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

Abstract Exploration activities outlined a shear zone-hosted gold
and base metal occurrence in low-grade metasedimentary rocks near
the town of Workamba, Tigray State, northern Ethiopia. Gold
resources are estimated at ~2 t Au with concentrations reaching up
to 8 g/t Au in the ore. The geology of the study area is dominated
by foliated or sheared, lower greenschist facies metavolcanic and
metasedimentary rocks, which are correlated to the lower island-arc
metavolcanic rocks of the ~860 to 750 Ma Tsaliet Group and upper
carbonate and metasedimentary rocks of the ~740 Ma Tambien Group,
respectively. The Tsaliet and Tambien groups represent the southern
parts of the Arabian-Nubian Shield. The Tsaliet Group, which is
exposed in the north-western parts of the study area, consists
mainly of metavolcaniclastic rocks. The overlying slate, phyllite,
sericite-chlorite schists and marbles of the Tambien Group occupy
the south-eastern part of the area. A steeply dipping NE trending
ductile shear zone occurs in the metasediments along the contact
with the Tsaliet Group. It is intruded by monzogranite and
lamprophyre dikes or sills. The shear zone is correlated with an
early regional deformation event (D1), which also caused NE
foliations and ENE trending folds. A second regional deformation
phase (D2) resulted in NNE oriented weakly developed dextral shear
bands and N-S trending parasitic folds. NW striking brittle faults
postdate D1 and D2 ductile structures. Trace element patterns of
the metasedimentary rocks support findings of previously published
works suggesting that they are derived from the rocks of the
Tsaliet Group. Major, trace, and rare earth element characteristics
of the monzogranite dikes/sills permit the conclusion that they are
related to the ~620 to 520 Ma post-tectonic magmatism in the Tigray
region. The post-tectonic granitoids are resulted from
mantle-derived magmas, which are modified by fluid components of
the subducted slab or involvement of continental crust.
Petrographic and geochemical results of the lamprophyres show that
they are calc-alkaline in nature. The monzogranite dikes/sills have
208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values between 37.176 to
37.310, 15.508 to 15.525, and 17.799 to 18.310, respectively. These
lead isotope ratios are compatible with a significant mantle
component in these rocks. Lead isotope ratios of the
metasedimentary and metavolcanic rocks at Workamba are
heterogeneous, suggesting that not all samples did retain their
original Pb isotope signatures. However, the original Pb isotope
composition of these rocks tends to be less radiogenic as those of
the post-tectonic dikes/sills. Field and petrographic observations
show that the metavolcanic rocks are locally affected by propylitic
alteration and sericitization. The metasedimentary rocks suffered
from pervasive silicification, sericitization, and carbonatization.
The monzogranite and lamprophyre dikes/sills locally experienced
pervasive sericitization, carbonatization, epidotization, and
chloritization. Two generations of quartz veins occur cutting
and/or parallel to D1 foliation on the Tsaliet and Tambien groups.
The first generation quartz veins are folded and less abundant as
compared to the unfolded second generations. The metasedimentary
rocks and the dikes/sills are also cut by calcite ±quartz veinlets
in the mineralized zone. Mass balance calculations of the
metasedimentary rocks suggest that Na2O, and Sr are removed by
hydrothermal fluids from the original composition of the rocks.
K2O, MnO, CaO, Ba, and Pb are variably added or removed. The
dikes/sills retained near original geochemical composition. The
shear zone hosted metallic mineralization occurs within the
metasedimentary rocks, in close spatial proximity to the
dikes/sills. It took place under brittle conditions. Pyrite,
sphalerite, galena, and chalcopyrite are the main sulfide phases,
whereas pyrrhotite, arsenopyrite, and chalcocite are minor
components. Gold is invisible and was only detected in geochemical
analysis. Gangues include calcite, sericite, and quartz. The
mineralized rocks are enriched in Au, As, Sb, and Ag. The Au/Ag
ratio of all analyzed samples is less than 10. The paragenetic
relationships show that pervasive sericitization and
chlorite-epidote (propylitic) alteration occurred before an early
quartz and/or calcite ±quartz veining event. The mineralization
occurred roughly concomitant with carbonatization and calcite
±quartz veining, after the intrusion of the monzogranite and
lamprophyre dikes/sills. Sulfur isotope signature of sulfides
(δ34S(VCDT) = 1.8 to 9.1 ‰) is compatible with the hypothesis that
S is derived from the country rocks through leaching by
hydrothermal fluids. The carbon isotope signature of hydrothermal
calcite (13C(VPDB) = –5.6 to +1.8 ‰) and that of a fluid in
equilibrium with this calcite (13C(VPDB) = -3.7 to +3.7 ‰) are
similar to those of Tambien Group carbonate rocks (13C(VPDB) =
-4.5 to 7 ‰). Therefore, the C is dominantly derived from the
Tambien Group. The calculated oxygen isotope signature (18O(VSMOW)
values between 5.1 to 8.7 ‰) of a hydrothermal fluid in equilibrium
with calcite is compatible with a fluid of magmatic or metamorphic
origin. The 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb values of
sulfides, which range between 36.94 to 36.99, 15.47 to 15.49, and
17.36 to 17.38 respectively, differ markedly from those of
monzogranite dikes/sills. The data implies that the mineralization
is unrelated to post-orogenic magmatism and that the Pb isotope
signature of the sulfides represents the original Pb isotope
signature of the metasedimentary and metavolcanic rocks. Overall
evaluation of S, C, and Pb isotopic data permits to conclude that
metals, sulfur, and other ore components are derived from the
country rocks through leaching by metamorphic fluids. Petrographic
studies of fluid inclusions show the presence of coexisting
liquid-only, liquid-rich and vapour-rich two phase fluids. The
fluid inclusions are predominantly secondary or pseudo-secondary in
nature. Microthermometric measurments show that all fluid
inclusions are of low-salinity (~1 to 8 wt. % NaClequiv.) aqueous
types. Average homogenization temperatures of fluid inclusions in
calcite are around 300ºC, i.e. close to temperatures found in
orogenic gold systems. The mineralization at Workamba was likely
generated by devolatilization and dehydration processes during
metamorphism of the Tsaliet and Tambien Group rocks. The major
collision orogeny which waned in Tigray at ~630 Ma might have
caused eviction of the mineralizing fluids from deep regions. These
mineralizing fluids were then channeled in the shear zone. Gold was
presumably transported as Au(HS)2- complexes at intermediate
oxidation states by near-neutral fluids, and sulfidation of host
rocks is proposed as the likely gold deposition mechanism. The sub-
to greenschist facies metamorphism of the Tambien and Tsaliet Group
rocks, the enrichment in Sb, and As, the brittle-ductile shearing
experienced by the rocks, and the mineralization under brittle
conditions suggest a depth of formation between 4 to 6 km.
Generally, the findings of this study suggest that the genesis of
metallic mineralization at Workamba is of orogenic gold type and
unrelated to post-orogenic magmatism in the area, although a close
spatial proximity to post-orogenic dikes/sills exists. The age of
the mineralization is estimated to be late Neoproterozoic.