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Abstract

The open pit of the Iriki kaolin mine is located on the western flank of Imuta volcano (see route map, day 3-1) and produced filler and coating material for paper until February 2001. Now mining from the pit has stopped but still maintains production and sales by importing material from China, which is then treated at the mine.

The kaolin zone is white and light gray and consists of kaoli- nite, dickite, quartz, and minor opal C-T (Fujii et al., 1989; Fig. 1). The smectite zone is yellow to pale green and consists of smectite, kaolinite, opal, and minor amounts of alunite.

The open pit of the Iriki kaolin mine is located on the western flank of Imuta volcano (see route map, day 3-1) and produced filler and coating material for paper until February 2001. Now mining from the pit has stopped but still maintains production and sales by importing material from China, which is then treated at the mine.

The kaolin zone is white and light gray and consists of kaoli- nite, dickite, quartz, and minor opal C-T (Fujii et al., 1989; Fig. 1). The smectite zone is yellow to pale green and consists of smectite, kaolinite, opal, and minor amounts of alunite.

Stratiform blankets of silicification within the acid alteration can be observed (Figs. 2, 3, and Figs. 4), and the distribution of the silica blankets look rather complicated at first sight (Fig. 1). However, they form several parallel sheets (Figs. 2 and 3), which indicates that they may have formed in paleoaquifers.

Acid alteration observed in the pit and surrounding area formed by steam heating approximately 300,000 to 400,000 years ago. The boiling of ascending fluids formed H2S-bear- ing steam that condensed into cool groundwater; the H2S oxidized to sulfate, forming acid water that reacted with surrounding rock to form a shallow blanket of kaolinite, alunite, cristobalite, and smectite (Sillitoe, 1993). Although the acid fluid readily dissolves volcanic glass and most minerals, it is generally accepted that Al tends to remain insoluble at a pH >2 (Stoffregen, 1987) and will therefore be fixed in Al-bearing minerals such as kaolinite and alunite (Hedenquist et al., 2000).

The silica blankets still contain some Al2O3, however— about 71 to 79 wt percent Sio2 and 12 to 18 wt percent Al2O3 (Iriki kaolin mine data). At the northern margin of the pit, thick silica blankets have few clasts and silicification becomes massive (Fig. 5). Going south and west, the thickness of the silica blanket becomes thinner and the ratio of silicified matrix to clasts becomes lower. This change indicates that the water/rock ratio, i.e., the volume of water flow, was higher in the northern part of the pit (which is near the Yamashita quartz vein that was mined in the early 1900s). Even in the massive silicified blanket, fine-grained kaolinite is still observed under the microscope. This fine-grained material means that the pH of the acid water was greater than 2 (pH 3-4) and/or that the flux of groundwater was not sufficient to dissolve all the aluminum from the rock. Silica could, however, precipitate within the pH range discussed here.

Fig. 1.

The Iriki kaolin pit, looking east. The distribution of the silica blankets looks rather complicated at first sight; however, they occur as several parallel sheets.

Fig. 1.

The Iriki kaolin pit, looking east. The distribution of the silica blankets looks rather complicated at first sight; however, they occur as several parallel sheets.

Fig. 2.

Silica blankets. Upper surface of the silica blanket is rather sharp, whereas the lower boundary is not clear. Sili- cification developed in the matrix of the volcanic breccia. Upper surface of the silica blanket is disturbed by a large boulder. Several other silica blankets are observed below.

Fig. 2.

Silica blankets. Upper surface of the silica blanket is rather sharp, whereas the lower boundary is not clear. Sili- cification developed in the matrix of the volcanic breccia. Upper surface of the silica blanket is disturbed by a large boulder. Several other silica blankets are observed below.

Fig. 3.

Silica blankets. The silica blanket has a N 45° E strike and 20° SW dip. The structure is almost parallel to the bedding plane observed in vol- caniclasitic rocks. Several pieces of carbonized wood are observed in the silica blanket. The aquifer seems to be controlled by the permeability of the host rocks.

Fig. 3.

Silica blankets. The silica blanket has a N 45° E strike and 20° SW dip. The structure is almost parallel to the bedding plane observed in vol- caniclasitic rocks. Several pieces of carbonized wood are observed in the silica blanket. The aquifer seems to be controlled by the permeability of the host rocks.

Fig. 4.

Thick silica blanket occurs at the northeast margin of the pit. The thickness is more than 3 m. Areas of massive silica show little evidence of relict textures reflecting primary clasts. The absense of these textures may indicate that dissolution of Al2O3 by acid water replacement by silica precipitation was greater in this area.

Fig. 4.

Thick silica blanket occurs at the northeast margin of the pit. The thickness is more than 3 m. Areas of massive silica show little evidence of relict textures reflecting primary clasts. The absense of these textures may indicate that dissolution of Al2O3 by acid water replacement by silica precipitation was greater in this area.

In the steam-heated acid environment, silicification may be characterized by significant aluminum concentrations as noted above. Because acidity in the steam-heated environment is mainly controlled by the oxidation of H2S rather than SO2, the level of acidity would be weaker than that due to hypogene magmatic condensates observed in the high- sulfidation environment. As such, characteristics of silicifi- cation could be important in distinguishing the origin of advanced argillic alteration.

We can also see much pyrite and marcasite in the following three kinds of occurrences in the pit: (1) pyrite dissemination in the clay alteration, (2) pyrite and marcasite band at marginal zone of argillized clasts in clay alteration (Figs. 5 and 3), and (3) pyrite and marcasite veinlets in clay alteration (Fig. 6). These pyrite and marcasite occurrences could have formed through reaction between H2S brought by steam and Fe scavenged from the host rocks.

Fig. 5.

a and b. Pyrite band rimming kaolinized clasts.

Fig. 5.

a and b. Pyrite band rimming kaolinized clasts.

Fig. 6.

a. Pyrite veinlets in the kaolin zone. Veinlets strike N 70° to 80° W and dip 60° to 80° SW. b. Pyrite veinlets in the kaolin zone.

Fig. 6.

a. Pyrite veinlets in the kaolin zone. Veinlets strike N 70° to 80° W and dip 60° to 80° SW. b. Pyrite veinlets in the kaolin zone.

References

Fujii
,
N.
Tsukimura
,
K.
Julio
,
J.M.
,
1989
,
Mode of occurrence and genetic processes of the Iriki kaolin deposit, southern Kyushu
:
Bulletin of the Geological Survey of Japan,
  v.
40
, p.
299
322
(in Japanese with English abstract).
Hedenquist
,
J.W.
Arribas
,
R.A.
Urien-Gonzalez
,
E.
,
2000
,
Exploration for epithermal gold deposits
:
Reviews in Economic Geology,
  v.
13
, p.
245
277
.
Sillitoe
,
R.H.
,
1993
,
Epithermal models: Genetic types, geometrical controls and shallow features
, in
Kirkham
,
R.V.
Sinclair
,
W.D.
Thorpe
,
R.I.
Duke
,
J.M.
, eds.,
Mineral Deposit Modeling
:
Geological Association of Canada Special Paper 40
 , p.
403
417
.
Stoffregen
,
R.
,
1987
,
Genesis of acid-sulfate alteration and Au-Cu-Ag mineralization at Summitville, Colorado
:
Economic Geology,
  v.
82
, p.
1575
1591
.

Figures & Tables

Fig. 1.

The Iriki kaolin pit, looking east. The distribution of the silica blankets looks rather complicated at first sight; however, they occur as several parallel sheets.

Fig. 1.

The Iriki kaolin pit, looking east. The distribution of the silica blankets looks rather complicated at first sight; however, they occur as several parallel sheets.

Fig. 2.

Silica blankets. Upper surface of the silica blanket is rather sharp, whereas the lower boundary is not clear. Sili- cification developed in the matrix of the volcanic breccia. Upper surface of the silica blanket is disturbed by a large boulder. Several other silica blankets are observed below.

Fig. 2.

Silica blankets. Upper surface of the silica blanket is rather sharp, whereas the lower boundary is not clear. Sili- cification developed in the matrix of the volcanic breccia. Upper surface of the silica blanket is disturbed by a large boulder. Several other silica blankets are observed below.

Fig. 3.

Silica blankets. The silica blanket has a N 45° E strike and 20° SW dip. The structure is almost parallel to the bedding plane observed in vol- caniclasitic rocks. Several pieces of carbonized wood are observed in the silica blanket. The aquifer seems to be controlled by the permeability of the host rocks.

Fig. 3.

Silica blankets. The silica blanket has a N 45° E strike and 20° SW dip. The structure is almost parallel to the bedding plane observed in vol- caniclasitic rocks. Several pieces of carbonized wood are observed in the silica blanket. The aquifer seems to be controlled by the permeability of the host rocks.

Fig. 4.

Thick silica blanket occurs at the northeast margin of the pit. The thickness is more than 3 m. Areas of massive silica show little evidence of relict textures reflecting primary clasts. The absense of these textures may indicate that dissolution of Al2O3 by acid water replacement by silica precipitation was greater in this area.

Fig. 4.

Thick silica blanket occurs at the northeast margin of the pit. The thickness is more than 3 m. Areas of massive silica show little evidence of relict textures reflecting primary clasts. The absense of these textures may indicate that dissolution of Al2O3 by acid water replacement by silica precipitation was greater in this area.

Fig. 5.

a and b. Pyrite band rimming kaolinized clasts.

Fig. 5.

a and b. Pyrite band rimming kaolinized clasts.

Fig. 6.

a. Pyrite veinlets in the kaolin zone. Veinlets strike N 70° to 80° W and dip 60° to 80° SW. b. Pyrite veinlets in the kaolin zone.

Fig. 6.

a. Pyrite veinlets in the kaolin zone. Veinlets strike N 70° to 80° W and dip 60° to 80° SW. b. Pyrite veinlets in the kaolin zone.

Contents

References

References

Fujii
,
N.
Tsukimura
,
K.
Julio
,
J.M.
,
1989
,
Mode of occurrence and genetic processes of the Iriki kaolin deposit, southern Kyushu
:
Bulletin of the Geological Survey of Japan,
  v.
40
, p.
299
322
(in Japanese with English abstract).
Hedenquist
,
J.W.
Arribas
,
R.A.
Urien-Gonzalez
,
E.
,
2000
,
Exploration for epithermal gold deposits
:
Reviews in Economic Geology,
  v.
13
, p.
245
277
.
Sillitoe
,
R.H.
,
1993
,
Epithermal models: Genetic types, geometrical controls and shallow features
, in
Kirkham
,
R.V.
Sinclair
,
W.D.
Thorpe
,
R.I.
Duke
,
J.M.
, eds.,
Mineral Deposit Modeling
:
Geological Association of Canada Special Paper 40
 , p.
403
417
.
Stoffregen
,
R.
,
1987
,
Genesis of acid-sulfate alteration and Au-Cu-Ag mineralization at Summitville, Colorado
:
Economic Geology,
  v.
82
, p.
1575
1591
.

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