shaun adams

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,

“One of the greatest discoveries a man makes, one of his greatest surprises, is to find he can do what he was afraid he couldn't.”

~ Henry Ford, 1901 ~