Detailed mineralogy northen Nigeria, Ririwai
Post on 10-Jan-2023
0 Views
Preview:
Transcript
GEOLOGICAL JOURNAL, VOL. 22, THEMATIC ISSUE, 403-427 (1987)
Accessory mineralogy of the Ririwai biotite granite, Nigeria, and its albitized and greisenized facies
R.A. Ixer, J.R. Ashworth and C.M. Pointer Department of Geological Sciences, Aston University, Aston Triangle, Birmingham, 84 7ET, U. K.
The Ririwai granitic ring complex has suffered a sequence of post-crystallization alteration and mineralizing processes including albitization and, close to thc Ririwai Sn-Zn lode, microclinization, greisenization, and quartz-cassiteritc-sulphide veining.
Within thcse rock types the opaque and accessory minerals of Zr , Hf. U , Th , Nb, Ta, Ti. Sn. and REE have a number of styles of occurrcnce. They occur in coarse-graincd discrete crystals associated with magmatic quartz or as aggregates of small crystals. which are often aligned along the cleavage of micas, or form complex intergrowths outside the micas.
Late stage, possibly magmatic processes within the biotite granite have precipitated Zr. U, Pb, Nb, Ta. Ti as complex intergrowths of uranium-bearing plumbopyrochlorc, columhite, ilmenite. TiO, minerals. and zircon. Minor t o trace amounts o f monazite, uranothorite. and cassiterite are also present in thesc granites.
Albitization has resulted in the crystallization of coarse-grained. early haematite intergrown with slightly later magnetite. ilmenite, and columbite. Both columbitc and ilmenite are highly zoned with respect to their iron and manganese, but not their niobium and tantalum contents. Zircon is often strongly zoned with uranium-rich cores and hafnium-rich rims and shows partial rcplacement and overgrowth by uranothorite and xenotime.
Cassiterite. columbite, zircon, and Fe, Mo. Pb, and Zn sulphides are the main non-silicates developcd during greiscnization. although thorium too, is mobile, crystallizing mainly within uranothorite. Columbite has a wide range of compositions with tantalum-rich (up to 27 wt% T a Z 0 5 ) and tungstcn- rich (up to 14.5 wt% WO,) varieties. Cassiterite is comnionly colotir zoned and this can be related to its iron and niobium contcnt. Zircon shows replacement by. o r enclosure within, uranothorite, which itself has later xcnotime margins.
The textural and chemical evidence suggests that each alteration process essentially dissolved, o r replaced. earlier phases and then reprecipitated them; but with compositions compatible with the ncw fluids. The dissolution of small accessory phases within. and the alteration of biotite, initially to chlorite, appear to have played important roles in the liberation of rare elements by thesc fluids.
KEY WORDS Alkaline granites Mineralization Acccssory minerals Uranium-thorium minerals
1. Introduction
The Ririwai complex is amongst the best studied of the Younger granite complexes of Nigeria. Its geology has been described by Jacobson el al. (1963) and Jacobson and MacLeod (1977), and information on petrology, geochemistry, and mineraliz-
0 1987 by John Wiley & Sons, Ltd 0072-1050/87/TI0403-25$12.50
404 R.A. IXER, J .R. ASHWORTH, A N D C.M.POINTER
ation are summarized in the comprehensive account of Kinnaird et al. (1985). The complex, which represents the roots of an eroded alkaline volcano, comprises an outer ring-dyke of fayalite granite porphyry, surrounding a peralkaline granite and a central biotite granite emplaced within a collapsed volcanic pile. Both granites have been altered by post-magmatic fluids which have locally converted them to albitites. In addition, the biotite granite has been modified by hydrother- mal fluids to produce microclinization, greisenization, and a braided quartz- greisen vein system, which is at its most extensive along the Ririwai lode. This cassiterite-sphalerite lode extends for more than 5 km in an east-west direction, and is entirely enclosed within the biotite granite (Kinnaird et ul. 1985). Within the biotite granite an inclined borehole L13 has penetrated to 450 m and encoun- tered albitites in its basal 40 m (Figure 1). These have been interpreted as belonging to an apical part of an underlying granite (Kinnaird et al. 1985).
Regional studies of the Younger Granite province by Bowden and Kinnaird (1978) have shown it to be a zinc-rich tin province, associated with minor uranium, niobium. and tungsten. Kinnaird (1984) has further subdivided the mineralization of the province into (1) disseminated tantalum-bearing columbite, cassiterite, and REE minerals associated with albitization, and later (2) cassiterite-sulphide vein- style mineralization associated with microclinization, greisenization, and quartz- vein infilling.
A preliminary description of the mineralogy and a paragenesis for the mineraliz- ation at Ririwai, given in Kinnaird et ul. (1985), concentrated upon the lode and its cassiterite-sulphide mineralization. It showed that the albitites, where the biotite has been extensively altered to lithian micas and finally to zinnwaldite, carry accessory columbite, minor cassiterite, thorite, xenotime, thorium-rich mon- azite and hafnium-rich zircon. The fluids were hot (with homogenization tempera- tures of 460 - 260°C) and sodium-rich. and introduced sodium and iron together with U , Th, Zr , Nb, and H R E E .
Microclinization is a local, minor phenomenon. The greisenization process has altered the feldspars of the biotite granite firstly to chlorite and then to lithian micas, fluorite, topaz, and sericite and the biotites to zinnwaldite. The fluids responsible were hot (with homogenization temperatures of 380 - 360°C) and acidic. and altered the bulk rock geochemistry by lowering the relative A1,03 content, and increasing the contents of Li, Fe, Sn, W, Pb, Zn, and Cu in the form of white mica, cassiterite, wolframite, and sulphides. Th, Ce, and Y contents were also increased as seen by the formation of monazite.
Bowden et ul. (1981) analysed bulk samples of the Ririwai biotite granite and albitites from L13, for uranium and thorium. MacKenzie et ul. (1984) described the distribution of uranium and thorium within wallrocks and vein material from the Ririwai lode, and suggested that uranium was concentrated during microcliniz- ation and was probably held in Hf-rich zircon, and in xenotime in association with Ce and H R E E , whereas thorium was concentrated during greisenization and held in thorite, Th-rich monazite, zircon, and cassiterite. Both sets of geochemical data are summarized in Table 1 which gives the bulk rock analyses for uranium and thorium from all the rock types at Ririwai.
This paper presents data on the accessory and opaque minerals found within the unaltered biotite granite and albitite (from borehole L13) and in microclinized and greisenized rocks next to the Ririwai lode, in order to describe more fully the processes of albitization and greisenization and their associated mineralization.
L 1
3 C
OR
E
1 2 3 4
KE
Y m
icro
gani
te
03
3 F
66 M
1 la
mel
lar
perth
ite g
eeny
bro
wn
mca
. stre
aks
01
haem
atite
. acc
esso
ry c
assi
terit
e an
d zi
rcm
Q45
F50
M5
patc
h pe
rtht
e-rim
med
by
clea
r al
bite
kha
krco
bcre
d m
a, w
ith a
cces
sory
sph
aler
ite z
ircm
and
CaS
SitW
ite
Q20
F65
MI5
pat
ch p
erth
te. e
uhed
al M
ush-
geen
ma
. rna
rgna
lly c
hbrit
ised
and
acc
esso
ry c
assi
terit
e
Q45
F50
M5
patc
h pe
flhte
. wve
text
tre
d b
raw
n m
a wth
hae
mat
ite a
long
cle
avag
es, f
luor
ite. c
assi
terit
e an
d zi
rcon
Q33
F66
M1
lameX
ar w
fthte
. zo
ned
brow
n to
gee
n m
a wth
acc
essm
y fl
rxit
e a
nd z
ircon
,04
0 F
60 M
1 Ia
meU
ar p
erth
ite. r
agge
d ha
emat
ised
ma
with
acc
essm
y m
Ona
zite
and
cas
site
rite
. tim p
erth
ite w
ith in
ters
titla
l alb
ite. z
mw
aldi
te r
Wc
w K-s
par n
Wttw
te. w
ith a
cces
scry
cas
site
rite.
fluo
rite.
hae
mat
ite a
nd s
phal
erite
mlc
rocl
mis
ed p
wth
ite. n
ters
titla
l abi
te. b
row
n m
a pa
rtla
ly h
aem
atls
ed and
rm
d by w
een
mca
.a30 F
65 M
5 do
mna
ntly
ab
te.
sane
mro
ch
e. geen
botit
e an
d pa
le z
mw
aldi
te w
ith tr
aces
of
haem
atite
,02
0 F
75 M
5 ch
essb
oard
altn
te w
th c
lear
rm
s an
d nt
erst
itlal
lath
s. and c
hbrit
ised
geen
mC
a
- Q30
F55
M15
mot
tled
feld
spar
. geen
ma
reph
cng
brow
n g
ey
zm
wal
cfite
rep
hcng
K- s
par
n p
erth
te
-01
0 F
90 M
tr ch
essb
oard
and
lath
altn
te m
ca re
phce
d by
chb
tlte
shea
ves and
calc
ite w
th a
cces
say
coC
mbi
te
0 12
F85
M2
ches
sboa
rd a
nd la
th a
bte.
chl
onte
agg
egat
es w
ith h
aem
ame,
c-te
and
zrc
m
. Q10
F90
Mtr
ches
sboa
rd an
d la
th a
lbite
. rag
ged
brow
n cM
ontis
ed m
a. w
ith a
cces
sory
cas
site
rite
and
zone
d z
rcm
coar
se-g
aned
bot
ite p
eflh
te g
afl
te
abiti
te b
and
gels
ense
d zo
ne
0 m
ed
mg
ain
ed
bio
tite
pert
trle
ga
nte
ab
mse
d g
afl
te w
tth a
tnnd
ant
mar
oriti
c ca
vite
s po
tash
rn
eta
mtr
sed
zone
ge
och
em
al S
anpl
e si
te
Figu
re I
. Li
thol
ogic
al lo
g of
bor
ehol
c L1
3 pa
ssin
g tr
om b
iotit
e gr
anite
int
o un
dcrl
ving
alb
itite
. D
epth
in
met
res.
Q,
quar
tz;
F. t
otal
fel
dspa
r; M
, tot
al m
ica
cont
ent.
From
J.
Kin
nair
d (u
npub
lishe
d da
ta).
P
0
wl
Tab
le 1
. B
ulk
rock
ana
lyse
s fo
r U
and
Th
(in
pp
m)
of r
ocks
fro
m R
iriw
ai
U r
ange
T
h ra
nge
Thi
U
Sam
ples
Bio
tite
gran
ite
L13
3&80
25
-72
0.83
-0.9
7 4
Alb
itite
L1
3 66
-8 1
69
-73
0.9(
&1.
05
2 M
icro
clin
ite
lode
1&
13
39-4
2 3.
25-3
.80
2 (l
ode)
(1
7-87
) (1
3)
Gre
isen
lo
de
19
98
5.16
1
(lod
e)
(43-
191)
(6
) B
iotit
e gr
anite
su
rfac
e 7-
1 0
41-8
3 5.
5-8.
30
1
Dat
a fr
om K
inna
ird
et a
l. (1
985)
and
Mac
Kcn
zie
CI t
rl.
(198
4); t
he l
atte
r ar
e gi
ven
in p
aren
thes
is
P ? P
P z U
Tab
le 2
. R
epre
sent
ativ
e an
alys
es o
f ur
aniu
m-
and
thor
ium
-bea
ring
pha
ses
in b
iotit
e gr
anite
. 9 zz
Sam
ple
Zirc
on
L13-
100
core
in
ner
zone
L
IS10
0 ou
ter
zone
L1
3-18
5 ou
ter
zone
L
lS31
5 ou
ter
zone
Th
orite
L1
3-31
5 in
clus
ion
L 13
- 185
in
clus
ion
L13-
305
disc
rete
Mon
azite
L
lS18
5 di
scre
te
LlS
315
crys
tals
Cof
Jini
te
L13-
100
sepa
rate
gra
ins
RE
E p
hase
L1
3-3
15
Pyro
chlo
re
L13-
305
thre
e se
para
te g
rain
s w
ithin
com
plex
in
terg
row
th
S8 1
core
rim
ZrO
z ~
62.4
6 52
.20
62.7
9 59
.50
60.8
0
2.30
7.
30
1143
-
0.09
4.01
4.
97
1 1.6
9 2.
86
nd
NW
s
19.7
7 19
.23
19.3
4 19
.49
19.1
8 32
.04
34.0
5
2.54
0.
96
0.35
0.
31
2.34
4.
88
0.91
1.
39
4.41
0.
40
0.21
0.
12
5.79
0.
76
1.21
0.
37
6.36
0.
06
0.29
0.
01
nd
76.9
0 5.
32
0.66
0.
60
53.3
2 14
.07
2.34
0.
74
60.1
5 3.
18
2.83
-
7.26
0.
04
0.07
nd
12
.16
0.32
0.
07
0.37
1.
94
69.2
0 0.
30
0.44
4.
94
64.7
8 0.
33
0.43
3.
29
57.7
5 0.
54
0.43
1.
35
75.7
3 0.
26
nd
2.51
0.
19
0.10
T
a,05
T
hoz
UO
, T
iO,
21.8
5 0.
38
15.4
2 2.
21
21.1
6 0.
39
14.3
7 2.
52
20.2
7 0.
41
17.4
0 2.
42
22.7
0 0.
32
19.3
9 2.
00
22.5
5 0.
47
13.6
6 2.
32
2.73
0.
45
7.19
6.
22
1.64
0.
71
3.60
7.
08
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
37.4
9 17
.65
32.0
7.
26.2
7
-
-
-
-
0.73
-
0.55
-
40.2
7 44
.52
Na,
O
La2
0i
1.1X
nd
1
44
nd
1.
44
nd
1.36
nd
1.
32
-
0.93
-
I .82
-
- - -
-
-
-
-
9.66
4.
90
-
-
-
-
5.49
Pb
O
26. I
3 28
.52
23.1
4 18
.67
28.5
7 31
.41
36.3
9
CaO
Fe
O
0.23
0.
42
1.52
1.
76
0.07
0.
37
0.02
nd
0.
01
0.23
0.67
1.
47
0.47
0.
13
0.74
0.
93
0.43
-
0.29
nd
0.81
04
)l
0.91
0.
03
0.97
1.
68
1.07
0.
45
0.60
nd
C
aO
FeO
0.89
3.
15
0.42
3.
60
0.43
3
44
0.49
3.
62
0.45
3.
32
0.54
1.
63
0.33
1.
49
MnO
Si
OL
PzO
s T
otal
0.11
32
.43
0.10
99
.91
0.53
26
.36
0.22
92
.11
nd
32.3
7 0.
09
100.
83
nd
32.6
5 0-
21
100.
51
0.04
30
.76
0.08
98
.64
nd
11.2
7 0.
44
99.0
3 nd
12
.50
0.43
91
.16
nd
14.7
4 1.
15
96.1
1
-
1.45
28
.24
102.
29
-
2.41
26
.30
104.
88
-
14-2
3 0.
12
90.9
9 -
14.0
6 0.
11
90.5
7 -
16.8
9 0.
19
94.1
6 -
14.9
2 0.
22
97.8
4
-
0.06
nd
93
.74
MnO
Si
O,
Ce,
O,
Tota
l
0.19
nd
-
91.1
7 0.
15
nd
- Y 1
.40
0.15
nd
-
88.4
4 0.
15
Ild
- 88
.19
0.14
nd
-
91.9
8 -
8.23
5.
05
96.4
2 -
4.03
5
41
96
.55
P
4
- n
ot d
eter
min
ed
nd.
not
detc
ctcd
S
81 A
rfve
dson
ite g
rani
te.
a
408 R.A. IXER, J.R. ASHWORTH, AND C.M.POINTER
2. Petrographical studies and mineral analyses
One polished block and polished thin section of several representative examples o f unaltered biotite granite, albitite, and greisenized granite were made. Wherever possible, material was chosen which previously has been described in the literature with published bulk rock geochemistry. From L13 borehole material, ten fresh biotite granites, two partially albitized granites (350, 385) , three albitites (41 1, 440. 445), and one incipiently greisenized granite (125) were chosen, as were fourteen samples o f microclinized and greisenized granite adjacent to the Ririwai lode. Lexans and autoradiographs were used for finding uranium and thorium sites in the microclinized and greisenized rocks.
Routine transmitted and reflected light petrographical studies were followed by SEM and wavelength-dispersive electron microprobe analysis of the accessory and opaque phases. Quantitative analyses were performed for Fe, Mn, Ti, Nb, Ta , Z n , Sn. W, U , T h , Hf, Zr, R E E , P , Ca , Si, and Y on appropriate minerals and using simple standards and full ZAF correction procedures. In common with other studies, poor totals were obtained for uranium- and thorium-bearing phases, due to their metamict nature, poor crystallinity. the presence of unknown quantities o f OH and F radicals, and possibly adsorbed HIO.
3. Results
3a. Accessory and opaque mineralogy of the unaltered biotite granite The biotite granite contains zircon, columbite, and TiO1 minerals accompanied
by minor amounts of monazite, thorite, ilmenite, marcasite, pyrite, sphalerite, galena, chalcopyrite, and cassiterite. Secondary haematite is ubiquitous.
Zircon is common and is found as interstitial equant grains (up to 500 p m in diameter) between quartz crystals, o r as smaller grains, within biotite (up to 50 pm in diameter) associated with columbite, ilmenite, TiO, minerals, uranothorite, and fluorite. Many display concentric zoning, which is seen by variations in reflectance. Other lower reflectance areas cross-cut this zoning and extend from the margin of the zircon towards its core. The lower reflectance areas are typically enriched in T h , U, Ca , Y, P, Fe, and Mn compared to areas of the zircon with higher reflectance. Thorite inclusions (up to 10 p m in size) occur in the outer zones of some zircons, while others show an outer inclusion-free zone, 20 p m wide, which is hafnium-rich with up to 6.4 wt% Hf02 (Table 2).
Thorite is less common than zircon. In addition to forming small inclusions in zircon. it occurs in micas as larger grains (up to 250 x 180 pm) in association with zircon, columbite, and fluorite. Paragenetically it is seen to be later than zircon, upon which it nucleates. Table 2 shows the thorite t o be variable in composition: some are uraniferous with up to 14.1 wt% UO,; some have up to 2.8 wt%, Y z 0 3 , and some are zirconium-rich with up to 27.1 wt% Z r 0 2 . Optically, there is no evidence to suggest that the high zirconia values a re due to the presence of zircon as an admixed phase.
Thorium-bearing monazite may contain up to 12.2 wt% T h o , and 0.3 wt% UO,. It forms lath-shaped crystals 100 x 30 p m in size. which are often collected into radiating aggregates 300 p m in diameter, but can reach 800 pm in length. Table 2 shows the monazite t o have variable ratios of its major rare earth elements, Ce. La, and Nd. Monazite is often intimately intergrown with an unidentified
ACCESSORY MINERALOGY OF THE RIRIWAI GRANITE 409
LREE mineral which also contains thorium (up to 3.5 wt% Tho?) , minor amounts of uranium (up to 0.2 wt% UO?) and calcium (up to 0.6 wt%) suggesting that it is bastnaesite or fluocerite. Texturally, the LREE mineral appears to be replacing monazite, and fluocerite replacing monazite is recorded from the similar Taghouaji complex in Niger (Perez 1985). All these uranium- and thorium-bearing minerals are stained with haematite and carry small, 2-10 pm diameter, inclusions of haematite, pyrite, and limonite.
Pyrochlore is rare. and was seen as part of a single complex columbite-iImenite-TiO,-zircon-pyrochlore-fluorite intergrowth 150 k m in diam- eter, where it occurs as poorly polished grains, 30 pm in diameter (Plate 1).
Table 2 shows that it belongs to the plumbopyrochlore species, and contains up to 0.5 wt% T h o , and 19.4 wtY0 UO,. The presence of pyrochlore in albitized alkali granite from Ririwai has been known for some time, but it has a different composition from that found in the biotite granite. Beer (1952) in partial analyses for pyrochlore from the albitized alkali granite shows it to have up to 4.3 wt% T h o z and up to 3.5 wtoh UO,. Analyses from this study (Table 2) suggest the pyrochlore found in the alkali granite is also a plumbopyrochlore but is more niobium-rich and significantly tantalum- and uranium-poorer than plumbopyro- chlores from the Ririwai biotite granite.
Columbite occurs as small grains (up to 30 pm diameter) surrounded by 5-15 km wide TiO, rims, and is seen within biotite, chloritized biotite, o r feldspar, usually aligned along cleavages. Similarly rimmed but larger columbite grains, up to 250 pm in diameter, which may be optically unzoned or faintly zoned, are found between quartz crystals; or are associated with the pyrochlore intergrowth. Table 3 shows that the discrete columbite crystals in quartz, and columbite associated with pyrochlore, are iron-rich and niobium-rich. Atomic Fe/(Fe + Mn) ratios are between 0.82 to 0.91 for all but two analyses, Nb/(Nb + Ta) ratios have a range of 0.76 to 0-99. There is no strong correlation between these two ratios, nor any chemical zoning from core to rim other than a slight increase of manganese and decrease of iron at the margins of single crystals. Other elements are present within limited ranges, with up to 2.6 wt% TiO?, and 1.7 wt% W03. Only the tantalum content has a wide range, beyond that given in Table 3, from 1.2 wt% to 23.7 wt% Ta,O,.
Ilmenite (which optically is pinker than columbite) is found in minor amounts, as small lath-shaped crystals up to 200 x 20 pm in size, occurring along the cleavage of the micas, or as large aggregates (300 x 200 pm) of typically curved crystals in quartz. I t also forms part of the intergrowth with pyrochlore. It is enclosed within columbite or TiO, mineral rims and is extensively altered to fine grained haematite and TiO, minerals. Table 3 shows the ilmenite to be iron rich but to contain significant amounts of manganese with Fe/(Fe+Mn) in the range 0.45 to 0.99. Although the manganese content is erratic, both the amount of ilmenite and its manganese content increase generally down the drill-hole towards the underlying albitites, with the maximum manganese content of 22.9 wt% MnO occurring at L13-350. Discrete ilmenite carries little niobium (<2 wt% Nb,05) or tantalum ( < 0 3 wt% Ta,05), but ilmenites which are rimmed by columbite have up to 12.4 wt% Nb,O,.
TiOz minerals (rutile and ?anatase) occur as discrete acicular crystals up to 200 x 4 pm in length, lying along the cleavages of mica, and as aggregates of equant twinned rutile crystals up to 600 p,m in diameter, and as rims around columbite and ilmenite. The TiOz minerals have up to 4.1 wt% FeO, 0.1 wt% MnO and 1.0 wt% Ta20S. Table 3 shows that they fall into two groups: a low-niobium
1 3
Plat
e 1.
Col
umbi
te (
C),
myr
mek
itica
lly in
terg
row
n w
ith s
ilica
tes.
has
nar
row
TiO
, rim
s (w
hite
, bot
tom
of
pho
togr
aph
and
top
of p
hoto
grap
h (T
)). I
t is
inte
rgro
wn
with
zir
con
(Z)
and
plum
bopy
roch
lore
(P
), B
iotit
e gr
anite
, L1
3-30
5m.
Ref
lect
ed l
ight
, oi
l im
mer
sion
. Sca
lc b
ar 5
0 m
icro
ns.
Plat
e 2.
Cof
fini
te (C
o) w
ith fi
ne-g
rain
ed a
dmix
ed s
phal
erite
(lig
hter
gre
y pa
tche
s) c
nclo
ses
mol
ybde
nite
la
ths.
Gan
gue
min
eral
s ar
e qu
artz
(0
) and
mic
a. B
iotit
e gr
anite
. L1
3-10
0m.
Rcf
lect
ed l
ight
. oi
l im
mer
sion
. Sc
ale
bar
SO m
icro
ns.
Plat
e 3.
Zon
ed z
irco
n. T
he
mid
dle
zone
con
tain
s ab
unda
nt u
rano
thor
ite,
pyri
te
and
haem
atite
in
clus
ions
. The
out
er z
one
is ha
fniu
m-r
ich
(lig
ht-
rey)
Pyr
ite (
whi
te) a
s pa
tche
s an
d ve
inle
ts is
pre
sent
ar
ound
zir
con.
Alb
itite
, L1
3-41
lm
. Sc
ale
bar
It0
mic
rons
.
Plat
e 4.
The
sam
e zi
rcon
. A
bund
ant
thor
ite i
nclu
sion
s (w
hite
) ar
c cl
earl
y se
en. F
ine-
scal
e rh
ythm
ical
zo
ning
of
the
oute
r ha
fniu
m-r
ich
zone
is
also
wel
l di
spla
yed.
Bac
k-sc
atte
red
imag
e. S
EM
. Sc
ale
bar
100
mic
rons
.
0
;d “2
5 7
Plat
e 5.
Zon
ed c
olum
bite
, dar
ker
zone
s ar
e ir
on-r
ich.
Hae
mat
ite
(whi
te.
H) a
nd T
iO,
min
eral
s (g
rey
with
in c
olum
bite
) ar
e as
soci
ated
. A
lbit
ite
L13-
440m
. R
efle
cted
lig
ht.
oil
imm
ersi
on.
Scal
e ba
r 50
m
icro
ns.
Plat
e 6.
Zir
con
(Z)
show
ing
repl
acem
ent
of i
ts i
nner
zon
es b
y m
ica.
It
is ov
crgr
own
by u
rano
thor
ite
(T)
whi
ch c
onta
ins
smal
l in
clus
ions
of
pyri
te (
whi
te s
peck
s). G
angu
e is
mic
a. I
ncip
ient
ly g
reis
eniz
ed
gran
ite
L13-
125m
. R
efle
cted
lig
ht.
Scal
e ba
r 60
mic
rons
.
Plat
e 7.
Tw
o eu
hedr
al z
irco
ns (
Z) a
re o
verg
row
n by
ura
noth
orit
c (T
) whi
ch,
itsel
f, i
s re
plac
ed b
y xe
noti
me
(X)
alon
g its
edg
es.
A e
uhed
ral
grai
n o
f ca
ssite
rite
(C
) is
pres
ent.
Lod
e gr
eisc
n R
S6 (
2).
SEM
imag
e. S
cale
bar
20
mic
rons
.
Plat
e 8.
Col
umbi
te c
ore
(dar
ker
grey
in
cent
re)
encl
osed
with
in s
ubhe
dral
TiO
, (l
ight
er g
rey)
. L
ode
grei
scn
N58
. R
efle
cted
lig
ht.
oil
imm
ersi
on.
Scal
e ba
r 10
0 m
icro
ns.
;f=
Tab
le 3
. R
epre
sent
ativ
e co
lum
bite
, ilm
enite
and
Ti0
2 m
iner
al a
naly
ses
in b
iotit
e gr
anite
N
..
Sam
ple
FeO
' M
nO
TiO
, N
h,05
T
a,O
, W
O,
Sn0
2 T
otal
Col
umbi
re
L13-
100
core
rim
L1
3-15
5 co
re
core
rim
LI
3-15
5 L1
3-20
5 co
re
rim
L13-
256
near
ilm
enitc
L1
3-25
6 m
yrm
ckite
L1
3-30
5 sy
mpl
ectit
e L1
3-35
0 co
re
rim
Ilm
enite
L1
3-18
5 la
th
lath
L1
3-25
6 co
re
rim
L13-
305
L13-
350
TiO
, m
iner
als
L13-
155
lath
la
th
L13-
155
near
by
smal
ler
grai
ns
L13-
205
two
adja
cent
gr
ains
in
biot
ite
L13-
350
larg
e gr
ain
L13-
256
lath
sam
e
18.0
4 16
.76
17.6
4 17
.27
17.0
3 18
.51
17.9
7 17
.77
18.4
6 18
.64
17.3
0 18
.37
17.6
8
24
3
2.19
2.
17
2.28
2.
53
2.21
2.
33
2.40
2.
22
2. I9
2.
50
2.09
3.
11
9.77
10
43
1.07
I .2
8 6.
60
22.9
0.73
1 .s
o 0.
82
0.78
0.
58
0.56
0.
49
0.55
2.
61
I ,32
1.
51
1.12
0.
79
72.0
7 58
.32
63.6
8 59
.94
68.1
0 74
.99
65.6
9 67
.21
74.7
1 75
.29
64.8
4 70
.74
6743
6.95
18
.42
16.1
1 19
.72
11.2
5 3.
19
13.5
3 11
.83
1.69
1.
78
12.8
3 7.
80
10.6
9
n d
1.71
-
0.78
I1
d
0.29
0.
34
-
0.23
nd
-
35.7
4 35
.22
45.4
9 46
.19
39.7
5 19
.2
52.0
6 52
.38
4141
48
42
49.5
6 53
.8
1.28
0.
55
12.3
6 2.
85
1.65
4.
9
0.14
0.
26
0.24
0
. 14
0.49
0.
2
1.62
2.
68
1.73
1.
83
1.67
3.
19
2.71
3.
10
3.48
0.1
1 0.
07
04
3
0.05
0
46
0.
0 I
0.04
0.
05
0.07
94.4
1 91
.16
89.6
4 84
.24
91.9
4 81
.64
90.3
8 83
44
82.5
7
3.42
3.
93
8.00
13
.59
4.89
15
.64
5.12
13
.80
14.4
9
0.61
0.
56
0.4
1 0.
61
0.64
0.
58
0.20
0.
17
0 51
Tot
al F
e ex
pres
sed
as F
cO
ACCESSORY MINERALOGY OF THE RIRIWAI GRANITE 413
group (9 5 wt% Nb2OS) and a high-niobium group (13.5 to 15.5 wt% Nb205). Petrographically these two groups are indistinguishable and, indeed, they are often found next to each other within the same mica crystal. TiO, rims about columbite are too narrow for uncontaminated analyses but the data suggest that the Ti02 belongs to the high-nobium group, whilst Ti02 rims about low-niobium ilmenites belong to the low-niobium group.
Other phases within the biotite granite are only found in minor amounts and probably have been introduced by later mineralizing fluids. Cassiterite is extremely rare and forms small euhedral crystals up to 60 pm enclosing relict columbite. Galena, which is very close to being pure PbS and in which silver is undetectable, is often intergrown with light-coloured iron-poor (up to 2.5 mol% FeS) sphalerite. Pyrite, marcasite, molybdenite, bismuthinite, and native bismuth are rare. In L13-100 a late-stage complex intergrowth, 200 IJ-m in diameter, of nearly pure end-member molybdenite, minor iron-poor sphalerite, and coffinite is seen within small voids in the granite (Plate 2). The coffinite is variable in composition (Table 2) but has up to 4.9 wt% Tho, and 11.7 wtYo ZrO, with only minor amounts of Y,OR (up to 0.5 wtY0).
3b. Accessory and opaque mineralogy of the albitites The albitites, which have resulted from intense sodium-metasomatism, have a
distinctive mineral assemblage which is characteristically coarse-grained. Kinnaird et al. (1985) described the accessory mineral assemblage as early zoned zircon and haematite laths, mixed ilmenite-haematite, columbite-haematite, and magnetite- haematite intergrowths and optically zoned columbite, together with finer-grained uranothorite and TiO, minerals. Cassiterite, sphalerite, galena, and pyrite are also present. Haematite, columbite, and zircon are the most abundant of these phases.
Minor amounts of 40-60 krn diameter zircon crystals are found within the biotite and chloritized biotite but most zircon occurs as euhedral, intensely and complexly zoned crystals up to 600 pm in diameter associated with quartz. The zones are between 30 and 100 km wide, have different reflectances, and some carry abun- dant, small (<15 pm diameter) inclusions of uranothorite, pyrite, haematite, and marcasite. Individual zircon crystals have inclusion-poor metamict inner zones with up to 6.1 wt% U 0 2 , surrounded by inclusion-rich near opaque zones (with up to 0.06 wt% UO, and 0.4 wt% Tho,, Table 4) which are themselves enclosed within inclusion-free outer zones of higher reflectance which are hafnium-rich (up to 14.0 wt% HfOJ with characteristic radial fractures (Plates 4 and 5 ) .
Uranothorite OCcUiS as small inclusions within zircon, and also forms incomplete rims around zircon. It has a variable uranium content (between < 5 4 and 28.9 wt% UO,) and variable amounts of Y203, usually less than 1 wt% but with a maxiumum of 5.9 wt% Y203. Minor amounts of thorium-bearing xenotime (up to 2.2 wt% Tho,) occur as small (15 pm) crystals within inner zones of zircon and as replacements of zircon. A thorium-bearing (up to 2.4 wt% Tho,) uniaxial LREE mineral occurs as inclusions (5 - 70 pm in diameter) within yttrofluorite which replaces zircon. Both its composition and optical properties are consistent with those of bastnaesite.
Haematite is the most abundant opaque phase, and forms discrete tabular crystals up to 350 X 200 km in size, often in radiating clusters in the micas. Much haematite occurs as complex intergrowths with magnetite, ilmenite, and columbite where it is extensively replaced by them along fractures or grain boundaries. The haematite contains up to 5.3 wt% Ti02 but only minor amounts of manganese, (normally 41.1 wt% MnO), and niobium (up to 0.8 wt% Nb2OS). tantalum
Tabl
e 4.
Rep
rese
ntat
ive
anal
yses
of
uran
ium
- an
d th
oriu
m-b
eari
ng p
hase
s fr
om
albi
tites
and
gre
isen
ed g
rani
te
Sim
ple
Zr0
2 H
fO?
Tho
2 U
02
Y20
3 C
aO
FeO
S
O2
P20
s To
tal
A lb
itite
s Zi
rcon
L1
3-41
1 in
ner
oute
r in
ner
oute
r Th
orite
L1
3-41
1 in
clus
ions
in
zir
con
Xeno
time
L13-
41 I
L1
3-41
1 R
EE
pha
se
in z
irco
n
61.3
4 53
.94
55.5
3 6 I
.so
2.30
- - 1.89
0.
20
- 0.57
5.65
0.
08
0.08
0.
12
nd
nd
32.3
6 0.
14
We7
7 14
.00
0.42
0.
06
0.24
nd
nd
32
.13
0.31
10
1.10
-
28-1
1 -
93.3
2 3.
07
0.51
6.
10
- -
2.94
0.
58
0.98
0.
38
-
- 31
.99
- 98
.37
1.40
72
.07
1.47
14
7 1.
06
2-04
15.33
0.59
97
.33
- 53
.30
28.9
2 0.
78
1-12
-
16.8
5 -
100*
97
- 80
.08
4.96
-
0.39
-
16-3
3 -
101.
75
- 53
4)o
19.9
7 5.
07
0.94
0.
73
1647
1.
15
99.2
7 -
58.8
2 23
.72
0.81
1.
47
0.77
15
-68
0.38
10
1*80
- 2.
19
0.04
42
.74
0.24
-
4.05
23
.88
(73.
14)
- 1.
65
0.19
1-
07
0.11
0.
61
0.48
nd
(75.
11)
plus
38.
02 C
e203
. 70.
31 L
a203
and
12.1
0 N
d20.
3
P ? R S
c, P
Gre
i,yen
s Zi
rcon
L1
3-12
5 da
rk c
ore
Hig
h R
%
LOW
R%
R
S14(
1)
RS6
(4)
Thor
ite
L13-
125
incl
usio
ns
RS6
(2)
over
grow
th
of t
hori
te
on z
irco
n Xe
notir
ne
RS6
(2)
over
grow
th
on t
hori
te
Cof
finir
e R
S6(4
) co
ffin
ite
59.3
3 4.
99
2.58
0.
51
59.3
2 3.
34
2.32
0.
63
44.6
2 2.
69
7.35
0.
96
62.3
2 4.
97
0.27
1.
17
63.9
6 2.
94
0.17
0.
41
24.7
4 1.
77
40.0
8 3.
00
29.2
0 2.
10
35.1
7 7.
33
2.38
-
47.4
3 23
.45
1.04
-
51.1
2 18
.57
0.55
-
63.5
7 4.
53
0.89
0.
10
3.91
0.
43
0.07
-
2.68
59
.94
0.04
-
2.90
64
.54
0.34
1.
28
5.07
0.
19
0.18
7.09
0.
88
6.82
5.
06
7.5
1
44.9
3 48
.30
11.6
1 9.
01
0.10
0.
17
0.61
n d
0.77
0.
84
1.05
1.
18
0.69
1.33
1.
66
2.21
30
.26
0.67
30
.31
1.93
24
.59
1.12
30
.84
-
33.6
6
0.62
19
.10
0.97
23
.78
-
13.8
4 -
12.9
1 0.
10
11.6
3
3.86
5.
34
-
13.0
5 -
14.2
0
0.21
10
0.53
0.
33
98.3
7 0.
50
88.3
2 -
100.
88
-
101.
32
1.07
98
.33
0.51
10
0.78
1.
23
96.2
0 1.
19
91.0
7 1.
96
90.5
4
27.0
(1 (
76.8
6)
23.6
8 (8
1.99
)
1.94
90
.62
0.72
93
.07
m 23
R%
, rc
flec
tanc
c
P
wl
t-
4 16
(normally up to 0.25 wt% Ta,Os) and trace amounts of tin (Table 5 ) . Ilmenite is present in minor amounts as small laths 60 x 10 pm in size along
cleavages in mica, and more commonly in intergrowths with haematite as tabular crystals up to 100 k m in length. The iron-manganese composition of ilmenite is variable, with two ranges of Fe/(Fe + Mn) ratios (0.24 to 0.25 and 0.86 to 0.88) suggesting that ‘single’ grains are a mixture of ilmenite and pyrophanite. Although optical zoning is seen in these ilmenite grains, neither colour nor reflectance variations correlate well with composition. Ilmenite contains up to 2.3 wt% Nb,O,, 0.9 wt% Ta,Oi, and trace amounts of tin. Locally, ilmenite is extensively altered to haematite and TiO, minerals. Magnetite, too, is intimately intergrown with haematite and replaces haematite, to give mixed grains up to 300 x 150 pm in size. Magnetite is manganese-poor, up to 0.4 wt% MnO, but has up to 3.5 wt‘X TiO, and trace amounts of niobium, up to 0.2 wt% Nb,O, and tantalum, up to 0.2 wt% TazOj, but no detectable tin or tungsten (Table 5 ) .
Small columbite crystals up to 40 pm in diameter are found in micas and feldspar, but most columbite is coarse-grained (up to 500 p m in diameter), euhedral, and optically highly zoned with brown cores and bluer, lower reflectance margins (Plate 6). Relict haematite is common within columbite. Unlike the columbite analyses obtained from the surface albitite samples at Ririwai (Kinnaird er ul. 1985), which showed a very restricted range of compositions, the present analyses show that columbite from the drill-hole albitites is extremely variable in composition. This compositional variation can be related to the mineral’s optical properties, for the inner, browner iron-rich cores have Fe/(Fe + Mn) ratios between 0.80 and 0.96 and the outer, lower reflectance manganese-rich rims between 0.16 to 0.40. The boundaries between iron-rich and manganese-rich columbite are sharp and distinct. The variation in niobium and tantalum contents is less extreme; the ratios Nb/(Nb + Ta) varies between 0.86 and 0.98 showing little correlation with the iron and manganese content of the columbites.
Small acicular crystals of Ti02 minerals up to 20 pm in length lie along cleavages of biotite and chloritized biotite, or form equant crystals up to 60 pm in diameter surrounding columbite and ilmenite. Table 5 shows the TiO, minerals to be niobium-poor with up to 4.7 wt% N b 2 0 5 together with up to 2.7 wt% Ta,O,. 0.2 wt% SnO, and 0.04 wt% WO,.
Minor amounts of pyrite and marcasite are associated with zircon and uranothor- ite frequently as rims to, or inclusions within them. Iron-poor sphalerite (up to 1.4 mol%, FeS) is intergrown with galena, both of which enclose haematite laths.
R.A. IXER, J.R. ASHWORTH, AND C.M.POINTER
3c. Accessory and opaque mineralogy of the microclinized and greisenized granites.
The mineralization, especially the sulphide phases, accompanying both processes has been extensively described in Kinnaird ef ul. (1985). Both processes have produced the same mineralogy which is essentially cassiterite as the main oxide phase accompanied by sphalerite, galena, molybdenite, chalcopyrite. pyrite, and marcasite. Zircon, columbite, wolframite, thorite, monazite, and TiO, minerals are also present, as are trace amounts of ilmenite. Fourteen samples of micro- clinized and greisened rocks adjacent to the Ririwai lode, and L13-125, an incipiently greisened granite from the drill-hole, were studied.
Zircons from L13-125 occur as 200 pm diameter crystals which have been extensively corroded, and typically show total replacement of their inner zones by silicates and fluorite to leave 30 pm wide outer zones with 30 pm uranothorite overgrowths (Plate 7). In material from the Ririwai lode the zircons are equant crystals up to 120 pm in diameter within quartz crystals and display fine-scale
Tab
le 5
. R
epre
sent
ativ
e an
alys
es o
f co
lum
bite
, ha
emat
ite,
ilmen
ite,
mag
netit
e an
d T
i02
min
eral
s of
th
e a
lbiti
tes
Sam
ple
FeO
Fe
,O,
MnO
T
iOL
N
b20i
Ta
,O,
WO
, Sn
O,
Tota
l
Hae
mat
ite
L13-
440
lath
with
in
colu
mbi
te
L13-
445
asso
ciat
ed
with
mag
netit
e L1
3-44
0 cl
ose
to
ilmen
ite
Ilmen
ite
L13-
440
inte
rgro
wn
with
hae
mat
ite
Mag
netit
e
Col
umbi
te
L13-
440
sing
le z
oned
cr
ysta
l
dark
da
rk
crys
tal
dark
da
rk
dark
L13-
445
L13-
445
sing
le z
oned
TiO
, m
iner
als
L13-
440
four
sep
arat
e gr
ains
in
chlo
rite
2.10
4.
34
447
5.17
3.
28
2.25
41.1
5 40
.09
11.2
7 10
.39
32.9
7
16.3
1 16
.40
16.4
9 6.
94
3.18
18
.39
16.9
1 7.
90
5.71
34
5
2.26
4.
02
4.32
4.
60
95.5
8 0.
06
89.2
4 0.
24*
90.8
0 0.
04
89.3
9 0.
05
93.7
4 0.
06
94.2
9 0.
85
1.36
6.
11
1.68
6.
67
1.04
34
.76
1.67
35
.58
62.5
8 0.
41
3.15
3.
51
3.95
12
.49
16.1
3 2.
65
3.60
11
.90
13.9
5 16
.39
0.35
0.
13
0.12
0.
12
2.27
4.
15
4.03
5.
26
2.39
2.
84
49.3
0 50
.62
50.9
7 50
.47
2.81
0.79
0.
50
0.18
0.
46
0.58
0.
56
0.16
0.
20
0.67
0.
88
88.5
6 88
.06
89.1
0 91
.77
0.02
04
7*
0.69
0.
29
0.84
0.
30
2.26
1.
07
0.44
0.63
0.19
63.3
4 70
.66
7645
69
.38
7 1 .4
8 74
.18
73.8
5 70
.24
71.3
6 75
.21
4.74
4.
68
4.25
2.
43
0.22
0.24
0.22
0.
21
0.88
0.
20
0.20
0.
39
0.1
1
16.2
1 9.
08
3.73
10
.5 I
7.37
4.
87
5.78
9.
73
7.53
3.
13
2.71
1.
79
I .24
0.65
100.
13
99.0
0 10
0.25
10
0’40
10
0.62
10
0.81
101.
06
100.
33
98.6
8 99
.13
99.0
7
99.8
0 I 0
0. I 5
10
0.40
99
.78
98.7
4 10
0.65
10
0.30
99
.97
99.2
2 98
.66
98.8
4 98
.84
99.1
4 99
.62
* ‘c
onta
min
atio
n’.
by s
econ
dary
X-r
ay g
ener
atio
n in
col
umbi
te i
s su
spec
ted.
In
haem
atitc
, ilm
enitc
. an
d m
agne
tite,
the
pro
port
ion
of F
eO t
o Fe
,O,
is ca
lcul
ated
on
the
basi
s of
the
sto
ichi
omet
ric
ratio
of
tota
l ca
tions
to o
xyge
n (2
:3 in
hac
mat
ite,
and
ilmen
ite.
3:3
in m
agne
tite)
. In
col
umbi
te a
nd T
iOz
min
eral
s, t
otal
Fe
is ex
pres
sed
as F
eO.
418 R . A . IXER, J .R. ASITWORTFT, AND C.M.POINTER
zoning. I n the white micas and chlorite they occur as aggregates (‘welded clumps’) up to 400 p m in diameter in association with columbite, cassiterite, and TiOz minerals. Compositionally, the zircons contain little uranium (up to 1.2 wt%) U 0 2 ) o r thorium (up to 0.3 wt% T h o z ) but have up to 5.0 wt% HfO, (Table 4).
Uranothorite in L13-125 overgrows zircon or infills dissolution voids within it. I t is zirconium-rich with up to 29.2 wt% ZrOz. In the greisened wallrocks of the lode uranothorite is widespread and forms elongated crystals up to 300 pni long i n quartz. In the white micas it forms part of the loose aggregates of zircon, monazite, xenotime, columbite, and cassiterite which are accompanied by fluorite; here uranothorite overgrows zircon and is itself replaced by xenotime (Plate 7).
Locally, cross-cutting veinlets carry abundant 60 pm long thorite crystals. Large uranothorites are optically zoned, with cloudy cores, but show little o r n o chemical zoning. Table 4 shows that the uranothorite in the greisens of the Ririwai lode has variable uranium contents with up to 23.5 wt% UO,, and contains significant amounts of other elements, notably yttrium (between 0.6 to 10.2 wt% Y203) and phosphorus (up to 3.0 wt% P,O,). but relatively little zirconium (up to 2.4 wt% ZrO,). Monazite is common as laths up to 600 X 160 pin in size between quartz and mica crystals. Xenotime is rare: i t replaces uranothorite and contains up to 3.9 wt% Thol and 0.4 wt% UOz. Trace amounts of L R E E minerals are associated with zircon, monazite, and yttrofluorite, and may belong to the bastnaesite- fluocerite groups of minerals.
All the uranium- and thorium-bearing phases are haematitically stained and are surrounded by pleochroic halos when in mica.
Columbite in L13-125 is seen as small crystals (up to 50 pm) surrounded by 5-10 p m wide TiO, mineral rims within mica, or typically, as relict crystals up to 200 pm, within very coarse-grained cassiterite. These relict columbites show a wide range of compositions with Fe/(Fe + Mn) ratios between 0.60 and 0.90 and Nb/(Nb + Ta) between 0.76 and 0.98; they also contain up to 1.0 wt% WO, and up to 0.5 wt%i SnO, (Table 6). Within the greisenized lode the columbites are small. up to 150 p m in diameter, enclosed within TiO, mineral rims, and have variable compositions with Fe/(Fe + Mn) between 0.64 and 0-90 and Nb(Nb + Ta) o f between 0.79 and 0.99.
TiO, minerals typically form overgrowths, up to 40 p m in width, on columbite (Plate 8) and contain significant amounts of niobium, up to 13.6 wt% Nb,05 (L13-125) and 8.5 wtYo Nb,O, (Ririwai lode); of tantalum, up to 1.2 wt% Ta,05 (L13-125) and 1.3 wt% (Ririwai lode), with up to 8.6 wt% W 0 3 and 0.5 wt%i SnO, next to the lode. Discrete laths of Ti02 up to 360 p m in length, and twinned rutile crystals up to 200 p m in diameter occur in the wallrocks of the lode. They have variable concentrations of niobium (2.4 to 10.7 wt% Nb2OS), tantalum (0.1 to 0.8 wt%, Ta,O,), tin (0.1 to 0.7 wt% SnO,), and tungsten (0.9 to 5.5 wt% WO,) but little manganese (up to 0.04 wt% MnO). In general, TiO, mineral overgrowths on columbite have higher minor element concentrations than do discrete Ti02 crystals.
Cassiterite is characteristic of the greisens and forms abundant small, 60 p m crystals. whereas larger crystals, up to centimetres in diameter, are complexly zoned and twinned, and carry relict columbite. Zoning is seen optically by lighter and darker internal reflections. Table 6 shows there to be a correlation between intensity of the body colour and the increase in iron and to a lesser extcnt, tantalum concentrations.
Rare laths, up to 300 x 60 p m in size, within quartz have been optically identified as wolframite. Trace amounts of ilmenite too, are present in quartz.
9 0 0
m
FcO
hl
nO 110,
NhZ
O,
I,i,O
, W
O.
SnO
, T
otd
4
Tab
le 6
. R
epre
sent
ativ
e an
alys
es o
f co
lum
bite
, Ti
O,
min
eral
s an
d ca
ssite
rite
fro
m g
reis
eniz
ed g
rani
te.
m 8 id
Sdm
ple
Col
um bi
te
L13-
125
colu
mbi
te
repl
aced
by
Cas
site
rite
RS1
4 ve
in:
in q
uart
z as
soci
ated
TiO
z in
qua
rtz
RS6
(3)
asso
c. T
iO,
TiO
z m
iner
uls
L 13
-1 2
s R
S14(
2) s
ingl
e ru
tilc
RS1
4(2)
sin
gle
RS1
4 ve
in.
over
grow
ths
Cas
site
rite
L1
3-12
5 re
plac
ing
dark
co
lum
bite
pa
lcr
pale
st
RS6
(3)
da
rk
pale
r pa
lest
R
S6 I
4 ve
in
dark
pa
ler
grai
n in
mic
a
grai
n in
mic
a
on c
olum
hitc
13.4
9 18
.53
12.0
9 16
.16
18.9
2 16
.15
17.1
3 13
.75
2.55
2.
49
5.45
7.
08
5.07
6.
89
6.09
6.
14
2.05
0.
55
0.17
0.
64
0.29
0.
10
0.85
0.
35
5.52
0.
35
2.10
0.
59
8.1
1 0.
57
1.98
0.
95
2.05
0.
80
5.70
2.
74
2.19
1.
92
7.04
1.
29
0.07
84
.75
0.04
81
.87
04
3
88.9
3 0.
04
76.7
4 0.
01
91.2
8 0.
04
80.5
2 0.
05
77.7
1 0.
13
76.8
6
nd
0.18
nd
0.
10
nd
0.05
04
)4
0.15
0.
04
0.16
nd
0.
04
nd
0.05
nd
0.
12
66.0
8 75
.88
7 I .3
8 52
.69
76.2
2 59
.27
54.6
4 73
.02
12.1
7 13
.61
3.8 1
10
.72
2.35
6.
98
7.4 I
8.
53
3.30
0.
57
0. I2
2.
39
0.33
0.
13
2.16
2,
26
11.6
3 2.
60
7.05
27
.07
2.17
1.
25
24.1
6 3.
82
1.22
0.
90
0.35
0.
78
0. 17
0.
41
0.70
1.
32
5.04
2.
16
0.52
0.
40
0.25
0.
32
0.50
1.
33
0.43
11
d 0.
04
0.46
nd
0.
35
0.28
tr
0.2'
) 0.
60
0.1
I 0.
66
0.45
0.
32
89.7
4 97
.35
09.2
9 96
.79
98.0
2 99
.68
95.9
3 98
.07
-
-
99.5
3 99
.79
99.4
8 10
0~30
10
0.16
99
.90
100.
9 I
98.9
2 10
0.76
98
.97
100~
17
100.
22
99.8
7 10
1 43
I0
1 .0
3 99
.87
100.
31
100.
73
IOO
.15
100~
41
99.0
9 10
0.27
99
.49
102.
13
*Tot
al F
e ex
pres
sed
a5 F
cO
tr.
trac
e
320 R.A. IXER. J .R. ASHWORTH, AND C.M.POINTER
Molybenite, as curved laths within mica, is associated with the oxide phases. Sphalerite up to two centimetres in diameter, is iron-poor and encloses trace amounts of pyrrhotine and chalcopyrite. Galena, pyrite, and marcasite occur as intergrowths about cassiterite.
Late stage coffinite is rare and was found in microclinized samples where it forms 30 to 150 p m diameter, poorly crystalline grains which are interstitial t o quartz. It has a variable composition (Table 4) with significant thorium, up to 3.5 wt% T h o 2 , yttrium up to 16.0 wt% Y,O, and phosphorus up to 3.2 wt% P20s .
4. Discussion
4a. Biotite granites This study confirms the observations of earlier workers that the biotite granite
contains accessory zircon and columbite with minor amounts of thorite and mona- zite (Kinnaird el d. 1985), but shows that these a re accompanied by ilmenite and pyrochlore. Phases associated with groundmass quartz and which are relatively coarse-grained, are interpreted as being magmatic, possibly late-stage magmatic rather than the result of hydrothermal alteration. However, the presence of chloritized biotite, of columbite (with TiO, rims) and ilmenite aligned along these altered mica cleavages, of altered zircon, trace amounts of cassiterite and sul- phides, coffinite with sulphides, and fluid inclusion data, all suggest that the granite has suffered pervasive hydrothermal alteration and mineralization.
Table 1 shows that the biotite granites within L13 are rich in uranium, with a ThiU ratio o f 0.8 to 1.0. Primary pyrochlore and late stage coffinite are both found within the biotite granite, and although neither is common they are the most significant uranium carriers. Minor amounts of uranium are found within uranothorite and, t o a lesser extent, within zircon and monazite. Thorium, too, is found within these phases but is mainly concentrated within uranothorite, but also in significant amounts in monazite (Kinnaird et al. 1985) and trace amounts in other REE minerals. The presence of coffinite with minor amounts of thorium is evidence that uranium and perhaps thorium have been remobilized within the granite. The lower uranium but similar thorium contents of the biotite granites from the surface, compared to those from drill-hole L13. show that uranium has been remobilized from the surface biotite granites and presumably lost by weathering.
Niobium, tantalum plus iron, and manganese a re found principally within col- umbite, within pyrochlore, and in minor amounts in ilmenite and TiO, minerals. Figure 2. which shows the plots of Nb/(Nb + Ta) against Fe/(Fe + Mn) for columbite, ilmenite, and TiOz minerals, suggests that there is a limited range of iron:manganese substitution but a wide range of niobium:tantalum substitution within columbite. Ilmenite shows a similar range of niobium:tantalum but a wider range of iron:manganese substitution. There is no obvious relationship between the two ratios.
Titanium is hosted in coarse-grained ilmenite, and in minor amounts within columbite and pyrochlore, but much occurs within the small TiO, grains often enclosing columbite, associated with the alteration of biotite. Semiquantitative analyses of biotite show them to have 0.2 wt% Nb20, and 0.1 wt% Ta,O, and 0.6 to 3.2 wt% T i 0 2 . It is suggested that these metals are released during alteration, especially chloritization, and are reprecipitated as columbite, followed
ACCESSORY MJNERALOGY OF THE RIRIWAI GRANITE
0.7--
"'Nb+Ta
10-
N'Nb+Ta
0.7-
421
- . Biotite Granite
u ' 2 I . Ail A A *: * *
:* 6c 6 0 3
Albitites J
Ilmenite
.*** 'G *G
*G
Greisenized Granite OG I 1 4 0.5 0.9 1.0
%e+Mn
Figure 2. Plot o f Fe/(Fe + Mn) ratio against Nb/(Nb + Ta) ratio for colurnbitc. ilmenite :ind Ti02 minerals f rom unaltered biotitc granite. albitites. and greisened granites.
by niobium-and tantalum-bearing TiO, minerals and finally more stoichiometric TiO,.
Figure 3 shows that the TiO, minerals in the biotite granite plot in two fields: most plot in a field which has the greatest niobium and tantalum contents of any TiO? from Ririwai and are interpreted as being the result of incipient alteration, but approximately 25 per cent plot towards the Ti corner of the diagram and may represent more intense alteration and differentiation of niobium and tantalum from titanium.
In the biotite granites, magmatic fluids have precipitated Nb, Ta, Ti, Pb, U , and Z r as columbite, pyrochlore, ilmenite, and zircon which are intergrown as complex symplectities. Zr , U, Th, Hf, and REE elements have precipitated in zircon associations. Later hydrothermal fluids have altered and redistributed the elements from biotite and also precipitated minor amounts of cassiterite, sulphides, and coffinite.
4b. Albitites This study shows that haematite, columbite, and zircon were formed extensively during albitization, accompanied by minor amounts of ilmenite, magnetite, cas- siterite, uranothorite, and xenotime. Chloritization of biotite was accompanied by crystallization of fine-grained TiO, minerals along the cleavages of the mica.
Table 1 suggests that both the bulk analyses and ThiU ratio for the albitites are similar to those of the biotite granite from L13. No discrete uranium minerals have been found in the albitites to explain the ratio, but the maximum uranium contents, both in zircon and uranothorite, are higher than those found in the biotite granites. Most of the thorium content of the albitites is within uranothorite,
122 R.A. IXER, J.R. ASHWORTH, AND C.M.POINTER
Ti
Biotite granite 0 Albitites 0 Greisen
80% T INb Ta\
Figurc 3. Triangular plot of Ti. Nb. and T a in TiO, inincrak from unaltered, albititized and gi-ei\ened granite.
and the high values within inner zones of zircon are due to small uranothorite inclusions. Many zircons have inner uranium-bearing core and hafnium-rich outer margin. Numerous zircons show dissolution accompanied by precipitation of fluor- ite, xenotime, and other REE minerals which carry minor amounts of uranium and thorium (mainly thorium).
The addition of iron by the fluids is evidenced by the abundant, early, coarse- grained haematite which is very close to being stoichiometric, followed by crystal- lization of magnetite, ilmenite, and columbite. Figure 2 shows that in both columb- ite and ilmenite there is a wide range in the iron to manganese ratio and this is optically seen by the zoning displayed by single crystals. Niobium to tantalum ratios are less variable, but the data show that the manganese-rich phase (pyrophanite) is more tantalum-rich than ilmenite. No correlation of iron to manganese and tantalum to niobium is evident, however, in columbite. Comparisons between the columbite found in albitite and those of the biotite granite show they plot in different fields, suggesting that none of the albitite columbites are inheritedirelict from the biotite granite.
Figure 3 shows that the TiO, minerals in the albitite carry less niobium and tantalum than most of those from the unaltered granites (and greisenized granite), which is consistent with the degree of alteration of the micas. Analyses of the TiO? show them to have significantly higher average manganese contents (0.18 wt% Mn) than those in either biotite granite or greisens (<O.OS wt% MnO). The mobility of the manganese in the albitites is well documented.
During albitization the fluids precipitated Z r with minor U , followed by Z r plus Hf; and later fluids crystallized Th and REE. Alongside this, the fluids precipitated Fe followed by Fe, Mn, Nb, Ti, Ta.
ACCESSORY MINERALOGY OF THE RIRIWAI GRANITE 423
4c. Greisens and microcline-rich rocks The present study shows that both cassiterite and sulphides were introduced
during greisenization, and that the greisens also carry minor amounts of zircon, monazite, uranothorite, xenotime, and columbite. The extensive alteration of the micas is accompanied by abundant fine-grained columbite and TiO,.
Table 1 suggests the bulk uranium contents of the greisens to be less than those of the biotite granites, but that the thorium content is enhanced, and hence the Th/U ratio is increased for the greisens. Bowden et ul. (1981) suggested that this enhanced ThiU ratio showed that uranium was mobile during greisenization, and that thorium was essentially fixed. The presence of paragenetically late yttrium- rich coffinite does suggest that uranium was relatively mobile during greisenization (or in post-greisenization times), but the presence of enhanced bulk rock thorium values together with the occurrence of abundant uranothorite (intergrown with cassiterite and within cross-cutting veinlets) suggests that it too, was mobile and crystallized during greisenization.
Coffinite is the only discrete uranium mineral which has been identified, although much of the uranium appears t o be within uranothorite. Both zircon and xenotime carry only minor amounts of uranium and an early uranium-rich zircon phase is absent. Uranothorite is the main thorium carrier; lesser amounts of thorium are found in coffinite, monazite, xenotime, and zircon. The uranothorite is characteristically zirconium-poor but has a wide range of yttrium contents, and in Figure 4 is seen to plot in a distinct field away from those found in unaltered biotite granites. Uranothorites in the incipiently greisened granites (L13-125) plot away from both those fields, and are both zirconium- and yttrium-rich. These distinctly different compositions suggest that the greisen uranothorites a re not
PLOT OF Zr02=Y203 FOR THORITE AND COFFlNlTE
+G
+ +
+ t
++
+ + 2 A * X
Biotite Granite + Uranothorite +GUranothorite for sample
113-125 +G+G 0 Coffinite
Microclinite o Coffinite
G+&?
+%G G* x Uranothorite
&&
+G+G +G
A Uranothorite
X
X A
A X X X X X x x
v l Y - " " " -r - n l - " I 0
2 4 6 8 10 12 14
Figure 4. Plot of wt'% ZrO, agains wt% Y , 0 7 for uranothoritc and coffinite in biotitc granite, albitites. and greisenizcd granite.
423 R . A . IXER, J.R. ASHWORTH, AND C.M.POINTER
inherited from the biotite granite, but result from greisening fluids. Monazite contributes to the whole-rock LREE content and xenotime, uranothorite, and yttrofluorite to the whole-rock yttrium content.
Niobium, tantalum, iron, and manganese are found together in columbite and TiO, minerals (which are almost exclusively found within the altered micas), and presumably also in ilmenite although this was too small for analysis. Figure 2 shows the iron to manganese ratios of columbite in greisens to be more variable than those in the biotite granite but less variable than those in the albitites. Niobium to tantalum ratios are, however, similar to those from the biotite granite columbites. Iron is also present within pyrite, marcasite. chalcopyrite, and, in minor amounts, in sphalerite, cassiterite, and Ti02 minerals.
Titanium is concentrated within the TiO, phases. which have variable amounts of niobium and tantalum. These are accompanied by significant amounts of tin and especially tungsten, when compared to their concentrations in the TiO, minerals from the albitites. From the present study it is not possible to show that the tin and tungsten were progressively released during the alteration of the micas, although this is likely.
Tin is found mainly within the abundant cassiterite, which characterizes the greisenization. It is zoned with respect to iron and niobium but also carries tantalum and titanium. The relationship seen at Ririwai, namely that the darkest zones are iron- and niobium-rich, has been found for cassiterite elsewhere (Greaves et al. 1971).
The incipiently greisenized granite, L13-125 clearly shows the dissolution of zircons, especially of their inner (U-rich?) cores, and the dissolutionheplacement of columbite by cassiterite and that fluids precipitating uranothorite were both zirconium- and yttrium- rich. Zircon, uranothorite, and xenotime within material from the greisens close to the Ririwai lode show that the fluids precipitated Zr, followed by Th plus minor U and Y , followed by Y with minor Th. The same fluids precipitated Nb and Ta followed by Sn, and by Zn, Pb, Mo, and W.
The accessory mineralogy and styles of mineralization of the Ririwai ring complex are shared by many other alkali granite complexes world wide, for example those o f the Arabian Shield (Drysdall et ul. 1984; Jackson 1986), as well as others in Nigeria (Kinnaird ef u I . 1985) and Niger (Kinnaird 1985).
In particular the mineralogy of the Ririwai biotite granite, its altered facies and later quartz-tin-sulphide mineralization is closely similar to that from the Tag- houaji ring complex in Niger, as described by Perez (1985), and in both complexes the role of post-magmatic fluids has been responsible, in a large part, for the variety of rock types and associated mineralization.
5. Conclusions Textural and paragenetic studies of the accessory and opaque minerals of the biotite granite and its altered varieties shows that they can be divided into two, according to the relationship between the ore phases and silicates.
1. Fine-grained colurnbite, ilmenite, and TiOz minerals are associated with the breakdown of biotite and liberation of Ti, Fe, Nb, Ta, Mn, and probably W and Sn, and their local reprecipitation along the new mica cleavage. An essentially similar association is seen in all the rocks. Analyses of the TiO, phases suggest that, with an increase in the alteration of the biotite, there is an increased differentiation of niobium and tantalum from titanium (this is represented in Figure 3).
ACCESSORY MINERALOGY OF T H E RIRIWAI GRANITE 425
2. Coarse-grained phases are typically associated with quartz, but here the association differs between the rock types. The biotite granite carries zircon, with columbite, minor ilmenite, and pyrochlore and late zirconium-rich coffinite; the albitites carry haematite, minor magnetite, iron-manganese zoned columbites and ilmenite; the greisens carry cassiterite plus iron, zinc, lead, and molybdenum sulphides, uranothorite, and late yttrium-rich coffin- ite. The textural and chemical evidence suggests that both alteration processes (albitization and greisenization) resulted in the destruction of earlier phases followed by their reprecipitation; but with different compositions compatible with the new fluids.
Table 7 summarizes the relative abundancies of the primary ore phases found in the biotite granite and its altered equivalents. Although the different fluids produced essentially very similar assemblages the proportions of the minerals vary markedly.
Overall, the following sequence of metal precipitation has occurred, as summa- rized in Figure 5.
Late granitic fluids, which may be magmatic, precipitated Zr, U , Nb. Ta, Ti, and Fe, albitizing fluids precipitated Fe, Mn. Nb, Ta, Zr, with minor U and Th, and greisenizing fluids precipitated Sn, Th, Zn, Pb, Mo, and Y .
Each process shows a fixed and similar paragenetic relationship between Zr, Hf, U , Th, and Y which is: Zr t U , Th and Hf; followed by Th ? U and Y; followed by Y? Th. This is seen as zircon crystals, overgrown by thoriteiuranothor- ite, overgrown by xenotime. The paragenetic position of monazite is unclear.
Uranium is concentrated early, and has its maximum concentration in the biotite granites where it forms discrete minerals. Some additional uranium may have been introduced during albitization, but it is lost during greisenization (and micro- clinization), and leached during surface weathering. Thorium too, may be further introduced during albitization, but its concentrations are enhanced during greisen- ization, and it is not lost by low temperature surface processes.
Table 7. Relative abundances of the accessory and opaque phases from Ririwai
Biotite granite Albitite Greisen
Zircon Uranothorite Xenotime Pyrochlore Monazite ‘Bastnaesite’ Coffinite Haematite Ilmenite Magnetite Columbi te TiOz minerals Cassiterite Wolframite Sulphides
major minor not seen rare minor rare rare rare minor not seen minor minor minor rare minor
major minor rare not seen rare rare not seen major minor minor major rare minor rare minor
major minor rare not seen minor rare rare rare minor not seen minor minor major minor major
426 R.A. IXER, J .R. ASHWORTH. AND C.M.POINTER
For all rock types. Fine-grained minerals along mica cleavages Biotite + Fe,Nb - Ti ?Ti
(Ta,Ti) (Fe, N b,Ta, Sn,W
Coarse-grained phases Biotite granite zircon association, Zr + Zr --+ Th + late U and coffinite (Th) (Hf) (U,Zr) (Th,Zr)
columbite, ilmenite (Ta) Nb,Ta,Pb,U, pyrochlore association FeTi + (Fe,Ti)
Al bitites
zircon association Zr ---f Zr + Th -+ (Th)
(Th)
FeNb
(Nb)
Y
(U) (Hf) (U) LREE
FeMnTi haematite, ilmenite Fe- (Nb) columbite association FeMnNb
(Ta )
Greisen lode zircon association, Zr + Th + Y - late U and coffinite (Th,Hf) (U ,Zr,Y) (Th) (Th, Y)
columbite, cassiterite FeNb 3 su I p hide associations ( M n ,Ta )
Sn + Zn,Mo,Pb,Fe, etc (Fe,Nb) sulphides
Figure 5 . Scquence ol precipitation of elements lrom ore fluids 21s deduced lrom paragenetic studies. Minor elements are in parenthches.
Acknowledgements. Peter Bowden and Judith Kinnaird are thanked for their encouragement and for supplying all the material. The staff at the SURRC are thanked for their help, in particular Dr Gus MacKenzie for the preparation of fission-track plates. Drs N. Jackson and P. Webb are thanked for many useful suggestions. C.M.P. acknowledges a NERC research studentship. Chris Gee typed the manuscript, and Sue Knox prepared the text-figures.
References
Beer, K.E. 1952. The petrology of some of the riebeckite-granitcs o f Nigeria. Repon Grological S u r w y U K iliomic' Energy Dirision, 116, HMSO London. 38 pp.
Bowden, P., Bennett, J.N., Kinnaird, J.A., Whitley, J.E., Abaa, S.I., and Hadzigeoraiou-Stavrakis, P.K. 1981. Uranium in thc Niger-Nigeria Younger granite province. Mirzernlogicul Mugazinc.. 44, 370-3x9.
- and Kinnaird. J.A. 1978. Younger granites of Nigeria - 21 zinc-rich tin province. Trrttls. Inrr. Min. Meiul. Sec!. B, B66Bh9. '
Drysdall, A.R., Jackson, N.J., Douch, C.J., Rarnsay, C.R., and Hackett, D. 1984. Rare metal mineralization relatcd to Prccambrian alkali granitcs in thc northwest Arabian Shield. Econornic G ~ o l o m . 79. 13661377.
Greaves,-'G., Stevenson, B.G., and Taylor, R.G. 1971. Magnetic ca5siterite from Herberton, North
ACCESSORY MINERALOGY OF T H E RIRIWAI GRANITE 427
Queensland. Australia. Economic Geology, 66, 4X(k-387. Jackson N.J. 1986. Mineralization associated with fclsic plutonic rocks in the Arabian Shield. Journccl
of’ Africun Errrih Science, 4, 2 13-227. Jacobson, R.R.E. and MacLeod, W.N. 1977. Geology of the Liruei. Banke. and adjacent younger
granite ring-complexes. B~l le t in of’ ihe Nigeriun Gcologicul Survey, 33, pp. 117 -, Snelling, N.J., and Truswell, J.F. 1963. Age determination5 in the geology of Nigeria with special
reference to the Older and Younger granitcs. OverseuJ Geol. Mirrcrrilogictrl Resmrch, 9, 168-1 82. Kinnaird, J.A. 1984. Contrasting styles 0 1 Sn-Nb-Ta-Zn mineralization in Nigeria. Jortrrrul o/Africu/ i
Eurih Science. 2, 81-90, - 1985. Hydrothermal alteration and mineralization of the alkaline anorogenic ring complexes of
Nigeria. Joitrnul of’ Africun Erirth Science. 3, 229-25 I . - Bowden, P., Ixer, R.A., and Odling, N.W.A. 1985. Mineralogy. grochemistry. and mineralization
01 the Ririwai complex. northern Nigeria. Joirrnul of’ Afiic~cn Eurrh %irnc?.3, 185-222. MacKenaie, A.B., Bowden, P., and Kinnaird, J.A. 1984. Combined neutron activation and particle
track itnalysis of element distribution i n a rock slice of mineralized granire. Jotirriol of Radiounulylical c r n r l N u c l ~ a r Cheniis/rj Articles, 82, 331-352.
Perez, J.B. 1985. Nouvelles donnc;.s sur lr coniplcw grurii/ryrrr, cinorogenique de Tughouji (R6puhlique rl i i NigPr) influence des fluides uii cows dcz lu cri\/u/li>uiion. Unpublished PhD thesis, University o f Nancy. 317 pp.
top related