Gold mineralization in the Um El Tuyor area, South Eastern Desert, Egypt

This thesis reports petrographic, mineralogical and geochemical
data that constrain the hydrothermal alteration and gold
mineralization in the Um El Tuyor area, SE Egypt. In order to
distinguish the characteristics and envisage the genesis of gold
mineralization in the study area, insights into the geologic
context, structural evolution and geochemistry of the country rocks
are provided. The geochemical investigations have been integrated
with field and petrographic relationships, along with Landsat (TM)
imagery interpretations to better constraints on the tectonic
setting of the basement complex in the study area. The particular
geographic location near the intersection of two major high strain
zones, namely the Allaqi-Heiani suture and Hamisana Shear Zone,
played a crucial role in the deformation history of the Um Tuyor
area. The basement complex cropping out in the Um El Tuyor area is
part of the Neoproterozoic Allaqi-Heiani ophiolitic belt,
comprising allochtonous ophiolitic thrust slices and detached
sheets, island arc volcano-sedimentary-plutonic assemblages, and
syn-orogenic and post-orogenic intrusions. The ophiolitic rocks
exhibit field and geochemical characteristics that make them akin
to the supra-subduction zone ophiolites, formed most likely in a
back-arc basin. The island arc assemblage comprises mainly
calc-alkaline metavolcanic-plutonic rocks and back-arc pelitic
metasediments. Early calc-alkaline granite intrusions tapered along
the foliation and thrust planes during the orogenic episodes,
whereas less fractionated tholeiitic olivine gabbro and
peraluminous monzogranite encompass a course of post-orogenic
plutonism evolved in a within plate setting. An early period of
crustal shortening (Dm) involved transportation and overriding of
huge ophiolitic sheets from the north to south is manifested by
major thrust faults and imbricate ophiolitic thrust slices.
Regional folds and pervasive foliation cleavage signify a NE-SW
compressional regime (D2) superimposed on the thrust fabrics. A
third deformation increment is indicated by the presence of
abundant NNW-trending major folds and left-lateral faults
superimposed on the older structural fabrics (D3). D4 records an
episode of transcurrent deformation yielded slip reactivation of
the pre-existing NW-trending faults and formation of discrete shear
zones, one of which accommodates gold mineralization in the study
area. Finally, a weak shear strain (D5) is indicated by the
intersecting fault and joint trends traversing the post-orogenic
rocks. Regional metamorphism was coeval with deformation, and
peaked under conditions of amphibolite facies during D2.
Geothermobarometry calculations point to temperatures of 534-561oC
and pressure of 5.26-6.20 kbar for the peak of the metamorphic path
of Um El Tuyor basement. Gold is mainly confined to the quartz
veins and less commonly to narrow domains of the next
quartz-sericite alteration zones. Field, microscopic and microprobe
observations suggest that hydrothermal alteration in the Um El
Tuyor mine area was post-peak metamorphism, and syn-kinematic with
local shearing. The main auriferous veins in the Um El Tuyor mine
area consist of massive, partially recrystallized, or laminated
quartz ±carbonate. The quartz-carbonate veins are essentially
fault-fill bodies, which have been fractured and re-filled with
milky to grey laminated quartz in later stages of the geothermal
system. The laminated quartz veins contain narrow elongate slivers
of the host pelitic rocks (composed essentially of
chrorite-sericite-sulphides±graphite), assumed to have been peeled
off and incorporated during incremental (crack-seal) vein growth.
vii Gold occurs as inclusions or within the lattice in arsenopyrite
and arsenian pyrite, commonly in association with subordinate
sphalerite, chalcopyrite, and pyrrhotite in the auriferous quartz
veins. Another, high fineness type of gold fills microfractures in
sulphides and quartz, and/or occurs as dispersed blebs and globules
in domains of pervasive alteration, particularly where sericite and
carbonate are intergrown. In the mine area, a metal zonation
extends from an inner Fe-As-Zn-Au ±Pb±Ag±Te bearing veins through
an intermediate Fe-As-Cu rich wallrocks to a distal halo enriched
in Fe-Cu-Co, and Ni. A three stage hydrothermal alteration model
(initial, transitional and advanced) is proposed for the Um El
Tuyor ore-forming hydrothermal system, on basis of the field and
microscopic observations and the electron microprobe data. The
initial stage involved hydrolysis of the wallrocks in presence of a
near acid fluid, whose pH was buffered by the wallrock mineralogy.
The transitional stage involved also hydrolysis reactions, but was
dominated by carbonatization, sulphidation and redox reactions. The
latter reduced the interacting fluids, particularly where the fluid
: rock ratio was low and provided favourable conditions for gold
deposition. It is interpreted that sulphidation has affected the
gold solubility via changes in oxygen fugacity through redox
reactions. The advanced stage was most likely a phase of intense
sericitization (after chlorite), which consumed K+, liberated H+,
and buffered the solution pH. Cation-exchange reactions were
limited to the time when favoured Na+ activity and temperature
conditions promoted deposition of albite. Sulphidation remained
operating through this stage, and the un-buffered conditions were
locally attained under high fluid/rock ratios. Compositional zoning
of the auriferous pyrite and arsenopyrite crystals, along with the
presence of patchy pyrrhotite and sphalerite inclusions in these
crystals, and the absence of these features in pyrite and
arsenopyrite of the late sulphide assemblage may imply incipient
low oxygen fugacity conditions during gold deposition. Further,
compositional zoning of the large arsenian pyrite crystals, from
barren cores to auriferous margins, is considered as a function of
redox reactions involving oxidation of Au and reduction of As.
Fluid inclusion studies revealed heterogeneous entrapping of
immiscible aqueous and carbonic fluids in the Um El Tuyor
auriferous quartz veins. Criteria including the primary and
secondary modes of occurrence of the carbonic inclusions,
coexistence of inclusions with carbonic and aqueous fluids of
variable relative proportions, densities, filling degrees, partial
homogenization temperatures, and bulk compositions are considered
evocative for phase separation as gold deposition mechanism in the
Um El Tuyor auriferous quartz veins. The initially homogenous ore
fluid was likely a low salinity aqueous-carbonic solution (±1-2
mol% CH4 or N2), which started to separate into two phases and
precipitate gold when conditions attained ~340°C at ~1.5 kbar (at
depth of ~6 km under lithostatic condition). Destabilization of
gold-sulphur complexes through interplay of cooling, redox state
variation, pH changes, and decrease in sulphur fugacity should have
contributed in gold deposition in the auriferous veins. The entire
gold-base metal mineralization and quartz veining event extended
over conditions of 170-429oC at 0.9-2.1 kbar respectively,
equivalent to depths of 3-8 km and compatible with crustal
conditions of greenschist metamorphism and brittle–ductile
transition. This wide range of pressure probably represents the
total fluid pressure regime within the shear zone from the
formation of the auriferous quartz veins to periods of continuous
pressure decrease during uplift, including a sudden pressure
decrease occurred as a consequence of incremental opening of the
fissure, followed by filling by newly deposited quartz.