Transcript Slide 1

Subject : Economic geology
Prepared by:
Dr. Ahmed Mohamed Zayed
E mail: [email protected]
3/7/2012
th
7
lecture
Hydrothermal processes
What is the meaning of hydrothermal solutions, how they
form, how they react with the rocks through which they
pass, and how they deposit their carried constituents are
topics of interest to economic geology.
 Ore forming fluids can be subdivided into:
1) Magmatic Hydrothermal
2) Seawater - meteoric water (rain, river, and/or lake water
(groundwater) - connate water (water included within interstitial
pore spaces of sediment as it is deposited)
4) Mixing of 1and 2
4) Metamorphic fluids
Hydrothermal is a descriptive term rather than genetic term
 Hydrothermal fluids are parts of geothermal system in which
fluids circulate
 A hydrothermal system with recharge and discharge channels is
referred to as open system, ones with only discharge channels are
.called closed systems.
 In hydrothermal systems there are 5 sources for the water:
1) Seawater
2) meteoric water (rain, river, and/or lake water (groundwater)
3) Connate water (water trapped in sediments and breccias at time
of formation)
4) Metamorphic water-especially common at transition from
greenschist to amphibolite grade due to dehydration reactions
4) Magmatic
Source of metals in hydrothermal fluids have 3 origins:
1) Rocks or sediments through which fluids pass and interact
2) Magmas
3) Combination of 2-mixing in geothermal systems
Magmatic Hydrothermal Fluids
 Magmatic-hydrothermal fluids originate from magmas as they
cool and crystallize at various levels of the earth’s crust, and are
responsible
for
a
wide-range
of
ore
deposits
 At some stage, either early or late in the crystallization history of a
felsic magma, it will become water saturated resulting in the
exsolution of an aqueous fluid which forms a chemically distinct
phase in the silicate melt (this is called water- or vapor –saturation)
 This aqueous phase will be in chemical equilibrium with the
igneous melt.
 The aqueous fluid is composed mainly of water plus significant
contents of CO2, SO2, H2S, NaCl, KCl, FeCl, CaCl, HCl, HF, and of
course a wide variety of metals.
 Magmatic-hydrothermal fluids, once exsolved and rise can move
directly into the near surface environment with little interaction with
geothermal waters, or they may become thoroughly mixed with
geothermal waters and this will lead to different ore deposits with
different geological characteristics.
 Five ore types of hydrothermal ore deposits can be distinguished
according to T, P and the geologic relation under which they are
formed :1) Hypothermal
2) Mesothermal
3) Epithermal
4) Telethermal
5) xenothermal
1) Hypothermal deposits
 They are formed at high T (300-500 °C) and great depths
 They are characterized by textures and structures that indicate
the occurrence of well developed replacement.
 Conection to the surface is impeded
 Characteristic minerals are Au, Wolframite (iron manganese
tungstate mineral), pyrrhotite, pentlandite, Scheelite (calcium
tungstate mineral CaWO4), Pyrite, Chalcopyrite, Sphalerite, Galena,
Stannite, Cassiterite, Uraninite, and Cobalt ( pyrite is the most
common mineral)
1) Mesothermal deposits
 They are formed at moderate T (200-300 °C) and P
 They have tenuous connection to the surface
 The characteristic deposits are Copper, Pb, Zn, Ag and Au while
the characteristic minerals are Chalcopyrite, bornite (Cu5FeS4),
Galena
Wolframite, pyrrhotite, pentlandite, Scheelite, Pyrite,
Sphalerite and chalcocite
3) Epithermal deposits
 They are formed at shallow depths and low T (100-200 °C)
 They are in the form of filling vein, irregular branching fissures,
stockworks, breccia pipe.
 replacement is less common
 the country rocks near the epithermal vein are extensively altered while
the vein walls may be sharply defined
 Characteristic minerals are silver gold, stibnite (sulfide mineral with
Sb2S), cinnabar (HgS)and native mercury
4) Telethermal deposits
 They are formed at shallow depths and low T (<100 °C)
 They are formed from hydrotherml fluids that have migrated for long
distance from their magmatic source, so they lost most of their heat and
their potential to react chemically with the surrounding rocks.
 some geologists believe that the telethermal deposits are the products of
meteoric water.
 Common minerals are sphalerite, galena, chalcopyrite, pyrite, native
copper, oxides of uranium, vanadium and copper.
5) Xenothermal deposits
 They resulted from plutons intruded in shallow depths which expelled
fluids of high T in low pressure environment , and this cause metals of
the fluid to undergo rapid cooling and hence the mineralization load of
these fluids are deposited over short distance and hence we find low
and high T minerals side by side (ie. confusing paragenetic sequence )
 most xenothermal deposits are associated with volcanic and taffaceous
rock of recent age.
Characteristic minerals of this group is complex because low T mineral
such as Ag are found side by side with the high T wolframite.
Mineral sequence
 Crustification :it is a characteristic feature in the cavity filling
deposits in which the ore is build up in successive layers or crusts
by crust, where the younger crust is deposited on an older one
 The cause of such sequence is related to the decreasing of the
mineral solubility in the solution in accordance with the decrease
in T, P, where the least soluble mineral is deposited first while the
most soluble mineral is deposited last
Examples of hydrothermal deposits in Egypt
 Hypothermal deposits
1) Cassiterite (Tin oxide SnO2) quartz vein with or without
wolframite eg in Igla, Nwibi and El Muilha (central Eastern
Desert )
2) Wolframite quartz vein in Abu Kharif, Umm Bissilla, Zarget El
Naam and Qash Amer
 Mesothermal deposits
1) Chalcocite which represent the copper mineralization is recorded
in Samara in Sinai
2) gold quartz vein in Atalla, El Fawakhir, Umm Rus, Barramiya,
Hamash etc (Eastern Desert )
 Epithermal deposits
1) The most important epithermal deposits are cinnaber (HgS) and
stibnite which is rare in Egypt.
 Telethermal deposits
 The most important telethermal deposits are lead Zinc deposit ,
which occur in the Miocene rocks at Umm Gheig
Textures of hydrothermal ore deposits :
A- Replacement textures:
Replacement is the process of almost simultaneous solution
and deposition by which a new mineral of partly or totally
different chemical composition may grow in the body of an
old mineral or mineral aggregate. According to this
definition, replacement is accompanied by very little or no
change in the volume of the rock. However, in practice, this
process is accompanied by expansion or contraction (and it
has proven quite challenging to write balanced chemical
reactions representing replacement textures in which the
volume of the products and reactants is the same!).
Replacement is more common at high T and P where open
spaces are very limited or unavailable, and fluid flow is
rather difficult. It also depends largely on the chemical
composition and reactivity of both the host rock and the
hydrothermal solution.
B- Open space filling textures
 Open space filling is common at shallow depths where
brittle rocks deform by fracturing rather than by plastic flow.
At these shallow depths, ore bearing fluids may circulate
freely within fractures, depositing ore and gangue minerals
when sudden or abrupt changes in P and/or T take place. As
such, open space filling textures will be different from those
resulting from replacement, and a set of criteria may be used
to identify this process. Nevertheless, many hydrothermal ore
deposits form by the combined effects of replacement and
open space filling, which requires a lot of caution in textural
interpretation.
 Cavity openings and fillings under subaerial karstification
result in the formation of typical open space filling textures
Criteria for identifying open space filling processes:
1- Many vugs and cavities
2- Coarsening of minerals from the walls of a vein to its centre
3- Comb structure: Euhedral prismatic crystals growing from
opposite sides of a fissure symmetrically towards its centre
develop an interdigitated vuggy zone similar in appearance to
that of the teeth of a comb .
4- Crustification: Crustification results from a change in
composition and/or physicochemical conditions of the
hydrothermal solution, and is represented by layers of different
mineralogies one on top of the other.
5- Symmetrical banding
6- Matching walls: If an open fissure has been filled without
replacement, the outlines of opposite walls should match