MINING GEOLOGY, 35(4), 289•`293, 1985

Sulfur Isotope Ratio of Framboidal Pyrite in Kuroko Ores

from the Ezuri Mine, Akita Prefecture, Japan*

Kosei KOMURO** and Akira SASAKI***

Abstract: The "relict" framboidal pyrite in the Ezuri kuroko ores, which is considered as the most primitive phase

in the mineralization (KOMURO, 1984), is remarkably enriched in light sulfur isotope (ƒÂ34S CDT=ca.-20•ñ) as com-

pared with other sulfides fixed and recrystallized during later stages (ƒÂ34S CDT=-0.3•`+7.9•ñ). The framboidal

pyrite is thought to be the product of biologically mediated sulfur from the Miocene sea under euxinic environment.

1. Introduction

In a recent work of textural characterization

of the Ezuri kuroko ores, KOMURO (1984)

concluded that a variety of textures observed

in the massive ores of the deposits can be ar-

ranged into a systematic uni-directional evolu-

tion series, i.e., framboidal and botryoidal

textures•¨concentric and banded colloform

textures•¨euhedral to anhedral intergrowth

textures, which can also be correlated, respec-

tively, to the three main stages of mineraliza-

tion, i.e., Fe stage•¨Cu-Fe stage•¨Zn-Pb

stage. Hence, it is very likely that in the most

primitive framboidal ores may be preserved a

certain important geochemical information on

the initial condition of the kuroko-forming

processes.

In nature, framboidal pyrite is most com-

mon in the sedimentary environment where

bacteriogenic sulfur is available, and it is wide-

ly accepted that the recognition of such a

biogenic sulfide is most convincingly made by

sulfur isotopic examination (e.g. SWEENY and

KAPLAN, 1973). Thus, mudstones of the Green

Tuff formations which host kuroko deposits

in the Hokuroku basin are found to contain many framboidal pyrites of clearly bacteri-ogenic origin (NAKAJIMA and SASAKI, 1985). However, it has been also well established that framboidal pyrite is not necessarily the pro-duct of biological activity and that the purely inorganic process can produce this texture

(e.g. FARRAND, 1970). Under the circum-stances, it seems very intriguing to look the isotopic composition of sulfur in the Ezuri ores, particularly in the primitive framboidal

phases.

2. Samples

Samples used in the present study are listed in Table 1. They are polished specimens representative of the stratiform massive kuroko ores at 45N5L*1, their petrographic characteristics having been already described in detail (KOMURO, 1984). Samples of the

group A are from the upper part of the zone of "black ore with pebble-like yellow ore"

(Zone 2) and those of the group B include the lower part of the Zone 2 and the upper part of the zone of "black ore" (Zone 3). Following two samples were added for the present study. The sample C is representative of the "powdery yellow ore and the sample D of

the "disseminated pyritic siliceous ore", both of which are overlain by the massive kuroko ores mentioned above.

The samples B-c and B-d are the key samples of this study. As seen in Figure 1, a

Received on June 15, 1985: accepted on July 27, 1985.*A part of this study was presented at the Annual

Meetings of Society of Mining Geologists of Japan,

held on Jan. 31, 1985.**Institute of Geoscience

, University of Tsukuba,

Ibaraki 305, Japan.***Geological Survey of Japan

, Yatabe Ibaraki 305,

Japan.

Keywords: Sulfur isotopes, Kuroko ores, Ezuri mine,

Framboidal texture.

*1 45-Jo , 43-Go, L-170 (See, SATO and SASAKI, 1980, p93, Fig. 3).

289

290 K. KOMURO and A. SASAKI MINING GEOLOGY:

Table 1 ƒÂ34S values of kuroko ores from the Ezuri mine.

Abbreviations: py, pyrite; bn, bornite; cp, chalcopyrite; sp, sphalerite; gn, galena; tet, tetrahedrites; ba, barite; qz,

quartz; f, framboidal; c, recrystallized colloform; e, euhedral to anhedral and irregularly shaped; *: refer to KOMURO (1984).

Fig. 1 Photomicrographs of polished sections of samples B-c and B-d. Scale bar is 0.1 millimeter. Abbreviations

are the same as in Table 1. Framboidal pyrite with interstitial bornite. Note the euhedral to anhedral pyrite grains

are scattered in places (2) and some of the framboids are more or less recrystallized, accreted together (1, 2).

Petrographic characteristics have been described in detail (KOMURO, 1984; P255, Part B-1).

number of tiny pyrite framboids are well

preserved. However, it should be noted that euhedral to anhedral pyrite grains are scat-

tered in places and also that some of the fram-

boids are more or less recrystallized, accreted

together and rimmed by overgrowing col-

loform pyrite to display somewhat evolved tex-tures. Modal analysis by the point-counting

method under a microscope revealed that the

relative abundance of the primitive fram-

boidal pyrite to all the pyrite present is

estimated to be about 40% in the sample B-c and 49% in the sample B-d.

3. Experimental

Because of the extremely small grain size

and intricate textures of the samples, complete

mineral separation with any direct physical

method is impractical. Therefore, polished

thin sections of about 0.4 millimeter thick are

first cut out of the samples. From the thin sec-

tions are then carefully dug up a proper quanti-

35 (4), 1985 Sulfur Isotope Ratio of Framboidal Pyrite from the Ezuri Mine 291

ty of concentrates of the objective minerals. The concentrates are ground thoroughly in an agate-mortar and put to the stepwise chemical treatments as described below to extract the sulfur in different mineral phases separately.

Firstly, the sulfur in sphalerite and galena

can be obtained as H2S by treating the sample

with warm 2N HC1. The residue here may con-

tain pyrite, chalcopyrite, bornite and barite as

possible sulfur-bearing phases, of which chal-

copyrite, bornite and barite are easily decom-

posed with a boiling mixture of HI, H3PO2

and HC1 ("Thode solution")(THODE et al.,

1961) to liberate their sulfur as HIS, leaving

pyrite almost unattacked. Pyrite sulfur is,

then finally, extracted from the residue as HIS

by reacting with tin(II)-strong phosphoric

acid ("Kiba reagent") at 280•}10•Ž (SASAKI

et al., 1979). Through all above treatments the

liberated H2S is quantitatively precipitated as

ZnS in zinc acetate solution, then converted to

Ag2S by adding silver nitrate solution.

The Ag2S samples thus obtained are finally

converted to SO2 for the mass spectrometric

analysis according to the method of ROBINSON

and KUSAKABE (1975). The isotopic measure-

ment is made with a McKinney type mass spec-

trometer (90•K sector, 20 cm radius). The results

are expressed in the conventional ƒÂ34S (CDT)

per mil scale. The experimental accuracy is

estimated to be •}0.2 per mil or better.

4. Results and Discussion

The analytical results are given in Table 1

and schematically shown in Fig. 2.

Different textures of pyrite in the kuroko

ores are found to be associated with different

sulfur isotope values. Samples dominantly of

euhedral to anhedral pyrite (A-b, A-c, B-e, C,

D) have the positive ƒÂ345 values ranging from

+3.7 to +7.9 per mil, while those containing

considerable amount of framboidal pyrite (B-

c, B-d) show the negative ƒÂ34S values of -4.8

and -8.3 per mil. Samples dominated by

recrystallized colloform pyrite (A-d, A-e. B-b)

have the values ranging from +3.4 to +6.6

per mil, similar to those of the euhedral to

anhedral pyrite. However, the isotopic nature

of original colloform pyrite before recrystal-

lization remains unknown.

Fig. 2 Sulfur isotopic variation in kuroko ores

from the Ezuri mine. Abbreviations are the same

as in Table 1.

Fig. 3 The inferred sulfur isotope ratio of fram-

boidal pyrite from the Ezuri mine. Abbreviations

are the same as in Table 1. For legend see Fig. 2.

As easily deduced from these results and the

textural characteristics of samples B-c and B-

d, the primitive framboidal pyrite in the ores

must be remarkably enriched in light isotope

32S . Applying the modal analysis data men-

tioned earlier, and assuming the mean ƒÂ34S

value of all the recrystallized pyrite grains to be

+5 per mil, the ƒÂ34S value for the framboidal

292 K. KOMURO and A. SASAK: MINING GEOLOGY:

pyrite is estimated roughly to be around -20 per mil (Fig. 3).

Chalcopyrite with irregularly shaped in-

tergrowth texture (A-a, B-a) has ƒÂ34S values of

+4.9 and +5.3 per mil, similar to those of

euhedral to anhedral pyrite. Composite

samples of sphalerite and galena with irregular-

ly shaped intergrowth texture (A-c, A-e, B-b,

B-e, B-f, C, D) have ƒÂ34S values ranging from

-0 .3 to +4.2 per mil. The values are seen to

vary in parallel with those of coexisting

recrystallized pyrite, though the fractionation

between sphalerite and galena is unknown at

the moment. A similar parallelism has been

described from Shakanai No. 1 deposit by

KAJIWARA (1971).

The estimated ƒÂ34S value of the primitive

framboidal pyrite is found to be about 40 per

mil lighter than the value for the inferred

Miocene oceanic sulfate with a ƒÂ34S level of

around +21 per mil (CLAYPOOL et al., 1980).

It is well known that in the sediments of

modern euxinic basins such as Black Sea,

Baltic Sea and Santa Barbara Basin, southern

California, the biologically-mediated sulfide

sulfur some 40 per mil or more lighter than the

source seawater sulfate is observed (e.g.

GOLDHABER and KAPLAN, 1974). Such a

biogenic sulfur has recently been recognized in

framboidal pyrites from the mudstones of the

Green Tuff formations of the Hokuroku basin

and their ƒÂ34S values are about 50 per mil

lighter than the Miocene oceanic sulfur (NAKA-

JIMA and SASAKI, 1985). It thus appears most

probable that the framboidal pyrite in the

Ezuri ore is also the product of such biological

activities.

The ƒÂ34S values of sulfides with varying

evolved textures such as euhedral to anhedral

and irregularly shaped intergrowth textures,

however, are all quite different from those of

the framboidal pyrite, and are similar to the

available data for other kuroko deposits

(SASAKI, 1974; OHMOTO et al., 1983). Judging

from the apparent continuity observed in the

textural evolution series of the Ezuri ore

(KOMURO, 1984), this isotopic discontinuity bet-

ween the "primitive" (framboidal) pyrite and

the "later" sulfides is rather astonishing. So

far as the present isotopic results are con-

cerned, it appears more likely that the majority of sulfur in the later processes has not been related to the sulfur in the framboidal pyrite but has come from a different source which still remains unknown. In any case, more works are needed to clarify the details of the sulfur history in the deposits.Acknowledgements: The authors wish to ex-

press many thanks to Prof. T. Fujn and Dr. Y. KAJIWARA for constractive discussions and reading the manuscript.

References

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35 (4), 1985 Sulfur Isotope Ratio-of Framboidal Pyrite from the Ezuri Mine 293

THODE, H. G., MONSTER, J. and DUNFORD, H. B. (1961): chim. Acta, 25, 159•`174.

Sulphur isotope geochemistry. Geochim. Cosmo-

餌釣鉱山黒鉱鉱石中のフランボイダル黄鉄鉱の硫黄同位体比

小室光世 ・佐 々木昭

要 旨:餌 釣黒鉱鉱床 の組織 の検討 に基づ けば,多 様な組

織 は一連 の時間的変化系列 を構成 してお り,主 要な鉱化

期は以下 のごと くまとめ られる(KOMURO, 1984):I.フ

ラ ンボイ ダル黄鉄鉱 の形成,II.同 心 円状黄鉄鉱 ・斑銅

鉱の形 成,III.既 存鉱物 の再結 晶,閃 亜鉛鉱 ・方鉛鉱 ・

四面銅 鉱に よる交代,黄 銅 鉱へ の相変化.硫 黄 同位体比

の組織依存性 を検 討 した結果,同 位体 比は各組織,つ ま

り各鉱化期に対応 して変化 してお り,最 も初 生的 とみ な

し得 るフランボイ ダル黄鉄鉱は,他 の組織 を呈する硫化

物 の一般 的な値(δ34S=-0.3～+7.9‰)と 比較 して顕著

に軽 い硫 黄32Sを 濃 縮 して い る こ とが 判 明 した(δ

34S≒-20‰).フ ランボイ ダル 黄鉄鉱 のδ34s値 は,中 新

世 の海水硫酸 の値 よ り約40‰ 程度軽 く,嫌 気的環境下で

堆積物表層付近 でのバ クテ リア による硫酸イオ ンの還 元

に起 因すること を示唆 している.