MECHANICS OF PLASTIC DEFORMATION> PLASTIC DEFORMATION IS BY FLOW; THE PROCESSES MAY BE:1.INTERGRANULAR MOVEMENT A) DISPLACEMENT TAKES PLACE BETWEEN INDIVIDUAL GRAINS ALONG GLIDE PLANE B) INDIVIDUAL CRYSTALS AND GRAINS OF ROCK MOVE UNDER STRESS C) INDIVIDUAL GRAINS MAINTAIN THEIR SHAPE AND SIZE D) EACH GRAIN MOVES & ROTATES RELATIVE TO EACH OTHER-

D) GRANITOID ROCKS - LARGER CRYSTALS ARE BROKEN INTO SMALLER SPHERICAL GRAINS & ROTATE RELATIVE TO EACH OTHER(GRANULATION)

2. INTRAGRANULAR MOVEMENT A) DISPLACEMENT WITHIN INDIVIDUAL

CRYSTALS B) SLIPPING TAKES PLACE ALONG GLIDE

PLANES C) ATOMIC STRUCTURE CONTROLS POSITION

& NUMBER OF GLIDE PLANES; GLIDE PLANES ARE RELATED TO SYMMETRY OF MINERAL.

TYPES OF GLINDING PLANES1.TRANSLATION-GLIDING A) LAYERS OF ATOMS SLIDE INTERATOMIC RELATIVE TO

OTHER LAYERS B) SHAPE IS CHANGED; CRYSTAL LATTICE IS UNCHANGED2. TWIN-GLIDING A) LAYERS OF ATOMS SLIDE A FRACTION IN AN

INTERATOMIC DISTANCE RELATIVE TO ADJACENT LAYERS

B) DISPLACED PART BEARS TWINNED RELATIVE TO UNDISPLACED PART

TRANSLATION-GLIDING. A. ARRANGEMENT OF ATOMS BEFORE GLIDING. B. ARRANGEMENT OF ATOMS AFTER GLIDING ALONG PLANES G1 G1 & G2 G2

TWIN-GLIDING. A. ARRANGEMENT OF ATOMS BEFORE GLIDING. B. ARRANGEMENT OF ATOMS AFTER GLIDING ON GLIDE PLANES G1G1, G2G2, G3G3 AND G4G4.

C) SHEETS OF ATOMS DON’T SLIP ALONG GLIDE PLANES IN ANY DIRECTION

GLIDE DIRECTIONS ARE GLIDING LINES OF DIRECTION OR MOVEMENT

RECRYSTALLIZATION – PLASTIC DEFORMATION WITHOUT CHANGE IN SHAPE, E.G. LIMESTONE TO MARBLE

A) NUMBER OF CRYSTALS PER UNIT VOLUME DECREASES; SIZE OF INDIVIDUAL CRYSTALS INCREASES

B) UNDER DIFFERENTIAL PRESSURE CONDITIONS, SOLUTION & RECRYSTALLIZATION OF ROCK IS SHORTHENED OR LENGTHENED

LIMESTONE TO MARBLE

RIECKE PRINCIPLE – 1) SOLUTIONS IN PORE SPACES OF ROCKS DISSOLVE OR OCCUR MOST READILY UNDER GREATEST EXTERNAL PRESSURE 2) RECRYSTALLIZATION OCCURS MOST READILY AT POINTS OF LEAST EXTERNAL PRESSURE. RP IS APPLICABLE TO RECRYSTALLIZATION OF METAMORPHIC ROCKS. RECRYSTALLIZATION FACILITATES PLASTIC DEFORMATION

> MAJOR TYPES OF DEFORMATION ARE 1) ELASTIC, 2) PLASTIC AND 3) RUPTURE

DESCRIPTION OF FOLDSFOLDS – WAVELIKE UNDULATION CAUSED BY

BENDING OF ROCKS USUALLY PRODUCED BY COMPRESSIVE STRESSES; A BEND IN THE ROCK LAYERS

ATTITUDE OF BEDS• STRIKE – TREND OR DIRECTION OF STRATA

OR BEARING OF THE HORIZONTAL LINE ON THE PLANE OR DIRECTION OF A LINE FORMED BY THE INTERSECTION OF THE BEDDING AND A HORIZONTAL PLANE.

2) Dip- angle between the bedding & a horizontal plane

PARTS OF FOLDS• AXIAL PLANE(SURFACE) – PLANE THAT

DEVIDES FOLD SYMMETRICALLY, USUALLY BISECTS THE LIMBS

• LIMBS(FLANKS) – SIDES OF THE FOLDS• FOLD AXIS – LINE OF MAXIMUM OF THE

FOLD• CREST – LINE ALONG THE HIGHEST PART OF

THE FOLD• CRESTAL PLANE – SURFACE FORMED BY THE

CRESTS

TROUGH – LINE OCCUPYING THE LOWEST PARTS OF THE FOLD

• TROUGH PLANED – PLANE CONNECTING TROUGH LINES

• HINGE LINE – CONNECTS POINTS OF MAXIMUM FOLD CURVATURE

• INFLECTION POINT – LIMB POINT WHERE SENSE OF CURVATURE CHANGES

•

 

> PARTS OF FOLD (DRAW DIAGRAMS) Page 36 > NOMENCLATURE OF FOLDS

FOUR GENERAL TYPES OF FOLDS1.ANTICLINE – LIMBS DIPPING AWAY FROM

EACH OTHER & AWAY FROM THE FOLD AXIS2.SYNCLINE – LIMBS DIPPING TOWARDS EACH

OTHER & TOWARD THE FOLD AXIS3.DOME – CIRCULAR OR ELLIPTICAL CONVEX

FOLD WITH LIMBS DIPPING TOWARD AWAY FROM THE CENTER

4.BASIN – CIRCULAR OR ELLIPTICAL CONCAVE FOLD WITH LIMBS DIPPING TOWARD THE CENTER

> ANTICLINE & SYNCLINE CAN BE:1) SYMMETRIC – LIMBS DIP IN OPPOSITE

DIRECTIONS AT EQUAL ANGLE & AXIAL PLANE IS VERTICAL

2) ASSYMETRIC – LIMBS DIP IN OPPOSITE DIRECTIONS AT DIFFERENT ANGLES & AXIAL PLANE IS INCLINED

3) OVERTURNED - LIMBS DIP IN THE SAME DIRECTION AT DIFFERENT ANGLES & THE AXIAL PLANE IS INCLINED

4) RECUMBENT - OVERTURNED FOLD WITH HORIZONTAL LIMBS AND AXIAL PLANE

5) PLUNGING – FOLD AXIS IS PLUNGING AT AN ANGLE RATHER THAN BEING HORIZONTAL

OTHER VARIETIES OF FOLDS WHICH MAY BELONG TO ANY OF THE FOUR TYPES OF FOLDS

1.ISOCLINAL – TWO LIMBS OF FOLD DIP AT EQUAL ANGLES IN THE SAME DIRECTION

2.FAN FOLD – BOTH LIMBS ARE OVERTURNED3.CHEVREON FOLD – AXES ARE SHARP &

ANGULAR

4. MONOCLINE – BEDS INCLINED IN A SINGLE DIRECTION

5. STRUCTURAL TERRACE – DIPPING STRATA ASSUME A HORIZONTAL ATTITUDE

6. HOMOCLINE (Gk, “SINGLE INCLINATION) – STRATA DIPPING IN ONE DIRECTION AT A UNIFORM ANGLE

7. CLOSE OR TIGHT FOLD – DEFORMATION IS INTENSE TO CAUSE FLOWAGE OF MORE MOBILE BEDS

8. OPEN FOLD – FLOWAGE DOES NOT TAKE PLACE

DRAG FOLD – MINOR FOLDS ARE FORMED WHEN A COMPETENT BED SLIDES PAST THE INCOMPETENT BED

STRCTURAL CONTROLS ON MINERALIZATION• STRUCTURAL FEATURES (FAULTS, FRACTURES,

FOLDS) IN ROCKS CAN INFLUENCE ORE DEPOSITION 1) HEAT & PRESSURE - FORCES THAT CAN CAUSE

DEFORMATION OF ROCKS : 1.1 FOLDING – BENDING OF ROCKS 1.2 FAULTING – FRACTURING AND DISPLACEMENT 1.3 SHEARING – SLIDING PARALLEL TO THE PLANE OF CONTACT BETWEEN TWO ROCKS

1.4 COMPRESSION – COLLIDING TOGETHER OF TWO ROCKS 1.5 EXTENSION- SEPARATING OR INCREASING THE DISTANCE BETWEEN TWO ROCKSHYDROTHERMAL DEPOSITS – HYDROTHERMAL

FLUIDS CIRCULATE & LOCALIZE THE DEPOSITS ALONG THE STRUCTURES.

• PRE-MINERAL STRUCTURES – REFER TO STRUCTURES FORMED PRIOR TO MINERALIZATION

• POST MINERAL STRUCTURES – REFER TO STRUCTURES FORMED AFTER MINERALIZATION EVENTS

Figure 10 – 6.  Fracture systems in rocks overlying an igneous intrusion.   A & B:  radial fractures above a circular intrusion. C & D:  longitudinal fractures above an elliptical intrusion (from Emmons, 1937).

Figure 10 – 7.  Cross section of Ft. Knox granite-hosted gold deposit, Fairbanks District, Alaska, showing late-stage shear zones containing high grade gold mineralization ( 1.0 ounce per ton) (after Bakke, 1991).