C。ncentrati。ns 。f G。 - GODAC Data Site -NUUNKUI

Concentrations of Gold and other Ore Metals
in Volcanics Hosting the Pacmanus
Seafloor Sulfide Deposit
Roger MOSS*1
The
sulfide deposit is the subject of an ongoing
cite melt, whereas
Zinc, appears to behave
cobalt behaves
compatibly,
with Si02 in the dacites. A single sample
lower
Sb 0.17 ppm,
and shows
metal contents, suggesting
similar metal contents as the dacites could produce
hydrothermal
Co 7 ppm,
enriched in the da-
a significant negative
of basaltic andesite shows
the volcan-
metal contents of
Zn 84 ppm,
incompatibly, becoming
S, As, Zn, Sb and Ba contents than the dacites. PACMANUS
similar primitive mantle normalized
whether
precipitates. Average
dacitic volcanics include Ba 318 ppm, Au 3 ppb. As 3 ppm,
Key
seafloor volcanogenic
to the iimited data set of metals in seafloor volcanics, and to determine
18 ppm, and S 0.02%.
A. BINNS*2
investigation with two principal aims : to
ics could provide the metals found in the hydrothermal
Cu
Raymond
metal content of volcanic rocks in the vicinity of the PACMANUS
massive
add
Steven D. SCOTT*1
correaltion
higher Au, Co and Cu and
sulfides and dacites show
that leaching of a source with
the relative metal
contents
seen in the
precipitates.
words :PACMANUS,
metal content,volcanics,dacite,volatiles
･ ■
＊ Ｉ Marine Geology Research Laboratory. Department
＊ ２ CSIRO Division
JAMSTEC
Ｊ． Deep Sea Ｒｅｓ･，１３ i 1997！
■ ㎜
㎜
■
ｏ
ｆ Geology, University ｏ
ｆ Toronto
ｏ
ｆＥｘplol･ation and Milling
２５
1. Introduction
The
PACMANUS
Manus
Basin
(Fig.
deposit
cruise
(Binns
Binns
et aI., 1996),
&
Flux
volcanogenic
PACMANUS
; Scott &
massive
venting,
et al., 1996a,
edifice known
each
with
(up
and
and lower
minimum
Ridge,
and
is
(Auzende
was
there has been much
the likelihood of magmatic
all of the metals to VMS
; Urabe
&
convincing
discussion concerning
fluids contributing some
hydrothermal
Murumo,
1995 ; Yang
it remains
et al.,1996).
1991
systems
arguments
; de
there are
for such a process in some
necessary to evaluate whether
tent of the host
(Urabe,
; Stanton, 1991
& Scott, 1996). Although
or
volcanic
sequence
cases,
the metal con-
is sufficient to
resultant
differences
in
tirely discarded. To date, there have been few studies
etal. 1996).
associated
high
with
(45-63 °C) vent-
observed
et
Both
during
al., 1996a).
the
The
sulfides, iron-manganese
oxide mounds
(Binns et al., 1996). The
and bacterial mat
comprising
Pual Ridge are dominated
by da-
Fig. 1 Simplified tectonic and bathymetric map of the Manus
(modified from Benes et al.,1994).
2 ５８
the two
produce
crusts,
volcanics
between
of
precipitates include both zinc-rich and cop-
massive
levels, generally
& Scott, 1993 ; Waters
Recently,
1987
of a
at deeper
basaltic andesite is
of the ridge, and to the north and south of the ridge
Ronde,
as Pual
temperature
water),
cruise
Manus-
b).
to 262 °C) venting
(shimmering
hydrothermal
the
lobate andesite and
to the northeast.
character
precipitates (Binns
ManusFlux
active
and
occurring
a different
hydrothermal
per-rich
6500"
arms
(Binns
of at least four distinct sites
activity and with
smokers
1995;
sulfide deposit. Since
hydrothermal
ing
Binns,
"Shinkai
to be comprised
temperature
present
I
is a currently
"Yokosuka"
volcanic
known
mainland
the PACMANUS
lies near the bathymetric
Y-shaped
black
Guinea
Pillowed
several cruises have visited the site, in-
the 1995
of active
1993
PACMANUS
cruise (Auzende
now
New
in 1991, during
Scott,
initial discovery,
cluding
is situated in the eastern
east of the Papua
1). Discovered
seafloor
cite with lesser rhyodacite
a deposit before a leaching process
can be en-
the metal content of modern
of
seafloor volcanics (Keays
&
Fryer, 1994
Scott, 1976
Terashima
examined
modern
; Cawood
and
; Doe, 1994
;
et al.,1994), and none of these studies have
felsic volcanics. The
attraction
of studying
seafloor volcanics lies in the fact that, if proper
ly selected, they can represent the most pristine
rocks as
sociated with massive sulfide deposits, and give valuabl<
information
regarding
hydrothermal
Basin region, showing
the evolution
system. An
of a
magmatic
investigation of the
ore meta
the location of the PACMANUS
JAMSTEC
deposit
Ｊ． Deep Sea Res., 13 (1997)
content
of volcanic samples
MANUS
from the vicinity of the PAC-
deposit was therefore undertaken
limited
a total acid digestion at Activation Laboratories
Ltd, Ancaster, Ontario.
data set of metals in seafloor volcanics, but in par-
ticular, to investigate the concentration
canics associated
This
to add to the
lowing
ongoing
MANUS
with a seafloor massive
investigation
an extensive
of metals in volsulfide deposit.
has included
collection of dredged
the analysis of
samples
from
and other areas of eastern Manus
et al.,1996a,
b, 1997).
ples collected during
2. Sample
All samples
"Shinkai
we
Basin (Moss
report results for sam-
the Manusflux
Collection and Analysis
submersible
6500"
All but one of the samples
lected
Basin, as part
used in this study were
col
in the vicinity of the four vent sites comprising
the PACMANUS
of Pual
mafic
deposit on the crest and eastern flank
Ridge.
The
remaining
basaltic andesite
sample,
from
299-R-01,
lobate flows
with
is a
glassy
on the floor of a valley immediately
east of Pual ridge, at 2134
m depth. A description of the
samples
1.
is given
Dacite
collected by the
during the 1995 "Yokosu-
cruise to the Manus
Description
rinds, outcropping
cruise.
used in this study were
6500"
ka" "Shinkai
Here
PAC-
3. Sample
in Table
observed
in outcrop
consists
of blocky
that are lightly to moderately
sedimented
Large
in some
blocks of dacite observed
flows
(Photo
1).
areas, appear
to
et al.,1996b). A series of five dives (297, 299, 301, 304
along on the top of the flow. In
rafts that were carried
hand specimen, the dacite is typically very fresh, has a
&
glassy
of the Japan/France
305) were
NUS
New
performed
Starmer
Program
(Auzende
in the vicinity of the PACMA-
vent sites,of which three resulted in the collection
of volcanic samples.
In order
changes
cles
appearance
tend
to minimize
the
effect of post-eruption
in the gold content of the volcanics,
used for analysis (cf. Connors
et al.,1993). Gold
al., 1996).
sured
free
are
neutron
activation analysis (INAA)
using 200 milligram
ability,
samples
following
of Stix
(1992).
Major
cence
the
technique
&
sulfur by the LECO
infrared spec-
trometric technique, and Cu, Ag and Pb by ICP-MS
JAMSTEC
Ｊ. Deep Sea Ｒｅｓ･， 13 (1997)
variable vesicularity. Vesia result
of flow, and
Examination
rare
(Photo
was
2). Such
is essential
to be used to draw
either in magmas
(Waters
et
of thin sections of dacite en-
glass to be analyzed
were
are
unaltered
and
glass,
if results of the analyses
conclusions
or
unaltered,
regarding
metal avail-
in volcanic products.
Gorton
elements were analyzed by X-ray fluores-
spectroscopy,
elongated,
of phenocrysts,
and other trace elements were analyzed by instrumental
shows
distributed in the samples
that the
phenocrysts
and
to be
homogeneously
unaltered
glassy volcanic rocks, selected by visual examination,
were
be
fol
4. Results
A totalof twelve samples of Pual Ridge volcanics
from the
vicinityof the PACMANUS
vent siteshave
2 ５９
Table ２ Whole rock and selected trace element analyses of glassy volcanic rocks.
been analyzed for gold and other ore metals (Table 2
With only one exception,
）.
ａ sample of basaltic andesite
゛alues that are similar to andesites (mean
rhyolites (mean
２.7 ppm)
２.０ ppm)
given by Stanton
(1994).
from the southern flank of Pual Ridge, the samples are
Similarly･ antimony varies from 0.12 to 0.24 ppm and
dacitic in composition. The barium content of the PAC-
averages 0.17 ppm close to the average abundance in an-
MANUS dacites ranges from 299 (basaltic andesite)
333 parts per million (ppm)
and averages 318 ppm.
This average is at the low end of the wide range of
to 798 ppm (average
1994).
desite of 0.2 ppm (Stanton,
８ ０
t ２３ PPm and averages
２６０
= 421) for dacitic lavas from
modern island arcs (Stanton,
varies from
to
Gold in the dacites
２ to ７ ppb （Ｆｉｇ. 2）.Ｎｏ significant correla-
tions are observed between gold and other trace metals
less than the average of
Stanton
shows
(1994).
1994).
and
Copper ranges from
１８ ppm (Fig. 2
）
・significantly
３０ ppm for arc dacites given by
Copper in the PACMANUS dacites
ａ strong･ significant negative correlation with
zinc (Table 3j｡
The samples have
ａｎ average zinc content of 84 ppm･
considered here･ nor are there any significant correla-
greater than the mean zinc content
ｏ
ｆ arc dacites （59
tions between gold and the major elements Si, Fe and
ppm),
ｏｆ MORB and arc
Mg
basalts of 84 ppm and 77 ppm respectively (Stanton,
２６θ
f Table 3). The dacites contain 2 t
０ ３ ppm As.
and closer to the mean values
JAMSTEC
Ｊ. Deep Sea Res., 13 (1997)
Fig. 2 Frequency
distributions
of selected metals in PACMANUS
volcanics.Shaded sample is basaltic andesite, the remain-
der are dacite.
1994). The
zinc content of the dacites is positively cor-
related with Si(>2 (Fig. 3), but shows
tive correlations with Fe2C>3, MgO,
showing
significant nega-
Co and Cu. Besides
a negative correlation with zinc, cobalt shows a
significant negative correlation with SiO? (Fig. 3). and
JAMSTEC
J. Deep Sea Res., 13 (1997^
positive correlations
with
respect, the behaviour
to that of zinc. The
NUS
arc dacites of 12 ppm
and
MgO.
In
this
of cobalt in the dacites is opposite
mean
dacites is 7 ppm,
FegO^
cobalt content
approximately
given by Stanton
of the PACMA-
half the mean
(1994).
for
Sulfur in
261
Fig. 3 Scatter
the dacites varies from
average
of 0.02%.
served between
here (Table
No
0.01 to 0.04%,
and
gives an
significant correlations are ob-
sulfur and any of the metals considered
3).
ent trace metal concentrations
Au
(15 ppb) and Cu (406) are significantly enriched in
the basaltic andesite compared
to the PACMANUS
da-
quite differ-
than the dacite (see Fig.
higher Au, Co and Cu and lower S, As, Zn, Sb
and Ba contents. For this reason, and because
it is only-
one sample, the basaltic andesite was not included in cor262
relation calculations, nor in plots of metal content. Both
cites.
As expected, the basaltic andesite shows
2) having
plots of selected metals against Si02.
5. Discussion
Magmatic
and
hydrothermal
processes
known
to
affect the metal content of volcanic rocks include the following
(Connors
et al., 1993
JAMSTEC
; Doe,
1994
; Togashi
&
J. Deep Sea Res., 13 (1997")
Terashima,
1997)
:
concentration
the degree
magma
of metal in the source
of partial melting
;
and crustal assimilc
degassing
of magma,
hydrothermal
and seawater
this study
of the original metal content could be lost
Primitive
mantle
in press). Such
alteration
glassy
volcanic rocks i:
content of the volcanics, and allows an esti
mate
to be made
With
the exception
here
do
of the metal
content
normalized
S1O2, Fe2C>3 or MgO
significant correlations
(Table
3), suggesting
plots of metal content
potential source
witl
that, at leas
plots for PACMANUS
similar "negative"
a relative enrichment
Au, whereas
rocks
and
should
difference
however
of silver compared
to Pb and
a relative depletion (Fig.
in the normalized
Ag
content
be treated with caution, as the Ag value
for PACMANUS
The
sulfides and da-
slopes, but the dacites
the sulfides show
4A). This
of the magma
of zinc and cobalt, the metals consi
not show
cites show
show
limits the effect of the post eruption processe
on the metal
for the dacite
deposits from different districts(Barrie & Hannington,
both pre- and syn-eruption;
use of fresh unaltered
dered
melt, as much
are useful for comparing
tion;
The
be considered a minimum
by pre-eruptive degassing.
;
differentiation, mixing
canics, should
dacites is based on only one analysis.
broadly
parallel patterns
of the dacites and the
for these metals, there is littleeffect from differentiatioi
sulfides are consistent with leaching of metals from
processes.
source
ing
Zinc, appears
to behave
incompatibly,
show
a significant positive correlation with Si02, and nega
tive correlations
with
Fe203
and MgO.
trast to the findings of Stanton
the Solomon
(1994)
This
(1994)
is in con
for felsic lavas o:
Islands, but agrees with the findings of Dot
for the behaviour
of zinc in MORB.
The
strong
positive correlations of cobalt with Fe2C>3 and MgO,
strong
negative
anc
correlations with Si02 and Zn indicate
that cobalt is behaving
compatibly
None
significant correlations with sul
of the metals show
fur, suggesting
by
that metal
distribution is not controlled
possible submicroscopic
It has been
deposits
volcanic
that the metal
rocks
(Franklin
; Barrie & Hannington,
possible
contributions
the hydrothermal
tions of melt
Pual
Ridge
et al., 1981
inclusions
andesite,
and
that a volatile phase
the magma,
and that the volatiles degassed
of the magma
(Yang
has produced
similar results (Yang,
served
among
the metals
Gold
J. Deep
precipitates (Moss
et al.,
1996b, 1997).
In terms of ore metal content, dacites from the PACMANUS
mon
area are similar to arc volcanics from the SoloIslands (Fig. 4B), which
previously
is in agreement
with the
noted arc-like signature of Pual Ridge volcan-
ics (Binns & Scott, 1993 ; Scott & Binns, 1995 ; Binns et
Back
arc basalts from
the Japan Sea both show
the Lau
Basin and
depletions in Zn relative
by the PACMANUS
and Solomon
is a depletion in Cu relative to Zn, whereas
canic suites considered
show
patIsland
dacites
the other vol-
a general enrichment
in Cu
relative to zinc (Fig. 4B).
from
in
on eruption
of dacites
Zn,
in the obto partition
here for the PACMANUS
Sea Res., 13 (1997)
in the hydrothermal
volcanics. A distinctive feature of PACMANUS
pers. comm.).
is also known
to provide the metal concentrations
this is
present
reported
previous
tern showed
into the volatile phase, and so concentrations
metals, presented
JAMSTEC
was
& Scott, 1996). Analysis
volatile phase.
strongly
these
were
mechanism
observed
6. Conclusions
of vesicles in the andesitic
shown
and Fe
calculations indicating the viability of a
leaching
with
; Doe,
investiga-
phenocrysts
glass, have
Cu
mass
balance
is consistent
to Au and Cu, in contrast to relatively flatAu-Zn-Cu
volcanic fluid to
are ignored. Recent
in pyroxene
conclusion
in the
those volcanics, and
of a metal-rich
system
dacites. This
from
content of VMS
in press). However,
true only if metals are leached from
with a similar metal content as the PACMANUS
al., 1996b).
sulfides.
should reflect the metal content observed
associated
1994
suggested
in the dacite melt
a
of
vol-
The
ore metal concentrations
determined
in unaltered
glassy volcanic rocks from the vicinity of the PACMANUS
deposit are considered
to reflect minimum
metal
contents of the dacitic melt, due to the partitioning of
many
of these metals into a volatile phase. Only zinc and
cobalt show
any significant correlations with indices of
differentiation Zinc behaves
richment
incompatibly,
showing
en-
in the glass with increasing silicacontent in the
263
in the dacite glass.
Primitive
mantle
normalized
contents
PACMANUS
dacites
show
broadly
similar patterns, although
three
to four orders of magnitude
canics. The
and
metal
hydrothermal
of the
precipitates
the sulfides are
enriched
over the vol-
similarity of the patterns indicates that leach-
ing of a substrate
with a metal
the
content similar to the da-
cites, would
provide
right
sulfides. The
ore metal content of the dacite is most
lar to that in arc volcanics
although
metal
from
the PACMANUS
ratios
the Solomon
volcanics
show
for the
simi-
Islands,
a distinct Cu
depletion.
Acknowledgments
Co-Chief
Urabe
Scientists Jean-Marie
are thanked
Auzende
for providing
opportunity
to participate
cruise. We
thank
"Yokosuka"
and
RAB
in the
the captain
and RM
crew
6500"
for their contribution
to the success
This
research
by grants
Sciences
and
funded
and Engineering
the Bank
of Toronto
of Nova
Doctoral
Research
Scotia to SDS,
Fellowship
1995
of the
R/V
chief pilot and his
team
was
with the
ManusFlux
and
the "Shinkai
and Tetsuro
of the cruise.
from
the Natural
Council
of Canada
and
by University
to RM.
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Ｊ． Deep Sea Ｒｅｓ･， 13 (1997)
Photo 1 PACMANUS
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