Characterization of Suswa Geothermal Prospect, Kenya, from Fumarole Gas

Geochemistry

By: Kipngok, J.K., Jill R.H., Malimo S.J., Ochieng L.A., Igunza G.M., Mwanyasi F., Bett E.K., Kangogo S.C. and Kanda I.K.

Date: 4th Nov., 2016 Venue: Addis Ababa

Outline

• Background Exploration Geochemistry of Suswa • Objectives for the 2015 work • Sampling and Analysis • Results: QA/QC • Non Condensible Gases • 3He/4He • Stable Isotopes of Water • Conclusion

Introduction

Geological Setting

• S1 - Lavas and pyroclastics • S2 – Pyroclastics (trachyte,

carbonatite, and trachybasalt composition tuffs, ignimbrites, and ash deposits)

• S3 – Agglutinate (trachytes) • S4 – Pyroclastics (trachyte pumice

lapilli tuffs with thin trachyte agglutinate flows)

• S5 – Lavas (trachyte) • S6 – Lavas (aphanitic phonolite

lava flows and ash‐fall deposits) • S7 – Pyroclastics ( • S8a – Lavas (anorthoclase‐phyric

phonolite) • S8b1 - Collapse Breccia (pit crater) • S8b2 - Collapse Breccia (island

block)

Geological Setting

• S8c – Lavas (anorthoclase‐phyric phonolite lava flows and one or more scoria cones)

• S8d - Landslide Deposits • S9 - Lavas (anorthoclase‐phyric

phonolite; the feldspar phenocrysts show substantial magmatic resorption)

• 1986-1987-UNDP and GOK by Ármannsson (ISOR) • KPLC 1993 • GDC 2012/2013 • Regional Exploration provided local meteoric water

lines

Background Geochemical Exploration

• Most results reported only CO2 and air

• 2012/2013 campaign did not reach the inner caldera

• For the few samples with reported H2S or CH4, gas geothermometers 230 to 280C

• CO2 geothermometer >300C in the inner caldera, < 300C outside the caldera (1986/7) to over 350⁰C (2012/3)

• Stable isotopes explained two possible sources of water (1986/7)

Results of Previous Geochemical Surveys

The Nature of Suswa Fumaroles

Outlet temperature: 90 -95C

• Resample the fumaroles (and additional new ones) in such a manner as to obtain gas chemistry and gas/steam measurements while minimizing the effect of air contamination.

• Sample available local groundwater • Evaluate the source of the Suswa Geothermal

Reservoir fluids • Evaluate the reservoir conditions indicated by fluid

chemistry • Provide geochemical contributions to the conceptual

model of Suswa

Objectives of 2015 Geochemical Work

• Sampled 20 fumaroles + 4 cold waters • Many low pressure and difficult to seal • Noncondensible gas in evacuated Giggenbach bottles

with NaOH and Zn acetate + CO and hydrocarbons (no NaOH)

• Analyzed 8 duplicate gas samples • Steam condensate for stable isotopes of water and

selected elements in bottles • Helium isotopes in copper tubes • 4 cold waters for stable isotopes and 3 for water

chemistry

Sample Collection

Sample Collection

Results: QA/QC

• CO2 and H2 mole % dry gas in duplicate samples by GDC and GNS compare reasonably well, but H2S does not.

• RPD (relative % difference) of gas/steam are high and variable.

• Focus on compare dry gas or gas ratios because other units such as

• For H2S, 2015 GDC data is not used in any cases where there is 2015 GNS .

• Stable isotopes of water and condensate in 2015 comparable with 1986/1987.

Sample CO2 H2S H2 CH4 N2

SWF-3-2015 67.4% 194% - 32.2% 168%

SWF-6-2015 136% - - 124% 47.7%

SWF-8-2015 33.7% 167% 3.9% 18.5% 104%

SWF-12-2015 79.1% 193% - 5.1% 10.3%

SWF-14-2015 191% - 180% 179% 27.6%

SWF-15-2015 59.1% 42.6% 45.3% 44.3% 16.7%

SWF-16-2015 119% 189% - 38.1% 14.6%

SWF-19-2015 123% 196% 198% 198% 118%

SWF-20-2015 49.3% - 55.2% 42.1% 30.7%

RPD Calcs 2015 GDC Data vs 2015 GNS Data

Results: QA/QC

• All gas samples contained air

• Used air corrections based on oxygen concentrations

• Did not use GDC 2015 H2S data

• Used GNS if available, or if not, GDC 2012/2013 results from the same fumarole

• Gas distribution indicates stronger geothermal signature within the inner caldera

• CO2/H2S

• <1000 within the inner caldera

• >10,000 at the outer rim of the outer caldera

• H2S, a more soluble gas, could be removed by condensation

Non condensable gas distribution

SWF-31158

SWF-8146

SWF-12706

SWF-15796

SWF-193579

SWF-16711

SSF-15910

SSF-24954

SSF-311567

SSF-43472

SSF-52399

SSF-63472

SSF-83987

SSF-96774

SSF-101381

SSF-116016

SSF-127671

SSF-134726

SSF-1410906

SSF-1512516

• Gas distribution indicates stronger geothermal signature within the inner caldera

• CO2/H2

• Not related to condensation

Non condensable gas distribution

• Both GNS and GDC data sets show CO2/CH4 lows in inner caldera, albeit a different order of magnitude.

Non condensable gas distribution

• Magmatic He strongest in inner caldera (SWF-19, -3 and -15)

Non condensable gas distribution

Ar

N2/100

10He

100

200

500

1000

2000

5000

10000

10

20

50

100

200

500

1000

1 0.2 0.1 0.05 0.02 0.010.5

GNS SWF-3 GNS SWF-12

GNS SWF-14 GNS SWF-15

GNS SWF-19 GNS SWF-6

GNS SWF-8 GNS SWF-16

GNS SWF-20N2/He N2/Ar

He/Ar

air

asw*

crust

*Note: "asw" denotes "air-saturated groundwater."

**Note: For samples with He concentrations reported below the detection limit (DL); half of the DL i s used for plotting. DL = 0.001 mmol/100molH2OSamples Below DL:GNS SWF-6GNS SWF-8GNS SWF-16GNS SWF-20

***Note: All Plotted Samples have been Corrected for Air Contamination

He-Ar-N2 Trilinear Plot (Giggenbach, 1991)

• 250-350C (GNS)-280-350C (GDC) inside the inner caldera: most likely 250-290⁰C and if you include CO2, 325C

• 120-250C (GNS) from fumaroles outside the caldera: most likely 190-225C

Gas Geothermometry

Sampling Site Lab

H2-CO2

(C)1

CO2-H2

(°C)4

H2S-CO2

(°C)1

H2-Ar

(°C)5

CO2-N2

(°C)3

CH4-CO2

(°C)4

Inner Caldera GNS 339 295 251 351 290 280

Outer Caldera GNS 189 213 248 116 226 336

Inner Caldera GDC (2015) 334 281 320 - 314 >350

Outer Caldera GDC (2015) - - - - 282 >350

Outer Caldera

GDC

(2012/13 - - 213 - 230 >350

1: Nehring and D'Amore, 1984 2: Giggenbach, 1980 3: Arnorrson, 1987, using air corrected data 4: Arnorrson and Gunnlaugsson, 1985 5: Giggenbach and Goguel, 1989, using air corrected data

Liquid/Vapor Equilibrium

SWF-3

SWF-8

SWF-12

SWF-14

SWF-15

SWF-19

SWF-20

SWF-6

SWF-16

• Developed by Giggenbach

• Focused on gas ratios

• Reviewed several gas/gas and gas/water reactions

• Most consistent included Ar, H2 and CO2

• Inside the caldera, fumarole gases in equilibrium with vapor and liquid 200 to 270C

Log CO2/Ar versus log H2/Ar (Giggenbach, 1991)

Liquid/Vapor Equilibrium

Possibilities

• Gas separation occurring at high temperatures (250-300C)

• Equilibration happening at two-phase conditions

Diagram of log(CH4/CO2) vs. log(H2/H2O) (from Marini and Fiebig, 2005)

Inferences from NCG Distribution

• Gases related to high temperature interaction with rock or gas-gas reactions CO2, H2S, H2, CH4 are present inside the moat area but only (predominantly) CO2 in the outer caldera

• Gases both more and less soluble than CO2, so condensation not only mechanism

• Upflow is in the eastern and western moat areas in the vicinity of SWF-8 and -15, and SWF-3 and -14

• Temperatures of the hydrothermal fluid source of fumarole discharges are most likely 250-290⁰C in the inner caldera and 190-225⁰C

• Gas ratios suggest that both vapor and liquid may be present in the hydrothermal system

Isotopes: Distribution of 3He/4He

• Inner caldera: R/Ra values >6 indicating magmatic influence

• Outer caldera dominated by air

SWF-61.23

SWF-36.50

SWF-86.25

SWF-12.19

SWF-51.30

University of Rochester R/Ra

Isotopes: Distribution of ẟD

• Distinguishes Inner and Outer Caldera Steam SWF-5

-30.9

SWF-1-36.1

SWF-2-23.9

SWF-3-21.6

SWF-6-84.5

SWF-7-21.2

SWF-8-12.3

SWF-11-32.3

SWF-12-12.2

SWF-14-28.7

SWF-15-23.1

SWF-16-51.7

SWF-4-36.1

SWF-9-56.5

SWF-10-74.5

Isotopes: Distribution of ẟ18O

• Distinguishes Inner and Outer Caldera Steam

SWF-5-8.09

SWF-1-8.25

SWF-2-6.53

SWF-3-6.02

SWF-6-15.89

SWF-7-5.81

SWF-8-3.25

SWF-11-7.50

SWF-12-5.58

SWF-14-6.83

SWF-15-6.15

SWF-16-10.85

SWF-4-8.15

SWF-9-10.88

SWF-10-13.08

Isotopes: ẟD vs ẟ18O

• Distinguishes Inner and Outer Caldera Steam

100 oC

140 oC

180 oC 220 oC

260 oC 300 oC

-90.0

-80.0

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

-18.00 -16.00 -14.00 -12.00 -10.00 -8.00 -6.00 -4.00 -2.00 0.00

δD

eu

teri

um

δ Oxygen-18

Suswa - Stable Isotopes - 2015 Sampling (WETlab); 1986-1987 Sampling (ISOR)

2015: Outside Caldera 2015: Inside Caldera 1986-1987: Inside Caldera

1986-1987: Outside Caldera Meteoric Water Samples Global MWL

Central Kenyan Meteoric Water Line

Fractionation Trends↓ 100 oC to 300 oC ↓

Steam

Liquid

Water Source based on Stable Isotopes

1. Deep, approximately 260°C water that has risen to near surface in the moat and boiled at the surface (100°C), then condensed as it travels laterally;

2. Local meteoric water which has been heated to 260°C with minimal water/rock interaction, boiled in the reservoir, with steam moving to the surface in the vapor phase;

3. Mixtures of meteoric water and steam that is then re-boiled.

1. ≥260°C deep hot water has δ18O=-1.5 to +0.5 and δD=-2 to +6) (single stage or continuous boiling) boils at ~100°C, produces steam of east and west inner caldera

2. meteoric water (approximately δ18O=-4.5, and δD=-22), positive(+2) δ18O shift boiling in reservoir: steam δ18O ≈ -3, and δD ≈ -19 if the boiling occurred at 300C, and δ18O ≈ -5, and δD ≈ -26 if the boiling occurred at 200C, either indicate reservoir vapor

3. Range of isotopes in outer caldera to be related to different meteoric water sources: condensation more likely

Model of Boiling and Condensation

Favored Model • Upflow at the east and western

ends of the Island Block • Reservoir extends below parts of

the ring fault zone and Island Block-throughout the inner caldera

• Outflow below buried lava in northern outer caldera in lava-tuff sequence along faults

• Similar outflow to the south but shallower

Models

Alternate Model • Upflow at east and west ends of the

Island Block • Reservoir extends below little or all

of the inner caldera • Hot northward outflow below deep

moderate conductor or cool outflow in or above the lava

• Alternates do not include a very small system narrowly restricted to upflows

Models

Conclusion

• Geothermometers (those based on ratios without air-related gases) suggest 250C to 290C.

• Reservoir fluids are two-phase, evidence of equilibrium with both liquid and vapor between 200 and 270C.

• A magmatic component in the eastern and western inner caldera fumaroles (3He/4He ratios). The source of noble gases in the rest of the fumaroles is air.

• Upflow occurs in the inner caldera: eastern (SWF-8) and western (SWF-3): lower CO2/H2S, lower CO2/CH4 values, higher ẟD and ẟ18O, high 3He/4He, evidence of sulfurous alteration.

Conclusion

• The source of the fumarolic steam at Suswa could be local meteoric water as sampled east of the project area, heated to ≥260°C by deep circulation with some relatively minor positive shift in ẟ18O, then boiled at relatively high temperatures (220 to 300C).

• Isotopic and gas/ratio data suggest that the steam discharging on the outer caldera and the outer rim could be the result of partial condensation (roughly 10% with continuous condensation) of steam similar to that discharging within the inner caldera (moat).

Introduction

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