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The distribution of gold in volcanic rocks of eastern Iceland
Chemical Geology, 48 (1985) 17--28
17
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE DISTRIBUTION OF GOLD IN VOLCANIC ROCKS OF EASTERN ICELAND MARCOS
Z E N T I L L I I, R O B E R T R. B R O O K S ', J O H A N N H E L G A S O N ' , D O U G L A S E. R Y A N ~ and H A N F E I Z H A N G ~
i Department of Geology, Dalhousie University, Halifax, N.S. B3H 3J5 (Canada) 2Department of Chemistry, Biochemistry and Biophysics, Massey University, Palmerston North (New Zealand) 3Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, N.S. B3H 3,]5 (Canada) (Received September 20, 1983, revised and accepted May 18, 1984)
Abstract Zentilli, M., Brooks, R.R., Helgason, J., Ryan, D.E. and Zhang, H., 1985. The distribution of gold in volcanic rocks of eastern Iceland. Chem. Geol., 48: 17--28. Gold contents are reported for basaltic lavas and dykes from the Tertiary succession of eastern Iceland. In unsaturated (olivine-normative) tholeiites Au decreases in concentration with degree of differentiation. However, in saturated (quartz-normative) and more differentiated varieties, Au shows a high degree of positive correlation with incompatible elements such as Y and Zr. This behaviour suggests that in the latter rocks Au is not held in early crystallized phases but is enriched in the residual melt so that it follows the trend of the incompatible elements. Thus basaltic andesites and silica-rich icelandites contain twice as much Au (x = 4.1 ng/g-1) as basalts. This general trend appears to follow an erratic but concomitant enrichment of S in the rocks. Oxidation and zeolite alteration have led to mobilization and to enrichment of Au in volcaniclastic units and in the porous top of a lava flow. It is suggested that in older volcanic piles of Icelandic type the most suitable source rocks for Au mineralization could be in the more evolved quartz-normative tholeiites (ferrobasalts), and basaltic andesites and icelandites, particularly dykes and volcaniclastic units. Loosely-held Au is readily mobilized from volcanic glass and sulfides due to weathering, burial metamorphism and hydrothermal alteration. Therefore only the freshest, glassy rocks should be expected to retain their primary Au concentrations.
1. Introduction Until the recent development of rapid and highly sensitive analytical methods for gold (Sighinolfi and Santos, 1976; Fryer and Kerrich, 1978; Brooks et al., 1981a,b, 1982), few reliable quantitative data existed on the geochemical behaviour and distribution of Au in volcanic rocks (Crocket, 1974). Boyle (1961) has shown that Au in hydrothermal deposits may be derived by alteration of mafic rocks, and other studies of mafic rocks as potential sources of Au mineralization (Gottfried et al.,
0009-2541/85/$03.30
1972; Tilling et al., 1973; Keays and Scott, 1976) indicate the necessity of determining the normal background Au contents for relatively fresh volcanic rocks. The provision of this need has proved to be elusive because of distinct differences in the Au content of volcanic rocks from different tectonic environments and even within a given tectonic environment, such as different segments of the Mid-Ocean Ridge system (Keays and Scott, 1976). Some of these differences may originate from magmatic processes related to tectonic environment or from subsequent inter-
© 1985 Elsevier Science Publishers B.V.
18 action with seawater and hydrothermal fluids. It appears to be a consensus that mafic volcanic rocks have more Au than felsic volcanic rocks (e.g., Boyle, 1979). Also, in most rock suites it appears that Au is not concentrated in the residual silicate melt during magmatic differentiation (Gottfried et al., 1972; Tilling et al., 1973}. This interpretation is in accord with data on rocks clearly related by magmatic differentiation, e.g. those of some diabase-granophyre sheets or those of layered ultramafic complexes such as the Skaergaard intrusion (Vincent and Crocket, 1960; Gottfried et al., 1972). The exceptions are found in reports by Volarovich and Shilin (1971) and Korobeynikov (1981) who indicate a gradual increase in Au content from gabbros to aplites, and in recent studies of ophiolite complexes (Sighinolfi and Gorgoni, 1977; Dupuy et al., 1981), indicating that in non-cumulate ultramafic rocks, Au was enriched in the melt relative to the residue. Keays and Scott (1976) have suggested that in mid-ocean ridge basalts Au shows two affinities: (1) for sulfide phases; and (2) for early separated oxide and silicate phases such as Cr-spinel and olivine. Some of the Au in igneous rocks is probably irregularly dispersed in the native state or in sulphides, whereas the remainder of the Au, which is rather uniformly distributed, probably occurs in rock-forming minerals {Borisenko et al., 1972). Although Au abundances for rocks from the world ridge system have been published (Engel et al., 1965; Anoshin and Yemelyanov, 1969; Laul et al., 1972; Crocket, 1974; Crocket and Teruta, 1977), we are not aware of similar data for Icelandic rocks. The purpose of this paper is to report on 54 Au determinations in samples from subaerial lavas, dykes and volcaniclastics from eastern Iceland. Planned as a pilot project, the samples represent a wide range of chemical composition (basalt to icelandite), of stratigraphic position (over 2.9 km) and state of alteration (zeolite-free to epidote alteration). Unlike most other regions where basalts are studied in this context, hydrothermal altera-
tion of volcanic rocks in this part of eastern Iceland is believed not to have involved seawater (Hattori and Muehlenbachs, 1982). 2. Geological background The samples were obtained from an area of westerly
19
14o 051W
~_~v..=~. _./ AREA
REGIONALSTRIKEEWDIP OF~AVAS
SITE
65°OI'N
///"/
B~REIDDALUR
HOLMATINi~I~J65°041N ~ 1
REYDARFJORDUR
0 I 2 3 4 5kJIomefres
65o01'N
14o'05'W
14o25'W
Fig. 1. Location map of the analyzed samples in the Tertiary volcanic province of eastern Iceland.
atomizer coupled to a Varian ® AA4 75 atomic absorption spectrophotometer with automatic background correction. Instrumental conditions are given in Table I. It is possible that this procedure does not extract all of the Au, particularly t h a t bound up in the silicate lattice, but as most Au will exist in the u n b o u n d form, this is not likely to be a serious problem. No accuracy and precision studies have been done with this m e t h o d for rocks with Au concentrations in the 1--10-ng-g -~ range. However, we feel that the data are internally consistent and potentially important, and warrant publication at this stage. Instrumental conditions (Table I) were the same used by Brooks et al. (1981b, 1982). A relative error of <5% and a precision of + 3.8% determined by analysis of standard rocks at the 18-#g-g-~ level were reported by Brooks et al. (1981b). For the present study, a precision better than +30% is estimated for samples in the 0.5--1.9ng-g-~ level (detection limit for Au is 0.3 ng g-~), on the basis of comparison of adjacent samples with nearly-identical chemical compositions. The relative error is probably around this level as well. These estimates for the precision and accuracy include the sampling error which for Au is probably the major proportion of the total erorr. Sulfur was determined turbidimetrically by the m e t h o d
TABLE I Standard operating conditions for the determination of gold using flameless atomic absorption by means of a heated graphite atomizer Argon flow rate
Drying step Charring step Atomization step Analysis line Lamp current Sample volume Recorder scale factor (Y) Recorder scale factor (X)
50 cm 3 rain. -~ with 7-s stop during atomization step 30 s at 120°C 30 s at 1000°C 7 s at 2300°C 242.8 nm 10 mA 20 #1 5 mV in. -1 100 s in.-'
o f Shapiro (1973); accuracy should be close to + 5%. Other elements have been analyzed by a combination of analytical techniques (XRF, AA, DNA), or quoted from Gibson et al. (1982) and Flower et al. {1982), where a large number of analyses of rocks from groups A and B are tabulated and discussed. It is k n o w n that a higher ratio of Fe203/ FeO of oxidized lavas can strongly affect the results o f normative calculations. C.I.P.W. norms have been computed adjusting the ratio so t h a t Fe203/(Fe203 + FeO) = 0.15, which should approach the original value. As a mea-
20 sure of differentiation, we have chosen the FM index, defined by Wood (1978) as the ratio [(FeO* + MnO)/(FeO* + MnO + MgO)] × 100, where all oxides are in wt.% and FeO* refers to total Fe expressed as FeO.
>(y
t '.~
71
6~ A •
|:M M M
J
4. Results and discussion
(
QT
I
=3 i
•
I M
21!
•
I.
•
•
•
M~O
•
l i •e
•
r: +241
J
113
•
EI]M
•
rl:
~"
•
jM
51 (X~excluded} OIO-O05(PS)
p=
Lm/ i . . . .
•
_.
+ . . . .
i
.
.
.
.
T . . . .
70
60
, . . . .
•
80
. . . .
+ . . . . .
90
+C'O
FM INDEX
4.1. Group A (I.R.D.P. lavas) These are samples of seven olivine-normative tholeiites, seven quartz-normative tholeiites, three basaltic andesites and one icelandite (squares in Figs. 2 and 3). They span 2500 m of stratigraphic succession from the exposed cliff section south of the I.R.D.P. drillsite (Fig. 1) to a depth of 1300 m in the drillhole. The topmost lavas are fresh and unzeolitized, hydrothermal alteration increases with depth, and below a depth of 1200 m, alteration is marked by mineral assemblages of epidote -+ quartz + prehnite + chlorite + albite (Mehegan et al., 1982). Andradite--epidote assemblages are found in some samples, suggesting maximum basalt alteration temperatures of 350--300°C (Exley, 1982). The mean Au content for all 18 lavas is 1.68 -+ 1.42 ng g-1 (ppb) but the distribution is biased by two anomalously high values of 5 ng g-1 (units 136.1 and 178.2). If these are excluded, the mean is 1.27 + 0.81 ng g-~ which is still twice as high as the average values given by Tilling et al. (1973) for oceanic tholeiites, or for the interiors of pillow lavas from the Mid Atlantic Ridge at latitude 25 ° to 29°N (0.52--0.24 ng g-~; Keays and Scott, 1976), and is intermediate between values for oceanic tholeiite and Hawaiian tholeiite (Tilling et al., 1973). Statistical data for Au correlations with other constituents are shown in Table III. Classification of the probability function, r,
D • OT
l
' IC X c CLASTI,: !JNITq
+I
Concentrations of Au and other data are shown in Table II. Rock classification is based on field observations and the C.I.P.W. norm. Some of the results are displayed graphically in Figs. 2 and 3.
B •
Fig. 2. Gold vs. differentiation F M index (as defined in the text). Gold analyses and rock types as in Table II. M indicates samples from a " m i d d l e " stratigraphic group at depths between 640 and 1300 m in the I.R.D.P. core (Gibson et al., 1982). F M index for rocks from groups A and B calculated from analyses in Flower et al. (1982). P . S . = possibly significant (Brooks, 1972).
7!
AI2 f~
I
LEGE+ ]
+t
I
5~
+ • • .o+ I
Ii M
I M
E
&+
+
(
QT
/
I
C •
e
~
•
+! M
|
M
,e ~
t" •
•
20
~
r=+347
_ C
Im
TI
~
40
%
~
•
n= 51 p= 0 0 8 + 0 0 1 ( S )
+i~
d f~ , 60
F
-T .... 80
~
, I00
Ymg/g (ppm) Fig. 3. Gold vs. yttrium. Analyses for Y for groups A
and B from Gibson et al. (1982). Legend as in Fig. 2. S = significant (Brooks, 1972).
into divisions of significance follows that of Brooks (1972). The most striking result is the unexpected positive correlation (in particular when two anomalously high values are omit-
21 TABLE II
TABLE II (continued)
Concentration of gold (ng g-I = ppb) in Icelandic volcanic rocks
Unit No. Type
Unit No. Type
Au
972 958 828 766 716 52.1 78.1 86.2 114.1 127.2 136.1 178.2 183.1 214.2 220. 7 270.1 285.4 300.1
QT IC QT OT BA OT OT QT QT QT QT QT BA OT BA OT OT QT
1.1 3.4 0.6 0.8 2.5 0.5 1.0 0.5 1.2 1.9 5.0 5.0 1.8 0.9 0.8 0.5 1.6 1.2
(B) I.R.D.P. dykes: 988 944 761 744
1.1 33.2 103.2 213.3 290.2
QT QT BA QT QT QT QT OT (transitional) QT
(D) Holmatindur lava flows (cont.): RF36 RF35
(A) I.R.D.P. lava flows:
2.0 0.9 12.0 0.9 1.6 2.4 1.9 1.6 4.1
Au
RF9 RF8 RF7 RF6 RF5 RF4 RF2 RF1 RF0
OT OT OT OT QT QT OT OT OT OT OT
Averages Olivine-normative tholeiites Quartz-normative tholeiites Basaltic andesites and icelandite All rocks, except clastics Clastic units (A) I.R.D.P. lava flows (B) I.R.D.P. dykes (D) Holmatindur lava flows
1.9 1.1 0.7 0.5 0.4 0.4 2.7 1.1 0.8 1.9 1.4 n
Range
~
25
0.5--5.0
1.45 + 0.95
21
0.4--6.2
2.00 + 1.60
5
0.8--12.0
4.10 ± 4.52
51 3
0.4--12.0 2.2--7.6
1.93 ± 1.92 5.23 ± 2.76
18 9
0.5--5.0 0.9--12.0
1.68 ± 1.42 3.04 ± 3.49
24
0.4--6.2
1.69 ± 1.29
S.D.
OT ffi olivine-normative tholeiite; QT = quartz-normative t h o l e i i t e ; B A = basaltic andesite.
(C) I.R.D.P. clastic units :
86.1T 86.1B 86.2T
fine red sediment 5.9 fine red sediment 7.6 basaltic breccia, amygdaloidal 2.2
(D) Holmatindur lava flows: RF88 RF87 RF86 RF46 RF45 RF44 RF43 RF42 RF41 RF40 RF39 RF38 RF37
QT QT QT (transitional) QT QT OT OT OT OT OT OT OT OT
2.7 6.2 4.1 1.2 1.6 1.6 0.8 2.7 1.6 1.5 1.3 1.3 1.1
t e d ) , b e t w e e n A u and SiO2, Rb, Zr a n d Y (Fig. 3) a n d t h e negative c o r r e l a t i o n b e t w e e n A u a n d MgO, F e O a n d Ni. This c o u l d be interp r e t e d t o r e p r e s e n t an increase o f A u w i t h m a g m a t i c d i f f e r e n t i a t i o n (Fig. 2). T h e highest values (5 ng g-l) f o u n d in samples 1 3 6 . 1 a n d 1 7 8 . 2 c o r r e s p o n d t o altered plagioclase p h y r i c lavas, w h i c h in t e r m s o f t e x t u r e a n d a l t e r a t i o n are indistinguishable f r o m o t h e r s in t h e g r o u p . Perhaps significantly, b o t h samples are f r o m a " m i d d l e " stratigraphic g r o u p o f lavas (M in Fig. 2) recognized b y G i b s o n et al. ( 1 9 8 2 ) as having relatively high Zr/Y, C e / Y b a n d higher e l e m e n t a l Sr a b u n d a n c e s .
22 T A B L E III Statistical d a t a for selected gold c o r r e l a t i o n s w i t h o t h e r c o n s t i t u e n t s o f Icelandic volcanic r o c k s Reference
Population
n
Au vs.
A
I.R.D.P. lavas
16 *~ SiO: FeOto t MgO CaO Na~O Rb Zr Y Th U Cu Zn
B
I.R.D.P. d y k e s
9
C
I.R.D.P. clastic u n i t s
3
D
H o l m a t i n d u r lavas
r
P
Significanoe*
+0.73 -0.53 -0.63 0.64 +0.50 +0.75 +0.75 +0.78 +0.50 +0.55 0.40 +0.59
0.001--0.01 0.02--0.05 0.001--0.01 0.001--0.01 0.02--0.05 <0.001 <0.001 <0.001 0.02--0.05 0.02--0.05 0.1 0.01--0.02
S* S S* S* S S** S** S** S S PS S
SiO 2 MgO Rb Zr Y Th U Cu
+0.81 -0.75 +0.90 +0.89 +0.89 +0.69 +0.88 -0.33
0.001--0.01 0.02--0.05 <0.001 0.001--0.01 0.001--0.01 0.02--0.05 0.001--0.01 >0.10
S* S S** S* S* S S NS
FeO MgO Na:O K:O Mn Cu S
+0.99 -0.96 -1.0 +0.95 -0.99 +0.77 +0.77
0.02 >0.01 0.001--0.01 >0.10 0.05--0.] >0.10 >0.10
S NS S* NS PS NS NS
24
Rb Zr Y Cu elevation
--0.38 -0.38 -0.33 +0.54 +0.68
0.05--0.10 0.05--0.10 0.05--0.10 0.001--0.01 0.001--0.01
PS PS PS S* S*
24 13
Y Y
-0.37 +0.55
0.05--0.10 0.02--0.05
PS S
Y SiO 2 MgO
+0.78 +0.46 -0.54
A, B and D:
olivine-normative tholeiites quartz-normative tholeiites basaltic a n d e s i t e s a n d icelandite
4 4 4
>0.10 >0.10 >0.10
NS NS NS
*' NS = not significant; PS = possibly significant; S = significant; S* = highly significant; S** = very highly significant. , 5 T w o samples w i t h 5 ng g-' have b e e n eliminated from this group only.
23
4.2. Group B (I.R.D.P. dykes)
CORE
-503m-
This group consists of nine samples (triangles in Figs. 2 and 3) of nearly-vertical basaltic dykes (one olivine-normative, seven quartz-normative tholeiites, one basaltic andesite) that trend NNE and are approximately normal to the flows. The dykes belong to the Breiddalur Dyke Swarm (Fig. 1) and are in general conspicuously less altered than the lavas. The average Au content of all dykes is 3.04 ng g-t with the median at 1.9 n g g -t. Again, one high value (12 ng g-t, sample 761) affects the mean. This sample is of basaltic andesite composition (55.32% S i O ) a n d has high values of LILE. If this sample is omitted, the mean is 1.93 + 1.02 ng g-t, slightly higher than that of the lavas they intrude. As with group A (I.R.D.P. lavas) a very significant positive correlation is evident between Au and SiO2, Rb, Zr, Y (Fig. 3) and U; and a negative correlation of Au with MgO. Even excluding the anomalously high Au value, the correlations are "possibly significant" (Brooks, 1972).
4.3. Group C (I.R.D.P. clastic units) The Au content of samples of this set are shown stratigraphically in Fig. 4. Lava unit 86.2 is a greyish~oTeen fine-grained aphyric slightly amygdaloidal basalt listed in Table II. Upwards, the flow becomes reddish brown and has a scoriaceous brecciated top with some clastic matrix. The transition to the overlying clastic unit (86.1) is gradual, from a breccia with scoriaceous clasts to a reddishbrown massive to bedded sand-to-silt-size tuff. Chemically, the clastic units are enriched in Au, Cu and the alkalis, but depleted in MgO and CaD with respect to the underlying lava flow. There is a positive correlation between Au and total Fe, K20, S and Cu, and a negative one with MgO, Na20 and Mn (Table III). The Au content appears to have increased significantly in the more porous levels. Probably this enrichment is related less to weathering than to hydrothermal processes, as suggested
A. SiO. F + O ~ g O C o O , ~ # ~ .
~
n(l/g(pl~)%
% ,ml %
%
M.O Co % (mg/~)l}pm
S
•
.
=/o
5.9 50.1 17.0 3.2 4.8
3.7
1.0
21
67
020
7.6 48.6 18.1 2.7
1.9 2,4
]2
360
310
.34
50
.006
VVV V VVVl
:'ZZ2[---1--I .........
i
-504m-
-~:---__~____~_______~__2__2 49.1
_ .~__~__~_~ b__~__~_AI _-~--~-~- -I -505m-
_~__~__~1 _v___~__v_I
AM vAl V A V A Vl VVAVV} ~V ~ Vl v v v'vl vvv vvvvl vvv -506m- VVVVl V V. [~ VVVVV VVVV
14.8
3.5
3.7 5.1
7.2
0.1
LEGEND VOLOA.,CL.S.,C SEO,ME.+..EOO,S. 8.0+..
S.LT + . E E . , S . o.e
0.5 49.5 13.9 4.7 9.0
3.5 0.2
.54
IO
.004
Fig. 4. Distribution of gold and other elements in volcaniclastic rocks overlying a basalt flow at ~ 504-m depth in the I . R . D . P . c o r e .
by the local presence of visible pyrite crystals in the pores. At this depth in the drillhole, lava flows contain assemblages including smectite, chlorite, calcite, quartz, laumontite, epidote and locally garnet (Mehegan et al., 1982). The Au content of the volcaniclastic units is comparable to those reported by Anoshin et al. (1969) for silt and volcanic sediments in the Atlantic Ocean near Iceland (mean 4.2 ng g-t).
4.4. Group D (Holmatindur lavas) This set consists of 24 samples (circles, in Figs. 2 and 3) of tholeiitic basalts from the b o t t o m part (RF0 to RF9), median (RF35 to RF46) and near the top (RF86 to RF88) of a cliff section at Holmatindur (Fig. 1). The mean Au content of group-D lavas 1.69 n g g -1) is identical to that of group-A lavas (1.68 ng g-t) and lower than that of group-B dykes. Here too, there seems to be a rather ordered distribution of values around the mean, and some anomalously high values. In contrast with groups A and B, Holmatindur lavas do not show a positive correlation
24
between Au and Y and Zr and Rb, but a negative one instead (Table III). Also, the mean Au content increases stratigraphically from sea level to the top of the cliff ("elevation", Table III). Both primary and secondary effects may be responsible for the higher Au values. The top lavas consist predominantly of silica-saturated tholeiites and are the least affected by burial metamorphism. It is possible that these top lavas display their "prim a r y " Au content, which elsewhere has been modified by alteration. 5. Discussion The contrasting behaviour of Au between groups A or B, where Au shows a positive correlation with SiO2, Y and Zr, among others, and group D, where the trend is reversed, is surprising. One might ask what is specifically different in the rocks of group D. These rocks are generally richer in magnesia and hence have relatively lower F M index values. Also, rocks that occur ~ 400 m above sea level in the Holmatindur section (samples R F 3 5 to RF46) are intersected at ~ 1450-m depth in the I.R.D.P. core (Helgason and Zentilli, 1982), and thus the latter are bound to be more altered and metamorphosed. However, it is generally accepted (e.g., Wood, 1978; Gibson et al., 1982) that Zr and Y are among the least mobile incompatible elements, and their abundances and concentration ratios are considered to be relatively stable during low-grade hydrothermal metamorphism. Thus, the sympathetic behaviour of Au with Zr and Y appears to reflect a primary, or magmatic trend. When the chemistry of the rocks is compared, groups A and B include a much wider range of compositions (especially SiO2, MgO) with a predominance of saturated (quartz-normative) basalts, and several more silicic varieties, whereas those in group D are predominantly silica-undersaturated (olivine-normative) tholeiites and plot on the left-hand side of Fig. 2. As shown in Table II, olivine-normative tholeiites contain relatively less Au than quartz-normative
tholeiites and show an overall negative correlation with Y (Fig. 3), and this trend is "possibly significant". In contrast, quartz-normative tholeiites of groups A and B indicate a significant sympathetic relationship between Au and Y. For basaltic andesites and icelandires, which contain erratic but relatively high Au, the relationship seems to follow that of saturated rocks. Unfortunately insufficient samples of felsic rocks were analyzed during this pilot project to confirm the trend. Au shows a weak (not significant) positive correlation (+0.163) with S (Table III). The mean S content increases rather erratically from olivine-normative tholeiites (x = 0.006 + 0.008%) through quartz-normative tholeiites (x = 0.023 -+ 0.038%) to basaltic andesites and icelandites (E = 0.031 + 0.040%). Dykes are relatively richer in S (x = 0.049 + 0.049%), confirming the c o m m o n occurrence of dis.
.
.
.
.
......... q
I
LEGEND
120 i
Au ng/g (ppb) ] •
>;8
• 19-38 h)O I
[
i
! i
-I
1
080 1 I
i 060-t
040- i
020 i !i J ooo'"
r= +270 n= 5I p= O.05(PS)
• • •
•nil
.
L'"'~;: .2"
60
70
8O
90
!O0
FM INDEX
Fig. 5. S u l f u r vs. d i f f e r e n t i a t i o n F M i n d e x , w i t h indic a t i o n o f gold levels. N o t i c e t h e s u b s t a n t i a l enrichm e n t o f sulfur in r o c k s w i t h F M i n d e x b e t w e e n 74 a n d 81 ( f e r r o b a s a l t s a n d basaltic andesites; Wood, 1 9 7 8 ) . P.S. = possibly significant ( B r o o k s , 1972).
25 persed sulfides in m a n y of the d y k e margins. Also, S displays a (significant) positive correlation {+0.284) with total Fe-oxide (FeO*), which has also been noticed in submarine basalt glasses (Czamanske and Moore, 1977). In fact, the highest S values are found in rocks with an FM index between 74 and 81 (Fig. 5), generally associated with evolved quartz-normative tholeiites (ferrobasalts; Wood, 1978) and basaltic andesites. A few of the high Au values occur in rocks rich in S, but not all, reflecting the erratic nature of the distribution o f both elements in the rocks. Experimental work by Mironov et al. (1980) suggests that during differentiation Au should follow the path of S. The trend of Au depletion during differentiation noted in the olivine-normative tholeiites could be interpreted to reflect the removal of Au in early immiscible sulfide phases. However, it is possible that some of the Au was separated from the basaltic magma into early crystallized chrome-spinels or olivine. As crystallization proceeds, the tholeiitic magma becomes enriched in Fe and S (Haughton et al., 1974; Czamanske and Moore, 1977). If ilmenite or magnetite start crystallizing, however, the magma m a y release some of its S as immiscible sulfide liquid (Haughton et al., 1974). If the magma is in motion at this stage, it is possible that this immiscible sulfide (and whatever Au has been partitioned into it) m a y not sink and separate, but be carried with the magma as droplets (Haughton et al., 1974), thus leading to erratic distribution of sulfides and of discrete Au-bearing particles. Changes in the oxygen fugacity in the magma, such as those induced by assimilation of water, diffusion of hydrogen out o f the magma, foundering of cooled oxidized roof rocks from the magma chamber, and even direct assimilation of oxygen during eruption, could account for the sudden appearance of sulfides (Haughton et al., 1974). Although the strong influence of S on the behaviour of Au appears probable, the high degree of correlation between Au and some of
the incompatible elements, such as Y, Zr and Rb in the more evolved rocks, indicates that Au is enriched in the residual liquids and m a y be finally trapped in late accessory minerals and the glassy mesostasis, besides sulfides. During cooling and quenching of the magmas, sulfides may continue to be evolved; Au may be partitioned into these sulfide droplets or unstable compounds (Mironov et al., 1980). It is known that certain sulfides at high temperatures will accept substantial amounts of Au in solution, but this Au is exsolved on cooling (Barton, 1969). Therefore, Au may be present in the native form, even if it was originally separated from the melt in sulfides. During cooling, oxidation and alteration of the lavas, the path of S and Au may be quite different. Some of the sulfur may have oxidized to sulfate, as evidenced by the presence of anhydrite in the volcanic pile (Exley, 1982) or may have left the lavas in the volatile phase. Sigurdsson (1982), for example, suggests that only ~15% of the sulphur was retained in volcanic deposits of the Laki basaltic eruption in Iceland, the remainder having been released to the atmosphere. Perhaps S is preferentially retained in intrusive dykes, by a pressure-related process similar to the inhibition of degassing of deep-sea basalts by hydrostatic pressure (Moore and Fabbi, 1971). In this context it appears relevant to compare our results with those o f Keays and Scott (1976) who found the Au content of unaltered glass rims of tholeiitic pillow basalts to be five to seven times higher than the crystalline interiors of the pillows, from which the Au would have been leached during alteration. S follows a similar trend. They interpreted the "excess" Au in the glass rims, above the baseline levels of the crystalline interiors, to represent the a m o u n t of Au that would have precipitated on loosely bound sites. Such sites would have included products of deuteric alteration, grain boundaries, mesostasis phases, and sulfides (Keays and Scott, 1976). Some of the high Au values found in dykes and fresh rocks of this study (> 3 ng g-l)
26 could represent such loosely bound Au in glass and mesostasis that escaped alteration or mobilization. There is little d o u b t that Au has been mobilized as indicated by the relative Au enrichment of a porous oxidized amygdaloidal flow t o p and inter-lava volcaniclastic sediment. The precipitation of soluble Au species carried in percolating fluids, in the presence of sulfides or Fe-oxides and -hydroxides, are well
6. Conclusions
(1) The average Au content of Tertiary lavas and dykes from eastern Iceland is higher by a factor of 2 to 3 than the average values given for oceanic ridge and rise tholeiites, b u t similar to that of Hawaiian tholeiites and continental basaltic provinces (Tilling et al., 1973). However, the Au values found are comparable to those of unaltered glass rims of
pillow basalts from the Mid-Atlantic Ridge at 26 ° and 30°N (Keays and Scott, 1976). (2) The average Au content in the studied rocks increases from lowest in olivine-normative tholeiites, to highest in Fe-rich quartznormative tholeiites (ferrobasalts), and basaltic andesite and icelandite. This trend appears to follow an erratic b u t concomitant enrichment of S in the rocks, suggesting a related evolution of these elements in the magmas. (3) In quartz-normative tholeiites, basaltic andesites and icelandites, Au shows surprising positive correlations with Y, Zr and other indices of magmatic differentiation, suggesting that Au has been systematically partitioned into the melt. In contrast, olivine-normative tholeiites display a negative correlation with Y and Zr, suggesting that Au has been retained in early separated phases. (4) This study confirms the tendency of Au to occur in basalts both in relatively low and systematic background levels, probably disseminated in primary phases, and in anomalously high erratically-distributed concentrations, probably loosely-held in primary deuteric or secondary phases as has been suggested for other suites (Fritze and Robertson, 1969; Keays and Scott, 1976). In order to be representative, the size of a sample to be analyzed for Au should be large enough to take this inhomogeneity into account. (5) Au has been enriched in the porous, oxidized and zeolitized amygdaloidal t o p of a basalt flow and in overlying volcaniclastic units. This relative enrichment, shared by Cu, suggests that hydrothermal fluids had the ability to transport and redistribute metals during hydrothermal alteration and metamorphism of the volcanic pile, despite the apparent lack of involvement of seawater in the system. (6) In the absence of high-magnesia Cu--Nisulfide-enriched basaltic rocks (Keays and Scott, 1976), i.e. in Icelandic-type volcanic piles, the most suitable source rocks for Au mineralization produced by leaching during alteration or metamorphism would be the more evolved, quartz-normative tholeiites (ferroba-
27 salts), a n d basaltic andesites a n d icelandites, i n c l u d i n g d y k e s a n d volcaniclastic units. (7) Because glass is m e t a s t a b l e a n d r e a d i l y devitrified, a n d A u held in sulfides is easily m o b i l i z e d d u r i n g w e a t h e r i n g , burial a n d hydrothermal alteration, only the freshest of lavas, p y r o c l a s t i c s a n d glassy d y k e m a r g i n s s h o u l d be e x p e c t e d t o r e t a i n t h e i r p r i m a r y A u concentrations. Acknowledgements T h e a u t h o r s a c k n o w l e d g e s u p p o r t o f this w o r k b y grants A - 9 0 3 6 t o M.Z. a n d A - 0 3 0 9 t o D . E . R . f r o m t h e N a t u r a l Sciences a n d Engin e e r i n g R e s e a r c h Council ( N S E R C ) o f Canada. We t h a n k t h e f o l l o w i n g c o n t r i b u t i o n s : Shirish Parikh p e r f o r m e d s o m e o f t h e c h e m i c a l analyses o t h e r t h a n gold, D o u g Meggison d r a f t e d t h e figures, a n d N o r m a M c K e i g a n p a t i e n t l y t y p e d several versions o f t h e m a n u s c r i p t . T h e m a n u s c r i p t has b e n e f i t t e d f r o m critical reading b y P r o f e s s o r s J.A. Walker a n d W.S. F y f e , a n d b y an u n i d e n t i f i e d reviewer. T h e a u t h o r s , h o w e v e r , t a k e full r e s p o n s i b i l i t y f o r t h e interpretations and the data.
References Anoshin, G.N. and Yemelyanov, D., 1969. Gold in igneous rocks of the Atlantic Ocean as determined by neutron-activation analysis. Dokl. Akad. Nauk U.S.S.R., 189: 1107--1110. Anoshin, G.N., Emel'yanov, E.M. and Perezhogin, G.A., 1969. Gold in the modern sediments of the northern part of the Atlantic Ocean Basin. Geochem. Int., 6: 897--905. Barton, P.B., 1969. Solubility of gold in sulfide minerals. In: Experimental Geochemistry, Geological Survey Research, 1969. U.S. Geol. Surv., Prof. Pap. G50A, A108. Boriskenko, L.F., Miller, A.D. and Fisher, E.I., 1972. The abundance of gold in ultrabasic rocks. Geochem. Int., 9: 121--128. Boyle, R.W., 1961. The geology, geochemistry, and origin of the gold deposits of the Yellowknife district. Geol. Surv. Can., Mere., 310: 1--193. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geol. Surv. Can. Bull., 280: 1--584.
Brooks, R.B., 1972. Geobotany and Biogeochemistry in Mineral Exploration. Harper and Row, New York, N.Y., 290 pp. Brooks, R.B., Chatterjee, A.K. and Ryan, D.E., 1981a. Determination of gold in natural waters at the parts-per-trillion (pg cm -3) level. Chem. Geol., _ 33: 163--169. Brooks, R.B., Holzbecher, J., Ryan, D.E., Zhang, H.F. and Chatterjee, A.K., 1981b. A rapid method for the determination of gold and silver in sulfide ores and rocks. At. Spectrosc., 2: 151--154. Brooks, R.B., Chatterjee, A.K., Smith, P.K., Ryan, D.E. and Zhang, H.F., 1982. The distribution of gold in rocks and minerals of the Meguma Group of Nova Scotia, Canada. Chem. Geol., 35: 87--95. Crocket, J.H., 1974. Gold. In: K.H. Wedepohl (Editor), Handbook of Geochemistry, Vol. 11. Springer, Berlin, Ch. 79. Crocket, J.H. and Teruta, Y., 1977. Palladium, iridium and gold contents of mafic and ultramafic rocks drilled from the Mid-Atlantic Ridge, Leg 37, Deep Sea Drilling Project. Can. J. Earth Sci., 14: 777--784. Czamanske, G.K. and Moore, J.G., 1977. Composition and phase chemistry of sulfide globules in basalt from the Mid-Atlantic Ridge rift valley near 37°N lat. Geol. Soc. Am. Bull., 88: 587--599. Dagley, P., Wilson, R.L., Ade-Hall, J.M., Walker, G.P.L., Haggerty, E.S., Sigurgeirsson, T., Watkins, N.D., Smith, P.J., Edwards, J. and Grasty, R.L., 1967. Geomagnetic polarity zones for Icelandic lavas. Nature (London), 216: 25--29. Dupuy, C., Dostal, J. and LeBlanc, M., 1981. Distribution of copper and gold in ophiolites from N e w Caledonia. Can. Mineral., 19: 225--232. Engel, A.E.J., Engel, C.G. and Haven, R., 1965. Chemical characteristics of oceanic basalts and the upper mantle. Geol. Soc. Am. Bull., 76: 719--734. Exley, R.A., 1982. Electron microprobe studies of Iceland Research Drilling Project high-temperature hydrothermal mineral geochemistry. J. Geophys. Res., 87: 6547--6558. Flower, M.F.J., Pritchard, R.G., Brem, G., Cann, J.R., Delaney, J., Emmerman, R., Gibson, I.L., Oakley, P.J., Robinson, P.T. and Schmincke, H.-V., 1982. Chemical stratigraphy, Iceland Research Drilling Project, Reydarfiordur, eastern Iceland. J. Geophys. Res., 87: 6489--6510. Friedleifsson, I.B., Gibson, I.L., Hall, J.M., Johnson, H.P., Christensen, N.I., Schmincke, H.-U. and Schoenharting, G., 1982. The Iceland Research Drilling Project. J. Geophys. Res., 87: 6359-6362. Fritze, K. and Robertson, R., 1969. Precision in the neutron activation analysis for gold in standard rocks G-1 and W-1. In: J.R. DeVoe (Editor), Modern Trends in Activation Analysis. Natl. Bur. Stand. U.S., Spec. Publ., 312: 1279--1283.
28 Fryer, B.J. and Kerrich, R., 1978. Analysis of precious metals at ppb levels in rocks b y a combined wet chemical and flameless atomic absorption method. At. Absorpt. Newsl., 17: 4---6. Gibson, I.L., 1966. The crustal structure of eastern Iceland. Geophys. J.R. Astron. Soc., 12: 99--102. Gibson, I.L., Kirkpatrick, R.J., Emmerman, R., Schmincke, H.-V., Pritchard, G., Oakley, P.J., Thorpe, R. and Marrincr, G.F., 1982. The trace element composition of the lavas and dikes from a 3 km vertical section through the lava pile of eastern Iceland. J. Geophys. Res., 87: 6532--6546. Gottfried, P., Rowe, J.J. and Tilling, R.I., 1972. Distribution of gold in igneous rocks. U.S. Geol. Surv., Prof. Pap. 727, 42 pp. Hattori, K. and Muehlenbachs, K., 1982. Oxygen isotope ratios of the Icelandic crust. J. Geophys. Res., 87: 6559--6565. Haughton, D.R., Roeder, P.L. and Skinner, B.J., 1974. Solubility of sulfur in mafic magmas. Econ. Geol., 69: 451--462. Helgason, J., 1982. Magnetostratigraphy of the exposed lava section east of the IRDP drill hole in Reydarfjordur, Eastern Iceland. J. Geophys. Res., 87: 6405--6418. Helgason, J., 1983. Structural relationships and magnetostratigraphy of the volcanic succession and the Breiddalur dyke swarm in Reydarfjordur, eastern Iceland. Ph.D. Thesis, Dalhousie University, Halifax, N.S., 260 pp. Helgason, J. and Zentilli, M., 1982. Stratigraphy and correlation of the region surrounding the IRDP drill hole 1978, Reydarfjordur, Eastern Iceland. J. Geophys. Res., 87: 6396--6404. I.R.D.P. (Icelandic Research Drilling Project), 1979. Crust of oceanic affinity in Iceland (I.L. Gibson (Editor)). Nature (London), 281 : 347--351. Keays, R.R. and Scott, R.B., 1976. Precious metals in ocean-ridge basalts: Implications for basalts as source rocks for gold mineralization. Econ. Geol., 71: 705--720. Korobeynikov, A.F., 1981. Gold fractionation in crystallizing magma. Dokl. Akad. Nauk S.S.S.R., Earth Sci. Sect., 258: 193--196. Laul, J.C., Keays, R.R., Ganapathy, R., Anders, E. and Morgan, J.W., 1972. Chemical fractionations in meteorites. Geochim. Cosmochim. Acta, 36: 329--345. Mehegan, J.M., Robinson, P.T. and Delaney, J.R.,
1982. Secondary mineralization and hydrothermal alteration in the Reydarfjordur drill core, eastern Iceland. J. Geophys. Res., 87: 6511--6524. Mironov, A.G., Al'muknamedov, A.I., Medvedev, A.Ya. and Krendelev, F.P., 1980. Geochemistry of gold in melts of basalt composition (from experimental data). Geochem. Int., 15: 23--35. Moore, J.G. and Fabbi, B.P., 1971. An estimate of the juvenile sulfur content of basalt. Contrib. Mineral. Petrol., 33: 118--127. Shapiro, L., 1973. Rapid determination of sulfur in rocks. J. Res. U.S. Geol. Surv., 1: 81--84. Sighinolfi, G.P. and Gorgoni, C., 1977. Gold distribution in the Ivrea--Verbano gabbroic complex. Chem. Geol., 20: 99--107. Sighinolfi, G.P. and Santos, A.M., 1976. Determination of gold in geological ~ m p l e s at parts per milliard levels by flameless atomic-absorption spectroscopy. Mikrochim. Acta, 2: 33--40. Sigurdsson, H., 1982. Volcanic pollution and climate: the 1783 Laki eruption. Trans. Am. Geophys. Union (Eos), 63: 601--602. Tilling, R.I., Gottfried, D. and Rowe, J.J., 1973. Gold abundance in igneous rocks: bearing on gold mineralization. Econ. Geol., 68: 168--186. Vincent, E.A. and Crocket, J.H., 1960. Studies in the geochemistry of gold, I. The distribution of gold in rocks and minerals of the Skaergaard intrusion, east Greenland. Geochim. Cosmochim. Acta, 18: 130--142. Volarovich, C.P. and Shilin, N.G., 1971. Gold behaviour during crystallization of granitoid magma on the example of the Miocene gabbro-granodiorite formation in central Kamchatka. Int. Geochem. Congr., Moscow, Abstr. Rep., 2: 664--665. Walker, G.P.L., 1959. Geology of the Reydarfjordur area, eastern Iceland. Q.J. Geol. Soc. London, 114: 367--393. Watkins, N.D. and Walker, G.P.L., 1977. Magnetostratigraphy of eastern Iceland. Am. J. Sci., 277: 513--584. Wood, D.A., 1978. Major and trace element variations in the Tertiary lavas of eastern Iceland and their significance with respect to the Iceland geochemical anomaly. J. Petrol., 19: 394--436. Wood, D.A., Gibson, I.L. and Thompson, R.N., 1976. Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland. Contrib. Mineral. Petrol., 55: 241--254.
17
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
THE DISTRIBUTION OF GOLD IN VOLCANIC ROCKS OF EASTERN ICELAND MARCOS
Z E N T I L L I I, R O B E R T R. B R O O K S ', J O H A N N H E L G A S O N ' , D O U G L A S E. R Y A N ~ and H A N F E I Z H A N G ~
i Department of Geology, Dalhousie University, Halifax, N.S. B3H 3J5 (Canada) 2Department of Chemistry, Biochemistry and Biophysics, Massey University, Palmerston North (New Zealand) 3Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, N.S. B3H 3,]5 (Canada) (Received September 20, 1983, revised and accepted May 18, 1984)
Abstract Zentilli, M., Brooks, R.R., Helgason, J., Ryan, D.E. and Zhang, H., 1985. The distribution of gold in volcanic rocks of eastern Iceland. Chem. Geol., 48: 17--28. Gold contents are reported for basaltic lavas and dykes from the Tertiary succession of eastern Iceland. In unsaturated (olivine-normative) tholeiites Au decreases in concentration with degree of differentiation. However, in saturated (quartz-normative) and more differentiated varieties, Au shows a high degree of positive correlation with incompatible elements such as Y and Zr. This behaviour suggests that in the latter rocks Au is not held in early crystallized phases but is enriched in the residual melt so that it follows the trend of the incompatible elements. Thus basaltic andesites and silica-rich icelandites contain twice as much Au (x = 4.1 ng/g-1) as basalts. This general trend appears to follow an erratic but concomitant enrichment of S in the rocks. Oxidation and zeolite alteration have led to mobilization and to enrichment of Au in volcaniclastic units and in the porous top of a lava flow. It is suggested that in older volcanic piles of Icelandic type the most suitable source rocks for Au mineralization could be in the more evolved quartz-normative tholeiites (ferrobasalts), and basaltic andesites and icelandites, particularly dykes and volcaniclastic units. Loosely-held Au is readily mobilized from volcanic glass and sulfides due to weathering, burial metamorphism and hydrothermal alteration. Therefore only the freshest, glassy rocks should be expected to retain their primary Au concentrations.
1. Introduction Until the recent development of rapid and highly sensitive analytical methods for gold (Sighinolfi and Santos, 1976; Fryer and Kerrich, 1978; Brooks et al., 1981a,b, 1982), few reliable quantitative data existed on the geochemical behaviour and distribution of Au in volcanic rocks (Crocket, 1974). Boyle (1961) has shown that Au in hydrothermal deposits may be derived by alteration of mafic rocks, and other studies of mafic rocks as potential sources of Au mineralization (Gottfried et al.,
0009-2541/85/$03.30
1972; Tilling et al., 1973; Keays and Scott, 1976) indicate the necessity of determining the normal background Au contents for relatively fresh volcanic rocks. The provision of this need has proved to be elusive because of distinct differences in the Au content of volcanic rocks from different tectonic environments and even within a given tectonic environment, such as different segments of the Mid-Ocean Ridge system (Keays and Scott, 1976). Some of these differences may originate from magmatic processes related to tectonic environment or from subsequent inter-
© 1985 Elsevier Science Publishers B.V.
18 action with seawater and hydrothermal fluids. It appears to be a consensus that mafic volcanic rocks have more Au than felsic volcanic rocks (e.g., Boyle, 1979). Also, in most rock suites it appears that Au is not concentrated in the residual silicate melt during magmatic differentiation (Gottfried et al., 1972; Tilling et al., 1973}. This interpretation is in accord with data on rocks clearly related by magmatic differentiation, e.g. those of some diabase-granophyre sheets or those of layered ultramafic complexes such as the Skaergaard intrusion (Vincent and Crocket, 1960; Gottfried et al., 1972). The exceptions are found in reports by Volarovich and Shilin (1971) and Korobeynikov (1981) who indicate a gradual increase in Au content from gabbros to aplites, and in recent studies of ophiolite complexes (Sighinolfi and Gorgoni, 1977; Dupuy et al., 1981), indicating that in non-cumulate ultramafic rocks, Au was enriched in the melt relative to the residue. Keays and Scott (1976) have suggested that in mid-ocean ridge basalts Au shows two affinities: (1) for sulfide phases; and (2) for early separated oxide and silicate phases such as Cr-spinel and olivine. Some of the Au in igneous rocks is probably irregularly dispersed in the native state or in sulphides, whereas the remainder of the Au, which is rather uniformly distributed, probably occurs in rock-forming minerals {Borisenko et al., 1972). Although Au abundances for rocks from the world ridge system have been published (Engel et al., 1965; Anoshin and Yemelyanov, 1969; Laul et al., 1972; Crocket, 1974; Crocket and Teruta, 1977), we are not aware of similar data for Icelandic rocks. The purpose of this paper is to report on 54 Au determinations in samples from subaerial lavas, dykes and volcaniclastics from eastern Iceland. Planned as a pilot project, the samples represent a wide range of chemical composition (basalt to icelandite), of stratigraphic position (over 2.9 km) and state of alteration (zeolite-free to epidote alteration). Unlike most other regions where basalts are studied in this context, hydrothermal altera-
tion of volcanic rocks in this part of eastern Iceland is believed not to have involved seawater (Hattori and Muehlenbachs, 1982). 2. Geological background The samples were obtained from an area of westerly
19
14o 051W
~_~v..=~. _./ AREA
REGIONALSTRIKEEWDIP OF~AVAS
SITE
65°OI'N
///"/
B~REIDDALUR
HOLMATINi~I~J65°041N ~ 1
REYDARFJORDUR
0 I 2 3 4 5kJIomefres
65o01'N
14o'05'W
14o25'W
Fig. 1. Location map of the analyzed samples in the Tertiary volcanic province of eastern Iceland.
atomizer coupled to a Varian ® AA4 75 atomic absorption spectrophotometer with automatic background correction. Instrumental conditions are given in Table I. It is possible that this procedure does not extract all of the Au, particularly t h a t bound up in the silicate lattice, but as most Au will exist in the u n b o u n d form, this is not likely to be a serious problem. No accuracy and precision studies have been done with this m e t h o d for rocks with Au concentrations in the 1--10-ng-g -~ range. However, we feel that the data are internally consistent and potentially important, and warrant publication at this stage. Instrumental conditions (Table I) were the same used by Brooks et al. (1981b, 1982). A relative error of <5% and a precision of + 3.8% determined by analysis of standard rocks at the 18-#g-g-~ level were reported by Brooks et al. (1981b). For the present study, a precision better than +30% is estimated for samples in the 0.5--1.9ng-g-~ level (detection limit for Au is 0.3 ng g-~), on the basis of comparison of adjacent samples with nearly-identical chemical compositions. The relative error is probably around this level as well. These estimates for the precision and accuracy include the sampling error which for Au is probably the major proportion of the total erorr. Sulfur was determined turbidimetrically by the m e t h o d
TABLE I Standard operating conditions for the determination of gold using flameless atomic absorption by means of a heated graphite atomizer Argon flow rate
Drying step Charring step Atomization step Analysis line Lamp current Sample volume Recorder scale factor (Y) Recorder scale factor (X)
50 cm 3 rain. -~ with 7-s stop during atomization step 30 s at 120°C 30 s at 1000°C 7 s at 2300°C 242.8 nm 10 mA 20 #1 5 mV in. -1 100 s in.-'
o f Shapiro (1973); accuracy should be close to + 5%. Other elements have been analyzed by a combination of analytical techniques (XRF, AA, DNA), or quoted from Gibson et al. (1982) and Flower et al. {1982), where a large number of analyses of rocks from groups A and B are tabulated and discussed. It is k n o w n that a higher ratio of Fe203/ FeO of oxidized lavas can strongly affect the results o f normative calculations. C.I.P.W. norms have been computed adjusting the ratio so t h a t Fe203/(Fe203 + FeO) = 0.15, which should approach the original value. As a mea-
20 sure of differentiation, we have chosen the FM index, defined by Wood (1978) as the ratio [(FeO* + MnO)/(FeO* + MnO + MgO)] × 100, where all oxides are in wt.% and FeO* refers to total Fe expressed as FeO.
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4. Results and discussion
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4.1. Group A (I.R.D.P. lavas) These are samples of seven olivine-normative tholeiites, seven quartz-normative tholeiites, three basaltic andesites and one icelandite (squares in Figs. 2 and 3). They span 2500 m of stratigraphic succession from the exposed cliff section south of the I.R.D.P. drillsite (Fig. 1) to a depth of 1300 m in the drillhole. The topmost lavas are fresh and unzeolitized, hydrothermal alteration increases with depth, and below a depth of 1200 m, alteration is marked by mineral assemblages of epidote -+ quartz + prehnite + chlorite + albite (Mehegan et al., 1982). Andradite--epidote assemblages are found in some samples, suggesting maximum basalt alteration temperatures of 350--300°C (Exley, 1982). The mean Au content for all 18 lavas is 1.68 -+ 1.42 ng g-1 (ppb) but the distribution is biased by two anomalously high values of 5 ng g-1 (units 136.1 and 178.2). If these are excluded, the mean is 1.27 + 0.81 ng g-~ which is still twice as high as the average values given by Tilling et al. (1973) for oceanic tholeiites, or for the interiors of pillow lavas from the Mid Atlantic Ridge at latitude 25 ° to 29°N (0.52--0.24 ng g-~; Keays and Scott, 1976), and is intermediate between values for oceanic tholeiite and Hawaiian tholeiite (Tilling et al., 1973). Statistical data for Au correlations with other constituents are shown in Table III. Classification of the probability function, r,
D • OT
l
' IC X c CLASTI,: !JNITq
+I
Concentrations of Au and other data are shown in Table II. Rock classification is based on field observations and the C.I.P.W. norm. Some of the results are displayed graphically in Figs. 2 and 3.
B •
Fig. 2. Gold vs. differentiation F M index (as defined in the text). Gold analyses and rock types as in Table II. M indicates samples from a " m i d d l e " stratigraphic group at depths between 640 and 1300 m in the I.R.D.P. core (Gibson et al., 1982). F M index for rocks from groups A and B calculated from analyses in Flower et al. (1982). P . S . = possibly significant (Brooks, 1972).
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~
, I00
Ymg/g (ppm) Fig. 3. Gold vs. yttrium. Analyses for Y for groups A
and B from Gibson et al. (1982). Legend as in Fig. 2. S = significant (Brooks, 1972).
into divisions of significance follows that of Brooks (1972). The most striking result is the unexpected positive correlation (in particular when two anomalously high values are omit-
21 TABLE II
TABLE II (continued)
Concentration of gold (ng g-I = ppb) in Icelandic volcanic rocks
Unit No. Type
Unit No. Type
Au
972 958 828 766 716 52.1 78.1 86.2 114.1 127.2 136.1 178.2 183.1 214.2 220. 7 270.1 285.4 300.1
QT IC QT OT BA OT OT QT QT QT QT QT BA OT BA OT OT QT
1.1 3.4 0.6 0.8 2.5 0.5 1.0 0.5 1.2 1.9 5.0 5.0 1.8 0.9 0.8 0.5 1.6 1.2
(B) I.R.D.P. dykes: 988 944 761 744
1.1 33.2 103.2 213.3 290.2
QT QT BA QT QT QT QT OT (transitional) QT
(D) Holmatindur lava flows (cont.): RF36 RF35
(A) I.R.D.P. lava flows:
2.0 0.9 12.0 0.9 1.6 2.4 1.9 1.6 4.1
Au
RF9 RF8 RF7 RF6 RF5 RF4 RF2 RF1 RF0
OT OT OT OT QT QT OT OT OT OT OT
Averages Olivine-normative tholeiites Quartz-normative tholeiites Basaltic andesites and icelandite All rocks, except clastics Clastic units (A) I.R.D.P. lava flows (B) I.R.D.P. dykes (D) Holmatindur lava flows
1.9 1.1 0.7 0.5 0.4 0.4 2.7 1.1 0.8 1.9 1.4 n
Range
~
25
0.5--5.0
1.45 + 0.95
21
0.4--6.2
2.00 + 1.60
5
0.8--12.0
4.10 ± 4.52
51 3
0.4--12.0 2.2--7.6
1.93 ± 1.92 5.23 ± 2.76
18 9
0.5--5.0 0.9--12.0
1.68 ± 1.42 3.04 ± 3.49
24
0.4--6.2
1.69 ± 1.29
S.D.
OT ffi olivine-normative tholeiite; QT = quartz-normative t h o l e i i t e ; B A = basaltic andesite.
(C) I.R.D.P. clastic units :
86.1T 86.1B 86.2T
fine red sediment 5.9 fine red sediment 7.6 basaltic breccia, amygdaloidal 2.2
(D) Holmatindur lava flows: RF88 RF87 RF86 RF46 RF45 RF44 RF43 RF42 RF41 RF40 RF39 RF38 RF37
QT QT QT (transitional) QT QT OT OT OT OT OT OT OT OT
2.7 6.2 4.1 1.2 1.6 1.6 0.8 2.7 1.6 1.5 1.3 1.3 1.1
t e d ) , b e t w e e n A u and SiO2, Rb, Zr a n d Y (Fig. 3) a n d t h e negative c o r r e l a t i o n b e t w e e n A u a n d MgO, F e O a n d Ni. This c o u l d be interp r e t e d t o r e p r e s e n t an increase o f A u w i t h m a g m a t i c d i f f e r e n t i a t i o n (Fig. 2). T h e highest values (5 ng g-l) f o u n d in samples 1 3 6 . 1 a n d 1 7 8 . 2 c o r r e s p o n d t o altered plagioclase p h y r i c lavas, w h i c h in t e r m s o f t e x t u r e a n d a l t e r a t i o n are indistinguishable f r o m o t h e r s in t h e g r o u p . Perhaps significantly, b o t h samples are f r o m a " m i d d l e " stratigraphic g r o u p o f lavas (M in Fig. 2) recognized b y G i b s o n et al. ( 1 9 8 2 ) as having relatively high Zr/Y, C e / Y b a n d higher e l e m e n t a l Sr a b u n d a n c e s .
22 T A B L E III Statistical d a t a for selected gold c o r r e l a t i o n s w i t h o t h e r c o n s t i t u e n t s o f Icelandic volcanic r o c k s Reference
Population
n
Au vs.
A
I.R.D.P. lavas
16 *~ SiO: FeOto t MgO CaO Na~O Rb Zr Y Th U Cu Zn
B
I.R.D.P. d y k e s
9
C
I.R.D.P. clastic u n i t s
3
D
H o l m a t i n d u r lavas
r
P
Significanoe*
+0.73 -0.53 -0.63 0.64 +0.50 +0.75 +0.75 +0.78 +0.50 +0.55 0.40 +0.59
0.001--0.01 0.02--0.05 0.001--0.01 0.001--0.01 0.02--0.05 <0.001 <0.001 <0.001 0.02--0.05 0.02--0.05 0.1 0.01--0.02
S* S S* S* S S** S** S** S S PS S
SiO 2 MgO Rb Zr Y Th U Cu
+0.81 -0.75 +0.90 +0.89 +0.89 +0.69 +0.88 -0.33
0.001--0.01 0.02--0.05 <0.001 0.001--0.01 0.001--0.01 0.02--0.05 0.001--0.01 >0.10
S* S S** S* S* S S NS
FeO MgO Na:O K:O Mn Cu S
+0.99 -0.96 -1.0 +0.95 -0.99 +0.77 +0.77
0.02 >0.01 0.001--0.01 >0.10 0.05--0.] >0.10 >0.10
S NS S* NS PS NS NS
24
Rb Zr Y Cu elevation
--0.38 -0.38 -0.33 +0.54 +0.68
0.05--0.10 0.05--0.10 0.05--0.10 0.001--0.01 0.001--0.01
PS PS PS S* S*
24 13
Y Y
-0.37 +0.55
0.05--0.10 0.02--0.05
PS S
Y SiO 2 MgO
+0.78 +0.46 -0.54
A, B and D:
olivine-normative tholeiites quartz-normative tholeiites basaltic a n d e s i t e s a n d icelandite
4 4 4
>0.10 >0.10 >0.10
NS NS NS
*' NS = not significant; PS = possibly significant; S = significant; S* = highly significant; S** = very highly significant. , 5 T w o samples w i t h 5 ng g-' have b e e n eliminated from this group only.
23
4.2. Group B (I.R.D.P. dykes)
CORE
-503m-
This group consists of nine samples (triangles in Figs. 2 and 3) of nearly-vertical basaltic dykes (one olivine-normative, seven quartz-normative tholeiites, one basaltic andesite) that trend NNE and are approximately normal to the flows. The dykes belong to the Breiddalur Dyke Swarm (Fig. 1) and are in general conspicuously less altered than the lavas. The average Au content of all dykes is 3.04 ng g-t with the median at 1.9 n g g -t. Again, one high value (12 ng g-t, sample 761) affects the mean. This sample is of basaltic andesite composition (55.32% S i O ) a n d has high values of LILE. If this sample is omitted, the mean is 1.93 + 1.02 ng g-t, slightly higher than that of the lavas they intrude. As with group A (I.R.D.P. lavas) a very significant positive correlation is evident between Au and SiO2, Rb, Zr, Y (Fig. 3) and U; and a negative correlation of Au with MgO. Even excluding the anomalously high Au value, the correlations are "possibly significant" (Brooks, 1972).
4.3. Group C (I.R.D.P. clastic units) The Au content of samples of this set are shown stratigraphically in Fig. 4. Lava unit 86.2 is a greyish~oTeen fine-grained aphyric slightly amygdaloidal basalt listed in Table II. Upwards, the flow becomes reddish brown and has a scoriaceous brecciated top with some clastic matrix. The transition to the overlying clastic unit (86.1) is gradual, from a breccia with scoriaceous clasts to a reddishbrown massive to bedded sand-to-silt-size tuff. Chemically, the clastic units are enriched in Au, Cu and the alkalis, but depleted in MgO and CaD with respect to the underlying lava flow. There is a positive correlation between Au and total Fe, K20, S and Cu, and a negative one with MgO, Na20 and Mn (Table III). The Au content appears to have increased significantly in the more porous levels. Probably this enrichment is related less to weathering than to hydrothermal processes, as suggested
A. SiO. F + O ~ g O C o O , ~ # ~ .
~
n(l/g(pl~)%
% ,ml %
%
M.O Co % (mg/~)l}pm
S
•
.
=/o
5.9 50.1 17.0 3.2 4.8
3.7
1.0
21
67
020
7.6 48.6 18.1 2.7
1.9 2,4
]2
360
310
.34
50
.006
VVV V VVVl
:'ZZ2[---1--I .........
i
-504m-
-~:---__~____~_______~__2__2 49.1
_ .~__~__~_~ b__~__~_AI _-~--~-~- -I -505m-
_~__~__~1 _v___~__v_I
AM vAl V A V A Vl VVAVV} ~V ~ Vl v v v'vl vvv vvvvl vvv -506m- VVVVl V V. [~ VVVVV VVVV
14.8
3.5
3.7 5.1
7.2
0.1
LEGEND VOLOA.,CL.S.,C SEO,ME.+..EOO,S. 8.0+..
S.LT + . E E . , S . o.e
0.5 49.5 13.9 4.7 9.0
3.5 0.2
.54
IO
.004
Fig. 4. Distribution of gold and other elements in volcaniclastic rocks overlying a basalt flow at ~ 504-m depth in the I . R . D . P . c o r e .
by the local presence of visible pyrite crystals in the pores. At this depth in the drillhole, lava flows contain assemblages including smectite, chlorite, calcite, quartz, laumontite, epidote and locally garnet (Mehegan et al., 1982). The Au content of the volcaniclastic units is comparable to those reported by Anoshin et al. (1969) for silt and volcanic sediments in the Atlantic Ocean near Iceland (mean 4.2 ng g-t).
4.4. Group D (Holmatindur lavas) This set consists of 24 samples (circles, in Figs. 2 and 3) of tholeiitic basalts from the b o t t o m part (RF0 to RF9), median (RF35 to RF46) and near the top (RF86 to RF88) of a cliff section at Holmatindur (Fig. 1). The mean Au content of group-D lavas 1.69 n g g -1) is identical to that of group-A lavas (1.68 ng g-t) and lower than that of group-B dykes. Here too, there seems to be a rather ordered distribution of values around the mean, and some anomalously high values. In contrast with groups A and B, Holmatindur lavas do not show a positive correlation
24
between Au and Y and Zr and Rb, but a negative one instead (Table III). Also, the mean Au content increases stratigraphically from sea level to the top of the cliff ("elevation", Table III). Both primary and secondary effects may be responsible for the higher Au values. The top lavas consist predominantly of silica-saturated tholeiites and are the least affected by burial metamorphism. It is possible that these top lavas display their "prim a r y " Au content, which elsewhere has been modified by alteration. 5. Discussion The contrasting behaviour of Au between groups A or B, where Au shows a positive correlation with SiO2, Y and Zr, among others, and group D, where the trend is reversed, is surprising. One might ask what is specifically different in the rocks of group D. These rocks are generally richer in magnesia and hence have relatively lower F M index values. Also, rocks that occur ~ 400 m above sea level in the Holmatindur section (samples R F 3 5 to RF46) are intersected at ~ 1450-m depth in the I.R.D.P. core (Helgason and Zentilli, 1982), and thus the latter are bound to be more altered and metamorphosed. However, it is generally accepted (e.g., Wood, 1978; Gibson et al., 1982) that Zr and Y are among the least mobile incompatible elements, and their abundances and concentration ratios are considered to be relatively stable during low-grade hydrothermal metamorphism. Thus, the sympathetic behaviour of Au with Zr and Y appears to reflect a primary, or magmatic trend. When the chemistry of the rocks is compared, groups A and B include a much wider range of compositions (especially SiO2, MgO) with a predominance of saturated (quartz-normative) basalts, and several more silicic varieties, whereas those in group D are predominantly silica-undersaturated (olivine-normative) tholeiites and plot on the left-hand side of Fig. 2. As shown in Table II, olivine-normative tholeiites contain relatively less Au than quartz-normative
tholeiites and show an overall negative correlation with Y (Fig. 3), and this trend is "possibly significant". In contrast, quartz-normative tholeiites of groups A and B indicate a significant sympathetic relationship between Au and Y. For basaltic andesites and icelandires, which contain erratic but relatively high Au, the relationship seems to follow that of saturated rocks. Unfortunately insufficient samples of felsic rocks were analyzed during this pilot project to confirm the trend. Au shows a weak (not significant) positive correlation (+0.163) with S (Table III). The mean S content increases rather erratically from olivine-normative tholeiites (x = 0.006 + 0.008%) through quartz-normative tholeiites (x = 0.023 -+ 0.038%) to basaltic andesites and icelandites (E = 0.031 + 0.040%). Dykes are relatively richer in S (x = 0.049 + 0.049%), confirming the c o m m o n occurrence of dis.
.
.
.
.
......... q
I
LEGEND
120 i
Au ng/g (ppb) ] •
>;8
• 19-38 h)O I
[
i
! i
-I
1
080 1 I
i 060-t
040- i
020 i !i J ooo'"
r= +270 n= 5I p= O.05(PS)
• • •
•nil
.
L'"'~;: .2"
60
70
8O
90
!O0
FM INDEX
Fig. 5. S u l f u r vs. d i f f e r e n t i a t i o n F M i n d e x , w i t h indic a t i o n o f gold levels. N o t i c e t h e s u b s t a n t i a l enrichm e n t o f sulfur in r o c k s w i t h F M i n d e x b e t w e e n 74 a n d 81 ( f e r r o b a s a l t s a n d basaltic andesites; Wood, 1 9 7 8 ) . P.S. = possibly significant ( B r o o k s , 1972).
25 persed sulfides in m a n y of the d y k e margins. Also, S displays a (significant) positive correlation {+0.284) with total Fe-oxide (FeO*), which has also been noticed in submarine basalt glasses (Czamanske and Moore, 1977). In fact, the highest S values are found in rocks with an FM index between 74 and 81 (Fig. 5), generally associated with evolved quartz-normative tholeiites (ferrobasalts; Wood, 1978) and basaltic andesites. A few of the high Au values occur in rocks rich in S, but not all, reflecting the erratic nature of the distribution o f both elements in the rocks. Experimental work by Mironov et al. (1980) suggests that during differentiation Au should follow the path of S. The trend of Au depletion during differentiation noted in the olivine-normative tholeiites could be interpreted to reflect the removal of Au in early immiscible sulfide phases. However, it is possible that some of the Au was separated from the basaltic magma into early crystallized chrome-spinels or olivine. As crystallization proceeds, the tholeiitic magma becomes enriched in Fe and S (Haughton et al., 1974; Czamanske and Moore, 1977). If ilmenite or magnetite start crystallizing, however, the magma m a y release some of its S as immiscible sulfide liquid (Haughton et al., 1974). If the magma is in motion at this stage, it is possible that this immiscible sulfide (and whatever Au has been partitioned into it) m a y not sink and separate, but be carried with the magma as droplets (Haughton et al., 1974), thus leading to erratic distribution of sulfides and of discrete Au-bearing particles. Changes in the oxygen fugacity in the magma, such as those induced by assimilation of water, diffusion of hydrogen out o f the magma, foundering of cooled oxidized roof rocks from the magma chamber, and even direct assimilation of oxygen during eruption, could account for the sudden appearance of sulfides (Haughton et al., 1974). Although the strong influence of S on the behaviour of Au appears probable, the high degree of correlation between Au and some of
the incompatible elements, such as Y, Zr and Rb in the more evolved rocks, indicates that Au is enriched in the residual liquids and m a y be finally trapped in late accessory minerals and the glassy mesostasis, besides sulfides. During cooling and quenching of the magmas, sulfides may continue to be evolved; Au may be partitioned into these sulfide droplets or unstable compounds (Mironov et al., 1980). It is known that certain sulfides at high temperatures will accept substantial amounts of Au in solution, but this Au is exsolved on cooling (Barton, 1969). Therefore, Au may be present in the native form, even if it was originally separated from the melt in sulfides. During cooling, oxidation and alteration of the lavas, the path of S and Au may be quite different. Some of the sulfur may have oxidized to sulfate, as evidenced by the presence of anhydrite in the volcanic pile (Exley, 1982) or may have left the lavas in the volatile phase. Sigurdsson (1982), for example, suggests that only ~15% of the sulphur was retained in volcanic deposits of the Laki basaltic eruption in Iceland, the remainder having been released to the atmosphere. Perhaps S is preferentially retained in intrusive dykes, by a pressure-related process similar to the inhibition of degassing of deep-sea basalts by hydrostatic pressure (Moore and Fabbi, 1971). In this context it appears relevant to compare our results with those o f Keays and Scott (1976) who found the Au content of unaltered glass rims of tholeiitic pillow basalts to be five to seven times higher than the crystalline interiors of the pillows, from which the Au would have been leached during alteration. S follows a similar trend. They interpreted the "excess" Au in the glass rims, above the baseline levels of the crystalline interiors, to represent the a m o u n t of Au that would have precipitated on loosely bound sites. Such sites would have included products of deuteric alteration, grain boundaries, mesostasis phases, and sulfides (Keays and Scott, 1976). Some of the high Au values found in dykes and fresh rocks of this study (> 3 ng g-l)
26 could represent such loosely bound Au in glass and mesostasis that escaped alteration or mobilization. There is little d o u b t that Au has been mobilized as indicated by the relative Au enrichment of a porous oxidized amygdaloidal flow t o p and inter-lava volcaniclastic sediment. The precipitation of soluble Au species carried in percolating fluids, in the presence of sulfides or Fe-oxides and -hydroxides, are well
6. Conclusions
(1) The average Au content of Tertiary lavas and dykes from eastern Iceland is higher by a factor of 2 to 3 than the average values given for oceanic ridge and rise tholeiites, b u t similar to that of Hawaiian tholeiites and continental basaltic provinces (Tilling et al., 1973). However, the Au values found are comparable to those of unaltered glass rims of
pillow basalts from the Mid-Atlantic Ridge at 26 ° and 30°N (Keays and Scott, 1976). (2) The average Au content in the studied rocks increases from lowest in olivine-normative tholeiites, to highest in Fe-rich quartznormative tholeiites (ferrobasalts), and basaltic andesite and icelandite. This trend appears to follow an erratic b u t concomitant enrichment of S in the rocks, suggesting a related evolution of these elements in the magmas. (3) In quartz-normative tholeiites, basaltic andesites and icelandites, Au shows surprising positive correlations with Y, Zr and other indices of magmatic differentiation, suggesting that Au has been systematically partitioned into the melt. In contrast, olivine-normative tholeiites display a negative correlation with Y and Zr, suggesting that Au has been retained in early separated phases. (4) This study confirms the tendency of Au to occur in basalts both in relatively low and systematic background levels, probably disseminated in primary phases, and in anomalously high erratically-distributed concentrations, probably loosely-held in primary deuteric or secondary phases as has been suggested for other suites (Fritze and Robertson, 1969; Keays and Scott, 1976). In order to be representative, the size of a sample to be analyzed for Au should be large enough to take this inhomogeneity into account. (5) Au has been enriched in the porous, oxidized and zeolitized amygdaloidal t o p of a basalt flow and in overlying volcaniclastic units. This relative enrichment, shared by Cu, suggests that hydrothermal fluids had the ability to transport and redistribute metals during hydrothermal alteration and metamorphism of the volcanic pile, despite the apparent lack of involvement of seawater in the system. (6) In the absence of high-magnesia Cu--Nisulfide-enriched basaltic rocks (Keays and Scott, 1976), i.e. in Icelandic-type volcanic piles, the most suitable source rocks for Au mineralization produced by leaching during alteration or metamorphism would be the more evolved, quartz-normative tholeiites (ferroba-
27 salts), a n d basaltic andesites a n d icelandites, i n c l u d i n g d y k e s a n d volcaniclastic units. (7) Because glass is m e t a s t a b l e a n d r e a d i l y devitrified, a n d A u held in sulfides is easily m o b i l i z e d d u r i n g w e a t h e r i n g , burial a n d hydrothermal alteration, only the freshest of lavas, p y r o c l a s t i c s a n d glassy d y k e m a r g i n s s h o u l d be e x p e c t e d t o r e t a i n t h e i r p r i m a r y A u concentrations. Acknowledgements T h e a u t h o r s a c k n o w l e d g e s u p p o r t o f this w o r k b y grants A - 9 0 3 6 t o M.Z. a n d A - 0 3 0 9 t o D . E . R . f r o m t h e N a t u r a l Sciences a n d Engin e e r i n g R e s e a r c h Council ( N S E R C ) o f Canada. We t h a n k t h e f o l l o w i n g c o n t r i b u t i o n s : Shirish Parikh p e r f o r m e d s o m e o f t h e c h e m i c a l analyses o t h e r t h a n gold, D o u g Meggison d r a f t e d t h e figures, a n d N o r m a M c K e i g a n p a t i e n t l y t y p e d several versions o f t h e m a n u s c r i p t . T h e m a n u s c r i p t has b e n e f i t t e d f r o m critical reading b y P r o f e s s o r s J.A. Walker a n d W.S. F y f e , a n d b y an u n i d e n t i f i e d reviewer. T h e a u t h o r s , h o w e v e r , t a k e full r e s p o n s i b i l i t y f o r t h e interpretations and the data.
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