Structural controls on lode gold deposits

transcript

Bruno Lafrance:Bruno Lafrance:Bruno Lafrance:Bruno Lafrance:

Bruno LafranceMineral Exploration Research Centre

Laurentian UniversityManitoba Open House Gold Workshop 2008

Outline

1. Definition of lode gold deposits

2. Formation of fractures and veins

3. Evolution of fault zones

4. Structural controls on mineralization

5. Summary and Conclusions

Outline

Magnitude Average Rupture Rupture Relative

slip length area frequency

M8 ∼4 m ∼100 km ∼104 km2 N yr-1

M7 ∼1 m ∼30 km ∼103 km2 ∼10 N yr-1

M6 ∼40 cm ∼10 km ∼102 km2 ∼102 N yr-1

M5 ∼10 cm ∼3 km ∼10 km2 ∼103 N yr-1

M4 ∼4 cm ∼1 km ∼1 km2 ∼104 N yr-1

M3 ∼1 cm ∼300 m ∼105 m2 ∼105 N yr-1

M2 ∼4 mm ∼100 m ∼104 m2 ∼106 N yr-1

M1 ∼1 mm ∼30 m ∼103 m2 ∼107 N yr-1

from van der Pluijm and Marshak (1997)

Multiple fault reactivation events

Flin Flon, Manitoba

Amisk basalt

Missi Conglomerate

Narrow Z-folded quartz vein

Chloritic laminae

Clockwise dextral rotation of stretching lineation in Missi conglomerate

Slickenline

Chloritic lamina in thick quartz vein

Extensional vein

Outline

4. Common structural controls on mineralization

I. Dilational and contractional jogs along faults/shear zones

II. Intersection of two syn-mineralization structures

III. Intersection of faults/shear zones with highly competent and/or chemically reactive rocks

IV. In faults/shear zones along lithological contacts between competent and less competent rocks

V. Zones that plunge parallel to a stretching lineation

VI. Fold hinge zones and limbs

Modified after Cox (2005)

I. Dilational and contractional jogs along faults/shear zones

Ore shoots are parallel to the intersection between the extensional veins and shear veins.

Robert and Poulsen (2001)

Diabase

Sulphide-quartz vein

Diorite

Iron Formation

MetasedimentaryRocks0 100

Plan of the North Zone, 250-foot level, Hard Rock Mine

Komis Mine, La Ronge Domain, Saskatchewan

IV. In faults/shear zones along lithological contacts between competent and less competent rocks

F4 Folded carbonate-rich gold-bearing domain parallel to S2

Field of view is 4mm.

Upper Canada mine – Cross-section

V. Zones that plunge parallel to a stretching lineation

Timiskaming conglomerate

Stretching lineation

Nova Scotia GeologyNova Scotia Geology

Precambrian to Carboniferous:undifferentiated intrusive rocks

Precambrian

Cambrian to Ordovician

Halifax FormationGoldenville Formation

Cambrian to Early Carboniferous

Early Carboniferous

Triassic to Jurassic

Late Carboniferous

100 km

Avalon Terrane

Halifax

Chedabucto Fault

Meguma Terrane

Cobequid

Gold District

VI. Fold hinge zones and limbs

Courtesy of D. Kontak and R. Horne

Meguma Vein Array

Bedding Concordant veinsSaddle ReefsLaminated veinsEn echelon arraysMassive veins

Cross veinsAngular veins

Flexural folding: Flexural folding: beddingbedding--parallel parallel slipslip hinge dilationhinge dilation

Saddle Reefs … represent hinge dilation …product of flexural folding … late in fold development

Ovens Dufferin

En echelon bedding-concordant veins

Movement horizon

Laminated vein

en echelon veins

Conclusions1. The siting of lode gold deposits is controlled by the

geometry, orientation, and nature of the host structures and host rocks.

2. Lode gold deposits typically form along host structures, such as faults, which acted as channels for the flow of large volumes of hydrothermal fluids.

3. The deposits occurs in zones of high permeability such as:

(I) Fractured damage zones: including contractionaljogs, intersection between syn-mineralization structures, competent host rocks intersected or hosted by syn-mineralization structures, lithologicalcontacts, fold limbs

(II) dilational zones: including dilational jogs and saddle reef

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Structural controls on lode gold deposits

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