soroako serpentinisation
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FIELD DETERMINATION OF SERPENTINISATION AT SOROAKO
FIELD DETERMINATION OF SERPENTINISATION AT SOROAKO
JUNE 2002
Jacques Babineau
PLAN OF PRESENTATION
1. The olivine problem in the East Block coarse fraction (> 1 inch)
2. Classification of serpentinisation degree for field determination
3. Observations to be made on rock samples
4. Comparative rock show 1
5. Tests to be performed on rock samples
6. Comparative rock show 2
7. Practice on real rock samples
THE OLIVINE PROBLEM:WHY IS IT IMPORTANT?
•Unserpentinised peridotites are composed of 70 to 80% olivine
•East Block rocks are serpentinised at variable degrees: some olivine left
•Olivine melts at very high temperature (up to 1890 degrees Celsius)
•High olivine % in E-Block coarse fraction (> 1 inch) do not melt in furnace
•Unmelted olivine tends to block heat flow & slag tap in the furnace
•Chris Doyle has determined that <15% olivine is not a problem
•Chris Doyle has determined that >40% olivine is definitely a problem
PETROGRAPHIC OBSERVATION:SERPENTINE VS OLIVINE
Soroako Bedrock
y = -1.116x + 80.884
0.00
20.00
40.00
60.00
80.00
100.00
0.00 20.00 40.00 60.00 80.00 100.00
Observed Modal Serpentine (%)
Ob
serv
ed M
od
al O
livin
e (%
)
West type 3
West type 2
West type 1
Hybrid
East -6"
East -18"
All
Linear (All)
Field ofSerp > 40%
Ol < 40%
Field of Serp > 60%
Ol <15%
FIELD CLASSIFICATION OF SERPENTINISATION
• VERY LOW (OR ABSENT) <5% - Serpentine undetectable
• LOW 5 to 39%
• MODERATE (MEDIUM) 40 to 59%
• HIGH 60 to 100%
Note: Serpentinisation is gradational and the properties of the serpentinised rocks to be observed are also gradational
SERPENTINISATION:WHAT IS HAPPENING TO THE ROCK?
OLIVINE SERPENTINE + MAGNETITE
OPX SERPENTINE + MAGNETITE
•Grain size decreases
•Magnetic susceptibility increases
SERPENTINISATION DETERMINATION OBSERVATIONS: WHAT DO YOU SEE?
1. COLOR ON FRESH BROKEN SURFACE
- Gets darker with increasing serpentinisation
2. ASPECT ON FRESH BROKEN SURFACE: GLASSY TO DULL
- Gets dull or amorphous with serpentinisation
3. COLOR ON DRY DRILL CORE AND SAW-CUT SURFACES
- Gets darker with increasing serpentinisation
4. TEXTURE ON DRY DRILL CORE AND SAW-CUT SURFACES
- Dark serpentine stringers increase in thickness and proportion
- grain size & % of matrix in stringers decrease with serpentinisation
TEXTURE ON DRY DRILL CORE AND SAW-CUT SURFACES
LOW SERPENTINISATION (30%)LAILA HARZBURGITERX307115 40% OL
SERPENTINE STRINGERS WITH FINE-GRAINED OLIVINE SUB-GRAINS
THIN SECTION VIEW
CROSSED NICOLS
cm
COLOR & ASPECT ON FRESH BROKEN SURFACES
VERY LOW0% SERP
RX307103
LOW20% SERPRX307126
MEDIUM50% SERPRX307113
Light color
Glassy &
Crystalline
aspect
HIGH80% SERPRX307127
5 mm
Dark colorDull
amorphous
aspect
VERY LOW0% SERP
RX307120
COLOR & ASPECT ON FRESH BROKEN SURFACES
LOW10% SERPRX307160
MEDIUM45% SERPRX307112
HIGH60% SERPRX307125
1 c
m
LOW20% SERP
RX307126
COLOR AND TEXTUREON DRY (WET?) CORE & SAW CUT SURFACES
VERY LOW0% SERP
RX307104
MEDIUM50% SERP
RX307116
HIGH70% SERP
RX307156
Light colorno
serpentine stringersNo fine-grained matrix
Dark colorserpentine stringers
fine-grained matrix
COLOR AND TEXTUREON DRY (WET?) CORE & SAW CUT SURFACES
VERY LOW0% SERP
RX307101
LOW20% SERP
RX307161
MEDIUM45% SERP
RX307112
HIGH90% SERP
RX307146
SERPENTINISATION DETERMINATION TESTS:WHAT DO YOU FEEL AND SEE?
1. HARDNESS ON ANY FRESH SURFACE
- Decreases with serpentinisation from hard (up to 7) to soft (2.5)
2. ROUGHNESS (TOUCH) ON FRESH BROKEN SURFACE
- Decreases with serpentinisation from rough to smooth
- Highly serpentinised rocks may have a soapy or waxy touch
3. ATTRACTION TO A PEN-MAGNET
- Increases with serpentinisation from none (0) to medium (2.5)
4. MAGNETIC SUSCEPTIBILITY
- Increases with serpentinisation from 0 to 20, or higher?
4. DENSITY: Decreases with serpentinisation from roughly 3.4 to 2.5
COLOR & ASPECT ON FRESH BROKEN SURFACE
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
LOWSERPENTINISATION
10% SERPRX307160
15% SERPRX307138
20% SERPRX307126
30% SERPRX307115
COLOR & ASPECT ON FRESH BROKEN SURFACE
60% SERPRX307125
80% SERPRX307139 80% SERP
RX30712790% SERPRX307109
HIGH SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
COLOR & ASPECT ON FRESH BROKEN SURFACE
40% SERPRX307144
45% SERPRX307114
50% SERPRX307113
MEDIUM SERPENTINISATION
MEDIUM SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
COLOR & ASPECT ON FRESH BROKEN SURFACE
0% SERPRX307120
0% SERPRX307158
0% SERPRX307103
<5% SERPRX307101
VERY LOW /ABSENT SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
SUBHEDRALOPX
ANHEDRALCPX
COLOR AND TEXTUREON DRY CORE & SAW CUT SURFACES
80% SERP
RX30712460% SERP
RX30712590% SERP
RX307146
HIGH SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
COLOR AND TEXTUREON DRY CORE & SAW CUT SURFACES
30% SERP
RX30711520% SERP
RX30716120% SERP
RX307126
LOW SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
COLOR AND TEXTUREON DRY CORE & SAW CUT SURFACES
40% SERP
RX30714445% SERP
RX30711245% SERP
RX307114
MEDIUM SERPENTINISATION
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
VERY LOW / ABSENT SERPENTINISATION
0% SERP
RX3071010% SERP
RX3071040% SERP RX307103
0% SERP
RX307158
COLOR AND TEXTUREON DRY CORE & SAW CUT SURFACES
HOW MUCH SERPENTINISATION IN THESE ROCKS ?
FIELD DETERMINATION OF SERPENTINISATION TOOLS NEEDED
•Your eyes
•Your hands
•A knife or a nail
•A pen-magnet or/and a magnetic susceptibility-meter
•Use the combination of all the properties of the rock
FIELD DETERMINATION OF SERPENTINISATION AT SOROAKO:
CONCLUSION
•The serpentinisation degree is gradational between 2 end members
•The properties of the serpentinised rocks are also gradational
•There is an overlap of some of the properties between the classes
•Do not use only one property to determine the serpentinisation degree
•Use the combination of all the properties of the rock
THE END : LETS PRACTICE!
FIELD CLASSIFICATION OF SERPENTINISATION
• VERY LOW (OR ABSENT) <5%
• LOW 5 to 39%
• MODERATE (MEDIUM) 40 to 59%
• HIGH 60 to 100%
Note: Serpentinisation is gradational and the properties of the serpentinised rocks to be observed are also gradational