Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
Newest Topics:

Gold at Okote, Ethiopia

Viewpoints:

How CO2 inclusions form from aqueous fluids (UPDATED)

Understanding heterogeneous fluids : why gold is not transported in CO2 fluids

Gold-quartz deposits form from aqueous heterogeneous fluids: NOT from CO2 fluids

Inclusion shapes can prove heterogeneous FI trapping

Disproportional FI trapping from heterogeneous fluids explains gas-dominant systems

A discussion of H2 analysis by mass spectrometry

A mechanism to form H2 in the MS ioniser during analyses


News:

Kalgoorlie Au data

Sangan skarn Fe deposits, Iran

New model 205 decreptiometer

Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China


Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids


Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



 Interesting Conferences:

-----2020-----

Geoconvention 2020, Calgary Canada, May 11-15 2020

PACROFI 2020, Calgary Canada, May 18-20 2020

ACROFI 8,Townsville, Qld, Aust. June 16-18 2020

AOGS2020, Hongcheon Sth Korea, Jun 28-Jul 4 2020

6th Archean, Perth, W.Aust. July 14-16 2020

SEG2020, Whistler, BC, Canada, Sept. 15-18 2020


Comprehensive Geology Conference Calendar


Selecting the best grainsize for decrepitation analyses


Decrepitation analyses are carried out on crushed grains, preferably monomineralic. The optimum sample grainsize depends on the fluid inclusion sizes and the host mineral. Several samples have been analysed on multiple size fractions to ascertain the most suitable grainsize to use for routine analyses. Grainsizes of -1100+600 microns (-20+30 mesh/inch), -600+420 microns (-30+40 mesh/inch), -420+200 microns (-40+80 mesh/inch) and <200 microns (<80 mesh/inch) have been analysed on several different minerals to determine the best size fraction to use. Coarse grainsize samples tend to give erratic (noisy) decrepigrams probably because only fluid inclusions near the grain surfaces decrepitate. Very fine grainsize samples give very low decrepitation levels, probably because many fluid inclusions are destroyed during sample crushing. The preferred grainsize for decrepitation analyses was selected to be the -420+200 micron fraction. The following graphs show the decrepitation results of each of these 4 grainsizes on 2 samples of quartz, 2 samples of andesite and one sample of dolomite. In the andesite samples, the fluid inclusions are probably hosted in feldspars.

Quartz

2 samples of quartz from the Mt Pleasant Molybdenum prospect show that the highest response depends on the fluid inclusion population and varies for different populations within the same sample. For the low temperature population in sample A86925, the 2 coarse fractions (purple and green) give the highest response. But the response of the -420+200 micron fraction (blue) is good and this fraction usually gives less random fluctuations with temperature and better reproducibility. In the fine grained fraction (orange), many fluid inclusions have been destroyed in the sample preparation and the analytical response is unacceptably weak.

quartz sample, grainsize tests

quartz sample, grainsize tests

Andesite

In these andesite samples, the fluid inclusions are probably hosted in feldspar minerals.The best response is in the -420+200 micron fraction (-40+80 mesh) (blue curve). The response in the <200 micron fraction (-80mesh) (orange curve) is substantially less. In sample ACH697-26 the -420+200 micron fraction was analysed twice, analyses C64 and C73 (blue and yellow), and these duplicate analyses are in very good agreement and show that variations in the different grainsize fractions are real and not merely instrumental reproducibility. The -420+200 micron fraction is the best for routine analyses.

andesite sample grainsize tests


andesite sample grainsize tests



Dolomite

In dolomite, the 3 coarser grainsize fractions give similar decrepitation intensity, but the -420+200 micron fraction (blue curve) gives the smoothest (least random noise) plot and is the best size for routine analyses. The fine grainsize fraction of <200 microns (orange curve) again shows weak decrepitation due to destruction of many fluid inclusions during sample preparation.

dolomite sample grainsize tests

Conclusions


For routine  decrepitation analyses, the best grainsize fraction is the -420+200 micron fraction. Coarser grainsize fractions can give useful results, but fine grainsize samples are unacceptable due to loss of fluid inclusions during sample crushing.

However if you are trying to avoid interference from fluid inclusions in other analytical experiments then use of fine grained sample fractions could help.