PGE Geochemistry and Mineralogy of
Dunite, Chromitite, and Laterite Samples
from the Acoje Ophiolite Block, Philippines
Michelle Dossey
Geosciences, master's level (120 credits)
2023
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
,Abstract
Ni-laterites have the potential to become unconventional ore deposits for platinum group elements
(PGE). This study was conducted to determine enrichment trends of PGE as a result of the Ni-
laterization process. 6 samples were selected by mine workers from the protolith, saprolite, and
limonite horizons of the Ni-laterite profile from the Acoje ophiolite block, Luzon, Philippines, and sent
to Luleå University of Technology (LTU). 2 samples representing the protolith are described as dunite
having undergone serpentinization, 1 sample is a massive chromitite from the saprolite layer of the
laterite profile, 1 sample is a massive chromitite from the limonite layer of the laterite profile, and 2
samples are limonitic soils.
Total PGE contents of the investigated Acoje samples range from 161-1180 ppb with the highest
contents of PGE occurring in the limonite hosted chromitite, and the lowest contents in the saprolite
hosted chromitite. C1 chondrite-normalized patterns reveal distinct trends of the PGE in the different
sample types: dunite samples have a positive trend from Ir-Pd, the chromitite samples have a negative
trend from Ru-Pd with a negative Ir anomaly and the limonite samples have a strong positive trend
from Ir-Pd. Rare earth elements (REE) chondrite-normalized patterns of the samples show a negative
Ce anomaly in the limonite while the dunite and saprolite-hosted chromitite have negative Eu
anomalies. Cr# (Cr/[Al+Cr]) and Mg# (Mg/[Fe2++Mg]) were analyzed using automated mineralogy and
produced Cr# values ranging from 0.67 – 0.77 and Mg# values from 0.46 – 0.59.
17 platinum group minerals (PGM) were identified from the Acoje samples: 9 from the dunite, 1 from
the saprolite-hosted chromitite, 6 from the limonite-hosted chromitite and 1 from the limonite.
Laurite ((Ru,Os)S2) was identified in samples A-02 and A-07 and is the only primary mineral identified.
Secondary PGM, thought to have formed due to alteration processes during serpentinization, were
identified as alloys composed of: Cu-Pd, Cu-Pt, Pt-Ni-Cu, Pt-Fe, Ir-Ni-(Pt,Fe), and Cu-Pt-Au-(Pd-Ag).
PGM are small, measuring consistently <10 µm in diameter. Laurite occurs as inclusions in unfractured
chromite. PGM alloys in the dunite samples occur along the boundary of sulfide minerals or within
serpentine. PGM identified in the limonite-hosted chromitite occur along interstitial fractures within
chromitite or in a Fe-Al oxide matrix within pore spaces.
,Contents
Abstract .....................................................................................................................................................
Figures ..................................................................................................................................................... ii
Tables ..................................................................................................................................................... iv
Abbreviations and vocabulary ............................................................................................................... iv
Acknowledgement .................................................................................................................................. v
1. Introduction ........................................................................................................................................ 1
1.2 Goal of the study ........................................................................................................................... 1
2. Literature review ................................................................................................................................. 1
2.1 Geologic Setting ............................................................................................................................ 1
2.2 Ophiolites and podiform chromitites............................................................................................ 3
2.2.1 Chromitite classifications ....................................................................................................... 3
2.3 Ni-Co laterites ............................................................................................................................... 4
2.3.1 Geology and occurrence ........................................................................................................ 4
2.3.2 Ni-laterite profile ................................................................................................................... 4
2.3.3 Ni-laterite ore types ............................................................................................................... 4
2.3.4 PGE in Ni-laterites .................................................................................................................. 5
3. Materials and Methods ....................................................................................................................... 5
3.1 Quantitative element screening ................................................................................................... 6
3.2 SEM/EDS mineral analysis ............................................................................................................. 6
3.3 Cr# and Mg# .................................................................................................................................. 6
3.4 Hydroseparation and polished monolayers .................................................................................. 7
3.5 Software ........................................................................................................................................ 7
4. Results ................................................................................................................................................. 7
4.1 Whole rock geochemistry ............................................................................................................. 7
4.1.1 Dunite samples....................................................................................................................... 7
4.1.2 Chromitite samples ................................................................................................................ 9
4.1.3 Limonite samples ................................................................................................................. 10
4.1.4 Ultramafic Index of Alternation (UMIA)............................................................................... 11
4.2 Mineralogy .................................................................................................................................. 11
4.2.1 Chromite geochemistry........................................................................................................ 12
4.2.2 Silicates ................................................................................................................................ 14
4.2.3 Sulfides ................................................................................................................................. 17
4.2.4 Platinum group minerals ...................................................................................................... 18
5. Discussion.......................................................................................................................................... 24
5.1 Comparison with previous publications ..................................................................................... 24
i
, 5.1.1 Primary mineralogy .............................................................................................................. 24
5.1.2 UMIA .................................................................................................................................... 24
5.1.3 REE chondrite-normalized pattern ...................................................................................... 25
5.1.4 PGE chondrite-normalized pattern ...................................................................................... 26
5.2 Automated mineral identification .............................................................................................. 27
5.3 Platinum Group Minerals ............................................................................................................ 28
5.3.1 Mineral types and abundances ............................................................................................ 28
5.3.2 PGE mobilization .................................................................................................................. 28
6. Conclusion ......................................................................................................................................... 29
References ............................................................................................................................................ 30
Appendix A – A04 Hydroseparation and Monolayer ............................................................................ 33
Appendix B – Thin Sections ................................................................................................................... 39
Appendix C – Platinum group mineral elemental maps ....................................................................... 43
Figures
FIG. 1 (A) OVERVIEW OF LUZON, PHILIPPINES; (B) GEOLOGIC MAP BASED ON YUMUL (2001). THE ACOJE (RED) AND COTO (BLUE)
BLOCKS OF THE MASINLOC MASSIF WITH THE IBA FAULT (E-W TRENDING) AND SUBIC BAY FAULT (N-S TRENDING) DIVIDING THE
SAN-ANTONIO AND CABANGAN MASSIFS (GREEN), GEOLOGIC FEATURES ARE BASALT-DIABASE, GABBRO-TROCTOLITE,
HARZBURGITE, AND PYROXENE-GABBRONORITE; (C) LOCATION OF STUDY SAMPLES). ........................................................ 2
FIG. 2 EXAMPLE OF A LATERITIC PROFILE. DEPTH OF THE LIMONITIC LAYER IS DEPENDENT ON THE GEOLOGY OF THE PROTOLITH,
CLIMATE, AND TOPOGRAPHIC RELIEF. (BUTT & CLUZEL, 2013; AIGLSPERGER, ET AL., 2016) ............................................. 4
FIG. 3 SAMPLES RECEIVED FROM ACOJE. A-01 AND A-02 ARE DUNITE SAMPLES; A-04 AND A-05 ARE LIMONITE SOILS; A-07 AND A-
09 ARE CHROMITITES. .......................................................................................................................................... 6
FIG. 4 C1 CHONDRITE-NORMALIZED DIAGRAM OF REE (MCDONOUGH & SUN, 1995)............................................................. 9
FIG. 5 C1 CHONDRITE-NORMALIZED DIAGRAM OF PGE (NALDRETT & DUKE, 1980). (A) DUNITE SAMPLES; (B) CHROMITITE SAMPLES
WITH REFERENCE VALUES FROM BACUTA, ET AL. (1990); (C) LIMONITE SAMPLES. ......................................................... 10
FIG. 6 (A) AF-S-M PROVIDES A VISUAL DEPICTION OF MG AND SI LOSS AS ULTRAMAFIC MATERIAL WEATHERS FROM PROTOLITH TO
LIMONITE; (B) A-F-SM DIAGRAM DISPLAYS THE PREFERENTIAL DEVELOPMENT OF FE-RICH (LATERIZATION) VS AL-RICH
(BAUXITIZATION) OF THE WEATHERED SOIL. INCLUDES DATA FROM AIGLSPERGER, ET AL. (2016) AND AQUINO, ET AL. (2022).
..................................................................................................................................................................... 11
FIG. 7 MAPPING OF THE OXIDE WT.% CONTENT OF THE INDIVIDUAL CHROMITE CRYSTALS MEASURED IN THIN SECTIONS (IMAGE
BOTTOM RIGHT CORNER FOR REFERENCE). FROM LEFT TO RIGHT ON THE BOTTOM, SAMPLES A-01, A-02, AND A-07; TOP, A-
09. ................................................................................................................................................................ 12
FIG. 8 CR# [CR/(CR+AL)] VS MG# [MG/{MG+FE2+)], CENTER POINT OF EACH SAMPLE SPREAD REPRESENTS THE MEDIAN. INCLUDES
DATA SETS FROM ORBERGER & ALLEWELDT (1994) AND BACUTA ET AL. (1990) ARE EXAMPLES OF CHROMITITE FROM THE
ZAMBALES OPHIOLITE COMPLEX. POINTS PLOTTED IN THE TOP LEFT CORNER BY BACUTA ET AL. (1990) ARE SAMPLED FROM THE
COTO BLOCK AND ARE EXAMPLES OF HIGH-AL CHROMITITES (CR# < 0.6). ................................................................... 13
FIG. 9 OLIVINE WITH CHROMITE (OPAQUE) AND SERPENTINE VEINS; (LEFT) A-01; (RIGHT) A-02. ............................................. 14
FIG. 10 PYROXENE CLASSIFICATION BASED ON MORIMOTO (1989), SAMPLE A-09 PLOTTED WITH ORBERGER & ALLEWELDT (1994).
..................................................................................................................................................................... 15
FIG. 11 PYROXENE BSE IMAGES FROM SAMPLES FROM A-01; (A) EUHEDRAL MINERAL INCLUSION IN FRACTURED CHROMITE; (B)
LARGE, WEATHERED PYROXENE CRYSTAL LOCATED WITHIN THE SERPENTINE MATRIX WITH FRACTURED FILLED WITH SERPENTINE;
(C) PYROXENE IN SERPENTINE MATRIX WITH CA-O VEINS; (D) PYROXENE OCCURRING WITH CHROMITE AND FE-NI-CU SULFIDES.
..................................................................................................................................................................... 15
ii
Dunite, Chromitite, and Laterite Samples
from the Acoje Ophiolite Block, Philippines
Michelle Dossey
Geosciences, master's level (120 credits)
2023
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
,Abstract
Ni-laterites have the potential to become unconventional ore deposits for platinum group elements
(PGE). This study was conducted to determine enrichment trends of PGE as a result of the Ni-
laterization process. 6 samples were selected by mine workers from the protolith, saprolite, and
limonite horizons of the Ni-laterite profile from the Acoje ophiolite block, Luzon, Philippines, and sent
to Luleå University of Technology (LTU). 2 samples representing the protolith are described as dunite
having undergone serpentinization, 1 sample is a massive chromitite from the saprolite layer of the
laterite profile, 1 sample is a massive chromitite from the limonite layer of the laterite profile, and 2
samples are limonitic soils.
Total PGE contents of the investigated Acoje samples range from 161-1180 ppb with the highest
contents of PGE occurring in the limonite hosted chromitite, and the lowest contents in the saprolite
hosted chromitite. C1 chondrite-normalized patterns reveal distinct trends of the PGE in the different
sample types: dunite samples have a positive trend from Ir-Pd, the chromitite samples have a negative
trend from Ru-Pd with a negative Ir anomaly and the limonite samples have a strong positive trend
from Ir-Pd. Rare earth elements (REE) chondrite-normalized patterns of the samples show a negative
Ce anomaly in the limonite while the dunite and saprolite-hosted chromitite have negative Eu
anomalies. Cr# (Cr/[Al+Cr]) and Mg# (Mg/[Fe2++Mg]) were analyzed using automated mineralogy and
produced Cr# values ranging from 0.67 – 0.77 and Mg# values from 0.46 – 0.59.
17 platinum group minerals (PGM) were identified from the Acoje samples: 9 from the dunite, 1 from
the saprolite-hosted chromitite, 6 from the limonite-hosted chromitite and 1 from the limonite.
Laurite ((Ru,Os)S2) was identified in samples A-02 and A-07 and is the only primary mineral identified.
Secondary PGM, thought to have formed due to alteration processes during serpentinization, were
identified as alloys composed of: Cu-Pd, Cu-Pt, Pt-Ni-Cu, Pt-Fe, Ir-Ni-(Pt,Fe), and Cu-Pt-Au-(Pd-Ag).
PGM are small, measuring consistently <10 µm in diameter. Laurite occurs as inclusions in unfractured
chromite. PGM alloys in the dunite samples occur along the boundary of sulfide minerals or within
serpentine. PGM identified in the limonite-hosted chromitite occur along interstitial fractures within
chromitite or in a Fe-Al oxide matrix within pore spaces.
,Contents
Abstract .....................................................................................................................................................
Figures ..................................................................................................................................................... ii
Tables ..................................................................................................................................................... iv
Abbreviations and vocabulary ............................................................................................................... iv
Acknowledgement .................................................................................................................................. v
1. Introduction ........................................................................................................................................ 1
1.2 Goal of the study ........................................................................................................................... 1
2. Literature review ................................................................................................................................. 1
2.1 Geologic Setting ............................................................................................................................ 1
2.2 Ophiolites and podiform chromitites............................................................................................ 3
2.2.1 Chromitite classifications ....................................................................................................... 3
2.3 Ni-Co laterites ............................................................................................................................... 4
2.3.1 Geology and occurrence ........................................................................................................ 4
2.3.2 Ni-laterite profile ................................................................................................................... 4
2.3.3 Ni-laterite ore types ............................................................................................................... 4
2.3.4 PGE in Ni-laterites .................................................................................................................. 5
3. Materials and Methods ....................................................................................................................... 5
3.1 Quantitative element screening ................................................................................................... 6
3.2 SEM/EDS mineral analysis ............................................................................................................. 6
3.3 Cr# and Mg# .................................................................................................................................. 6
3.4 Hydroseparation and polished monolayers .................................................................................. 7
3.5 Software ........................................................................................................................................ 7
4. Results ................................................................................................................................................. 7
4.1 Whole rock geochemistry ............................................................................................................. 7
4.1.1 Dunite samples....................................................................................................................... 7
4.1.2 Chromitite samples ................................................................................................................ 9
4.1.3 Limonite samples ................................................................................................................. 10
4.1.4 Ultramafic Index of Alternation (UMIA)............................................................................... 11
4.2 Mineralogy .................................................................................................................................. 11
4.2.1 Chromite geochemistry........................................................................................................ 12
4.2.2 Silicates ................................................................................................................................ 14
4.2.3 Sulfides ................................................................................................................................. 17
4.2.4 Platinum group minerals ...................................................................................................... 18
5. Discussion.......................................................................................................................................... 24
5.1 Comparison with previous publications ..................................................................................... 24
i
, 5.1.1 Primary mineralogy .............................................................................................................. 24
5.1.2 UMIA .................................................................................................................................... 24
5.1.3 REE chondrite-normalized pattern ...................................................................................... 25
5.1.4 PGE chondrite-normalized pattern ...................................................................................... 26
5.2 Automated mineral identification .............................................................................................. 27
5.3 Platinum Group Minerals ............................................................................................................ 28
5.3.1 Mineral types and abundances ............................................................................................ 28
5.3.2 PGE mobilization .................................................................................................................. 28
6. Conclusion ......................................................................................................................................... 29
References ............................................................................................................................................ 30
Appendix A – A04 Hydroseparation and Monolayer ............................................................................ 33
Appendix B – Thin Sections ................................................................................................................... 39
Appendix C – Platinum group mineral elemental maps ....................................................................... 43
Figures
FIG. 1 (A) OVERVIEW OF LUZON, PHILIPPINES; (B) GEOLOGIC MAP BASED ON YUMUL (2001). THE ACOJE (RED) AND COTO (BLUE)
BLOCKS OF THE MASINLOC MASSIF WITH THE IBA FAULT (E-W TRENDING) AND SUBIC BAY FAULT (N-S TRENDING) DIVIDING THE
SAN-ANTONIO AND CABANGAN MASSIFS (GREEN), GEOLOGIC FEATURES ARE BASALT-DIABASE, GABBRO-TROCTOLITE,
HARZBURGITE, AND PYROXENE-GABBRONORITE; (C) LOCATION OF STUDY SAMPLES). ........................................................ 2
FIG. 2 EXAMPLE OF A LATERITIC PROFILE. DEPTH OF THE LIMONITIC LAYER IS DEPENDENT ON THE GEOLOGY OF THE PROTOLITH,
CLIMATE, AND TOPOGRAPHIC RELIEF. (BUTT & CLUZEL, 2013; AIGLSPERGER, ET AL., 2016) ............................................. 4
FIG. 3 SAMPLES RECEIVED FROM ACOJE. A-01 AND A-02 ARE DUNITE SAMPLES; A-04 AND A-05 ARE LIMONITE SOILS; A-07 AND A-
09 ARE CHROMITITES. .......................................................................................................................................... 6
FIG. 4 C1 CHONDRITE-NORMALIZED DIAGRAM OF REE (MCDONOUGH & SUN, 1995)............................................................. 9
FIG. 5 C1 CHONDRITE-NORMALIZED DIAGRAM OF PGE (NALDRETT & DUKE, 1980). (A) DUNITE SAMPLES; (B) CHROMITITE SAMPLES
WITH REFERENCE VALUES FROM BACUTA, ET AL. (1990); (C) LIMONITE SAMPLES. ......................................................... 10
FIG. 6 (A) AF-S-M PROVIDES A VISUAL DEPICTION OF MG AND SI LOSS AS ULTRAMAFIC MATERIAL WEATHERS FROM PROTOLITH TO
LIMONITE; (B) A-F-SM DIAGRAM DISPLAYS THE PREFERENTIAL DEVELOPMENT OF FE-RICH (LATERIZATION) VS AL-RICH
(BAUXITIZATION) OF THE WEATHERED SOIL. INCLUDES DATA FROM AIGLSPERGER, ET AL. (2016) AND AQUINO, ET AL. (2022).
..................................................................................................................................................................... 11
FIG. 7 MAPPING OF THE OXIDE WT.% CONTENT OF THE INDIVIDUAL CHROMITE CRYSTALS MEASURED IN THIN SECTIONS (IMAGE
BOTTOM RIGHT CORNER FOR REFERENCE). FROM LEFT TO RIGHT ON THE BOTTOM, SAMPLES A-01, A-02, AND A-07; TOP, A-
09. ................................................................................................................................................................ 12
FIG. 8 CR# [CR/(CR+AL)] VS MG# [MG/{MG+FE2+)], CENTER POINT OF EACH SAMPLE SPREAD REPRESENTS THE MEDIAN. INCLUDES
DATA SETS FROM ORBERGER & ALLEWELDT (1994) AND BACUTA ET AL. (1990) ARE EXAMPLES OF CHROMITITE FROM THE
ZAMBALES OPHIOLITE COMPLEX. POINTS PLOTTED IN THE TOP LEFT CORNER BY BACUTA ET AL. (1990) ARE SAMPLED FROM THE
COTO BLOCK AND ARE EXAMPLES OF HIGH-AL CHROMITITES (CR# < 0.6). ................................................................... 13
FIG. 9 OLIVINE WITH CHROMITE (OPAQUE) AND SERPENTINE VEINS; (LEFT) A-01; (RIGHT) A-02. ............................................. 14
FIG. 10 PYROXENE CLASSIFICATION BASED ON MORIMOTO (1989), SAMPLE A-09 PLOTTED WITH ORBERGER & ALLEWELDT (1994).
..................................................................................................................................................................... 15
FIG. 11 PYROXENE BSE IMAGES FROM SAMPLES FROM A-01; (A) EUHEDRAL MINERAL INCLUSION IN FRACTURED CHROMITE; (B)
LARGE, WEATHERED PYROXENE CRYSTAL LOCATED WITHIN THE SERPENTINE MATRIX WITH FRACTURED FILLED WITH SERPENTINE;
(C) PYROXENE IN SERPENTINE MATRIX WITH CA-O VEINS; (D) PYROXENE OCCURRING WITH CHROMITE AND FE-NI-CU SULFIDES.
..................................................................................................................................................................... 15
ii
