Heat Refraction Around Mineralized Bodies: Implications for Heat Flow Density Studies and Paleoclimatic Reconstruction (Poster)

Heat Refraction Around Mineralized Bodies :

Implications for Heat Flow Density Studies and

Paleoclimatic Reconstruction

Laurent Guillou-Frottier (1) and Jean-Claude Mareschal (2)

(1) BRGM, Mineral Resources, Orléans, FRANCE

(2) GEOTOP, UQAM, Montréal, CANADA

Presented at « Geothermics at the turn of the century », Évora, Portugal, April 3-7, 2000

Université du Québecà Montréal

Précisions

L’ensemble de ce travail a été inspiré par différents travaux, certains étant

réalisés dans les années 1996 et 1998, à l’extérieur (UQAM) et au sein du

BRGM, d’autres provenant de résultats obtenus au cours du projet européen

mené par Robert Gable (BRGM) dans les années 1994-1997 :

Références :

Guillou-Frottier et al., High heat flow in the Trans-Hudson Orogen, central Canadian

Shield, Geophysical Research Letters, 23, 3027-3030, 1996.

Gable et al., Geothermics, a new BMS exploration tool, extended abstract, presented

in Finland, 1997.

Guillou-Frottier et al., Ground surface temperature history in central Canada inferred

from 10 selected borehole temperature profiles, Journal of Geophysical Research, 103,

7385-7397, 1998.

1) Most of precise heat flow estimates are made with

temperature measurements in MINING boreholes.

Other methods are much less precise.

2) Some of mineralized bodies possess highly anomalous

thermal conductivity values, leading to heat refraction

effects.

3) Exemples of heat refraction effects :

massive sulfide deposits , quartzites , ash-flow calderas

4) Implications for paleoclimatic signatures inferred from

temperature profiles measured in mining boreholes.

5) Conclusions for the 21st century.

Content

Case of heat flow density measurements along the

Thompson Nickel Belt (Manitoba, Canada)

Correlation between high heat flow

values and high conductivity values.

No variation in temperature gradients

Analytical model

of heat refraction

by a vertical

conductive body

south north (see Guillou-Frottier et al.,

Geophys. Res. Lett., 1996, for details)

Thermal conductivity values related to mineralized bodies

1) From Mwenifembo, J. of Applied Geophysics, 1993

Thermal conductivities of some major rock types and sulfide minerals

2) From Gable et al. (Brite-Euram European Project), 1997

Case of the Masa Valverde massive sulfide deposit (Iberian Pyrite belt) -

borehole A3.

From this and the previous example, it turns out that sulfide minerals

and related rocks possess anomalously high thermal conductivities.

Case of the Masa Valverde sulfide deposit (Iberian pyrite belt)

Temperature gradient and

simplified lithology for

Borehole A8 at the Masa

Valverde site. (From

Gable et al., 1997).

« Normal zone » :

average gradient =

0.03°C/km

« Anomalous zone » :

average gradient =

0.007°C/km,

(zone where massive sufides (in red) are present)

Finite-element modelling for a highly conductive body (From Gable et al., 1997)

A B

Dep

th (

m)

Distance (m)

Two synthetic profiles (A and B) are presented in the next slide,

as well as the undisturbed temperature profile measured at the edge.

Heat refraction around the deep inclined conductive body

15 20 25 30 35 40

0

100

200

300

400

500

600

700

800

Temp (x=A)

Temp (x=B)

Temp (undisturbed)

Temperature (°C)

Dep

th (

m)

80

(mW/m²)

87

94

36

53

with k=3 W/m/K

Numbers along different parts of the profiles indicate heat flow values in mW/m².

Variations in heat flow estimates can reach a factor of 2 to 3 !

Heat refraction around a highly conductive body

0.0

0.5

1.0

0 1 2 3 4

0.50.5

1

1

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

Temperature

0.0

0.5

1.0

0 1 2 3 4

1.04

1.52

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

Gradient

0.0

0.5

1.0

0 1 2 3 4

0.96 1.04

0 3 6 9 12 15 18

Heat flow

k=0.6 k=10 k=1.0

dept

hde

pth

dept

h

In this analytical calculations,

a highly conductive body, with

an aspect ratio of 0.1, is in

contact with slightly insulating

rocks (left) and « normal rocks »

(right).

The excess heat accumulated in

the left side is laterally transfered

across the conductive body.

This transfer leads to anomalous

heat flow at the outer edge of

the conductive body, at depth

(see white arrow).

Cross-sections (x,z) of the temperature

field around a vertical conductive body.

Thermal conductivity are dimensionless,

as well as temperature, gradients and

heat flows.

S.Z

S.Z

TavuaCaldera

Emperor Mine

I.C

1 km

a) b)

Adapted from Ahmad et al., 1987, and Anderson and Eaton, 1990.

C.D

El Indiomine

Adapted from Jannas et al., 1990.

1 km

c)

I.C

1 km

Lomilla caldera

Rodalquilarcaldera

Adapted from Rytuba, 1994

Mediterranean Sea

e) f)

Toquima caldera complex

Round Mountain

Adapted from Sander and Einaudi, 1990

Citorekdepression

Cirotan

Adapted from Marcoux and Milési, 1994

C.D.

C.D.

FIJI CHILE

SPAIN

JAVA NEVADA

2 km

10 kmGoldHill

d)

Soledadcaldera

BOLIVIA

La Joya

Adapted from Redwood, 1987

5 km

Mt. Jeffersoncaldera

Ash-flow calderas and epithermal ore deposits

As it can be seen in these

examples, among others, ore

deposits are preferentially

located close to the major

faults (caldera border faults

or regional faults)

Next slide shows how the

presence of ash-flow units,

which are insulating rocks,

in contact with rather

conductive rocks (faults),

leads to heat refraction

effects at depth, favoring

anomalous heat transfer at

the outer edges of caldera

border faults.

Deflection of temperature profiles

inside and outside the conductive fault

1.3

1.4

1.5

1.6

1.7

dimensionless depth

dim

en

sio

nle

ss t

em

pera

ture

x=2.1

x=2.04x=2.0

x=

1.9

7

x=

2.0

3k

=2

.1

x=2.0 ; 2.04; 2.1

k=

0.3

k=

1.0

z=0.87 z=0.90 z=1.0

z=1

z=0

temperature gradient (x=2.04) = 2 x temperature gradient (x=2.1)

AT DEPTH :

Case of ash-flow calderas

Deflections of temperature and thermal gradient profiles

around a conductor (from analytical results) D

imen

sion

less

tem

per

atu

re

Dimensionless depth0 1

-0.25

0

0.15

x/b=0

x/b=0.15

a/b=0.2x/b=0.22

x/b=0.4

x

2a

b

0

i.e. x=25a !!

An apparent cooling signature is visible inside the conductor, and an apparent

warming signature is still present far from the conductor (see also next slide).

Inferred Ground Surface Temperature History (G.S.T.H.)

from synthetic temperature profiles around a conductor

Same calculations as before Inversion of deflected profiles

Apparent warming

Apparent cooling

T G

Heat flow data distribution versus the number of used boreholes

IHFC database - CMC(119 data (=25%) with no information)

0

50

100

150

200

250

300

350

400

1 2 3 4 5 6 7 8

Nb of boreholes per site

Nb

of

da

ta

4%

66%

IHFC database - AFRICA(84 data (=14%) with no information)

0

100

200

300

400

500

600

1 2 3 4 5 6 7 8

Nb of borehole per site

Nb

of

data

4%

82%IHFC database - EUROPE(411 data (=20%) with no information)

0

200

400

600

800

1000

1200

1400

1600

1800

1 2 3 4 5 6 7 8

Nb of borehole per site

Nb

of

data

3.4%

74%

IHFC database - U.S.A(967data (=22%) with no information)

0

500

1000

1500

2000

2500

3000

3500

1 2 3 4 5 6 7 8

Nb of boreholes per site

Nb

of

data

6%

69%

CMC=Canada-Mexico-Cuba)

Most terrestrial heat flow estimates have been made by using one single borehole. Due to highly

conductive bodies around mining boreholes, heat flow estimates might be in error by neglecting

refraction effects within the depth range of the heat flow determination.

IHFC=International Heat Flow

Commission

CONCLUSIONS

• Heat refraction effects are probably much more frequent than

it is usually thought, due to lateral conductivity contrasts.

• Deep blind orebodies can seriously affect the heat flow estimates

(variations reaching locally a factor of 2 to 3).

• Temperature profiles distorted by heat refraction can be

misinterpreted in terms of paleoclimatic signatures (warming or

cooling).

• 21st century : need to log for temperature in several boreholes per site.

Geothermics at the turn of the century, Évora, Portugal, 3-7 April 2000