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 CO 2, SO 2, H 2 S, 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 CaWO 4 ), 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 SnO 2 ) 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