Ore-Bearing Fluids - Their Origin & Migration

 

The four most important considerations in the formation of ore deposits are:

1. Source and character of ore-bearing fluids.

2. Source of the ore constituents and how they were obtained in solution.

3. Migration of ore-bearing fluids.  

4. Manner of deposition.

  For a closer examination the ore-bearing fluids are divided into four categories:  

1. Magmas and magmatic fluids

2. Meteoric waters

3. Connate waters  

4. Fluids associated with metamorphic processes.  

METEORIC WATERS: Water from the atmosphere is meteoric water.  It is especially important in supergene processes.  Temperature, and consequently solubility of minerals increases as this water  percolates down.  Meteoric water contains the dominant crustal elements viz., Na, Ca, Mg, SO4--,CO3-- etc, and it can get mixed with magmatic water.

CONNATE WATERS: Water trapped in sediments at the time of their formation (fossil water).  Contain Na, Cl, Mg, HCO3, Sr, Ba, and N compounds.  They have little direct relationship with ore-bearing  fluids except when the strata containing them are undergoing metamorphism.  When activated, they become strong solvents of metals since they contain chlorine.  They are thus one of the sources of  hydrothermal fluids.

FLUIDS ASSOCIATED WITH METAMORPHIC PROCESSES: Connate and meteoric waters are set in motion during metamorphism.  They are chemically reactive due to heat and pressure, and are therefore active ore carriers.  They leach metals from surrounding rocks and from those through which they move, and deposit them in areas of low temperature and pressure or reactive wall-rock.  Metamorphic waters can also be derived from the breakdown of hydrous minerals (clay minerals contain as much as 14% water).  These waters move down in advance of regional metamorphism (down the metamorphic gradient) or the intruding magma.

Migration of Aqueous Fluids at Great Depths:

Refer: Ore Deposits - Parks & MacDiarmid

Migration of Aqueous Fluids at Shalow Depths:

Refer: Ore Deposits - Parks & MacDiarmid

Migration of Metals in the Colloidal State:

A colloidal system consists of two phases:

  1. The dispersed phase - is the diffused phase

  2. The dispersion medium - in which the diffused phase is dispersed.

  • Colloidal particles range in size from those in true solutions to those in coarse suspension.  The limit of size are 10-3 to 10-7 cm (> solution < coarse suspension).

  • The colloidal material may be solid, liquid or gas and may be dispersed in one of these same phases.

  • In the study of ore transport, we are concerned with solids suspended in liquids or a gaseous medium.

  • A colloidal system consisting of solids dispersed in a liquid is called a Sol.

  • Colloidal particles have large surface areas per unit volume.  Ions absorbed on the surfaces of such particles control their behaviour.

  • If the particles absorb cations they become positively charged, if they absorb anions they become negatively charged.

  • These charges prevent the particles of the sol from coagulating or flocculating, but if an electrolyte is added the particles neutralize and flocculate.

  • Most sulfides and organic sols are negative, whereas most oxide and hydroxide sols are positive.  There are some exceptions, eg.  colloidal silica is negative.

  • Colloids are most stable in cool, dilute solutions and in the presence of a second (protective) colloid. Eg.  colloidal gold is stable below 150oC and coagulates between 150-250oC.  In the presence of colloidal silica this colloidal gold is stable upto 350oC.

  • It is difficult to explain the colloidal migration of metals in depth because the rocks at depth are dense and relatively impermeable.

  • Some geologists suggest that ore fluids change from solutions at depth to colloidal sols in the near surface environments.

  • Evidence of this is furnished by certain minerals which are found in forms that suggest flocculation from a sol.

Notes & Handouts

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