Gold Mineralization favorable depositional Environment

Gold Mineralization favorable depositional Environment, a case study from Atalla Gold mine, Central Eastern Desert, Egypt.

By Ahmed Hamdy Youssef
Introduction
The Arabian-Nubian Shield is extensively studied by several authors (Fritz et al., 2013; Stern, 1994; Stern, 2000; Stein, 2003; Stern and Abd El-Salam, 1998; Johnson et al., 2011; Stern et al., 2010a; Stern et al., 2010b; Lundmark et al., 2012). However most of these studies focused on the evolution of the shield during the early stages of the Pan-African orogeny. The late Pan-African processes received less attention from the researchers. However, it is well known that the gold ore was formed during this late stage of the evolution of the Arabian-Nubian Shield (Zohier, 2011; Zohier et al., 2015; Zohier et al., 2018; Botros 2002; Botros, 2004). As a result, the gold ore may be used as a key to put more constrains on the late stage evolution of the shield. However, the formation mechanism of the gold ore, in general and in particular the depositional stage of the ore is still unclear.

The Egyptian gold formed through three main stages: 1- leaching stage from its source rock which is mainly ophiolitic serpentinites (Abu-Alam et al., 2018) by mantle carbonaceous fluids (Boskabadi et al., 2016) or by granitic intrusion which served as a heat source driving the convective cells within the source rock (Botros, 2015); 2- transportation stage from its source rock along structures created by Najd Fault system; 3- depositional stage in favorable structural sites (e.g. shear zones) and/or felsic host rock (I-type granite). Understanding the full model of gold deposition will help in; 1- exploring for new gold prospects in the Eastern Desert, 2- understanding the late Pan-African processes.

However, not much information is known about the Egyptian gold depositional environment several hypotheses were postulated: 1- is it related to I-type granitic intrusions and the degree of sulphur solubility in the granitic magma?, 2- is it related to favorable structural sites (e.g. shear zones) with no contribution from the chemistry of the host rock? or 3- fractured I-type granite with well-developed shear zones is needed for gold deposition?
STATUS OF KNOWLEDGE
Evidence supporting the role of granite hypothesis
The formation of granitoid intrusion related gold mineralization is largely controlled by the degree of sulphur saturation in the melt (Yang, 2012). Subdividing them into two types: type 1 in sulphide undersaturated granitic melt in which gold is evolved directly from magmatic fluids; type 2 in sulphide saturated granitic magma which sulphur is introduced in the early stage of evolution of the magma forming sulphide minerals later destructed by late stage hydrothermal fluids with low pH and high oxidation state leaching gold and later depositing it in structural favorable sites (e.g. shear zones).
Evidence supporting Favorable structural sites hypothesis
Zohier et al., (2017) studied intrusion related gold mineralization in Atalla granite and related gold deposition to the shear zones created within the granite that acted just as a competent structural host.
Fractured I-type granite with well-developed shear zones Hypothesis
This hypothesis is not yet addressed as to have a gold deposit, it may need a felsic host with certain degree of sulphur solubility and suitable tectonic environment combined (Yang, 2012). Therefore, it may give a certain degree of understanding to the conditions prevailed during gold deposition.

However, most of Egyptian gold geologists (Zohier et al., 2017; Botros 2015) is convinced that gold deposition just needs a favorable structural site (e.g. shear zones) however they did not explain why most gold occurrences in the central Eastern Desert are hosted in I-type granitic intrusions (Botros 2003).

Geologic settings
Geologic settings of the Eastern Desert
The Eastern Desert (ED) (Fig. 1) of Egypt is a part of Arabian Nubian shield (ANS) which expose the largest tract of juvenile Neoproterozoic crust (e.g. Kroner & Stern, 2005). It formed by the assembly of several arc terranes which separated by suture zones (e.g. Abd El-Salam & Stern, 1996), that later overprinted by low temperature metamorphism of mostly greenschist facies (e.g. Abu El-Enen et al., 2016). According to structural criteria, rock units and age dating, the ED is subdivided into three major domains from older to younger Southern ED (SED), Central ED (CED) and Northern ED (NED) they are separated by major structural shear zones.
The study area is in the CED therefore it’s important to briefly review its evolution. Stern (2017) subdivided the rock units in the CED into infrastructure and the overlying superstructure (Fig. 2). The infrastructure represents the middle crustal levels rocks exhumed as metamorphic core complexes. The overlying superstructure composed of ophiolites, metavolcanics, granitoid rocks, molasse sediments and un-metamorphosed Dokhan volcanics.
The Najd fault system is a mega shear zone which affected the CED in the age between 620-560 Ma (Stern, 1985). It was formed due to an oblique convergence between east and west Gondwana in a NE-SW direction and it was responsible for the metamorphism of the superstructure into greenschist to upper amphibolite facies (Fritz et al., 2002) and the intrusion of granitic rocks and its associated gold mineralization (Zohier et al., 2014; Zohier et al., 2017).

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Fig.1.Simplified geological map of the Eastern Desert of Egypt and Sinai showing the gold occurrences, the distribution of the main geological units and the broad structural trends. Abu-Alam et al. (2018).

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Fig.2. Schematic diagram illustrating crustal structure and processes beneath the CED Stern, (2017).

Geologic setting of the study area
Atalla granite is a small granitic intrusion 0.4 Km2 in the central Eastern Desert (Fig. 3) formed around 615 ± 9 Ma (Zohier et al., 2018) and classified by (Ahmed et al., 2003) as monzo-syno, I-type granite later invaded by several gold bearing quartz veins. In addition to the granite, other rock units can be observed: older to younger; serpentinites and related rocks, metavolcanics, metasediments, post-Hammamat felsite.
The area was affected by the Najd fault system that later formed the famous Atalla shear zone striking NW-SE in a sinistral sense of shearing (Akawy, 2007). The Ar-Ar dating 600.8 ± 5.5 Ma of hydrothermal white mica disseminated in gold bearing quartz veins indicates post intrusion gold mineralization overlapping with the end of Najd fault system (Zoheir et al., 2018). However, uncertainty in the decay constant involved in Ar-Ar dating method make the absolute age is a hard mission to accomplish (Min et al., 2000). Therefore, depending on Ar-Ar methods for determining the mineralization age is doubtful and full of uncertainties.

centertop
Fig.3. geologic map of Atalla granite pluton and its surrounding country rocks (modified after Abu-Alam et al. 2018).

Goals
The main goal of this study is to identify the gold depositional environment and to discriminate between hypothesis 1 (granite role), hypothesis 2 (structural role) and hypothesis 3 (fractured I-type granite). In this context, the study will aim to:
Identify the physico-chemical conditions (P-T-XO2) of the crystallization of the granites and the associated gold/sulfide mineralization as well the degree of sulphur solubility and saturation in the granite.

Study the alteration zones noticed within the granite around the auriferous
quartz veins.

Understand the structural evolution of the study area.

Suggest a tectonic model that may be used to explain the gold depositional environment.

Methods
While our hypotheses include new model for the depositional environment of gold in the Eastern Desert, our approaches involve an integration between structurally oriented field work, petrography and geochemical analysis. These data will provide constraints about the tectonic setting and the proper host rock characters associated with gold deposition.

Field work
Field work has already been done and carful structural measurements for the orientation of gold bearing quartz veins and the general structure trend of the area has been recorded for 1- creating geological map for the area of study and 2- identify the tectonic setting of gold deposition either related to extensional or compressional regimes.

Representative samples (around 160 samples) were collected from different rock units in the study area with special focus on the granite and its associated quartz veins. Further profiles were taken across the veins in order to collect the alteration products of the host rock.

Laboratory work
Laboratory work will include five aspects;
Detailed petrographic studies for 70 thin sections for the different rock units and the alteration zones and 12 polished sections has been made.
Fluid inclusions and microthermometric studies to identify the conditions prevailed during gold enrichment and the physico-chemical conditions of the crystallization of the granites.
Measuring oxygen isotopes in the vein samples to identify fluid source and S isotopes in the mafic minerals in granite samples to identify sulphur source.
Major, Trace and Rare Earth Elements analysis for the granite (15 samples) and its alteration zones (25 samples) to identify the petrogenetic origin of the granite and the effect of the fluids on the country rock.
Mineral chemistry using Electron microprobe for the granite and the alteration samples.

Thesis Time plan
Jan. Fab. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec.

2018 Preparation for field work Field Work Detailed Petrographic studies
2019 Geochemical Analysis Interpreting the data
2020 Writing and publishing at least one research paper Writing M.sc Thesis
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