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Hebei province, China
Gold deposits hosted in Syenite
A fluid inclusion acoustic
decrepitation study.
A number of gold deposits occur in an area about 200 Km north
west of Beijing, near the northern margin of the north china craton.
The deposits at Dongping, Huangtuliang and Hougou were visited and
samples collected for acoustic decrepitation studies during
September,
2005. These deposits occur within middle Palaeozoic Syenite or
Archaean
basement host rocks and the mineralisation event at Dongping mine
has
recently been dated as Jurassic and clearly unrelated to the host
syenite emplacement event. (Hart et. al, Mineralium Deposita, V37,
p.
326-351).
A total of 34 individual rock samples was collected from 7
locations.
Multiple samples are collected at each location to provide
information
on small scale homogeneity, as well as ensuring unbiased
representation
at each location. Ideally, monomineralic quartz samples are
collected,
but quartz was sometimes scarce in these mines and many whole rock
samples of combined quartz and feldspar were also used. Quartz
is
only heated to 620 C during the analysis but when other host mineral
phases are present the samples are heated to 800 C.
The main observation from my results is that the depositional fluids
in
all of these mines lack significant amounts of CO2. If CO2
is present in the fluids, the
decrepitation results show a distinct peak between 200 and 300 C, (explanation) and as seen in the graph of samples from archean gold
deposits.
This
lack of CO2 also occurs at Muruntau and the Motherlode
deposits in
California and so I infer these deposits share a similar fluid
genesis.
In general, fluids derived from metamorphic zones greater than lower
greenschist facies would be expected to contain CO2.
The formation temperature of the Dongping deposit is about
350 C (the beginning of decrepitation) while the Hougou deposit may
have formed from a slightly hotter fluid at about 370 C which also
contained a trace of CO2. Although the Dongping and
Hougou
fluids
are reasonably similar, note the very low decrepitation in quartz at
Huangtuliang. This mine had much less quartz veining and I had some
trouble getting many samples of quartz there. The 2 samples I did
obtain both had very low inclusion abundances. This quartz did not
seem
to be sheared and looked somewhat cherty and this deposit probably
formed at a much lower depth or from a vapour dominant fluid system
-
either condition gives much lower decrepitation intensity.
However, its formation temperature was still about 350 C.
Conclusions
Several fluid inclusion studies on these mines mention the presence
of
CO2 rich fluids in the inclusions, inferring the
importance
of such fluids in the ore genesis. However, these deposits actually
formed from fluids with very low CO2 contents and
only
a few of the samples show very low partial pressures of CO2
in their fluids. When performing manual microthermometry studies it
is
all too easy to focus on the peculiar inclusions and obtain a
distorted
impression of the fluid system. The decrepitation data, being the
results of many more individual samples and many thousands of
inclusions in each sample give a much more realistic representation
of
the fluids, which are not actually CO2 dominant at all in
these mines.
The low levels of CO2 in the parent fluids of these
deposits
indicates that the main fluid source is unlikely to be of
metamorphic
origin, unless it is barely much more than diagenesis, as such
fluids
would be expected to contain significant amounts of CO2.
Porphyry copper style deposits (high level magmatic) also lack
significant CO2 in their fluids and perhaps shallow
magamtic
systems are the source of the fluids. This would agree with the
inferred relationship between the age of the mineralisation and
"late
Jurassic to early Cretaceous crustal-melt granite dykes and stocks"
in
the region as reported by Mao et. al., Economic Geology, V98, 2003,
p
517-534.
In view of the low CO2 contents of the fluids in these
deposits, I infer that the preferred exploration target is for
structurally favourable regions related to and/or close to the late
intrusives in the district. These late stage intrusives seem to be
the
fluid source for these deposits, rather than any metamorphic fluids.
Dongping Mine
Samples were obtained from the #70 body underground workings, both
within the main ore zone which was about 30 metres thick here, as
well
as just outside the grade cutoff limit (1 gm/tonne) of the ore
zone. These 3 samples of quartz show no low temperature
decrepitation at all and lack CO2. However 2 of
these
samples had contained carbonate and were washed in acid to remove
this
prior to analysis. The formation temperature of the quartz is about
350
C, the start of significant decrepitation.
Sample details:
H1850 Sample# 1864A Ch5FT7 0.5g -420+200
Dongping underground #70 orebody, 30m wide section, many qtz
veins, milky white qtz vein with minor Kfeldspar
H1851 Sample# 1864B Ch5FT7 0.5g
-420+200
ACID WASHED (Carbonate present)
Dongping underground #70 orebody, 30m wide section, many qtz
veins, milky white qtz vein trace CR, much S=
Four more samples from the ore zone were of a mixture of quartz and
feldspar, with quartz being dominant. The peak at 590 C is caused by
the alpha -> beta phase transition in quartz. The decrepitation
response of the feldspars has not interfered with the response from
inclusions in quartz and the formation temperature is still seen to
be
about 350 C with no CO2 present.
Three more samples from the ore zone show no distinct quartz phase
transition peak at 600 C and have only weak silicification. However
the
formation temperature can still be determined as about 350 C.
Three samples of quartz from the margin of the economic ore zone,
just
outside the 1 gm/tonne cutoff point are probably indistinguishable
from
the quartz collected well within the ore zone (sample 1864 above).
However 1865A (red graph) does have a lower than usual formation
temperature of about 300 C. It is possible that there is a
temperature
decrease as we progress outside the ore zone, but there are
insufficient samples in this study to be sure of this.
Sample details:
H1853 Sample# 1865A Ch5FT7 0.5g -420+200
Dongping underground #70 orebody, just outside ore zone (1gm cutoff)
thick qz, coarse milky white qtz vein
H1854 Sample# 1865B Ch5FT7 0.5g -420+200
Dongping underground #70 orebody, just outside ore zone (1gm cutoff)
thick qz, coarse milky white qtz vein, CR, stress fractures,
boxworks after S=
H1855 Sample# 1865C Ch5FT7 0.5g -420+200
Dongping underground #70 orebody, just outside ore zone (1gm cutoff)
thick qz, coarse semi transl white qtz vein, CR
Several samples were taken from the ore trucks waiting at the shaft.
It
is impossible to be sure of the precise origin of these samples, but
the 2 quartz samples (red and green) show minor decrepitation around
250 C and traces of CO2 are inferred to be present in
this
quartz. The quartz formation temperature is still about 350 C.
Sample
1863C was analysed twice (blue and magenta) and seems to be
comprised
entirely of feldspar. Note that the instrument reproducibility of
the 2
analyses of the same sample, which were actually done on separate
days,
is quite good.
Sample details:
H1848 Sample# 1863A ch5ft7 0.5g -420+200
Dongping underground - ore on trucks at shaft loading station,
Coarse milky white fractured qtz
H1849 Sample# 1863B Ch5FT7 0.5g -420+200
Dongping underground - ore on trucks at shaft loading station,
Coarse semi translucent white qtz, haematitic fractures
H1869 Sample# 1863C Ch5FT7 0.5g
-420+200 Trace CO3=???
Dongping underground, ore from trucks at shaft loading point,
Silicified dark grey syenite with thin quartz veins and S= vein
Samples from the old surface workings once again lack any CO2
and have a formation temperature of about 350 C.
Sample details:
H1856 Sample# 1866A Ch5FT7 0.5g -420+200
Dongping opencut, surface workings on discovery outcrop,
coarse
milky white qtz, trace Kfeld
H1857 Sample# 1866B Ch5FT7 0.5g -420+200
Dongping opencut, surface workings on discovery outcrop,
coarse
milky white qtz
H1858 Sample# 1866C Ch5FT7 0.5g -420+200
Dongping opencut, surface workings on discovery outcrop,
coarse
milky white qtz, S= (gn, not py!)
H1859 Sample# 1866D Ch5FT7 0.5g -420+200
Dongping opencut, surface workings on discovery outcrop,
coarse
milky wh qtz, sulphides and CR
Huangtuliang mine
At this mine, quartz was quite scarce and only 2 clean quartz
samples
were collected, the remainder being feldspar dominated. The 2 quartz
samples (red and green) both had anomalously low decrepitation
intensity. The hand sample descriptions show them to be a cherty
quartz
rather than a typical vein quartz. This low decrepitation intensity
may
be due to a much higher level of emplacement, or a vapour dominant
fluid system at this mine. The fluids here are quite different to
the
other 2 mines in this study.
Sample details:
H1860 Sample# 1867A Ch5FT7 0.5g -420+200
Huangtuliang opencut from ore crusher feed, fine grained
grey cherty qtz vein, much haematite stains
H1861 Sample# 1867B Ch5FT7 0.5g -420+200
Huangtuliang opencut from ore crusher feed, fine grained
grey cherty qtz
H1878 Sample# 1867C Ch5FT7 0.5g
-420+200u ACID WASHED (Carbonate present)
Huangtuliang opencut, crusher feed samples, brown medium
grained
syenite, haematitic throughout
H1879 Sample# 1867D Ch5FT7 0.5g -420+200u
Huangtuliang opencut, crusher feed samples, brown medium
grained
syenite, strongly haematitic
These feldspar dominated samples still show decrepitation commencing
at
about 350 C, and it seems that despite the differences in the
fluids,
the deposit was still formed at about 350 C.
Sample details:
H1880 Sample# 1868A Ch5FT7 0.5g
-420+200u trace CO3=??
Huangtuliang underground, samples from orepass, only minor
qtz,
very coarse grained pink/grey syenite, minor S=
H1881 Sample# 1868B Ch5FT7 0.5g
-420+200u trace CO3=??
Huangtuliang underground, samples from orepass, only minor
qtz,
coarse grained pink/grey syenite, moderate disseminated S=, coarse
kfeld
H1883 Sample# 1868C Ch5FT7 0.5g
-420+200u ACID WASHED (Carbonate present)
Huangtuliang underground, samples from orepass, only minor
qtz,
Pink coarse syenite (Kfeld) with trace S=
Hougou mine
There was an abundance of quartz in stockwork veins in this mine,
and
many of the samples show minor decrepitation around 250 C,
indicating
the presence of small amounts of CO2 in the fluid
inclusions. The start of intense decrepitation also seems to
be a
little higher than the other mines and the formation temperature of
this quartz is about 370 C.
H1866 Sample# 1869E Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink
syenite, semi translucent coarse white qtz, much Kfeld
intergrowths
H1867 Sample# 1869F Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink
syenite, semi translucent grey-white fine grained qtz, much
Kfeld
intergrowths
H1868 Sample# 1869G Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink
syenite, semi-transl & milky white qtz, fractured, bands
of
Kfeld
Comparison mines
Other gold mines which lack significant CO2 occur at
Muruntau (red and green) and the Motherlode in California (blue).
However, most Archaean deposits have quite high CO2
contents
in the fluid systems as seen in these examples from the Abitibi
district in Canada (magenta and yellow) and from the Kalgoorlie
region
in West Australia (cyan).
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
