Geochemical and mineralogical study of the volcanic lake and thermals springs associated with the Sorik Marapi

Study of the chemical and isotopic compositions of the volcanic lake and thermal springs associated with the Sorik Marapi volcano (Sumatra, Indonesia). The objective of this study is the understanding of the hydrothermal system associated with the volcano as a help of eruption forecasting.

Sorik Marapi is a 2145 meters high andesitic stratovolcano of Sumatra (Indonesia).  This volcano is characterized by a large hydrothermal system and frequent phreatic eruptions.

Photo of the crater lake of Sorik Marapi (July 2003)

The summit crater contains a acidic hot lake (37.5°C) (pH=1.1) with an unusual acid sulfate-chloride composition characterized by a sulfate chloride ratio of 0.5 much lower than usually observed in most volcanic lakes (1.5).  The lake is also characterized by a low TDS (3.77g/l) and a high enrichment in d³⁴ S  (15‰).  This suggests that an important part of sulfates is the result of the disproportionation of magmatic sulfur dioxide  gas.  The geochemistry of the lake water can be interpreted as the result of the direct condensation of volcanic gases at the level of sub-aquatic fumaroles.  Moreover, the recycling of lake water in the hydrothermal system is limited, perhaps even non-existent.

Relative concentration of anions species in volcanic lakes: Ta : Taal (Delmelle et al., 1998),  Ku : Kusatsu (Ohba et al., 1994), An : Santa Ana (Bernard et al., 2004), Tin/Tam : Keli Mutu (Pasternack and Varekamp, 1994), Ij : Ijen (Delmelle and Bernard, 1994), Po : Poas (Rowe et al., 1995), Pu : Kawah Putih (Sriwana et al., 2000), Sv : Soufrière St Vincent (Sigurrdsson, 1977), Kelut (Bernard and Mazot, 2004), Qu : Quilotoa, Sa : Segara Anak (Aguilera et al., 2000), Ny : Nyos, Mo : Monoum (Kusakabe et al., 1989),  Ma : Maly Semiachik (Takano et al., 2000).

Based on their compositions, the hot springs from the North-East flank of the volcano can be divided in two groups: (a) acid sulfate-chloride (ASC) waters and (b) steam-heated or steam-condensate (SH, SC)waters.
These ASC springs are characterized by very high values of D_HSO4-H2S (≈27‰) and sulfate-chloride  ratio ≈1.5 which suggests that the ASC fluids are the result of the deep condensation of magmatic gases.  In addition, the linear correlation between conservative elements (Cl, B, Li, Mg) can be explained by the dilution of ASC fluids with meteoric waters during their ascent to the surface.
The SH and SC fluids probably result from the boiling of the ASC hydrothermal system.  In fact, the chloride are absent in these fluids and the d³⁴HSO₄ value (-3.60‰) is very low, this suggests that H₂S  is the only precursor of dissolved sulfates in those waters as it is typically observed when a hydrothermal system is boiling: only H₂S and carbone dioxide  can accompany water in the vapor phase.

above: photo of one of the acid sulfate chloride hot springs.

left: relative concentration of anions in the lake and hot springs of Sorik Marapi volcano.

All these geochemical information allow us to construct a model of the hydrothermal system where magmatic fluids emitted by a sallow intrusion condense to create the ASC reservoir.  However, at the top of the magma chamber, the gases are sufficiently hot for going directly to the surface and be dissolve in the lake.
A better knowledge of the hydrothermal system allow us to select the ASC hot springs as the most useful springs for getting information on the phreatic or magmatic evolution of the Sorik Marapi volcano, the ASC waters being the result of condensation of magmatic fluids. 

References:

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