SHAUN ADAMS
M[Earth]Sci
SCHOOLOF EARTH, ATMOSPHERIC
AND ENVIRONMENTAL SCIENCES.THE UNIVERSITY OF MANCHESTER
THE ORIGIN AND ASSOCIATED MINERALIZATION OF ALLUVIAL
GOLD: THE LEADHILLS DISTRICT, SCOTLAND.
CONTENTS
• BACKGROUND
• GEOLOGY
• MINERALISATION
• PREVIOUS GOLD EXPLORATION
• SAMPLING
• DATA PRESENTATION
• CONCLUSIONS
BACKGROUND - What?
AIMS
• Identify mineralizing processes • Summarize the distribution of gold• Identify economically viable sites for gold
extraction.
EXECUTION
• Assessing published geological, geochemical and geophysical datasets, for an indication of gold distribution..
• Executed a Geochemical survey, on local stream sediments.
• Performed Microchemical analysis of gold grain(s) found.
BACKGROUND - Why? • Lead – mined from 13th century to 1959.
• Gold – alluvial gold extraction since the sixteenth century.
• Nuggets as large as 0.98kg
• 118 ounces of Leadhills gold- Scottish crown jewels • Longcleugh - favored spot, with
gravels between the glacial till and bedrock on river banks most amicable gold hosts (Leake et al., 1998).
• Gold has been recorded to be restricted to a 20cm layer at the base of the glacial till (Leake et al., 1998).
BACKGROUND – Where?
Moine Thrust
Great Glen Fault
Highland Boundary Fault
Southern Uplands Fault
Iapetus Suture
SOUTHERN UPLANDS TERRANE
(SUT)
MIDLAND VALLEY TERRANE
(MVS)
GRAMPIAN HIGHLANDS
NORTHERNHIGHLANDS
Sub Division Of Scottish Terranes
The Southern Uplands are subdivided into three fault banded tectno-stratigraphic units: THE NORTHERN, CENTRAL AND SOUTHERN BELTS (Peach and Horne 1899).
BACKGROUND – Where?
GEOLOGY- Study Area
MINERALISATION The Leadhills and Wanlockhead
veins display two stages of mineralization:
1st stage - is widespread consisting of quartz with associated pyrite, muscovite, albite and gold precipitated into faulted and brecciated Ordovician greywackes.
Main stage - large scale lead-zinc-copper-silver veins of the region (Temple, 1956).
PREVIOUS EXPLORATION
• 4 main types and 1 sub-type Leake et al., (1998)
• Type 1 and 1a - major systems show WSW trend.
– PPK and KKF, age doesn’t control occurrence.
– Shear zone related mineralisation
• Type 2 – NW trending tertiary dykes
• Type 3 – detrital deposits within ultra-mafic rocks.
• Type 4 – Permian red bed associated, (Thornhill basin).
SOURCES
SAMPLE SITESSEDIMENT SAMPLES FROM 2 RIVER SYSTEMS:
SYSTEM A
• Tracts A1-4
• PPF & KKF
• Leadhills Fault Zone
• Tertiary Dykes
• Pb veins
SYSTEM B
• Tracts B1-5
• PPF
• Fardingmullach fault
• Pb and Qtz veins
SAMPLING METHODS
Wet Sieves – divided the sediment into 5 fractions D>2000μm, C 2000-720μm, B 720-500μm, A 500-180μm, F<180μm.
Suction pump Crowbar
‘PAYSTREAKS’ Zones of rapid velocity transition – where heavy minerals are naturally deposited in the river.
RESULTS
• GRAIN SIZE DISTRIBUTIONS
• LITHOLOGICAL DESCRIPTIONS
• X-RAY FLUORESCENCE – MAJORS– MINORS
• ESEM IMAGES
RESULTS
• GRAIN SIZE DISTRIBUTIONS
• LITHOLOGICAL DESCRIPTIONS
• X-RAY FLUORESCENCE – MAJORS– MINORS
• ESEM IMAGES
1
10
100
0 200 400 600 800
G R A I N SI ZE (um)
SS3 SS4 SS5
A1
GRAIN SIZE - System A
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
SS21 SS24 SS8 SS7 SS6 SS5
A2
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
SS22 SS24 SS8 SS7 SS6 SS5
A3
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
SS23 SS24 SS8 SS7 SS6 SS5
A4
A3
A4
A2
A1
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
CU
MU
LA
TIV
E F
RE
QU
EN
CY
(%
)
SS9 SS10 SS11 SS12 SS13 SS25 SS26 SS27
B1
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
CU
MU
LA
TIV
E F
RE
QU
EN
CY
(%
)
SS17 SS15 SS14 SS13 SS25 SS26 SS27
B2
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
CU
MU
LA
TIV
E F
RE
QU
EN
CY
(%
)
SS19 SS18 SS14 SS13 SS25 SS26 SS27
B4
GRAIN SIZE - System B
B2
B3
B4
B5B1
1
10
100
0 500 1000 1500 2000
GRAIN SIZE (um)
CU
MU
LA
TIV
E F
RE
QU
EN
CY
(%
)
SS16 SS15 SS14 SS13 SS25 SS26 SS27
B3
RESULTS
• GRAIN SIZE DISTRIBUTIONS
• LITHOLOGICAL DESCRIPTIONS
• X-RAY FLUORESCENCE – MAJORS– MINORS
• ESEM IMAGES
LITHOLOGY + XRF MAJORSTract A2
•Al-Si-Fe rich
• large concentration of lithology B (metamorphics and shales).
• Peaks in Pb, S and Ba in the D fraction at ss7 and ss9. Tract B1
• Al-Si-Fe rich.
•Increase of Na correlates with an increase in B fraction from ss25-27.
• Higher concentrations of Si and K then system A, silica peaks correlate with lithology A.
• Metals favour grain size and show consistent pattern for full tracts (titanium).
KEY POINTS – Grain Size and Lithology + XRF Majors.System A
• Coarser fractions in A2 and A3 then A4.• Rapid increase in grain size ss7 (500-2000μm).• Large concentration of Low grade Metamorphics and Shales in
sediments. • Lead, sulphur and Barium peaks at ss7- restricted to the 720-
2000μm fraction.
System B
• Increase in fraction (500-2000μm) from ss9-10 and ss11-12.• ss17-15 - huge increase in 500-720μm fraction.• ss19/20-14 – increase in 720-2000μm fraction. • Higher concentrations of Si and K then system A, silica peaks correlate with sandstone rich sediments (Portpatrick formation).• Metals favour larger grain sizes
RESULTS
• GRAIN SIZE
• LITHOLOGICAL DESCRIPTIONS
• XRF – MAJORS– MINORS
• ESEM IMAGES
XRF MINOR- System ATract A1
Tract A2-4
• High concentrations of Fe and Pb.
•Fe gradual increase
•Peak at ss4 (all)
•Diminish by ss5
Arsenic
•A4 – source of Arsenic
• diluted by A3 and A4
•Peak at ss8 <20ppm
•Nothing at ss7Tract A2- minimum dashed line, A3 - large dashed, A4 – solid
•ss7 peak in Leadhills Base Metals (ppb >50,000ppm), gold path finder elements excluding Arsenic.
•ss6 trough of all above, increase to ss5.
XRF MINOR- System BTract B1 •System B - vastly depleted in Leadhills base metals in comparison to A. Peak in Pb at ss10 close to the Pb vein, Zn and Cu exhibit contrasting peaks from ss10-13
•Gold path finders – peak in unison at ss11 after quartz vein, peaks in As and Cr at ss13 and contrasting peak of Sb at ss25. Small increase of Sb after ss26 after the tertiary dyke.
XRF MINOR- System BTract B2-5
B4,5
B2,3
Tract B3 + B4 - Block lines, Tract B2, B5 - Dashed
•Peak in Sb and Pb at ss15.
•Correlative trough of Pb and Sb at ss18.
•Substantial decrease in Sb when all tracts coalesce.
• The most significant Pb deposits - Tract B5.
•Tracts contains highest concentrations of Cd.
XRF- Glacial Till
CO
NC
EN
TR
AT
ION
(%
)
MAJOR
MINOR
•Correlate to stream sediment analysis – high concentrations of Si and Al.
•High concentrations of gold path finder elements.
•Grey – larger concentration of Leadhills base metals and gold path finder elements then Red.
RESULTS
• GRAIN SIZE DISTRIBUTIONS
• LITHOLOGICAL DESCRIPTIONS
• X-RAY FLUORESCENCE – MAJORS– MINORS
• ESEM IMAGES
ESEM IMAGES
CONCLUSIONS • The initial mineralization of gold in the area has progressed
through episodes of demineralization, large scale glaciation and redistribution by fluvial systems.
• The widespread nature of lead-zinc mining has created a smear, resulting in the blurring out of precious metals and creating a heavy skew on grain size distribution.
• XRF data indicates that System A is the more fertile system in terms of bulk metal composition
• Leadhills base metals concentrate in the larger fractions - fracture dimensions in rock, durability.
• A significant source for gold in the system is likely to be indicative of glacial sediments.
• Other potential gold sources could relate to tertiary dykes (Leake et al., 1998), or by the main stage mineralization redistributing gold from the initial stage.
• The relatively high concentrations of As, in fluvial system B could indicate that Au-As type mineralization as seen in the Glendinning deposit (Baron and Parnell, 2005) is prevalent in the area.
POTENTIAL SITES FOR GOLD MINERALISATION• Vein gold – within the Leadhills Shear Zone upstream from ss7
•Remobilization of gold from early veins in the Leadhills area, by the main stage mineralization that formed the major Lead-Zinc-Copper-Silver veins.
Gold associated with Au-As mineralization in quartz veins, elevated Arsenic levels are the main justification for this which could potentially be accommodated by Portpatrick Formation and glacial tills.
REFERENCESBaron, M., Parnell, J. (2005), Fluid evolution in base metal sulphide
mineral deposits in the metamorphic basement rocks of southwest Scotland an Northern Ireland, Geological Journal, V 40:1, Pp 3 - 21
Leake, R.C., Chapman, R.J., Bland, D.J., Stone, P., Cameron, D.G., Styles, M.T. (1998) The origin of alluvial gold in the Leadhills area of Scotland: evidence from interpretation of internal chemical characteristics. Journal of Geochemical Exploration, 63, 7-36.
Pattrick, R.A.D., Russell, M.J. (1989) Sulphur isotopic investigation of Lower Carboniferous vein deposits of the British Isles. Mineralium Deposita, 24, 148-153.
Peach, B. N., & Horne, J. (1899) The Silurian Rocks of Britain.Vol. I, Scotland. Mere. Geol. Surv. U.K.
Temple, A.K. (1956) The Leadhills-Wanlockhead lead and zinc deposits. Transactions of the Royal Society Edinburgh: Earth Sciences, 63, 85-113.
Woodcock, N. H. & Strachan, R. (2000). Geological History of Britain and Ireland. Oxford: Blackwell Science. pp19-36,112-123,
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