PREPARED BY:

VARINDRA SAINI

MBM ENGINEERING COLLEGE JODHPUR

S.NO. TOPIC PAGE NO.

1

INTRODUCTION

1

2

ORIGINS OF HYDROTHERMAL SOLUTION

1

3

OTHER FLUIDS IN THE EARTH’S CRUST AND THEIR ORIGINS

2

4

THE MOVEMENT OF HYDROTHERMAL FLUID IN THE EARTH’S CRUST

3

5

NATURE OF ORE FORMING SOLUTIONS

4

6 DISSOLUTION OF METAL IN SOLUTIONS TO FORM DEPOSITS

4

7

PRECIPITATION MECHANISMS FOR METALS IN SOLUTION

5

8

DIFFERENT TYPES OF HYDROTHERMAL DEPOSITS

6-11

9

Cl COMPLEXATION OF Zn

12

10

CONCLUTION

13

In addition to magmaticfluids, there are four other major

water types on or near the Earth’s surface. Although they

may allhave had similar origins, each of these fluid

reservoirsis different in terms of its composition and

temperature and will, therefore, play differentroles in the

formation of ore deposits. The major water types are defined

as sea water, meteoric water, connate water and metamorphic

water,listed typically in order of increasing depth (and

temperature) in the crust. A fifth fluid reservoir,where

waters are derived from a mixing of two or more other water

types, is also described below,specifically because mixed

fluids can be very important in certain ore-forming

environments.

In order to be effective as a mineralizing

agent,hydrothermal fluids need to circulate through the

Earth’s crust. The main reason for this is that

they need to interact with large volumes of rock in

order to dissolve and transport the metals required to

form hydrothermal ore deposits. The flow of an ore

fluid should preferably be focused so that the dissolved

constituents can be concentrated into an accessible

portion of the Earth’s crust that has dimensions

consistent with those of a potential ore body

Fluid inclusions in mineral grains preserve samples of hydrothermal solutions. Upon

cooling, the hydrothermal brines separate into solid (usually NaCl,gas (CO2 + CH4) and aqueous phases.The temperature at which the fluid was trapped can be determined by

heating the sample and measuringthe temperature at which gas + liquid recombine

It is possible to stabilize many different metal–ligand complexes under wide ranging conditions in natural hydrothermal solutions, thereby promoting the kind

of solubility levels required to make effective oreforming fluids. Once a metal is in olution, however, it then needs to be extracted from that fluid and concentrated in a

portion of the Earth’s crust that is sufficiently estricted and accessible to make an economically viable ore body. It is obvious that a wide range of precipitation

mechanisms are ikely to be effective since any mechanism that will destabilize a metal–ligand complex and, therefore, reduce the metal solubility, ill cause it to be deposited in the host rock through which the hydrothermal solution is passing.

At shallow crustal levels ore deposition will take place by open space filling, whereas deeper down where porosity is restricted, replacement of existing minerals

tends to occur. Decrease in temperature is the factor that, intuitively, is regarded as the most obvious way of promoting the precipitation of metals from hydrothermal

fluids. At depth, however, temperature gradients across the structures within which fluids are moving tend to be minimal and metal precipitation will be neither

efficient nor well constrained to a particular trap zone. Deposition of metals in such a case is achieved more effectively by changing the properties or composition of the hydrothermal fluid. If ore solution occurs by metal–chloride complexing

then precipitation could occur very efficiently by increasing the pH of the ore fluid.

Hydrothermal deposits occur in a variety of shapes and sizes .the most common forms are veins and cavity fillings.

VEINS

Veins are narrow ,elongated or tabular shaped economic minerals occurring within

host rock. Veins are further classified into following types:

Fissure-veins

Ladder-veins

Gash-veins

Stock works

CAVITY FILLINGS

Cavity fillings are ore deposits that get deposited from the hydrothermal fluids

inwell defined open spaces ( cavities ) other thanalong cracks or fractures available

in the host rock.

Saddle reef is an intresting cavity filling .In saddle reef cavities are associated with

folded rocks and minerals get deposited in such cavities

VEINS :

Fissure veins

Ladder veins

Gash veins

Stockwork

CAVITY FILLINGS :

Saddle reef

Widely disseminated

vein networks of

Sulfide Cu Deposis,

Waraz area,

NE-Iraq

Kurdistan Region

Hydrothermal

vein deposits

Note: the distribution of hydrothermal ore

deposits are irregular in the host rocks

Formation of hydrothermal deposits

Zn+2 + Cl- = ZnCl+

Zn+2 + 3Cl- = ZnCl3

-

Zn+2 + 2Cl- = ZnCl2

Zn+2 + 4Cl- = ZnCl4-2

Zn(H2O)6 + nCl = ZnCln + 6H2O

pK = -0.2; H = 43.3 kJ/mol

pK = -0.25; H = 31.2 kJ/mol

pK = 0.02; H = 22.6 kJ/mol

pK = -0.86; H = 5.0 kJ/mol

Thus at the end of the project we can conclude that Hydrothermal mineral

deposits are those in which hot water serves as a concentrating,

transporting, and depositing agent. They are the most numerous of all

classes of deposit.

It is interesting to know that Hydrothermal deposits are never formed from

pure water, because pure water is a poor solvent of most ore minerals.

Rather, they are formed by hot brines, making it more appropriate to refer

to them as products of hydrothermal solutions. Brines, and especially

sodium-calcium chloride brines, are effective solvents of many sulfide and

oxide ore minerals, and they are even capable of dissolving and

transporting native metals such as gold and silver

Hydrothermal deposit various useful and precious elements like gold ,

copper , tungsten,molybdenum and to some extent silver , lead and zinc

etc. Formation of hydrothermal ore deposits is linked not only to the generation of

significant volumes of fluid in the Earth’s crust, but also to its ability to circulate

through rock and be focused into structural conduits (shear zones, faults, breccias,

etc.) created during deformation. The ability of hydrothermal fluids to dissolve

metals provides the means whereby ore-forming constituents are concentrated in

this medium. Temperature and composition of hydrothermal fluids (in particular

the presence and abundance of dissolved ligands able to complex with different

metals), together with pH and fO2, control the metal-carrying capability of any

given fluid. Precipitation of metals is governed by a reduction in solubility which

can be caused by either compositional changes (interaction between fluid and rock,

or mixing with another fluid), or changes in the physical parameters (P and T) of

the fluid itself. Economically viable hydrothermal ore deposits occur when a large

volume of fluid with a high metal-carrying capacity is focused into a geological

location that is both localized and accessible, and where efficient precipitation

mechanisms can be sustained for a substantial period of time.

INTRODUCTION TO ORE FORMING PROCESS :by Laurence robb

ENGINEERING GEOLOGY

:by Praveen singh