the mixed salt system

fig:
Dissolution of halite and sylvite as a function of temperature (Ts)

Freezing
All phase changes occurring below room temperature. Freezing is wholly
complimentary to heating and each inclusion should be frozen and heated to complete the microthermometric runs.
Data obtained on freezing
refer to
gross fluid composition and density.
For multicomponent fluids
the normal sequence during freezing is L+V⇔ S+L+V ⇔ S+V.
The presence of additional component extends the temperature interval over
which solidification (and melting) takes place. The melting temperature is a
direct function of composition–thus fluid composition can be determined
provided appropriate experimental data is available.
Inherent problem–
Reluctance to freeze due to metastability, the extent of which is inversely
proportional to inclusion size.

Freezing point is depressed in different binary H2O- salt systems

Salinity is measured by measuring the final ice–melting temperature (Tm)
on re-heating the completely frozen inclusion.


Since the fluid comp. is difficult to establish, Tm is conventionally reported
as wt % NaCl eq. (The amount of NaCl which would produce an equivalent
lowering) with an assumption that in a mixed Ca-Na-K-Mg chloride solution
the error in estimation of salinity using freezing pt. dep. method of NaCl eq.
is <5% when studying salt–H2O systems the temperature of first melting
(Tfm)
can be determined, if proper care is taken to observe the temperature
of first appearance of liquid.
Tfm should correspond to the eutectic
temperature from which, the gross fluid comp. can be determined.

 composition                       Eutectic Temp( degree C)
H2O- NaCl- CaCl2- MgCl2   -57
H2O- NaCl- CaCl2               -52
H2O- CaCl2                         -49.5
H2O- FeCl2                         -35
H2O- MgCl2                       -33.6
H2O- NaCl- KCl                  -23
H2O- NaCl                          -21.2
H2O- KCl                            -10.6


NON-AQUEOUS CO2 (± CH4) SYSTEMS

The CO2 SYSTEM
Carbonic inclusions are present in variety of hydrothermal ores and pegmatites. Commonly they contain CH4 and rarely H2S.
Hence phase equilibria in the pertinent systems are prerequisites for interpretation of microthermometric data of these incl.

Sequence of phase changes in a pure CO2 inclusion during freezing- heating run finally homogenizing as liquid
CO2 (LCO2 + VCO2 →LCO2)
Different Stages
at 20 degree C               L+V (2x size). Initial stage
at  -100 degree C            Solid CO2+ V (.0001x size)
at  -56.6 degree C           L+V (4x size)
at 27 degreeC                 L+V(x size)
at 29 degree C                L CO2            Final stage


Density and homogenization temp. (Th,CO2) relationship in carbonic incl. Density is calculated from the observed CO2 homogenization temperature, depending on the nature of homogenization


 FIG:Phase relations in the CO2- CH4 system at sub- ambient temperature. The triple point of CO2 is lowered with increasing CH4 (Tm,CO2 in equilibria with LCO2 and VCO2 is depressed)







 Fig: Large primary incl. in quartz containing a vapor bubble, large cubic halite
crystal and two unidentified daughter (or captive) phases (a and b)







P-T Estimation by intersection of data obtained from different inclusions study


Coeval and cogenetic aqueous and carbonic fluids



Fluid Evolution



Example I
Greenstone- hosted Au- mineralization at Larafella, Burkino
Faso, West Africa
The lode Au mineralization is associated with low- saline CO2- rich
inclusion


Example II

alteration zone
At Hutti, the mineralized quartz veins are associated with
low saline aqueous inclusions (H2O+ Salt) ,
carbonic inclusions (CO2 ±CH4) and
aqueous- carbonic (H2O+ CO2 ± CH4 + NaCl) inclusions








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