III. Manifestations of High-Temperature Systems

With respect to the topography of surrounding volcanic rocks and their inferred heat sources:

Manifestations of hydrothermal systems hosted by high standing volcanic centers (with an inferred local cooling pluton as their heat source)

Manifestations of hydrothermal systems in rather flat terrain transferring heat from inferred extensive hot crust or plutons

Manifestations associated with high-temperature systems over extensive hot crustal rocks in a plate collision regime

A. Manifestations of Hydrothermal Systems Associated with High Standing Volcanic Centers

Low

• average of permeability (k) of the order of <1 to 3 millidarcy

Moderate

• average of permeability (k) of the order of 3 to 10 millidarcy

High

• average of permeability (k) of the order of >10 millidarcy

Depending on permeability of the reservoir rocks and their surroundings and the extent of recharge, three distinct types of reservoirs:

There are three types of Hidrotermal Systems Associated with High Standing Volcanic Centers:

1. Liquid Dominated System

2. Natural Two-Phase System

3. Vapor Dominated System

A. Manifestations of Hydrothermal Systems Associated with High Standing Volcanic Centers

a. All manifestations over the central part of the reservoir derive from ascending steam that is discharged by fumaroles and from minor steaming (hot) ground.

b. Condensed steam, with oxidised H2S gas, feeds minor hot acid springs.

c. Steam and CO2 can discharge together through hot mud pools d. Gas discharge fetures (CO2 and rarely with H2S). E.g.: Kaipohan

(Philipines) and Putizza (Italy). e. Warm springs in downslope with bicarbonates water (HCO3) f. Outflows of neutral pH chloride water g. Hot springs and hot pools (outflows discharge in valley or at lower

elevations) h. Some hot water and steam (spouting springs or geyser) i. Warm springs and seepages, some with travertine (CaCo3

deposits) in furthe downstream. E.g.: Hakone system (Japan)

1. Manifestations of Liquid Dominated Systems

Conceptual model of a liquid dominated. The model is based, in part, on the Palinpinon system (Philippines); the heat source

is a cooling pluton. (Modified from Hochstein, 1990.)

Examples for Liquid Dominated Systems:

Palinpinon and Tongonan (Philipines)

Berlin (El Salvador)

Momotombo (Nickaragua)

Cisolok and Cisukarame (Java)

Sipoholon (Sumatra)

Songwe (Tanzania)

a. Extensive areas of steaming ground with minor fumarolic activity. E.g.: Olkaria prospect (Kenya)

b. Heat transfer from the huge reservoir to the surface by ascending steam that condense at shallow depths. (mantaining dominantly conductive heat transfer to the surface)

c. Some minor warm springs that discharge small amounts of condensate.

d. Examples: Eburru prospect (Kenya Rift Valey), Namarumu (Kenya), Aluto (Ethiopian Rift)

2. Manifestations of Natural Two-Phase Systems

Simplified model of a high-temperature steaming ground system with a natural two-phase (coexisting liquid and vapor) reservoir beneath a broad volcanic center in a semiarid environment showing the restricted variety of

surface manifestations in this setting. The model has some affinity to the Olkaria system (Kenya) and many other similar systems in the East African Rift Valley.

a. Heat transfer is dominantly by steam ascending from the top of thick concealed layer with condensates of almost neutral pH bicarbonate water, that together with intense alteration, may act as confining cover.

b. Steaming ground and fumaroles are common

c. Minor acid condensates form “muddy” hot pools and small acid lakes with very low mass discharge.

d. The low permeability of the rock surrounding these reservoirs

e. There are no neutral pH chloride springs on the lower flanks.

f. Examples: Kamojang and Darajat (Java)

3. Manifestations of Vapor-Dominated Systems

Conceptual model of a vapor dominated system beneath a broad, high standing volcanic complex. The reservoir has a condensate layer on its top. Heat transferred within the reservoir is discharged at the surface by steam and hot condensates (bicarbonate waters). The model has some similarity to the Kamojang system

(Java); the heat source is a cooling pluton. (Modified from Hochstein, 1990.)