hebei, China gold

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).

Hebei geology map

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.

Decrepitation analysis procedures and meaning of the results.

Summary

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.

Dongping Ore zone

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=

H1852 Sample# 1864C Ch5FT7 0.5g -420+200 ACID WASHED (Carbonate present)
Dongping underground #70 orebody, 30m wide section, many qtz veins, milky white qtz vein, CR, Kfelspar. thin vein

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.

Dongping ore zone

Sample details:

H1871 Sample# 1864E Ch5FT7 0.5g -420+200
Dongping underground, # 70 orebody, Silicified grey syenite , minor S=

H1872 Sample# 1864F Ch5FT7 0.5g -420+200
Dongping underground, # 70 orebody, Silicified grey syenite , trace Kfeld

H1873 Sample# 1864G Ch5FT7 0.5g -420+200
Dongping underground, # 70 orebody, Silicified grey syenite, fine grained, trace S=

H1875 Sample# 1864H Ch5FT7 0.5g -420+200u
Dongping mine, underground, ore body #70, ore grade, Silicified med grained pink/grey syenite, trace S=

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.

Dongping underground ore

Sample details:

H1870 Sample# 1864D Ch5FT7 0.5g -420+200
Dongping underground, # 70 orebody, Silicified grey syenite

H1876 Sample# 1864J Ch5FT7 0.5g -420+200u
Dongping mine, underground, ore body #70, ore grade, Silicified fine grained pink/grey syenite, spotted dark grey

H1877 Sample# 1864K Ch5FT7 0.5g -420+200u
Dongping mine, underground, ore body #70, ore grade, Silicified medium grained pink syenite, trace S=

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.

Dongping ore margin

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.

Dongping ore on trucks

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

H1882 Sample# 1863C Ch5FT7 0.5g -420+200u     REPEAT H1869

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.

Hougou underground

Sample details:

H1862 Sample# 1869A Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink syenite, semi translucent coarse white qtz, trace Kfeld

H1863 Sample# 1869B Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink syenite, semi translucent coarse white qtz, trace Kfeld

H1864 Sample# 1869C Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink syenite, semi translucent coarse white qtz, trace Kfeld

H1865 Sample# 1869D Ch5FT7 0.5g -420+200
Hougou mine, underground. much quartz stockwork, extensive pink syenite, semi translucent coarse grey qtz, Kfeld, trace S=

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).

Comparison mines

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