Nature and characteristics of metasedimentary rock hosted gold and base metal mineralization in

148 Pages·2009·13.24 MB·English

by  Solomon Gebresilassie Gebremariam

Nature and characteristics of metasedimentary rock hosted gold
and base metal mineralization in the Workamba area, central
Tigray, northern Ethiopia

A Thesis submitted to the Faculty of Geosciences at Ludwig-
Maximilians University, Munich in partial fulfillment of the
requirements for the Ph.D. degree

Solomon Gebresilassie Gebremariam

June 2009

Evaluators: Prof. Dr. Robert Marschik
Dr. Albert Hans Gilg
Date of Defence: 26.11.2009
Contents
1 Introduction…………………………………………………………………………………….. 1
1.1 Problem statement………………………………………………………………………….. 2
1.2 Geography of Tigray and study area……………………………………………………….. 2
1.3 Mineral potential of Ethiopia……………………………………………………………… 3
2 Geologic context……………………………………………………………………………… 6
2.1 Geodynamic evolution of the East African Orogen………………………………………. 6
2.1.1 Geology of the Arabian-Nubian Shield……………………………………………... 7
2.1.2 Geology of the Ethiopian basement rocks…………………………………………... 9
2.1.3 Geology of Tigray basement rocks……………………………………………….... 11
2.1.3.1 Stratigraphic classification…………………………………………………. 11
2.1.3.2 Regional tectonic structure………………………………………………… 16
2.2 Geology of the study area………………………………………………………………... 17
2.2.1 Tsaliet Group……………………………………………………………………… 19
2.2.1.1 Metavolcanic rock………………………………………………………….. 19
2.2.1.2 Quartz-sericite rock………………………………………………………… 19
2.2.2 Tambien Group…………………………………………………………………….. 19
2.2.2.1 Sericite-chlorite schist……………………………………………………… 19
2.2.2.2 Slate and phyllite………………..………………………………………… 20
2.2.3 Intrusive rocks……..……………………………………………………………….. 21
2.2.3.1 Monzogranite……………………………………………………………… 21
2.2.3.1 Lamprophyre……………………………………………………………..... 21
2.2.4 Structural elements of the study area………………………………………………. 23
2.2.4.1 Foliation…………………………………………………………………… 24
2.2.4.2 Shear zones....……………………………………………………………… 25
2.2.4.3 Folds………......…………………………………………………………… 26
2.2.4.4 Faults……….……………………………………………………………… 27
2.2.4.5 Joints, lineations and fractures…..………………………………………… 27
2.2.5 Quartz veins………………………………………………………………………... 28
2.2.6 Geology of the mineralized (drilled) zone…………………………………………. 30
2.2.7 Down-hole geology………………………………………………………………… 30
3 Geochemistry………………………………………………………………………………… 32
3.1 Analytical method………………………………………………………………………... 32
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3.2 Major and trace element abundance in metasedimentary rocks…..……………………... 32
3.3 Element mobility in metasedimentary rocks…..………………………..………...……... 32
3.4 Trace element patterns of metasedimentary rocks…..………………….………………... 38
3.5 Major, trace, and rare earth element abundance in intrusive rocks…….....……………... 40
3.5.1 Element abundance…………………………..…………………………………….. 40
3.5.2 Element mobility of the Workamba intrusive rocks…………..…………………… 41
3.5.3 Trace and rare earth element patterns of intrusive rocks…….…………………….. 45
4 Hydrothermal alteration and mineralization………………………………………………… 48
4.1 Hydrothermal alteration…………………………………………………………………. 48
4.2 Mineralization…………………………………………………………………………… 51
4.3 Paragenetic sequence……………………………………………………………………. 54
4.4 Geochemistry of mineralized rocks…………..…………………………………………. 56
4.5 Sulfide mineral chemistry…………………….…………………………………………. 57
5 Microthermometry…………………………………………………………………………… 59
5.1 Sampling and petrography……………………………………………………………….. 59
5.1.1 Liquid-only fluid inclusions (Type I)……………………………………………… 59
5.1.2 Liquid-rich two phase fluid inclusions (Type II)…………………………………... 59
5.1.3 Vapor-rich two phase fluid inclusions (Type III)...………………………………... 60
5.2 Nature of fluid inclusions…...…………………………………………………………… 62
5.3 Microthermometric measurements………………………………………………………. 62
5.3.1 Type I fluid inclusions…………………………………………………………….. 62
5.3.2 Type II fluid inclusions…………………………………………………………….. 63
5.3.3 Type IIIA and B fluid inclusions…………………………………………………... 64
5.4 Interpretation of the fluid inclusion data…………………………………………………. 64
6 Isotope geochemistry………………………………………………………………………… 67
6.1 Sulfur isotope analysis and result…...…………………………………………………… 67
6.2 Carbon and oxygen isotope analysis and result……………...…………………………... 69
6.3 Lead isotope analysis and result…...…………………………………………………….. 71

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7 Discussion…………………………………………………………………………………… 77
8 Conclusion and recommendation……………………………………………………………. 89
References……………………………………………………………………………………… 91
Annexes………………………………………………………………………………………… 102

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List of figures
Fig. 1.1 Location and access map………………………………………………………………... 1
Fig. 1.2 Mineral occurrences in Ethiopia………………………………………………………... 4
Fig. 2.1 Rock distribution in the Arabian-Nubian Shield………………………………………. 6
Fig. 2.2 Tectonic evolution of the East African Orogen………………………………………… 7
Fig. 2.3 Gondwana Supercontinent in early Mesozoic………………………………………….. 8
Fig. 2.4 Distribution of basement rocks in Ethiopia…………………………………………….. 10
Fig. 2.5 Distribution of Tsaliet and Tambien Groups in Tigray……….…………..……………. 12
Fig. 2.6 Stratigraphic section showing the Lower Tsaliet and Upper Tambien Groups…...….… 13
Fig. 2.7 Major tectonic structures existing in Tigray……………………………..…………….. 17
Fig. 2.8 Geological map of the study area………………………………………………………. 18
Fig. 2.19 Examples of exposed rock outcrops in the study area……………………………….... 20
Fig. 2.10 Photomicrographs showing identified minerals in thin section……..………………… 22
Fig. 2.11 QAP classification diagram of the intrusive rocks……………………………………. 23
Fig. 2.12 Structural map of the study area………………………………..……………………... 24
Fig. 2.13 Diagram showing relationships between S1 foliation, weakly developed
second shear deformation, mylonitization, and later brittle fault….…………..……… 24
Fig. 2.14 Photograph showing closely spaced quartz veins in a shear zone………..…………… 25
Fig. 2.15 Diagram displaying NE oriented brittle-ductile and NNE oriented
ductile shear zones……………………………….…..…..…………………….……… 26
Fig. 2.16 Photographs showing D1 and D2 folds..……………………………………………… 27
Fig. 2.17 Photograph showing E-W trending joints and fractures….…………………………… 28
Fig. 2.18 Photographs displaying second generation quartz veins occurring parallel
to or cutting the foliation.………………………..……………………….…………… 28
Fig. 2.19 Geological map of the mineralized zone……………………………………………… 29
Fig. 2.20 Showing types of exposed rocks and exploration structures
in the mineralized zone................................................................................................... 30
Fig. 2.21 Geological borehole sections displaying down-hole geology………………………… 31
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Fig. 3.1 Isocon diagrams demonstrating element mobility in metasedimentary
rocks………..………………………………………………………………….………. 33
Fig. 3.2 Trace element patterns of metasedimentary rocks of the study area along
with that of nearby metavolcanic and metasedimentary rocks……….………………. 39
Fig. 3.3 Isocon diagrams displaying element mobility in intrusive rocks……….…..…………. 42
Fig. 3.4 Major element discrimination diagrams for the intrusive rocks………………………... 44
Fig. 3.5 Trace element patterns of intrusive rocks………………………………………………. 45
Fig. 3.6 Rare earth element patterns of intrusive rocks….……………………………………… 46
Fig. 3.7 Trace element discrimination diagrams of intrusive rocks….....………………………. 47
Fig. 4.1 Geology and alteration map of the study area………………………………………….. 48
Fig. 4.2 Photograph showing propylitic alteration occurrence in the study area………………... 49
Fig. 4.3 Photographs showing different alteration types that affected the
rocks………………………………………………………………………..…….……. 50
Fig. 4.4 Photomicrographs showing identified sulfides in the study area……………………….. 52
Fig. 4.5 Boreholes sections indicating the distribution of base metal sulfides in the rocks...…… 53
Fig. 4.6 Malachite occurrence on the cleavage planes of the metavolcanic rocks……………… 54
Fig. 4.7 Photomicrographs displaying paragenetic relationships between the sulfides.......…….. 55
Fig. 4.8 Paragenetic sequence of the sulfides and coexisting gangue and alteration
minerals……………………………………………………………..…………………. 56
Fig. 5.1 Showing the type of fluid inclusions occurring in the mineralized and
unmineralized quartz or quartz-calcite veins of the study area……………...………… 60
Fig. 5.2 Photomicrographs displaying the types of recognized fluid inclusions………………… 61
Fig. 5.3 Histograms displaying the ranges of ice melting temperature, salinity and
homogenization temperature of the heated/freezed inclusions………..………………. 63
Fig. 5.4 Photomicrograph showing coexisting liquid- and vapour-rich two phase fluid
inclusions in calcite……………………..……………………………………………… 65
Fig. 5.5 Homogenization temperature versus salinity diagrams of the different
types of heated/freezed fluid inclusions………………………………………………. 66
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Fig. 6.1 Diagram showing the position of sulfur isotope composition of Workamba
sulfides relative to that of sediment-hosted orogenic gold deposits elsewhere……..… 68
Fig. 6.2 Diagram displaying plot of carbon and oxygen isotope composition of Workamba
hydrothermal calcites together with that of nearby Tambien Group rocks……..…..… 70
Fig. 6.3 Lead isotope data plot of sulfides, feldspars and whole rock samples
relative to the growth curves and fields of Zartman and Doe (1981)…………………. 74
Fig. 6.4 Lead isotope composition of sulfides from Workamba plotted relative to the
discrimination diagrams of Stacey et al.(1980)..…………………………………….... 76
Fig. 7.1 Compilation of major geologic and tectonic events occurred in the study area
and Tigray region……….…………………………………………………………..…. 79
Fig. 7.2 Showing the crustal continuum model of orogenic gold deposits and the position of
Workamba gold and base metal mineralization with respect to it…..…………………. 83
Fig. 7.3 A summary of the ore forming processes and inferred depth of formation of the
mineralization at Workamba …………………………………………………………. 84

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List of tables
Table 2.1 Geochronological data of intrusive rocks in Tigray………………………………… 15
Table 3.1 Whole rock geochemistry data of Workamba metasedimentary rocks……...……… 34
Table 3.2 Selected major and trace element concentrations of Workamba and nearby
metasedimentary rocks used for estimation of element mobility………………….. 37
Table 3.3 Trace element ratios of the Workamba metasedimentary rocks along
with those of reference nearby metasedimentary rocks………………...………….. 38
Table 3.4 Whole rock geochemistry data of Workamba intrusive rocks……..………………... 43
Table 4.1 Whole rock geochemistry data showing concentrations of gold
and base metals in mineralized rocks…………………………...…………………. 57
Table 4.2 Representative electronprobe analysis results of pyrite, sphalerite
and galena from Workamba………………………………………………………... 58
Table 6.1 Result of sulfur isotope analysis……………………………………………………. 67
Table 6.2 Result of carbon and oxygen isotope analysis………………………………………. 69
Table 6.3 Result of lead isotope analysis of sulfides……………..……………………………. 73
Table 6.3 Result of lead isotope analysis of whole rock samples………………...……………. 73
Table 7.1 Comparison of the Workamba gold and base metal mineralization with the
characteristics of orogenic gold deposits elsewhere……………………………….. 85

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Acknowledgement

I would like to thank very much Deutscher Akademische Austauscht Dienst (DAAD) for
granting me a scholarship. The scholarship enabled me to study and live in Germany with my
family for the last four years. I would also like to acknowledge DAAD for covering the
expenses of my fieldwork. Generally, I run out of words to express the extraordinary financial
help I received from DAAD so that I finish my studies successfully.

I am very grateful of Prof. Dr. Robert Marschik, my supervisor, for the continuous and
extraordinary help, follow up, and guidance that he dedicated to me from the beginning to the
end of my studies. This work could not have been materialized without his significant input,
constructive criticisms, and fruitful discussions.

I would also like to thank very much the Department of Earth and Environmental Sciences of
Ludwig-Maximilians University for allowing me to have full access to various facilities, such
as laboratories, library, internet, printer and photocopy.

A lot of thank goes to Dr. Albert Hans Gilg for his incredible help and guidance especially
during the laboratory works of fluid inclusions. I am very thankful of the constructive
discussions I had had with him regarding interpretations of fluid inclusion data.

I am highly indebted to Dr. Frank Söllner for his guidance and unreserved help during the
energy dispersive spectrometry (EDS) analysis.

I would like to highly acknowledge Prof. Dr. Karl Thomas Fehr and Ms. Saskia Bernstein for
helping me during the electronprobe microanalysis.

My heartfelt acknowledgments go to Prof. Dr. Stefan Hölzl and my friends, Tobias Bauer and
Susanne Rummel, for their unreserved help and guidance during the lead isotope analysis.

I would like to thank very much Dr. Christoph Mayer (Geo-Bio-Center) for doing the carbon
and oxygen isotope analysis.

Ms. Madlen Fischer is highly acknowledged for preparing the thin and polished sections as
well as double polished sections for fluid inclusion studies.
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I am greatly indebted to the National Mining Corporation and its staff members, especially
Mr. Senbeto Chewaka, Kibret Sifeta, and Muluwork Ambaye for allowing me to work in their
project site and providing logistical support during the fieldwork. I would also like to thank
very much Mekelle University (my base University) for providing me a vehicle during the
field work.

I am very grateful of my friend, Dr. Dirk Küster (who was a staff member of Mekelle
University) for initiating the idea of doing Ph.D. in Germany and for all his helps and advises.

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