decrepitation of limestone skarn with tin at Gejiu , Yunnan, China

A decrepitation study of tin bearing limestone skarns, Gejiu, China.

August 2014

After the IAGOD conference in Kunming there was a field trip to the Gejiu tin mining district in SW China. Samples were collected from the operating tin mines and also from regional background areas remote from the mines. There was no quartz associated with the mineralisation and all samples were of various other minerals and of limestone. Fluid inclusion decrepitation of these other mineral species, particularly carbonates, is poorly understood, which limits the interpretation of this data.

Conclusions

The interpretation of decrepitation data on mixed, non-quartz mineral suites such as in this study is complex. However the results show that limestones, garnet, tourmaline and iron-oxides can all provide formation fluid information and discriminate between potentially mineralised and barren areas.

It is likely that these deposits at Gejiu formed in the temperature range from 350 to 450 C. There were very few samples in this study and a larger study would be required to provide better understanding and detail about the formation conditions and fluids.

The supposed basalt sample from the Kafang mine is neither a basalt nor of basalt derivation. Its lack of decrepitation suggests an unaltered sediment origin, but this is inconsistent with its mineralogy. Firm conclusions cannot be drawn from this single sample, but the model proposing a basalt origin for the local copper mineralization must be questioned more carefully.

See the summary for elaboration of these conclusions

Discussion and results

Publications about this mineralisation include:
Geology and vein tin mineralization in the Dadoushan deposit, Gejiu district, SW China. Yanbo Cheng et. al., Mineralium Deposita 2012, 47:701-712.
Geology and genesis of Kafang Cu-Sn deposit, Gejiu district, SW China. Yanbo Cheng et. al., Ore Geology Reviews, 2012, 48:180-196.

An enormous quantity of tin (in excess of 350,000 tons) has been mined from the many deposits in this district. Mining has taken place for several centuries and has been very intense over the last 100 years. Alluvial tin mining commenced some 2000 years ago.

Ores in this district are mostly vein type mineralisation hosted in Triassic carbonates over granite cupolas. The vein minerals include tourmaline, garnet, diopside, epidote, phlogopite, beryl and cassiterite. There are only minor sulphides present. Tin grades are up to 1.2% Sn. Almost no quartz was observed in the areas visited or samples collected.

Location of Gejiu in southern China, 250 Km south of Kunming.

These deposits are skarns within limestones, near to granites. Samples were collected from 2 active working mines at Kafang (underground) and Dadoushan (open pit) and also from an inactive small adit entrance at Baishachong, which was unique as the contact of the granite with the limestone skarn was visible here.

Location of mines and samples at Gejiu tin deposits. (1 minute grid lines)

gejiu sample locations

Three additional samples of unmineralised limestone were collected from far distant outcrops which are not associated with any mineralisation. These sample locations are near Dali and Lufeng, west of Kunming.

Location of regional background samples. (1 degree grid lines)

regional sample location map

Kafang underground mine

Within the Kafang mine a sample (2029A) of garnet from the skarn shows substantial decrepitation continuing to 800 C. There was much discussion about the other rock type, sample 2029B, which was stated to be a "basalt". But it reacted strongly with acid and much carbonate was present. The crushed grains were mostly of fine grained limestone, with substantial garnet and some tourmaline. There were no ferro-magnesian minerals observed. This sample is clearly not a basalt at all and probably not even an altered basalt. It may be a remnant sedimentary feature within the limestones, but it is unclear why it forms a distinctive unit and why it has no decrepitation. The lack of decrepitation is suggestive of an unaltered sedimentary unit, but the mineralogy shows that alteration has occurred.

The "basalt" is proposed to be the source of copper mineralisation within the mine, but this rock unit cannot be the copper source. Perhaps this individual sample was badly chosen, but it seems there is a problem with mapping of the supposed basalt in this mine and perhaps the entire district.

Kafang underground mine samples

Dadoushan open pit mine

At the Dadoushan open pit, cassiterite occurs within a fine grained skarn altered limestone which contains considerable tourmaline, mostly present as thin, fine grained bands. There are occasional large clumps of almost pure coarse grained tourmaline, and one of these was sampled (2031). These samples were from the actively worked ore zone.

The 2 samples of limestone show very intense high temperature decrepitation, which is often seen on carbonates. These 2 samples of limestone were collected within a few metres of each other and are surprisingly different. Decrepitation in carbonate minerals is possibly strongly affected by their ductility and cleavage and may not reflect the conditions within the primary skarn alteration fluid. Decrepitation of the tourmaline sample, 2031 (plotted below in red), is very similar to one of the limestones. This is unexpected if decrepitation in limestone is primarily a function of cleavage and ductility, as tourmaline is brittle and lacks strong cleavage, despite having well formed crystal structure. Tourmaline should retain primary fluid inclusions and decrepitation should provide a useful guide to the primary entrapment conditions of the parental fluid. These results suggest an alteration fluid event at about 350 - 380 C. The lower temperature of 300 C shown by sample 2030B may indicate a late stage event as the fluid system collapsed.
Dadoushan samples

Baishachong workings (inactive)

This location was sampled because the contact between the granite and limestone skarn is visible there. The samples collected were from dumps at the entrance to a small adit and may have come from widely separated locations within the workings. All of the samples were very ferruginous. During sample preparation each sample was separated into a magnetic and non-magnetic fraction, but only sample 2033 had enough of the non-magnetic fraction to allow analysis (2033B). All of the samples were only weakly magnetic, much less than pure magnetite and maghaemite may be dominant.

Haematite and magnetite are quite good host minerals for fluid inclusion decrepitation studies and have been used extensively (other examples here).

The decrepitation results show considerable variations between samples, suggesting highly variable fluid conditions within the deposit and a formation temperature of about 400 C.

The magnetic and non-magnetic fractions of sample 2033 show the host mineral dependence of decrepitation data, with much lower temperature decrepitation on the non-magnetic fraction. The non-magnetic fraction is not merely quartz as decrepitation of quartz does not continue above 600 C because of the transition from alpha to beta quartz at 573 C. Observation of this sample showed it to contain about 50% haematite grains with 15% quartz and 35% translucent yellowish grains of greasy appearance, suspected to be either andradite or epidote. There were also some rare garnet grains.

baishachong mag / nonmag

Limestone samples from skarn and regional background areas

The skarn limestone samples give intense decrepitation, which is a common feature of decrepitation of carbonate samples. In addition, the unmineralised crystalline limestone from Mt Changsha, near Dali, gave intense decrepitation. But the remote background sedimentary limestone samples from Dali and Lufeng gave no or only very small decrepitation.

It seems that the decrepitation response is strongly influenced by the degree of crystal formation, with non-crystalline samples having no decrepitation. However the crystal size in the Dadoushan skarn samples was only about 40 microns and yet there is intense decrepitation. Decrepitation of limestone samples is a function of both the primary thermal and fluid history as well as subsequent recrystallization and metamorphic events.

The 3 regional background limestone carbonates show very variable decrepitation. Sample 2035 is an unmineralised coarse crystalline marble and the decrepitation in this is due to the metamorphic recrystallization. The other 2 samples have very little or no crystals larger than about 10 microns and the low decrepitation is as expected from an unaltered sedimentary rock. The intense decrepitation often seen in carbonates may be caused by regional metamorphism and not just by hydrothermal or skarn alteration fluids.

regional limestones

Comparing decrepitation in different minerals

Garnet, tourmaline and magnetite (or mag-haematite) minerals are good hosts for fluid inclusions and decrepitation of these minerals can be useful in outlining alteration zones around mineralisation. A range of formation temperatures between 350 C and 450 C is suggested by the decrepitation of these minerals in the Gejiu district. Although skarn altered limestone also contains fluid inclusions and gives intense decrepitation it is difficult to interpret because carbonate minerals are ductile and have strong cleavage. The decrepitation is strongly influenced or even completely controlled by these mineral characteristics and it is not always obvious how to derive the original fluid temperatures from decrepitation of such carbonate samples.

Summary

Fluid inclusion studies are usually performed on quartz host minerals because normal microthermometric methods cannot be used on opaque minerals. By using baro-acoustic decrepitation it is possible to obtain some fluid inclusion data on the varied minerals in this region, including opaque minerals. However, it is more difficult to interpret decrepitation data from groups of dissimilar minerals such as those in this study. In particular, carbonate minerals are easily re-crystallized, which allows alteration of the fluid inclusions post-entrapment. Inclusion stretching, which is commonly observed within carbonate minerals and the relationship between crystallinity and decrepitation observed in these results are both major concerns which complicate the interpretation of baro-acoustic decrepitation within limestones and carbonates.

It is curious that in the Dadoushan pit the decrepitation of tourmaline in sample 2031 is similar to the nearby limestone sample 2030A. Tourmaline should preserve fluid inclusions well, in contrast to limestone. The similarity of decrepitation in this case indicates that, at least in some circumstances, limestone can preserve fluid inclusions.

Ferruginous samples such as those from Baishachong show diverse decrepitation patterns. Haematite / magnetite samples have been used extensively in other decrepitation studies and usually provide useful data. The variations seen at Baishachong probably indicate very variable formation conditions. As these samples were collected from spill at the adit entrance they could represent widely separated locations within the extent of the adit.

Low temperature sedimentary rocks such as chert show no decrepitation and this behaviour is also seen in the fine grained, un-recrystallised limestones from far distant locations near Dali. However, the coarse grained re-crystallized carbonate from Mt. Changsha, near Dali, shows intense decrepitation which is presumed to be related to fluids during a regional metamorphic event. Fluid inclusions within carbonates are expected to be reset to the conditions during the last thermal event.