Mother lode, California

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 (T_f) 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 CO₂deficient 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.

Back to main contents

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 T_fabout 400 C than the C and D subsamples, which have a T_fof 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