8/10/2019 Belousov1 (1)

1/9

1

PROCEEDINGS, Thirty-Seventh Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 30 - February 1, 2012SGP-TR-194

THE FORMATION OF HYDROTHERMAL-MAGMATIC SYSTEMS OF SKARN TYPE

Belousov V.I., Belousova I.V., Filippov Yu..

Institute of Volcanology and Seismology of the FEB RASBoulevard Piipa 9

Petropavlovsk-Kamchatsky 683006 Russiae-mail: bvi36@yandex.ru

ABSTRACT

Accompanied by intensive CO2 emission, submarinevolcanic activity at equatorial latitudes of the globecreates the necessary conditions for the formation ofcoral reefs. In the process of its evolution, the coral

head is getting compacted and turns into a waterimpermeable thickness (cap-rock), which plays arole of heat insulator. The big thickness of the caprock (up to 2km) leads to the rise in temperature inthe depths of hydrothermal-magmatic systems. Rockswith high content of silica can melt partially and formthe chambers of acid magmatic melts in the Earth'scrust as well as the secondary flows of volatilecomponents. H2O and CO2 are the main volatilecomponents.

The migration of CO2 in the hydrothermal-magmaticsystem occurs in the form of diffusion and jet streamsand is due to its weak chemical reaction activity in

water and silicate melts. The migration of the gasphase of CO2 in the water-bearing complex iscontrolled by the upper relative water impermeablehorizon, the formation of which is conditioned by theprocesses of acid and propylite metamorphism. Thebubbles of CO2 rise along the border of the gasimpermeable barrier which is usually tilted from thetop of the volcanic edifice to its periphery. Theycombine into jets and stimulate the boil in a verticalcolumn located above the apical part of the rock bodyat a depth of about 2km. Such uprising carbonatedhydrothermal column acts as a gas lift pump. As aresult of pumping of hydrothermal fluids, a hydraulicdepression contacting with the igneous convectivesystem appears in the water-bearing complex. Watervapor, which has high heat capacity, experiencestremendous heat losses in the magmatic convectivecolumn, immersed in the area of the thermal energygeneration in the lower crust and upper mantle. Thebalance in favor of strengthening of the diffusion fluxof CO2 is caused by the increase of the thickness anddecrease of the permeability of the upper waterimpermeable cover, which evolution is connectedwith the activities of the condensate water of thesurface formation. Rise in temperature of the water,

caused by the evolution of the insulating properties ofthe cover, is accompanied by its vaporization,degassing, phreatic and phreatic-magmaticexplosions. The connection between volcanicstructures and limestone organic formations,including coral structures, is not random, but

genetically determined. Such hydrothermal-magmaticsystems are characterized by great heat capacity,which manifests itself in the surface thermalmanifestations, in the places of exploitation ofgeothermal resources, and in the areas of the highintensity of minerals and ore deposits. The skarn typehydrothermal deposits are found in the depths of suchsystems.

This model describes, in particular, geological andhydrogeothermal situation which existed in the areasof Apennine Peninsula (Larderello Monte Amiatageothermal region) and California Cordillera(Geyzers-Clear Lake geothermal field).Papers should follow standard technical paper format,an abstract followed by the more detailedpresentation. The abstract should be typed on thisarea of the first page, with the presentation followingafter two lines of space.

SKARNS

Terminology, localization, petrologic-mineralogical characteristics

Einaudi (1982) describes skarns and ore skarns. Thefirst, which are distributed limitedly, were formed

along shale-limestone contacts duringmetamorphism. The latter are skarns which containthe ore mineralization. They were formed as a resultof the infiltration of the fluids produced by igneousintrusions. The classification of skarns may includethe type of the rock as well as the mineral aggregatesof substituted lithology. The terms of endo- andexoskarns relate to igneous rocks and carbonates,respectively. Einaudi (1982) subdivided exoskarnsinto Ca-skarns and Mg-skarns. Ca-skarns wereformed as a result of the substitution of limestone.

8/10/2019 Belousov1 (1)

2/9

They contain garnets (of andradite-grossulariteseries), clinopyroxenes (of diopside-hedenbergiteseries), wollastonite, scapolite, epidote andmagnetite. Magnesian skarns are the result of thesubstitution of dolomites. They are presented by suchminerals as diopside, forsterite, serpentine, magnetite,talc in the porous silica environments, and talc,

tremolite-actinolite, in silicon-enriched environments.Silica-pyrite skarns are the third type of skarns. Theyrelate to the ore formation stage associated with someporphyry deposits.

Ore skarns are classified according to thecomposition of the mineral aggregates. So there canbe Fe-skarns, Au-skarns, W-skarns, Cu-skarns, Zn-Pb-skarns, Mo-skarns and Sn-skarns. In porphyriticsystems, the hydrothermal changes and ore formationin carbonate rocks lead to the formation of the oreskarns which are economically important. The natureof the ore skarns in the conditions of the porphyriticsystems depends mainly on the content of carbonatesin the altered rocks, their permeability and structuralcharacteristics. Carbonate rocks decompose inaccordance with the reaction:CaCO3 + 2H

+= Ca2++ CO2+ H2O.

The type of the hydrothermal changes and oreformation also depends on the hydrothermal andmetasomatic processes. They are responsible for theformation of the different types of skarns. Einaudi(1982) called them skarn hornstones, calcium skarns,magnesia skarns and silicon-pyrite skarns. Skarnhornstones are formed as a result of thedecomposition of carbonates and dehydration of

carbonate strata without input of additionalcomponents. They are presented by wollastonite anddiopside hornstone. Formation of skarns is caused bythe various processes and is associated with the finalstage of the magmatic activization and hydrothermalphases of the magma intrusions into sedimentaryrocks. Cooling of the plutonic masses is accompaniedby the contact metamorphism and metasomatism ofthe enclosing rocks. Skarns are formed in thetemperature range of 700 to 200C and pressures of0.3 to 3 kbars. Mineralization of the metasomaticfluids varies from 10 up to 45 weight % NaCl eq. Thehydrothermal solutions came out of magma at anearly stage and mingled with meteor waters in

increasing amounts during cooling.

Tectonic position

Skarns occur in the areas of the manifestation ofmagmatism and the presence of carbonate rocks.Meinert et al. (2005) describes four main tectonicscenarios: 1- steep fall of the oceanic plate; 2-transitional low-angle subduction; 3- continentalsubduction and 4- continental rifting. Steep oceanicsubduction leads to the formation of Fe-, Cu- and Au-skarns connected with diorite and granodiorite

plutons. Transitional low-angle subduction may befavorable for Mo- and W-Mo-skarns connected withmonzonite and granitic plutons. Continentalsubduction relates to the tectonic positions for themajority of skarns which include Zn-Pb, Cu, Au, W,Mo deposits, usually associated with granodiorite andgranite plutons. Continental rifting, associated with

mantle plumes or upwelling asthenosphere, results inthe intrusion of granitic plutons and Sn-W skarns.

PORPHYRY HYDROTHERMAL-MAGMATICSYSTEMS

Burnham (1979) studied the hydrothermal-magmaticsystems formed during the cooling of the granodioritestocks containing 3 weight % of water. The intrusivebody is formed in subvolcanic environments. It wassupposed that at the initial stage of the coolingprocess the system was open. Which means that thevolatile components evolve through the fractureslocated above the pluton. Burnham notes that at a

later stage, when the hard shell forms from thecooling rocks, the intrusive body becomes a closedsystem. The maximum temperature inside the stock is1025C, and the 1000C isotherm is located at adepth of 2.5 km. It limits the part of the body 90% ofwhich is the melt. Above and to the other sides of1000C isotherm, the concentration of H2O in theremaining melt increases to the area where the melt issaturated with H2O up to 3.3 weight.%.

Using the experimental data, William-Jones andHeinrich (2005) consider that the phase of watervapor is an important agent in the transfer of metalsin hydrothermal systems. The volatile phase is

considered as a mineralized water fluid with thedensity less than critical. The fluid may be the brine(hypersaline) or water solution. Initially, it has amineralization > 26 weight % NaCl eq., and later