ores from the kamaishi mine, iwate prefecture, japan*

MINING GEOLOGY, 30(5), 265•`276,1980

Compositional Variation of Pentlandites in Copper Sulphide

Ores from the Kamaishi Mine, Iwate Prefecture, Japan*

Naoya IMAI**, Tadashi MARIKO***, Hiroaki KANEDA****

and Yoshihide SHIGA**

Abstract: Pentlandite has been found as a widespread minor or trace mineral in contact-metasomatic type copper

and iron-copper ores of the Kamaishi mine. The mineral usually occurs as microscopic grains up to 500 ƒÊm across,

being relatively abundant in the massive sulphide ore with cubanite- chalcopyrite and pyrrhotite-chalcopyrite

assemblages.

In this study, chemical compositions of the Kamaishi pentlandites with various modes of occurrence have

been determined using an electron microprobe. The pentlandites vary in their compositions over a wide range from

cobaltian pentlandite to cobaltpentlandite, although Ni to Fe atomic ratios are usually close to unity with few

exceptions. Their optical and other physical properties including X-ray data are also presented. The relationship

between the physical and chemical data is examined. Finally the variation in chemical composition of the

pentlandites is discussed in correlation with the difference in mode of occurrence and mineral assemblage.

1. Introduction

Pentlandites in the Sudbury-type nickel-

copper sulphide ores are usually characterized

by low values of Co content. For example, the

Sudbury pentlandites contain 0.009•`2.10 wt.

% Co (HAWLEY, 1962; MISRA and FLEET,

1973), and those in nickel-copper sulphide ores

from the Kotalahti and Hitura mines, Finland,

contain 0.16•`1.31 % Co (PAPUNEN, 1970).

On the other hand, the Co-rich varieties of

pentlandite, ranging in their composition from

cobaltpentlandite*1 to cobaltian pentlandite*1

(49.33•`18.84 wt. % Co) have been described

by KOUVO et al. (1959) from Finnish copper

sulphide ores. Subsequently, a cobalt-pent-

ladite with 54.1 % Co has been reported in the

ores having a peculiar mineral assemblage of

sulphides-arsenides from the vein deposits of

the Langis mine, Cobalt-Gowganda area, Can

ada (PETRUK et al., 1969). In Japan, an occur

rence of cobaltian pentlandite (15.8 % Co) was

reported by H. IMAI and FUJIKI (1963) from the

vein deposits of the Komori mine, Kyoto. Also,

a cobaltpentlandite (Co/Fe atomic ratio•¬9)

was confirmed in the massive iron-copper sul

phide ore from the Shimokawa mine, Hokkaido

(KATO and SATO, 1963).

An occurrence of pentlandite in copper sul

phide ores from the Shinyama ore deposit of

the Kamaishi mine was already reported by

Tsusua (1961). TAKEUCHI and YAMAOKA (1964;

1965) also found the mineral in copper sulphide

Received March 8,1980, in revised form September 29,1980.

*A part of this study was presented at the Autumn

Joint Meeting of the Mineralogical Society of Japan, Society of Mining Geologists of Japan, and the Japanese Association of Mineralogists, Petrologists and Economic Geologists, held in Kagoshima City, on October 19,1976 (IMAI et al., 1976b).

**Department of Mineral Industry , School of Science and Engineering, Waseda University, Ohkubo, 3-4-1, Tokyo 160.

***Institute of Earth Science , School of Education, Waseda University, Nishi-Waseda 1-6-1, Tokyo

160.****Department of Mineral Development Engineer

i ng, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Tokyo 113.

Keywords: Cobaltpentlandite, Cobaltian pentlandite, Kamaishi mine, Nippo ore deposit, Ohmine mine, Shinyama ore deposit.

*1 In this paper, conforming to H. IMAI and FIKOLI's

proposal the name of cobaltpentlandite is adopted for natural pentlandite with Co/Fe>1 and Co/Ni>1 in atomic ratios, and that of cobaltian pentlandite for those containing Co more than 3 weight percent with Co/Fe<1 and Co/Ni<1 in atomic ratios (H. IMAI and FUJIKI, 1963).

265

266 N. IMAI, T. MARIKO, H. KANEDA and Y. SHIGA MINING GEOLOGY:

ores from the Ohmine mine (now included in

the Kamaishi mine and is called the Nippo

working). However, no precise determination

of their chemical composition has been reported

yet.

In the course of our study on copper sulphide

ores of the Kamaishi mine (MARIKO et al.,

1973; IMAI et al., 1973; MARIKO et al., 1974;

SHIGA, 1975; IMAI et al., 1976a), we have made

an attempt to determine the chemical composi

tion of these pentlandites by an electron micro

probe. A preliminary investigation revealed

that the Kamaishi pentlandites vary in their

composition over a wide range with respect to

Co content (IMAI et al., 1976b). This paper

sums up all the results obtained to date, further

referring to the relation between the chemical

composition and physical properties, the cor

relation between the chemical data and the

mode of occurrence and mineral assemblage,

and the local variation of Co content in the

Kamaishi pentlandites. Crystal chemistry of

them is beyond the scope of this paper, and it

appeared in a separate paper (IMAI et al.,

1980).

2. Location and Outline

of the Ore Deposits

The Kamaishi mine is well known as one of

the largest producers of iron and copper ores

in Japan. The mine office is located at Kasshi

in Kamaishi City, approximately at lat. 39•‹15'

N, long. 141 •‹40'E. A Mineralized zone which

is called "West ore belt" of approximately 5

km (N-S) by 1.5 km (E-W), is situated in the

eastern part of the Kitakami mountains.

The ore deposits are of contact-metasomatic

type, occurring in the skarn zones which replace

mainly the calcareous and pelitic sediments of

Permo-Carboniferous age as well as the pre

existing "porphyrite" near the contact with

intruded granodiorites. The K/Ar ages of these

granodiorites which might presumably repre

sent the igneous activity to which the ore

deposits are genetically related range from 115

to 120 m.y. B.P., placing the time of crystalliza

tion late in the Cretaceous (SHIBATA and

MILLER, 1962; KAWANO and UEDA, 1965;

1967).

The ore deposits include two main ore types;

iron oxide ore and copper sulphide ore. The

former consists principally of magnetite occa

sionally with some sulphides, while the latter is

composed of pyrrhotite with some chalcopyrite

and cubanite, and rarely with magnetite. Some

orebodies, however, consist of the mixture of

magnetite and sulphides having an intermediate

character between the above two, called "iron

copper ore". Most of the iron ores of the mine

have been mined from the Shinyama ore de

posit, and considerable amounts of copper sulphide ores have come from the Shinyama

and Nippo ore deposits. The Nippo ore deposit

which had been mined by Rasa Industry Co.

under the name of Ohmine mine prior to 1971,

lies at about 5 km nortwest of the Shinyama ore

deposit. Geologic profiles of the Shinyama ore

deposit and the Nippo Fourth orebody are

shown in Figs.1 and 2, respectively.

The Shinyama ore deposit, the largest in the

mine consists of an iron oxide orebody and

seven copper sulphide or "iron-copper" ore-

Fig.1 Geologic profile through the Shinyama working

(after the data by the mine staff).FeB: Iron ore-body, 2B to 5B: Second to Fifth ore-bodies. Sixth and Seventh orebodies do not appear in

this profile.

30(5),1980 Compositional Variation of Pentlandites from the Kamaishi Mine 267

Fig. 2 Geologic profile through Fourth orebody (4B) at the Nippo working (after the data by the mine staff).

bodies in massive garnet and/or clinopyroxene skarns.

The iron oxide ores consist mostly of magnetite, locally accompanied with the impregnations or veinlets of chalcopyrite, pyrite and hematite. The principal ore-forming metallic minerals in the sulphide ores comprise, on the other hand, chalcopyrite, pyrrhotite and cubanite. In some places of barren zones, a small amount of pyrite occurs as disseminations or stringers. Under the ore microscope, the principal sulphides are found to be associated with various minor or trace minerals. They include Ni-and/or Co-bearing sulphides such as pentlandite, "argentian pentlandite", Ni-and Co-bearing mackinawite, siegenite and smythite as well as magnetite, sphalerite, marcasite and hematite.

The Nippo ore deposit comprising five copper sulphide orebodies occur in "breccia skarn"

(TAKEUCHI. and YAMAOKA, 1964;1965) and in

massive clinopyroxene-garnet and garnet

skarns. The principal ore-forming metallic

minerals in the Nippo copper sulphide ores

are pyrrhotite, cubanite and chalcopyrite, lo

cally with small amounts of magnetite, pyrite,

bornite and molybdenite. Minor or trace

minerals observable under the ore microscope

are Ni- and/or Co-bearing sulphides (pent

landite, "argentian pentlandite", Ni- and Co-

bearing mackinawite, smythite, siegenite and

millerite), marcasite, wittchenite, chalcocite,

covellite, electrum, native bismuth and ilmenite.

3. Mode of Occurrence of Pentlandite

Although pentlandite is found as a widespread

minor or trace mineral throughout the Shin

yama and Nippo sulphide ores, the mineral

tends to be more abundant in the massive ores

than in the disseminated, stockwork and

breccia ores. In weakly mineralized parts of

the barren skarns, hornfelses and igneous rocks,

pentlandite is almost or completely absent.

The pentlandites now in question vary in

grain size over a wide range from about 10 ƒÊm

to as large as 500 ƒÊm across. Individual grains

and aggregates show various shapes; subhedral

to anhedral (or oval), dots, "flames", lenses,

blades or rods (lamellae) and spindles, rosetts,

and "veinlets". The finer grains tend to occur

as "flames", spindles, minute en echelon lenses,

dots and lamellae within pyrrhotite and cu

banite and rarely within chalcopyrite. On the

contrary, the coarser grains tend to occur as

aggregates within pyrrhotite or to occupy the

spaces along the boundaries between pyrrhotite

and other minerals such as cubanite and chal

copyrite.

On the basis of a detailed observation of more

than one hundred polished sections, the Kama

ishi pentlandites are classified into three types

with respect to their textural form; 1) inter

stitital grains, 2) fine particles within other

sulphides and 3) "veinlets".

Interstitial grains

Most of the interstitial grains of pentlandites

lie along the grain-boundaries of pyrrhotite,

and also occupy the space between pyrrhotite

and massive or lamellar cubanite and rarely

chalcopyrite. This type of pentlandite is rel-


Fig.3 Photomicrographs of the polished sections, showing the mode of occurrence of Fe-Co-Ni

pentlandites in the copper sulphide ores from the Kamaishi mine.1 : Massive po-cp ore, Shinyama Fourth orebody, 450 mL, in air, Specimen no. 17; 2: Breccia

(stockwork) po-cp ore, Nippo Second orebody, 480 mL, oil imm., Specimen no. 2-9; 3: Massive po-cp ore, Nippo Fourth orebody, 250 mL, oil imm., Specimen no. 4-18; 4: Massive cp-cb-po ore, Shinyama Fourth orebody, 450 mL, in air, Specimen no. 39; 5: Disseminated or stockwork cp-cb ore, Nippo Third orebody, 380 mL, in air, Specimen no. 3-8; 6: Disseminated cp-bn ore, Nippo

Third orebody, 250 mL, in air, Specimen no. 102-4.Photomicrographs of 1 to 5 were taken under obliquely-crossed polars and 6 under one polar.Abbreviations; bn=bornite, cb=cubanite, cp=chalcopyrite, g=gangue minerals, pn=cobaltian pentlandite or cobaltpentlandite, po=pyrrhotite, sg=siegenite.


atively coarse-grained usually 500 ƒÊm or less

in size, and subhedral to anhedral in shape

(Fig. 3-1). This texture is most common in the

ores in which Co- and/or Ni-bearing sulphides

are notably concentrated. The grains sub

mitted to X-ray diffractometry which will be

described later belong to this type.

Fine particles within other sulphides

Within pyrrhotite grains, pentlandite occurs

as dots and blades or rods 20•`50 ƒÊm long and

as rosetts consisting of minute particles with

diameter up to 30 ƒÊm. The pentlandite blades

or rods may be observed to orientate parallel

to {0001} plane of pyrrhotite (Fig. 3-2). In

some cases, spindles or "flames" of them occur

along the twinning plane of pyrrhotite (Fig.

3-3).

On the other hand, the mode of occurrence

of pentlandites in massive and lamellar or lath-

shaped cubanites shows characteristic features;

they usually occur as spindles and blades or

stringers with narrow widths of 10 ƒÊm or less

and lengths from 50 to 200 ƒÊm (Fig. 3-4). In

some grains of cubanite, one or two prominent

sets of parallel blades of pentlandite may be

observed (Fig. 3-5). On rare occasions,

irregularly-shaped anhedral grains of pentlan

dite as large as 50 ƒÊm in diameter are embedded

in massive cubanite.

In general, pentlandites in chalcopyrite are

very rare and usually small in grain size, being

less than 50 ƒÊm across. However, in the Nippo

sulphide ore having a peculiar mineral as

semblage of chalcopyrite-bornite-pentlandite-

siegenite-millerite, pentlandites are intergrown

with chalcopyrite, and they have been altered

partially or completely to siegenite without

exception (Fig. 3-6). Most of the pentlandites

belonging to this type are considered to have

been exsolved from the host sulphides."Veinlets"

The "veinlets" of pentlandite ranging from

5 to 70 ƒÊm in width and attaining to about 400

ƒÊ m in maximum length, occur frequently along

the twinning planes of pyrrhotite grains in

massive cubanite and rarely in chalcopyrite.

These "veinlets" as well as interstitial grains

are likely to have been formed by accumula

tion of exsolved pentlandites from the host

sulphides due to subsequent migration (BRETT,

1964; MARIKO et al., 1974).

In some of the Sudbury-type nickel-copper

sulphide ores, pentlandites contain numerous

inclusions of pyrrhotite and chalcopyrite as

small blebs or rods (e.g., HAWLEY, 1962). In

contrast, few inclusions of chalcopyrite have

been observed within grains of the Kamaishi

pentlandites, although some of which are

replaced partially or completely by siegenite

as well as Ni- and Co-bearing mackinawite.

As stated before, pyrrhotite is one of the

major sulphide components, with which pent

landites are intergrown. MARIKO et al. (1974)

have revealed that the pyrrhotites in direct

contact with pentlandites in the Kamaishi

copper sulphide ores comprise (a) troilite-

hexagonal pyrrhotite assemblage, (b) hexa

gonal pyrrhotite and (c) hexagonal-monoclinic

pyrrhotites assemblage. They have also found

that pentlandites are scarcely found and most

of them are usually altered partially or com

pletely to siegenite within the aggregates of

monoclinic pyrrhotite.

4. Chemical Analysis

For the qualitative microanalysis of the

Kamaishi pentlandites in thirty two ore speci

mens, an Akashi electron microprobe ("Tro

nalyser", TRA-25) with 25•‹ X-ray take-off

angle and two-channel detecting system was

employed. The qualitative spot analysis in

dicated that the measurable elements were Fe,

Co, Ni and S, while the contents of other

elements such as Cu and Ag were less than the

detectable limits of the instrument. Linear-

scanning profiles obtained by electron micro

probe traverses under the characteristic X-rays

of FeKƒ¿, CoKƒ¿, NiKƒ¿, and SKƒ¿ confirmed

the absence of marked compositional zoning

within single grains.

In order to establish the chemical composi

tion of the material, the quantitative micro

analysis was performed on twenty four grains

in seventeen ore specimens listed in Table 1,

using a JEOL-50A electron microprobe with

35•‹ X-ray take-off angle and two-channel

detecting system. Instrumental setting for all

the measurements were ; accelerating voltage :


Table 1 Occurrence site, ore type and mineral assemblage of the copper and iron-copper ores from

the Kamaishi mine, containing pentlandites chemically analysed by electron microprobe.

*In this paper , minerals in each assemblage are arranged in order of decreasing abundance. Abbreviations in Tables 1 and 2 : ap=argentian pentlandite , bn=bornite, cb=cubanite, cp=chalcopyrite, mg=magnetite, mk=mackinawite, ml=millerite, pn=pentlandite, po=pyrrhotite , py=pyrite

, sg=siegenite.

Table 2 Electron microprobe analyses of pentlandites from the Kamaishi mine.

*The materials were X-rayed .

20 kV, specimen current : 1.4•~10-8 A on

Al2O3, size of beam spot : 4 ƒÊmƒÓ, the manner of

X-ray intensity measurement : fixed-time count

ing method for 10 sec. •~7 times, analyzing

crystals used: LiF for FeKƒ¿, CoKƒ¿ and NiK,ƒ¿

and PET for SKƒ¿. The following materials were

utilized as microprobe standards : homogeneous

synthetic troilite for Fe and S, synthetic millerite

for Ni and pure metallic cobalt for Co.

After the corrections for dead time and back-


Table 3 Chemical data for pentlandites from the Kamaishi mine.

ground, count rates were processed by means of

SHOJI'S programs which involve the ZAF cor

rections for matrix effects (YUI and SHOJI,

1976). The corrections followed the procedures

outlined by SWEATMAN and LONG (1969).

Table 1 gives a brief description of the

Kamaishi copper sulphide ore samples, the

Fe-Co-Ni pentlandites in which were chemi

cally studied in detail by electron microprobe

and Table 2 presents the final results of the

quantitative microanalysis. Each analysis rep

resents the result of five or more spot analyses.

Also, Table 3 gives atomic percents of metals,

atomic ratios of Ni/Fe and ‡”M*2/S, and the

corresponding chemical formulae calculated on

the basis of eight S atoms.

Figure 4 illustrates the triangle diagram,

showing the variation in chemical compositions

of the Kamaishi materials. From this figure as

well as from Table 2, it may be seen that Ni/Fe

atomic ratios of the Kamaishi pentlandites do

not vary significantly ranging from 0.81 to

1.07 close to unity, except for the material in

specimen no. 102 (anal. no. 21). Their com-

positional range coincides nearly with that of

Fig. 4 Triangular diagram showing the compositional variation of the Kamaishi Fe-Co-Ni pentlandites, together with that of the Finnish materials given by

Kouvo et al. (1959), as expressed by atomic percent of metals.Dashed-line represents the composition for which

Ni/Fe atomic ratio is equal to unity. Dash-dot lines represent the boundaries among pentlandite, cobaltian pentlandite and cobaltpentlandite. Analysis no.

(3) in Table 2 is omitted in the diagram because of its overlapping with others.

*2 In this paper, the symbol M denotes the metals of the

first transition series.


the Finnish pentlandites given by Kouvo et al.

(1959) as shown in Table 4.

Table 2 and Figure 4 indicate that pent

landites from the Shinyama ores are usually

characterized by the lower Co contents, cor

responding to cobaltian pentlandite, although

there is an exceptional occurrence of cobalt-

pentlandite from Seventh orebody which oc

cupies the nearest position to the Nippo ore

deposit.

On the other hand, in the Nippo ores, pent

landites vary in their Co contents from cobaltian

pentlandite to cobaltpentlandite. Higher Co

contents are seen in pentlandites in the breccia

and impregnated ores from Second and Third

orebodies emplaced in "breccia skarn" on the

upper levels and from upper part of Fourth

orebody replacing massive clinopyroxene or

clinopyroxene-garnet skarn on the deeper levels,

whereas lower Co contents are measured in

pentlandites from the lower part of Fourth

orebody.

The compositions of pentlandites are fairly

uniform regardless of the difference in the host

or associated minerals within a given ore

specimen. However, at a specific position of

any given orebody, pentlandites in the pyr

rhotite-chalcopyrite ores tend to have slightly

higher Co contents than those in the pyrrhotite-

cubanite ores.

Specimen no. 102-4 of the Nippo ores con

taining cobaltpentlandite rich in Ni with the

Ni/Fe atomic ratio of 1.74, is characterized by

the chalcopyrite-bornite-pentlandite-siegenite-

millerite assemblage, which is somewhat pe

culiar in the Kamaishi copper sulphide ores in

general as seen in Table 1. Of special interest is

that in the above Ni-rich cobaltpentlandite, the

‡” M/S atomic ratio (1.098) deviates signifi

cantly from the stoichiometric composition of

9/8 (1.125), and the metal-deficiency is ap

parent. This feature may be explained by the

electronic structures of Fe, Co and Ni atoms of

the first transition-metal elements (DONNAY

and SHEWMAN, 1972; RAJAMANI and PREWITT,

1973; IMAI et al., 1980).

5. Reflectance and Microhardness

The reflectance measurements in air for some

Fig. 5 Reflectance-dispersion curves for the Kamaishi Fe-Co-Ni pentlandites, as compared with that of a

normal Fe-Ni pentlandite in the Kamabuse-yama serpentinite, Kanto mountains.

* Analytical no. by electron microprobe as shown in

Table 2.

** Co/‡”M atomic ratio (Ni/Fe atomic ratio•¬1). K is

the curve for the Kamabuse-yama Fe-Ni pentlandite.

selected grains of the materials on freshly-

polished surfaces were performed with Olym

pus MMSK-RK multi-photometric micro

scope. All measurements were made against

WTiC standard provided by Carl Zeiss Jena

Co.

The reflectance-dispersion curves from four

grains of the Kamaishi materials are given in

Fig. 5, together with that of normal Fe-Ni

pentlandite in the Kamabuse-yama serpentinite

(IMAI et al., unpublished data) for comparison.

Also, the relationship between reflectance for

light with a wave length of 560 nm and Co con

tents is expressed diagrammatically in Fig. 6.

The reflectance evidently increases with in

creasing the Co/‡” Matomic ratio of the pent

landites, as has already been pointed out by

some previous investigators (e.g., BURNS and


Fig. 6 Relationship between Co/‡”M atomic ratio and

reflectance (R percent) in some Kamaishi Fe-Co-Ni

pentlandites, together with those for the Langis co

baltpentlandite and the Kamabuse-yama Fe-Ni

pentlandite (ƒÉ=560 nm).

Number corresponds to the analytical no. in Table 2.

L refers to the data for the Langis material given by

PETRUK et al. (1969), and K represents the value of

the Kamabuse-yama material.

VAUGHAN, 1970; RAJAMANI and PREWITT,.

1973).

The microhardness for selected grains of the

present materials were determined with an aid

of the AKASHI MVK-C microhardness tester.

The results thus obtained are shown in Fig. 7,

in terms of Vickers hardness number (VHN)

versus Co content. Kouvo et al. (1959) showed

that microhandness increased with increasing

Co content. As shown in Fig. 7, this trend is

also confirmed in the present study.

6. X-ray Diffraction Analysis

X-ray diffraction analysis for selected grains

of the Kamaishi pentlandites having different

Co contents was conducted using standard

Debye-Scherrer camera of 114.59 mm diameter

and Fe-filtered CoKƒ¿-radiation (ƒÉ=1.7902

A). Minute amounts of the powder were col

lected from the polished surfaces with a steel

needle under the ore microscope and attached

to the edge of glass fibres. The samples thus

prepared were X-rayed. To eliminate the errors

due to film shrinkage, Straumanis film-position

was employed. The intensities of the resolved

lines were estimated by both visual method and

Fig.7 Relationship between Co/‡”M atomic ratio and

Vickers hardness numbers (VHN) in some Kamaishi

Fe-Co-Ni pentlandites. The VHN was measured at a

50g load. The number corresponds to analytical no.

in Table 2.

Table 4 Unit-cell dimensions of some Fe-Co-Ni pent

landites.

Source: * Present study, ** Kouvo et al. (1959), *** Rajamani

and Prewitt (1973), + Petruk et al. (1969).

microphotometry.

All pentlandites now in question have similar

X-ray diffraction patterns which are nearly

identical with those of normal Fe-Ni pent

landites in literature and are in harmony with

the space group Oh5Fm3m. The unit-cell dimen

sions were determined by means of BRADLEY

and JAY'S extrapolation against cos2ƒÆ using

least squares method and the results are given

in Table 4, together with those from other

sources.

The unit-cell dimensions of solid-solution

series along the Co9O8-Fe,•¬4 ,5Ni•¬4.5S8 join de

crease with increasing Co content, although


KNOP et al. (1965) have found that the unit-cell

dimensions of synthetic Fe-Ni pentlandites are

larger than those of natural equivalents having

the same composition, and on heat treating the

lattices of natural one expand irreversibely.

Accordingly, the unit-cell dimensions of natural

Fe-Co-Ni pentlandites and those of synthetic

equivalents cannot be correlated with each

other (KNOP and IBRAHIM, 1961; KNOP et al.,

1965; PETRUK et al., 1969).

The relationship between unit-cell dimen

sions and Co contents in terms of the Co/‡”M

atomic ratio of naturally occurring pent

landites including the present Kamaishi ma

terials is shown in Fig. 8, from which linear

relationship between the two is recognizable.

The straight line obtained from the least squares

method may be expressed as follows :

a(A)=10.063-0.1447 (Co/‡”M).

This straight line extrapolates to Co/‡”M=0

with an intercept of 10.063 A which is close to

10.059 A of natural Fe-Ni pentlandite for

Fig.8 Relationship between Co/‡”M atomic ratios

and unit-cell dimensions in natural pentlandites.

Solid circles: Kamaishi Fe-Co-Ni pentlandites (Ni/

Fe atomic ratio•¬1). Open circles: Finnish pent

landites (Kouvo et al., 1959), Outokumpu pent

landite (RAJAMANI and PREWITT, 1973) and Langis

cobaltpentlandite (PETRUK et al., 1969).

which the Fe/Ni atomic ratio is equal to unity

(IMAI et al., 1980).The problems on the serious discrepancy re

cognized between the unit-cell dimensions of natural Fe(-Co)-Ni pentlandites and those of the synthetic equivalents with the same com

position were discussed in a separate paper (IMAI et al., 1980).

7. Summary and Conclusions

In summarizing the data on the Kamaishi

pentlandites given so far, the following conclusions may be drawn.

(1) As far as the Co content is concerned, the pentlandites now under investigation vary in their chemical composition over a wide range from cobaltian pentlandite to cobaltpentlandite, although the Ni/Fe atomic ratios are usually close to unity with few exceptions.

(2) In the copper sulphide ores from the Shinyama ore deposit, pentlandites are usually characterized by lower contents of Co, corresponding to cobaltian pentlandite. However, the materials from Seventh orebody (ironcopper orebody) situated near the Nippo ore deposit are characterized by high contents of Co, corresponding to cobaltpentlandite.

(3) In the copper sulphide ores from the Nippo ore deposit, the pentlandites from Second and Third orebodies in "breccia skarn" on the upper levels and from the upper parts of Fourth orebody in "massive skarn" on the deeper levels, are characterized by high values of Co content, corresponding to cobaltpentlandite. On the contrary, those from the lower part of Fourth orebody contain lower Co, corresponding to cobaltian pentlandite. Pentlandites in the Sudbury-type nickel-copper sulphide ores are characterized by low Co content. On the other hand, pentlandites associated with hydrothermal vein deposits of epithermal or mesothermal class belong usually to cobaltian

pentlandite or cobaltpentlandite (H. IMAI and FUJIKI, 1963; PETRUK et al., 1969). From this, it is supposed that the variation of Co content in

pentlandites from the Nippo ore deposit as noted above may represent the thermal gradient at the time of ore formation.

(4) The cobaltpentlandite in the Nippo ore


with chalcopyrite-bornite-pentlandite-siegenite

-millerite assemblage represents a metal-defi

cient species (‡”M/S atomic raio=1.098) in

which Ni/Fe atomic ratio (1.74) deviates sig

nificantly from unity (Ni-rich cobaltpent-

landite). This agrees well with the statements

by the previous workers that Ni/Fe atomic ratio

of pentlandites is increased in general way with

increasing Ni content to the bulk compositions

of sulphide assemblage (KNOP et al., 1965;

GRATEROL and NARDRETT, 1971; HARRIS and

NICKEL, 1972; RAJAMANI and PREWITT, 1973).

(5) The compositions of pentlandites are

fairly uniform, regardless of the difference of

the associated sulphide minerals within a given

ore specimen.

(6) At a specific position of any given ore

body, the composition of pentlandites in the

pyrrhotite-chalcopyrite ores tends to have

slightly higher Co content than those in the

pyrrhotite-cubanite ores.

(7) The pyrrhotites in direct contact with

pentlandites in the Kamaishi copper sulphide

ores include (a) troilite-hexagonal pyrrhotite

assemblage, (b) hexagonal pyrrhotite and (c)

hexagonal-monoclinic pyrrhotites assemblage.

In the last case (c), pentlandite are scarcely

found and most of them have been altered

partially or completely into siegenite.

Acknowledgements : The authors are grateful

to Professor S. TAKENOUCHI and Dr. T. SHOJI

of the University of Tokyo, and to Professor R.

OTSUKA of Waseda University for their kind

advices.

Thanks are also due to Professor T. NAKA

MURA and Mr. I. KINOUCHI of Waseda Uni

versity for their skilled technical assistance in

electronprobe microanalysis. The authors are

also indebted to the high-speed digital com

puter, HITAC 8800/8700 system installed at

the Computer Centre of the University of Tokyo

in the course of the ZAF corrections of the

microprobe data (Project No. 0358643002).

The authors express their sincere thanks to the

mining geologists of the Kamaishi Mine Office,

especially to Dr. S. HAMABE for their kind as

sistance during the field works.

This research has been supported in part by a

Grant-in-Aid for Fundamental Scientific Re

search from the Ministry of Education, Culture and Sciences in Japan, especially by Project No. 1431015 (1976/1977) awarded to the first author (N. I.), and Project No. 236043

(1977/1978) awarded to Professor S. TAKENOUCHI.

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岩手県釜石鉱山産銅硫化物鉱石中のペントランダイトに

おける化学組成の変化

今井直哉・鞠子正・金田博彰・志賀美英

要旨:釜石接触交代鉱床の銅および鉄銅鉱石中にはペン

トランダイトが広く認められる.このペントランダイト

は普通500μm以下の粒子として産し,キューバ鉱― 黄

銅鉱および磁硫鉄鉱―黄銅鉱の組合せを有する塊状硫化

物鉱石から比較的多量にみいだされる.

種々の産状を示すペントランダイトのEPMAによる

化学分析によると,そのNi/Fe原子比は,一部の例外を

除くとあまり変化せず,ほぼ1に近いが,Co含有量の

変化に富み,コバルトペントランダイトから含コバルト

ペントランダイトにわたる.また反射率,微小硬度およ

び格子定数の測定を行い,これらと化学組成との関係を

検討した.ペントランダイトの産状,鉱物組合せおよび

産出箇所と組成との関係についても考察を加えた.

ores from the kamaishi mine, iwate prefecture, japan*

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