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

How CO2 inclusions form from aqueous fluids

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:

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:


AGCC expo, Adelaide, Aust. Oct. 14-18 2018

-----2019-----

ECROFI, June 24-26, Budapest, Hungary

AOGS, Singapore, 28 Jul-2 Aug 2019

SGA, Glasgow Scotland, Aug. 27-30 2019


Comprehensive Geology Conference Calendar


Mother Lode, California, USA

Samples collected 1995

Geology:

The Mother lode is a zone approximately 200 Km long and 1 Km wide of quartz veins within the melones fault zone. The host rocks near the veins are typically extensively carbonate altered. The veins are mesothermal and erosion since emplacement has been estimated at 5-15 Km. (Landefeld, L, Bicentennial gold 88, Melbourne). Fluid inclusion data by Weir and Kerrick (Econ. Geol., V82, 328, 1987) shows typical fluids of 10 mol % CO2 and pressures of 1 - 2 Kbar ( 4-8Km ).
 

Summary:

In 1995, none of the mines were active and access to sample localities was difficult. Sampling was restricted to road cuttings and mullock from a few old diggings. It was not at all clear during sampling which quartz was associated with Au mineralisation, and which quartz was barren so it is possible that these samples all represent barren mullock. However, given that 30 separate samples were collected it is anticipated that at least some of them should be representative of the mineralisation stage.

The most obvious feature of the decrepitation data is the almost complete lack of CO2-rich fluids, indicated by the absence of  low temperature decrepitation peaks near 300 C. Only samples 1342 and 1384D shows such peaks, and these were both thought to be barren samples at the time of collection! None of the possibly mineralised samples show CO2-rich fluids! But the CO2-rich fluid compositions and high internal  pressures reported by the previous fluid inclusion work should give rise to a prominent low temperature decrepitation pattern.

There is also a range of fluid temperatures apparent in the decrepitation data, with one set of  samples beginning to decrepitate at either 360 C, and another set beginning to decrepitate at 400 C. In addition, some samples show intermediate decrepitation onset temperatures. This temperature of commencement of decrepitation is an approximate indicator of the formation temperature (Tf) of the host quartz. It may be possible to relate these temperature changes to variations in the mineralisation, but there is insufficient data and geological control of the samples in this study to reach a conclusion here.

Either the samples collected failed to represent the mineralised zone, due to access difficulties, or these deposits formed from CO2 poor fluids despite the strong carbonate metasomatism nearby. Perhaps the fluids were initially rich in CO2, but failed to deposit silica  (and gold) until later in their evolution and cooling, at which stage they were of low CO2 content but deposited quartz, gold and sulphides.  The early stage CO2-rich fluids would have caused the observed carbonate metasomatism of the nearby host rocks, and in so doing removed silica while adding  carbonate. The observed quartz veins were not deposited from this fluid, which was actually transporting silica away from the system.  As the system cooled, quartz became supersaturated and deposited from later, cooler and CO2 deficient fluids. The gold mineralisation appears to be related to this later fluid rather than the earlier CO2-rich fluid which caused the wall rock carbonate metasomatism.

This decrepitation data indicates a need for better sampling and perhaps a multiple stage quartz deposition model to understand the nature of the mineralising fluids in the Mother Lode deposits.
 
 

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Geology map with sample locations



 
 
 

Decrepitation Data


H934  Sample# 1341A  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Harvard mine Sonora, quartz reef
Very Milky white quartz, adjacent to sulphide mass

H935  Sample# 1341B  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Harvard mine Sonora, quartz reef
Milky white quartz with fractures

H936  Sample# 1341C  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Harvard mine Sonora, quartz reef
Milky white quartz, oxidized S= present

H937  Sample# 1341D  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Harvard mine Sonora, quartz reef
Coarse grain Milky white quartz, fuchsite? present
 
 

Sample 1342 contains CO2-rich fluid inclusions as seen by the decrepitation at low temperatures near 300 C. However this sample is not thought to be auriferous and mineralisation appears to be associated with a separate CO2 poor fluid event.

H938  Sample# 1342   USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Near Carson Hill mine, barren ladder/gash qz in seds
Semi-translucent quartz, some clear crystals
 

H939  Sample# 1343A  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Near 6 mile creek crossing, old shaft on lode, prob ore
Milky white vitreous quartz

H940  Sample# 1343B  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Near 6 mile creek crossing, old shaft on lode, prob ore
white, semi-translucent and grey quartz, with oxidized S=
 
 

In sample 1344, the B and C subsamples were visually identical but had markedly different decrepitation. Although it is not clear how this difference relates to mineralisation, decrepitation allows the discrimination of visually identical quartz samples.

H941  Sample# 1344A  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Angel's Camp, large old pit/stope
Semi transl pale gy qtz, horiz vein (buck?) clear qtz cavity infillings

H942  Sample# 1344B  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Angel's Camp, large old pit/stope
Milky grey qtz, ox S= stains, vertical vein in line of stope

H943  Sample# 1344C  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Angel's Camp, large old pit/stope
Milky grey qtz, ox S= stains, vertical vein in line of stope

H944  Sample# 1344D  USA95 Calif  0.5g  -420+200u
Mother lode, Calif,  Angel's Camp, large old pit/stope
Semi translucent white qtz, loose qtz in stope
 

H945  Sample# 1345A  USA95 Calif  0.5g  -420+200u
Mother lode, Calif, Sutter Ck, Lincoln project, dumps
Brecciated milky white qtz

H976  Sample# 1345B USA95 0.5g  -420+200
Mother lode Calif, Sutter Ck., Lincoln project adit dumps
Coarse milky white qtz

H977  Sample# 1345C USA95 0.5g  -420+200
Mother lode Calif, Sutter Ck., Lincoln project adit dumps
Coarse milky white qtz

H982  Sample# 1345D  USA95 0.5g  -420+200      Acid Washed
Mother lode Calif, Sutter Ck., Lincoln project, adit dumps
milky white qtz banded with graphitic shale
 
 

In sample 1346, the A and B subsamples have higher temperature inclusions with Tf about 400 C than the C and D subsamples, which have a Tf of about 360 C. Although all of these samples were thought to be from ore quartz and are visually almost identical, there are clear differences in the decrepitation and this quartz is obviously not all from a single genetic event..

H978  Sample# 1346A USA95 0.5g  -420+200
Mother lode Calif, Sutter Ck., Lincoln project, ore stockpile dumps
Coarse milky white qtz

H979  Sample# 1346B  USA95 0.5g  -420+200
Mother lode Calif, Sutter Ck., Lincoln project, ore stockpile dumps
Coarse milky white qtz with oxidised S= or siderite

H980  Sample# 1346C  USA95 0.5g  -420+200
Mother lode Calif, Sutter Ck., Lincoln project, ore stockpile dumps
Coarse milky grey qtz with tr oxidised S=, CR inclusions

H981  Sample# 1346D  USA95 0.5g  -420+200      Acid Washed
Mother lode Calif, Sutter Ck., Lincoln project, ore stockpile dumps
Coarse milky white qtz with tr oxidised S=, CR inclusions
 
 

Sample 1347 is a transported boulder from an unknown source, most likely from in the eastern gold belt. It has both an unusually high decrepitation temperature (430 C) and very low decrepitation activity and is clearly different to the mother-lode belt samples.

H983  Sample# 1347   USA95 0.5g  -420+200
Mother lode Calif, Moccasin Ck. Float from placer exploited creek, coarse boulder
white banded qtz, heavy brown stained, S=
 
 
 

Sample 1348 was collected from a road cutting, presumed to be unmineralised quartz, for comparison with the nearby Mary Harrison mine quartz (samples 1349 & 1350 below). Despite several visual differences from the mine quartz, it is not really possible to distinguish the road-cutting quartz from the mine quartz, and they have similar decrepitation temperatures. A more extensive suite of samples is needed in order to determine if we can discriminate between barren and background quartz within the Melones fault zone using decrepitation. It is also possible that this road cutting, which is only 1 mile from the Mary Harrison mine, is too close to be considered as background and is within an anomalous aureole around the mine.

 

H984  Sample# 1348A  USA95 0.5g  -420+200
Mother lode Calif, Road cutting at Coulterville, westernmost, GSN pub 11, p95, 1
Coarse milky white qtz, trace S=

H985  Sample# 1348B  USA95 0.5g  -420+200
Mother lode Calif, Road cutting at Coulterville, west + 1   GSN pub 11, p95, st1
Coarse milky white qtz, CR inclusions

H986  Sample# 1348C  USA95 0.5g  -420+200
Mother lode Calif, Road cutting at Coulterville, west + 2   GSN pub 11, p95, st1
Coarse milky white qtz, CR inclusions, Vug with clear qtz xtals

H987  Sample# 1348D  USA95 0.5g  -420+200
Mother lode Calif, Road cutting @ Coulterville, easternmost, GSN pub 11, p95, st1
Very Coarse semi translucent white qtz
 

H988  Sample# 1349A  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, near steep inclined shaft
Milky white cse qtz, CR inclusions

H989  Sample# 1349B  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, near steep inclined shaft
Milky gy-white cse qtz, CR inclusions
 

H990  Sample# 1350A  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, ridge crest silicification/stockwork
Pale Bn/white granular qtz

H991  Sample# 1350B  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, ridge crest silicification/stockwork
banded milky white qtz with much clear coarse qtz xtals

H992  Sample# 1350C  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, ridge crest silicification/stockwork
banded milky white and pl bn qtz, much fuchsite

H993  Sample# 1350D  USA95 0.5g  -420+200
Mother lode Calif, Mary Harrison mine, ridge crest silicification/stockwork
White brown and grey mottled qtz, with banding
 

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