Micro-inclusions in minerals provide evidence of ancient hydrothermal fluid compositions and conditions. There are three types of inclusions based on formation: primary during crystal growth, secondary along healed cracks after growth, and pseudo-secondary along internal cracks. Microthermometry examines phase changes like melting during heating to determine salinity, density, and trapping pressures and temperatures. Combined with experimental data for fluid systems like H2O-NaCl, it allows constraints on the P-T-X conditions of ore-forming hydrothermal fluids. Coexisting aqueous and carbonic inclusions can further narrow P-T through isochore intersection. Fluid evolution is also reconstructed from inclusion studies.

1. Fluid inclusions in Ore deposits
Aqueous biphase Aqueous polyphase
Carbonic (CO2±CH4) Hydrocarbon-bearing

2. Micro-geological systems, tiny cavities filled with representative of ore
forming fluid regimes-ANCIENT HYDROTHERMAL SYSTEMS or ore
deposits (modern hydrothermal systems).
Furnish data (composition, P, T, salinity, density)
Types : based on chronology
Primary: trapped during crystal growth
Secondary (trans-granular): formed after the crystal growth, along healed
cracks
Pseudo-secondary (intra-granular): occur along healed cracks and
fractures that terminate at the grain boundaries  cracks formed during
xl growth. PRACTICALLY FURNISH THE SAME DATA as primary inclusions
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

3. Assumptions
Homogeneous entrapment - proof: inclusions showing
apparently same phase ratios
No change in cavity volume after entrapment
Inclusions behaved as thermodynamically closed systems
L
V
Aqueous bi- phase
inclusion
Aqueous- carbonic
(gaseous) inclusion
LH2O
LCO2
VCO2

4. Any process, which interferes with the growth of a perfect crystal, may
cause trapping of primary inclusions
A period of rapid crystal growth forming a porous dendritic layer
succeeded by a slower growth controlled cystallizationn, thus covering
solid impervious layers and trapping many inclusions
If the material (nutrients) is supplied to the growing crystal 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
If some crystal-growing blocks grow faster than others the surface
becomes rough with many angular reentrants, which are filled in by
fluids during later growth periods yielding negative crystal cavities
Mechanisms of trapping

5. Phase changes since trapping
SHRINKAGE: leads to formation of the vapour bubble due to differential
shrinkage during cooling. Reversed in the lab by heating and
temperature (Th) can be determined. Types: L + V  L, L + V  V,
CRITICAL HOMOGENIZATION: by fading of the L- V meniscus
DAUGHTER MINERALS: Fluid could become saturated wrt some
dissolved electrolytes (salts) and they nucleate as daughter minerals
(NaCl, KCl, CaSO4 etc) dissolve (Ts,NaCl etc.) during heating – contrary to
captive phases
METASTABILITY: Inclusions are very small system even from the atomic
viewpoint. An inclusion of 10  size containing 30 solution of NaCl
may have only 10111012 molecules of NaCl. Similarly, an inclusion
containing 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. Examples: failure to nucleate ice, NaCl.2H2O,
CO25.75 H2O etc)

6. 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 in
pertinent systems to constrain the chemical (salinity, gross chemistry)
and physical (density, P, T) parameters of the fluid
Semi-quantitative constraints for comparing complex multi-
component natural fluids to simplified experimental systems
Microthermometry

7. H2O-NaCl system
For inclusion A (Tm= 10C), has a salinity of 13.9 wt. % NaCl
Wt. % NaCl = (1.78 Tm)  [0.042 (Tm)2]  [0.00057 (Tm)3]
For inclusions containing > 26.3 wt % NaCl must have a
halite daughter xl stable at room temp. Bulk salinity of such
incl. is calculated from the halite dissolution temperature
(Ts,NaCl)
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]
where Ts = Ts,NaCl/ 100

8. Freezing: inclusion composition
All phase changes occurring below room temperature
Freezing is complimentary to heating and each inclusion should be
frozen and heated to complete the microthermometric runs
Data obtained on freezing primarily refer to gross fluid composition
and density. For multi-component fluids the normal sequence during
freezing is L+V S+L+V  S+V. The melting temperature is a direct
function of composition–thus fluid composition can be determined
provided appropriate exptl. data (eutectic) are available
Inherent problem– Reluctance to freeze due to metastability, the
extent of which is inversely proportional to inclusion size

9. Eutectic first melting temperatures for selected
H2O- salt systems
SYSTEM Eutectic temp. (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

10. Sequence of photographs showing major phase changes
observed in biphase (L+V) in fluorite. The fluid composition is in
the H2O-NaCl-CaCl2 system

11. Homogenization temperature (Th) and isochore
P- T diagram for pure H2O showing the heating paths taken by two
inclusions A and B both homogenizing at the same temp (Th), but having
different bulk density. A: L+V L (d > dc) and B: L+V V (d< dc).
TtTh = pressure correction. Th is the minimum temperature

12. Influence of salinity on the boiling curve and slope of the
isochores in the H2O- NaCl system
With increasing density pressure correction increases

13. 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  no pressure correction
Fluid boiling

14. 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
Dissolution of daughter minerals

15. NON-AQUEOUS CO2 (CH4) SYSTEMS
VCO2
Sequence of phase changes in a pure CO2 inclusion
during freezing-heating run finally homogenizing as
liquid CO2 (LCO2 + VCO2 LCO2)
CO2 ice
20C 100C 56.6C 27C 29C

16. PURE CO2 SYSTEM
Density and homogenization temperature (Th,CO2) relationship
in carbonic inclusions
Density is calculated from the observed CO2 homogenization
temperature, depending on the nature of homogenization

17. CO2- CH4 SYSTEM
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)

18. Analyzing gaseous (± H2O) by Raman Spectroscopy
Presence of H2O and contents of CO2 and CH4 (XCO2, XCH4) can
be determined, which can be used to calculate density and
construct isochores in the CO2CH4 system

19. Coeval and cogenetic pure aqueous and carbonic fluid
inclusions
P-T estimation by Isochore intersection: FI thermobarometry

20. Fluid Evolution
Isothermal mixing: Metamorphic/ magmatic fluid with heated meteoric
water
Mixing with cooler- less saline fluid: Magmatic with metamorphic/
evolved connate water