Multistage gold mineralization in the Wa-Lawra greenstone belt, NW Ghana: The Bepkong deposit

Accepted Manuscript Multistage gold mineralization in the Wa-Lawra greenstone belt, NW Ghana: The Bepkong deposit Prince Ofori Amponsah, Stefano Salvi, Béziat Didier, Lenka Baratoux, Luc Siebenaller, Mark Jessell, Prosper Mackenzie Nude, Eugene Adubofour Gyawu PII:

S1464-343X(16)30151-0

DOI:

10.1016/j.jafrearsci.2016.05.005

Reference:

AES 2564

To appear in:

Journal of African Earth Sciences

Received Date: 6 November 2015 Revised Date:

21 April 2016

Accepted Date: 6 May 2016

Please cite this article as: Amponsah, P.O., Salvi, S., Didier, B., Baratoux, L., Siebenaller, L., Jessell, M., Nude, P.M., Gyawu, E.A., Multistage gold mineralization in the Wa-Lawra greenstone belt, NW Ghana: The Bepkong deposit, Journal of African Earth Sciences (2016), doi: 10.1016/j.jafrearsci.2016.05.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Multistage gold mineralization in the Wa-Lawra greenstone belt, NW Ghana: The

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Bepkong deposit

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Prince Ofori Amponsah1,2 *, Stefano Salvi1, Béziat Didier1, Lenka Baratoux1,3, Luc Siebenaller1,4, Mark Jessell1,5, Prosper Mackenzie Nude6, Eugene Adubofour Gyawu2

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1. Université de Toulouse, CNRS, Géosciences Environnement Toulouse, Institut de Recherché pour le Développement, Observatoire Midi-Pyrénées, 14 Av. Edouard Belin, F31400 Toulouse, France

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2. Azumah Resources Ghana limited, PMB CT452, Cantonments, Accra, Ghana

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3. IFAN Cheikh Anta Diop, Dakar, Senegal

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4. ONG-D “Le Soleil dans la Main” asbl, 48, Duerfstrooss, L-9696 Winseler, Luxembourg

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5. Centre for exploration Targeting, School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia.

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6. University of Ghana, Department of Earth Sciences, P.O. Box LG 58, Legon, Accra, Ghana.

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* Corresponding author: [email protected]

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Abstract

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The Bepkong gold deposit is one of several gold camps in the Paleoproterozoic Wa-Lawra

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greenstone belt in northwest Ghana. These deposits lay along the Kunche-Atikpi shear zone,

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which is part of the larger transcurrent Jirapa shear zone. The formation of these shear zones

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can be attributed to the general ESE-WNW major shortening that took place in the Wa-Lawra

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belt. Gold mineralization in the Bepkong deposit mainly occurs within graphitic shales and

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volcaniclastic rocks. The ore consists of four N-S trending lenticular bodies, plunging steeply

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to the south, that are lithologically and structurally controlled. Their shape and thickness are

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variable, though a general strike length of 560 m and an overall thickness of 300 m can be

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defined. An alteration mineral assemblage characterises the ore, and consists of chlorite-

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calcite-sericite-quartz-arsenopyrite-pyrite. Pyrite, as distinct from arsenopyrite, is not limited

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to the altered rocks and occurs throughout the area. At Bepkong, gold is associated with

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arsenopyrite and pyrite, which occur disseminated in the mineralized wall rock, flanking

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Type-1 quartz veins, or within fractures crossing these veins. Textural observations indicate

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the early formation of abundant arsenopyrite, followed by pyrite, with chalcopyrite, galena,

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sphalerite and pyrrhotite occurring as inclusions within pyrite and altered arsenopyrite.

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Detailed petrography, coupled with SEM, LA-ICP-MS and EMP analyses, indicate that gold

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in the Bepkong deposit occurs in three distinct forms: (i) invisible gold, mostly in

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arsenopyrite (ii); visible gold as micron-size grains within fractures and altered rims of

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arsenopyrite, as well as at the interface of sulphide grains; (iii) free visible gold in fractures in

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quartz veins and their selvages. We interpret the invisible gold to have co-precipitated with

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the early-formed arsenopyrite. The small visible gold grains observed within the sulphide

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interfaces, altered arsenopyrite, fractures and grain boundaries, are interpreted to have formed

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as a result of the dissolution and redistribution of the invisible gold during later alteration of

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arsenopyrite, which took place at lower temperatures during crenulation and fracturing

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accompanying late deformation, and was accompanied by pervasive pyritization of the wall

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rock.

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Keywords: invisible gold, gold remobilization, NW Ghana, Bepkong deposit, pyrite,

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arsenopyrite, fluid inclusions.

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1. Introduction

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Greenstone belts all over the world have been prospective regions for orogenic gold deposits

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for many decades now (Davis and Zaleski, 1998), and account for an important amount of the

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world's total gold production (a recent figure indicates about 13% of global production,

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corresponding to 15,920 metric tonnes of Au; Dubé and Gosselin, 2007). Well known world-

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class occurrences include Kalgoorlie in Australia, the Abitibi region in Canada, the Ashanti

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deposits in Ghana, and Mother Lode in the USA (e.g., Groves et al., 1998). These deposits

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are usually associated with first-order regional shear zones and are commonly situated on

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second- or third-order faults that splay off these first-order structures (Groves et al. 1988;

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Eisenlohr et al. 1989). In southern Ghana, gold mineralization has been identified with

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greenstone belts and has been reported to occur along broad shear zones and in narrow faults

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immediately adjacent to less deformed rocks (Appiah, 1991; Eisenlohr and Hirdes, 1992;

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Allibone et al., 2002; 2004). This region hosts some of the largest deposits of this kind, as

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exampled by the Obuasi mine (AngloGold Ashanti), the Ahafo mine (Newmont Mining

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Corporations), the Damang mine (Goldfields Limited), the Bibiani mine (Noble Mineral

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resources), and the Bogoso/Prestea which total over 40 million ounces (Oz) of gold.

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Therefore, the majority of geological studies on gold mineralization in Ghana have focussed

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on them (Junner, 1932, 1935, 1940; Kesse 1985; Milési et al., 1989, 1992; Leube et al., 1990;

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Blenkinsop et al.,1994; Davis et al, 1994 ; Oberthür et al., 1994, 1996, 1998; Mumin and

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Fleet, 1994; Hammond and Tabata, 1997; Klemd and Hirdes, 1997; Barritt and Kuma, 1998;

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Yao and Robb, 2000; Feybesse et al., 2006; White et al., 2015).

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However, with gold mining being the major driver of the Ghanaian economy, profitable (<1

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million ounces) albeit not exceptional gold deposits cannot be overlooked, making the

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Birimian in NW Ghana an important area to study. In this region, numerous gold exploration

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prospects have been identified, particularly in the N-S Wa-Lawra greenstone belt. These

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include the Kunche, Bepkong, Atikpi, Yagha, Basabli and Duri prospects (Fig. 1), although

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only Kunche (309,000 Oz measured and indicated resource) and Bepkong (113,000 Oz

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measured and indicated resource) are of economic importance. These two deposits are at a

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stage of advanced exploration to pre-production, by the Azumah Resources Limited

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Company. This paper investigates the gold mineralization and its relationships with

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deformation in the Bepkong deposit. We highlight the geological and structural controls on

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gold mineralization and reconstructs the ore genesis via mineralogical, geochemical, fluid

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inclusion studies and propose a metallogenic model for the gold mineralization.

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2. Regional geology of the Wa-Lawra Belt

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The N-S trending Wa-Lawra belt in NW Ghana is part of the Paleoproterozoic Birimian

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terrane of the West African Craton (WAC; Fig. 1) and is the only N-S trending belt in Ghana

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(Kesse, 1985; Samokin and Lashmanov, 1991; Pobedash, 1991; Roudakov, 1991). The belt is

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the southern portion of the larger Boromo belt in Burkina Faso (Béziat et al., 2000; Baratoux

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et al. 2011). It is composed of metamorphosed shales, greywackes, volcano-sediments,

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basalts, dacites, andesites, granites, para and ortho-gneisses and granitoids (Fig. 1).

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According to Feybesse et al. (2006), the Birimian rocks in Ghana were formed between 2250

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to 1980Ma.

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Regionally, the Wa-Lawra-Boromo belt is bounded to the west by the Diebougou-Bouna

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granitoid domain in Burkina Faso and to the east by Koudougou-Tumu granitoid domain

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(KTGD). The contact between the Wa-Lawra belt and the KTGD is marked by the Jang shear

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zone (Fig. 1).

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The Wa-Lawra belt can be subdivided into eastern and western parts. The boundary between

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the eastern and western parts is marked by the crustal-scale sinistral Jirapa shear zone (Block

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et al., 2015b) which extends into Burkina Faso. The western part of the Wa-Lawra belt is

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composed mainly of sediments (shales, greywackes, and volcano-sediments), volcanic rocks

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and intruded granitoids and the eastern part is mainly composed of granites, para and orthogneisses, rhyolites and granitoids (Amponsah et al., 2015).

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According to Baratoux et al. (2011) the volcanics and pyroclastic flows in the belt were

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emplaced between 2200 Ma to 2160 Ma. Detrital zircon age dating of the volcano-sediments

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in the Wa-Lawra belt gave ages older than 2140 Ma and syn-tectonic to late kinematic

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granitoid intrusion in the belt were emplaced around ~2150 and 2100 Ma, respectively

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(Agyei Duodu et al., 2009).

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The volcanic suites and sediments west of the Jirapa shear zone have been metamorphosed

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under greenschist facies conditions whilst the granitoids and para-ortho gneisses to the east

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have undergone amphibolite facies metamorphism. The metamorphic mineral assemblages

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associated to the greenschist facies metamorphism consist of chlorite, calcite and epidote

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whilst the amphibolite facies are garnet, plagioclase, clinopyroxene and hornblende (Block et

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al. 2015a).

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A polyphase deformation history has been proposed by Baratoux et al. (2011) and Block et

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al. (2015b) for the Boromo belt, including the Wa-Lawra belt. The first deformation event in

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N Ghana corresponds to N-S shortening under upper greenschist at the limit with blueschist

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to amphibolite facies conditions (Block et al., 2015b). This early deformation event was

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recorded in southern Ghana (Perrouty et al., 2012) and northern Burkina Faso (Tshibubudze

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and Hein, 2013; McCuaig et al., in review) but no unequivocal evidence was found in SW

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Burkina Faso (Baratoux et al., 2011). The D2 deformation phase (D1 according to Baratoux et

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al., 2011) formed N to NNW trending S2 penetrative foliation. The D2 event is marked by

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vertical dipping metamorphic penetrative foliation which is often parallel to primary bedding

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within the sediments. The metamorphic grade varies between the greenschist and amphibolite

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facies. Extensive domains reaching migmatite facies during D2 were found in the Bole region

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in north-western Ghana (Block et al., 2015a). Subsequent E-W shortening lead to the folding

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and overprinting of the S1 and S2 penetrative metamorphic foliation by a N-S trending

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schistosity S3. These structures were transected by S4 and S5 transcurrent shear zones which

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affected all the Birimian volcanic and volcano-sedimentary units and structured most of the

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syn-tectonic and early magmatic rocks. The D4 shear zones are N-S oriented sinistral while

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the D5 shear zones are NE oriented dextral, with shallow dipping lineations. Gold

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mineralization has been reported along these shear zones. The majority of the D4 and D5 shear

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zones formed under lower to upper greenschist facies conditions. Amphibolite facies ductile

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shear zones occur at the contact aureoles of syn-kinematic granitoids (Baratoux et al., 2011).

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The S4 and S5 shear zones are generally brittle to brittle-ductile compared to majority of S1 to

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S3, which are mostly ductile and schistose in nature, suggesting S4 and S5 shear zones

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operated at lower temperatures with respect to the previous deformation events. The D4 and

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D5 are associated to WNW-ESE and E-W shortening, respectively. Late E-W tension gashes

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and NE-oriented steeply dipping brittle faults formed under brittle conditions and E-W

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shortening and are ascribed to the D6 event (Block et al., 2015a). These late brittle structures

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have only limited extension. The veins sometimes contain chlorite, white mica or epidote

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minerals. The last deformation event D7 only affects highly anisotropic lithologies and is

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characterized by subvertical NW-striking crenulation cleavage related to NE-SW shortening.

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3 Methodology

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Gold mineralization at the Bepkong deposit does not appear in outcrop, as the area is entirely

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covered with transported alluvium and colluvium. All geological information was obtained

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from logging of reverse circulation (RC) drilling chips (which form the majority of the holes)

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and diamond drill core (DD) (only a very limited number of these exist) drilled by Azumah

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Resources Limited, and from outcrop samples collected during mapping of the surrounding

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areas within the Wa-Lawra belt. In all, 70 RC and DD bore-holes were logged, constituting 7

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E-W fence lines. A geological map was prepared by projecting the structural measurements

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and lithological data from the drill core logged to surface and was further constrained by

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interpreting conductivity and resistivity induced polarisation (IP) images processed by Sagax

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Afrique S.A in 2012, on behalf of Azumah Resources Limited.

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Mineralogical and textural relationships were investigated using optical microscopy,

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complemented by back-scattered electron (BSE) images obtained with a JEOL JSM 6360LV

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scanning electron microscope (SEM) equipped with a silicon drift detector analysis system, at

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the University of Toulouse, France.

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Spot analyses of pyrite and arsenopyrite crystals were performed by LA-ICP-MS at the

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Géosciences Montpellier laboratory (France), to determine trace element concentrations.

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These were performed using a Geolas (Microlas) Excimer ArF automated platform housing a

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193 nm Compex 102 nanosecond laser from LambdaPhysik, coupled with a high-resolution

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ThermoFinnigan (ELEMENT XR) ICP-MS. Data were acquired in the fast E-scan mode at

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low mass resolution using a flux of 15 J/cm2 at a frequency of 5 Hz and working with a spot

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size of 51 µm. Raw data were processed on-line using the GLITTER software package (e.g.,

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Van Achterbergh et al., 2001), and using several sulphide standards, i.e., pyrrhotite-Po-726

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(Sylvester et al., 2005), in-house chalcopyrite Cpy-RM (Velásquez et al., 2012) and MASS-1

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as external calibrators, and using 34S as the internal calibrator (e.g., Lorand and Alard, 2011).

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During analysis, the following isotopes were monitored: 33S, 34S, 56Fe, 57Fe, 59Co, 60Ni, 63Cu,

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the conditions described above, are between 0.01 to 0.1 ppm. Detection limits were

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calculated as three times the background standard deviation value and were converted to

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concentration units (ppm) with the yieldns parameter (e.g., Borisova et al., 2010). All

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calculated concentrations were comparable to the analytical precision limits of the in-situ

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femtosecond quadrupole LA-ICP-MS technique (< 15% RSD, e.g., Borisova et al., 2010) and

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the reference materials homogeneities (< 16% RSD, Velásquez et al., 2012).

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Forty arsenopyrite samples were analysed for their major elements (Fe, As, S) using an

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electron microprobe (EMP) at the University of Toulouse, France. A CAMECA SX50

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electron microprobe, with an accelerating voltage of 25 kv and current beam of 20nA was

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used. Chalcopyrite was used as the calibration standards for Fe and S and pure metals for Au,

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Ag and Sb.

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Fluid inclusion studies were preceded by careful petrography to identify fluid inclusions

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assemblages (FIA). Primary and secondary fluid inclusions were studied in 30-μm thick

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polished thin sections and doubly-polished slices of 150 μm thickness, using the criteria of

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Roedder (1984) and Goldstein and Reynolds (1994). Microthermometric measurements were

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performed at the University of Toulouse, following the procedures outlined by Roedder

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(1984) and Shepherd et al. (1985), using a Linkam THMGS 600 heating–freezing stage

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mounted on a BX-51 Olympus microscope. The stage was calibrated against synthetic pure

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H2O inclusions (0 and +374.1°C) supplied by SynFlinc and with natural pure CO2 inclusions

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(−56.6°C) from Campeirio (Ticino, Switzerland). Measurements below 0°C are accurate to

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±0.1°C, whereas at the highest temperature measured (~380 °C), they are accurate to ±1°C.

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Cryogenic experiments were carried out before heating, to reduce the risk of decrepitating the

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inclusions. Salinity (expressed as wt. % equivalent NaCl), bulk composition, and density data

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As, 82Se, 66Zn, 121Sb, 111Cd, 107Ag, 125Te, 197Au, 208Pb, and 209Bi. Typical detection limits, at

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were calculated using the modelling software of Bakker and Brown (2003) as well as the

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Flincor software (Brown and Hagemann, 1995).

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Raman spectroscopic measurements were made at the CEMES laboratory in Toulouse, using

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a Labram HR (Horiba Jobin Yvon) Raman spectrometer equipped with a Notch filter and a

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CCD detector cooled to −130°C by liquid nitrogen. The exciting radiation at 514.535 nm was

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provided by an Ar+ 108 laser (type 2020, Spectra-Physics) with a laser power of 200 mW at

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the source and ~20 mW at the sample. Spectra were collected with a grating of 1800

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lines/mm and a confocal ×50 objective, allowing a spectral resolution of 4 cm−1. The

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acquisition range was between 90 and 4,400 cm−1 in order to detect the maximum number of

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possible Raman active vibrational bands of the different dissolved and gaseous phases. Three

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minutes was typically required to measure the entire spectral range.

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4 Local Geology

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Within the Wa-Lawra belt, the Bepkong deposit is located on the N-S anastomosing Kunche-

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Atikpi shear zone (Amponsah et al., 2015), about 4.4 km west of the major N-S trending

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crustal-scale strike-slip Jirapa shear zone.

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4.1 Main Lithologies

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Rocks found in the Bepkong deposit are strongly sheared, metamorphosed graphitic shales

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(Fig. 2a), volcaniclastics (Fig. 2b, d) with graphitic interbeds, and microdiorite dykes (Fig.

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2c). The shales and volcaniclastics are locally interlayered.

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The shale is dark grey, fine grained and non-magnetic. It is mostly made up of quartz,

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graphite, and plagioclase (Fig. 3a, b) with accessory minerals such as rutile. A secondary

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assemblage of calcite, chlorite and sericite superimpose these minerals. The volcaniclastic

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rock is grey, fine-grained and are primarily composed of quartz and euhedral plagioclase

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feldspars, with accessory rutile. These minerals have been overprinted by a hydrothermal

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assemblage of calcite, chlorite and sericite (Fig. 3c, d). Replacement of the plagioclase

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feldspar by secondary calcite (Fig. 3c) is a common feature of the volcaniclastics in the

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Bepkong deposit. The volcaniclastic sediments exhibits graded bedding.

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The microdiorite which usually occurs as sheared dykes in the Bepkong deposit is composed

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primarily of phenocrysts of plagioclase and amphibole in groundmass composed primarily of

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plagioclase and minor white mica and accessory rutile, quartz and sphene (Fig. 3e, f). These

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minerals have been overprinted by calcite as a result of hydrothermal alteration.

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4.2 Deposit scale structures

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The combination of reverse circulation (RC) chips logging, drill core logging, and surface

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mapping in and around the deposit area and interpretations of ground geophysical induced

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polarization (IP) data, have helped with identification of three main deformation episodes in

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the Kunche-Atikpi shear zone (Fig. 4) where the Bepkong deposit is hosted. These

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deformation phases are DB1, DB2 and DB3. The subscript B is used to denote that the structures

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or structural history described in this section pertains to local scale deformation in the

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Bepkong deposit.

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4.2.1 DB1 deformation phase

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DB1 in the area (regional D4, Block et al., 2015a) is represented by N to NNW oriented

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steeply dipping sinistral transcurrent shear zone which denotes an E-W to ESE-WNW

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shortening. This deformation phase affected all the sediments, and the syn-DB1 magmatic

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plutons (Fig. 5a, b). This deformation event is manifested by N to NNW striking and

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vertically dipping penetrative foliation (SB1) with a sub-horizontal stretching lineation (Fig.

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5c) which trends 008˚ and plunges 20˚and parallel to S0 bedding in the sediments.

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SB1 is generally parallel to primary bedding in the sediments. This deformation is mainly

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observed within the Jirapa shear zone. Four km west of the Jirapa shear zone, in the deposit

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area, DB1 is characterized by a discrete anastomosing shear zone. This shear zone is a result

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of strain or deformation partitioning owing to the presence of lithologies with variable

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strength. It is marked by left stepping dilatational jogs, which have been named the Kunche-

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Atikpi shear zone in the study area. The anastomosing shear zones have developed an intense

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N to NNW striking and steeply dipping (78˚ to 80˚) SB1 foliation. This deformation is also

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marked by FB1 isoclinal folds (Fig. 5d) with vertical fold axis and axial planes parallel to the

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SB1 foliation with dips ranging from 76˚ to 82˚ to the west. These anastomosing shear zone

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systems visible at the deposit scale and have created dilatational zones which have permitted

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fluid movement within the sediments. This is shown by the syn-tectonic boudinaged quartz

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veins observed parallel to the foliation. The veins and veinlets usually exhibit asymmetric

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boudinage (Fig. 5e) along the anastomosing shear fabric suggesting sinistral sense of shear.

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The shales and volcaniclastic rocks in the Bepkong deposit exhibit brittle and ductile

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structures and this is attributed to the rheological properties of the rocks.

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4.2.2 DB2 deformation phase

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DB2 in the Bepkong deposit (regional D6, Block et al., 2015a) is marked by an E-W trending

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tension gashes (Fig. 5f, g) that crosscut the SB1 shear zone and are mostly quartz filled

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representing en echelon of quartz veins. This indicates an E-W shortening (the maximum

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compressive stress). The tension gashes are generally Z-shaped with a sinistral sense of shear.

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4.2.2 DB3 deformation phase

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The DB3 deformation phase observed in the Bepkong deposit (regional D7, Block et al.,

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2015a) is marked by an FB3 isoclinal, space cleavages and late brittle faults. FB3 isoclinal folds

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and crenulation cleavages (Fig. 2b, 5h) are at high angle to and overprint the initial SB1

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foliation and quartz veins filling DB1 shear zones. These FB3 are defined by vertical fold axes

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and an ENE-WSW axial planar SB3 foliation. E-W spaced cleavages cross-cut all the

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structures described above. The late NE and NW brittle faults (Fig. 5i) locally displace the

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DB1 shear zones and appear as broken fragments or as offsets in drill core. They are hard to

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find in the field due to the fact that there is no outcrop, but can be easily recognisable in the

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drill cores. This deformation phase is inferred to reflect N-S shortening.

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5 Ore body geometry and alteration

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Gold mineralization in the Bepkong deposit is limited to the dilational zones within the

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anastomosing N-NNW DB1 shear zones, and is associated with quartz veins contained mostly

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in volcaniclastic rocks and shales. The overall mineralized zone is 300 m wide with a strike

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length of 560 m. It trends N-S and plunges steeply to the south, with mineralization still open

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at depth after been modelled to a depth of 200 m. It consists of networks of veins forming

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four distinct parallel ore bodies of overall lenticular shape and variable thickness (Fig. 6).

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A pervasive alteration affects the sediments that host the mineralization; it can easily be

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observed on a macroscopic scale as one moves across the ore zone into the country rock,

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mostly by the elevated content of arsenopyrite and pyrite. Pyrite occurs in vein selvages and

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vein fractures, and also disseminated in the wall rock, whilst arsenopyrite is exclusively

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associated with the veins, where it occurs in the selvages and vein fractures. Other alteration

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minerals associated with the sulphides in the ore zone include chlorite, calcite, ankerite,

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quartz and sericite. The alteration zone has an irregular width, with it thickest part being

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about 300 m. The uneven size of the alteration is attributed mainly to the change in

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permeability and porosity of the host shales and volcaniclastics. This alteration zone is

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bounded by the graphitic alteration zone. In addition to forming the alteration assemblage,

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calcite and pyrite are ubiquitous within the rock suites in the Bepkong area.

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6 Quartz veins

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A single generation of quartz vein was described by Amponsah et al. (2015) in the Bepkong

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deposit. However, further drilling has permitted the identification of a second system of

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veins, associated with tension gashes. Although we could not observe clear crosscutting

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relationships between these two quartz vein systems, the second vein set could be related to

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the DB2 deformation and therefore postdate the first veins. Below, the two vein sets are

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described based on their infill type. They are designated as Type-1 and Type-2, respectively.

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Type-1 veins are milky in nature, crystalline, have variable thickness (Fig. 7) and are

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synchronous with the anastomosing DB1 shear zone. These veins are boudinaged (Fig. 7b)

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and brecciated (Fig. 7c), indicating that they formed under ductile to brittle conditions. The

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veins formed within the left stepping deformational jog during the late stages of the DB1

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deformation, which created open fracture systems. However, the veins do not contain vugs.

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Within and outside the dilatational zones, the thickness of these veins range from 0.4 to 3 m,

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and from 0.1 to 0.2 m, respectively. The quartz veins are composed predominantly of quartz,

303

plus calcite (5 %), sulphides (5%), ankerite (3 %), chlorite (3 %) and sericite (1 %). The

304

sulphides consist mainly of pyrite and arsenopyrite with minor chalcopyrite, pyrrhotite,

305

galena and sphalerite occurring as inclusions in the former sulphides. They occur in the vein

306

selvages, within the country rock (Fig. 7b) or, more locally, they fill fractures in the veins,

307

together with chlorite and sericite (Fig. 7d). Arsenopyrite crystals are commonly surrounded

308

by quartz fringes or well-developed pressure shadows, along the direction of shearing. The

309

quartz exhibits undulose extinction, indicating it has accommodated some strain. Textural

310

analysis of the quartz grains indicates mainly elongate blocky (Fig. 8a) and recrystallized

311

textures. The fine-grained quartz has serrated edges (Fig. 8b) indicating varying degrees of

312

dynamic recrystallization by the mechanism of grain boundary migration and sub-grain

313

rotation. Calcite and ankerite mostly exhibit interstitial textures within the recrystallized

314

quartz grain. Outside the ore zone, similar veins to Type-1 veins are observed, and are

315

composed mainly of quartz, pyrite and calcite.

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Type-2 quartz veins are associated with the DB2 deformation episode. These consist of en

317

echelon small quartz veins that fill DB2 tension gashes (Fig. 5f, g), and are made up of milky

318

quartz and contain traces of calcite. These veins are typically a few mm to few cm thick and

319

are not associated with any sulphides, ankerite or sericite. They do not contain gold

320

mineralization.

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7 Ore mineralogy

323

7.1 Sulphides

324

The principal sulphides that occur in the mineralized zone at Bepkong are arsenopyrite and

325

pyrite, plus minor pyrrhotite, galena, sphalerite and chalcopyrite (Figs. 9, 10, 11).

326

Arsenopyrite makes up about 30 % of the total sulphide population within the deposit and is

327

restricted to the mineralization zones, where it occurs within the altered host rock, quartz vein

328

selvages and fractures. It is medium grained (0.2 mm to 1 mm) with mostly anhedral shapes,

329

forming fragmented residual crystals usually overgrown by pyrite (Fig. 9a, 10a). BSE SEM

330

imagery reveals a primary crystallographic zoning, with alteration rims developing around

331

fracture and crystal margins (Fig. 10b). At Bepkong, the arsenopyrite has an average

332

composition of 23.8 wt. % S, 36 wt. % Fe and 41wt. % As (Table 1).

333

Pyrite is ubiquitous in the entire rock package at Bepkong. In rocks outside of the mineralized

334

zone, this sulphide is mainly subhedral to euhedral in shape and occur as disseminated

335

isolated crystals. In the mineralized zone, pyrite ranges from 0.5 mm to 35 mm in size, and

336

commonly overgrow arsenopyrite. It may display subtle fracturing (Fig. 10b). Under BSE

337

SEM imagery, the pyrite grains are uniform (Fig. 10c), with an average composition (Table

338

1) of 54 wt. % S, 45.7 wt. % Fe, 0.3 wt. % As. Chalcopyrite, pyrrhotite, galena and sphalerite

339

form mostly micron-size inclusions (up to 10 µm) within pyrite (Fig. 9b, 10a, 10d), in the

340

alteration rims of arsenopyrite (Fig. 11a), or in fractures in the latter (Fig. 11b).

341

The trace-element composition of the sulphides was determined by LA-ICP-MS (Table 2).

342

Pyrite generally has low concentration of trace elements when compared with arsenopyrite.

343

Invisible Au and Ag (as opposed to occasional inclusions intersected by the laser beam)

344

range, respectively, from 0.01 to 0.74 ppm (mean of 0.34 ppm) and 0.06 to 0.13 ppm (mean

345

of 0.10 ppm). Concentrations of Ni, Co, Sb, Se, Pb and Cu are low. Arsenopyrite is rich in Ni

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(19.5 to 712 ppm), Co (0.8 to 128 ppm), Sb (306 to 1225 ppm), Pb (0.01 to 336 ppm), Au

347

(0.82 to 37 ppm) and Ag (0.4 to 7 ppm).

348

7.2 Gold occurrence

349

In the Bepkong deposit, gold is mainly associated with arsenopyrite and pyrite in a form of

350

invisible gold (Table 2) and visible gold (Fig. 9c). It also occurs as free gold grains within

351

fractures of type-1 quartz veins (Fig. 12a) and within the altered country rocks that flank

352

Type-1 veins (Fig. 12b). Based on the LA-ICP-MS analyses, invisible gold occurs in

353

arsenopyrite and pyrite (Table 2), with the pyrite having much lower gold concentrations

354

compared to the arsenopyrite (Table 2; Fig. 13). Visible gold grains were found as micron-

355

size inclusions on the edges of pyrite grains, in the altered halos of arsenopyrite and within

356

fractures in arsenopyrite (Fig. 11a). The visible gold in the fractures of the arsenopyrite is

357

commonly accompanied by chalcopyrite and other sulphides (Fig. 11a, b). Rutile, albite and

358

other minerals are also locally present (Fig. 14a). Visible gold grains were also observed at

359

grain boundaries between pyrite and arsenopyrite (Fig. 14b).

360

7.3 Arsenopyrite geothermometry

361

In order to develop an integrated understanding of the temperature-pressure history of

362

mineralization in the Bepkong deposit, the arsenopyrite geothermometer together with fluid

363

inclusion studies are used, with the latter described in the section below.

364

To use the stability diagrams of Sharp et al. (1976; see also Sugaki et al., 1975; Clark, 1960a,

365

b; Kretschmar and Scott, 1976) to determine temperatures, the sulphides and the

366

sulpharsenides must attain equilibrium conditions without differential reequilibration (Morey

367

et al., 2008). BSE SEM imagery carried out on the arsenopyrite crystals in the Bepkong

368

deposit shows that the arsenopyrite crystals have primary cores, characterized by

369

crystallographic zoning, as well as rims and healed fractures, detectable under highly

370

contrasted BSE settings, suggesting dissolution and reprecipitation of this sulphide (Fig. 10b,

371

11a). Textural evidence suggests that pyrite precipitated at about the time of arsenopyrite

372

alteration, so we assume equilibrium between arsenopyrite rims/fractures and pyrite. From

373

electron microprobe analysis (EMPA) however (Table 1), there are only very small variations

374

in the concentrations of S, Fe and As between the altered zones and the primary arsenopyrite

375

grains.

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To determine the temperature in the Bepkong deposit, we used microprobe analyses on

377

mineralised arsenopyrite (Table 1). The elements analysed were S, Fe, Au, Cu, Ag, Pb, Bi,

378

As, Sb. Elements Au, Cu, Ag, Pb, Bi and Sb, when combined, gave a total concentration of

379

less than 1 wt. % and where therefore considered as minor elements and not relevant for the

380

geothermometric analysis. For temperature estimations, the atomic % of As were plotted on

381

the ƒ (S2) - Temperature diagram of Kretschmar and Scott (1976) and Sharp et al (1985) (Fig.

382

15). In order to distinguish between early (invisible) gold and the late visible gold (in

383

fractures and grain boundaries), we tried to separate analyses of arsenopyrite cores and rims.

384

However, as mentioned above, the two sets of values essential overlap; as a result, in Figure

385

15 we show an interval of T-fS2 values corresponding to an average range of the data, i.e.,

386

between 275 °C and 290 °C, which limit the conditions prevailing during alteration.

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8 Fluid inclusions

389

8.1 Fluid inclusion petrography

390

Fluid inclusion studies were performed on quartz crystals from the Type-1 mineralized quartz

391

veins and in pressure shadows (Fig. 16e) around the sulphides in the mineralized zone.

392

Petrographic studies, as outlined by Goldstein and Reynolds (1994), and subsequent

393

measurement of fluid inclusions thermometric properties were centred on closely associated

394

groups and trails of primary and pseudosecondary inclusions based on the genetic

395

classification by Roedder (1984).

396

Primary and pseudosecondary fluid inclusions (Fig. 16a, c) were observed mainly within

397

quartz crystals from the DB1 Type-1 quartz veins and also in pressure shadows. Primary

398

inclusions occur as isolated individuals or in small clusters, while pseudosecondary

399

inclusions are aligned along short planes that do not crosscut grain boundaries. Their sizes

400

range from less than a micron to about 35 µm.

401

Two types of fluid inclusion were identified based on the number of phases and degree of

402

filling at room temperature, phase variations during heating and freezing runs, and Raman

403

spectroscopic analysis. Type-1 fluid inclusions are CH4-H2O ± SO2 fluid inclusions (Fig. 16f)

404

and the least abundant within the mineralized zone in Bepkong. They usually occur as

405

clusters or in a trail within pressure shadows around arsenopyrite in the mineralized zone.

406

This fluid inclusion type is vapour (V)-rich with very little liquid (L), with the CH4 ± SO2

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vapour bubble generally occupying about 90-100 % of the total fluid inclusion. The

408

inclusions show irregular shapes and small sizes, ranging from 2 to 5 μm. Type-2 fluid

409

inclusions are H2O-CH4-CO2-SO2 fluid inclusions (Fig. 16b, d) and the most abundant fluid

410

inclusion type. These are two-phase L+V inclusions with the CH4-CO2-SO2 vapour bubble

411

generally occupying about 30 to 40 % of the total fluid inclusion, although it ranges from 10

412

to 90 %. This fluid inclusion type occurs as isolated primary inclusions or in

413

pseudosecondary trails, with sizes ranging from 5 to 30 μm.

414

8.2 Analytical results and fluid composition

415

Raman spectroscopic analysis were performed on all fluid inclusion types, to obtain the

416

composition of the fluids. Spectra show that the V-phase in Type-1 inclusions is composed of

417

CH4 ± SO2, with traces of CO2 detected in a few inclusions, whilst the L and V phases in

418

Type-2 fluid inclusion are composed of H2O ± SO2 and CH4-CO2-SO2, respectively (Fig. 17).

419

A total of about 40 fluid inclusions each for Type-1 and Type-2 fluid inclusions were

420

analysed in the Type-1 quartz veins and 15 fluid inclusions were analysed from pressure

421

shadows (data shown in Table 3). The lower temperature phase changes (i.e., the total

422

freezing, ice nucleation and melting, clathrate dissolution and methane homogenization

423

temperatures) were measured first to curtail the possibility of fluid inclusion decrepitation

424

during the high-temperature heating experiments (about 70% of the inclusions decrepitated

425

above 300 °C).

426

The supercritical methane bubble in the Type-1 fluid inclusions separated into a L and a V

427

phase at temperatures below -80 °C. When cooled to the temperature of the liquid nitrogen,

428

the methane froze and during reheating it attains the triple points between -184.4 °C to -180

429

°C. About 90 % of the fluid inclusions studied gave values close to the triple point of pure

430

CH4, (-182.5 °C). While the measurements below the triple point of pure CH4 can be

431

attributed to the presence of small amounts of SO2, as detected by Raman spectroscopy, the

432

values above the triple point were probably due to the difficulty in observing the phase

433

transition precisely, due to the very low temperatures of this phase transition, close to the

434

physical limit of the stage. CH4 homogenization temperatures ranged from -84.2 °C to -80

435

°C, with most inclusions displaying critical behaviour (-82.6 °C). The data above this value

436

are due to the presence of minor amounts of SO2. Clathrate melting temperatures ranged from

437

14 °C to 17 °C. These temperatures are within the range of clathrate melting temperatures for

438

CH4 (10 °C to 19 °C; Diamond, 2003). Total homogenization temperatures ranging from 310

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°C to 370 °C where measured. The densities, calculated using the Flincor computer program

440

(Brown and Hagemann, 1995) for the Type-1 inclusions, are at about 0.45 g/cm3.

441

During experiments with Type-2 fluid inclusions, the carbonic phase displayed ice melting in

442

a range between -74 °C to -56.8 °C. These temperatures, which are near the triple point of

443

CO2, suggest more important amounts of this gas with respect to CH4 in these fluid

444

inclusions. The departure from the triple point of pure CO2 is explained by the significant

445

CH4 present. Clathrate melting temperature occurred from 8 °C to 16 °C. Homogenization

446

temperatures of the carbonic phase ranged from 18 °C to 25 °C, while total homogenization

447

temperatures from 320-342 °C. The proportion of gases calculated from Raman and

448

microthermometric data are: 0.36 to 0.61 XCH4, 0.03 to 0.4 XCO2, and 0.01 to 0.29 XSO2.

449

Salinities for Type-2 fluid inclusions, calculated from clathrate dissolution temperatures,

450

range from 2.27 to 3.42 wt. % eq. NaCl, with densities ranging from 0.39 g/cm3 to 0.58 g/cm3

451

(BULK and ISOC of Bakker, 2003).

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9 Discussion

454

9.1 Structural control on gold mineralization and timing relationships

455

The initial pervasive deformation (DB1) that was observed at the deposit scale, is

456

characterized by N-S trending anastomosing shear zones that define an initial SB1 foliation

457

with steep dips; it is the main structural event that occurred in the Wa-Lawra belt. The

458

anastomosing pattern is interpreted to result from strain partitioning due to the rheological

459

difference between the shale and the volcaniclastic rocks. The FB1 and the LB1 are all parallel

460

to SB1 and are all probably part of the same transcurrent sinistral system. The FB1 folds are

461

asymmetric, non-cylindrical, tight to isoclinal, with an azimuth on the curvilinear axis at

462

about 5 to 22°, with an FB1 axial plane plunging north. These folds were formed during the

463

DB1 deformation event as a result of continual WNW-ESE shortening.

464

The anastomosing shear zones have created several dilatational zones or jogs during the DB1

465

deformation phase on the Wa-Lawra belt. The Bepkong deposit sits on one of these

466

dilatational zones. They have created discrete open system within an overall partly open wall

467

rock system that permitted channelized fluid flow with substantial flux from in and out of the

468

wall rock, hence the sulphidation observed within the wall rocks.

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The DB1 shear zone observed within the study area is part of the 40 km N-S Kunche-Atikpi

470

shear zone (Amponsah et al., 2015), which in itself is part of the crustal scale N-S

471

transcurrent shear system called the Jirapa shear zone in the Lawra belt (Baratoux et al.,

472

2011; Block et al., 2015). The formation of the anastomosing N-S Kunche-Atikpi shear zone

473

can be attributed to the general WNW-ESE major shortening event (Regional D4 event;

474

Block et al., 2015b). The mineralized shear system is part of the long-lived Jirapa shear zone

475

system. The spatial association of the mineralized or auriferous shear zone (Kunche-Atikpi

476

shear zone) and the regional transcurrent shear zone (Jirapa shear zone) is typical of orogenic

477

gold settings described by McCuaig and Kerrich (1998) and Goldfarb et al. (2001).

478

Amponsah et al. (2015) observed that the parallelism of the Type-1 veins in the ore shoots

479

within the DB1 shear zones, and the occurrence of gold in altered rock around the deformed

480

quartz veins clearly indicate a control of the anastomosing shear zone on the gold

481

mineralization in the Bepkong deposit. The maximum age for the deposition of the volcano-

482

sedimentary units of the Wa–Lawra belt is 2139 ± 2 Ma (U–Pb zircon ages; Agyei Duodu et

483

al., 2009). The onset of the DB1 (i.e., regional D4) took place after 2130 Ma, which is defined

484

by a U–Pb age constraint on D2, from metamorphic monazite (Block et al, 2015a). The DB1

485

deformation event took place relatively late in the Eburnean tectonic history in Northern

486

Ghana. The E-W orientation and brittle character of the DB2 veins suggest that they are

487

associated with the regional D6 event (Block et al., 2015a). The DB3 structures formed under

488

N-S shortening can be correlated with the last regional deformation phase D7 (Block et al.,

489

2015a).

490

9.2 Sulphide paragenesis and gold mineralization

491

Gold formation in the Bepkong deposit is associated with the local DB1 deformation event,

492

whereby gold-bearing hydrothermal fluid infiltrated dilatational zones in the host rocks. This

493

lead to the formation of Type-1 quartz veins, accompanied by alteration by chlorite + sericite

494

+ ankerite + quartz + calcite + sulphides, which took place adjacent to these veins, creating

495

alteration selvages as well as pervasively throughout the host rocks containing the ore body.

496

Textural relationships presented in this study indicate that arsenopyrite formation took place

497

at an early stage in the hydrothermal history of the deposit. This first generation is expressed

498

by the arsenopyrite cores that show primary crystallographic zoning (Fig. 10b). At a later

499

stage, arsenopyrite grains underwent important breakup and fracture development, as well as

500

alteration along the rims and on fracture margins (e.g., Figs. 10b, d, 11a). Pyrite on the other

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hand, forms euhedral to subhedral crystals and is chemically very homogeneous. These

502

features indicate a simple precipitation history, and that it formed relatively late since it did

503

not undergo fracturing and it systematically overgrows arsenopyrite broken-up grains. It can

504

also be noted that pyrite occurs throughout the alteration zone, whereas arsenopyrite is

505

limited to the veins and their alteration halos. Chalcopyrite, sphalerite, galena and pyrrhotite

506

occur as inclusions within pyrite and in the alteration rims of arsenopyrite, plus in fractures

507

within the latter, which suggest that these sulphides formed during alteration of arsenopyrite,

508

before or during onset of pyrite precipitation.

509

Trace element LA-ICP-MS analyses (Table 2; Fig. 13) show that arsenopyrite has high

510

concentration of Ni, Co, Te, Sb and Pb. Enrichments in Ni and Co have been widely reported

511

for arsenopyrite (e.g. Obuasi; Fougerouse et al., 2016a), and at Bepkong likely reflect

512

circulation of the hydrothermal fluid in the surrounding volcanic sediments and shale. High

513

concentrations of Se have been reported by Cook et al. (2013) in arsenopyrite, who

514

considered it a substitution for S in the sulphide structure. The shale is the probable source

515

for Se.

516

LA-ICP-MS analysis, SEM imagery, detailed mineralogical studies coupled with EMP

517

analysis, indicate that the occurrence of gold in the Bepkong deposit is in three distinct forms:

518

(i) invisible gold in arsenopyrite and, to a much lesser extent, pyrite; (ii) micron-size visible

519

gold inclusions in pyrite, within the altered rims of arsenopyrite and in fractures in this

520

sulphide, and at the interface between pyrite and arsenopyrite; (iii) free visible gold within

521

fractured quartz veins and altered vein selvages. These different modes of occurrences

522

indicate a probable complex gold mineralization evolution path.

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9.3. Invisible Gold

525

Several workers have discussed the ways in which invisible gold forms in sulphides.

526

According to Boyle (1979), gold precipitates in sulphides via the substitution reaction, where

527

Au substitutes for As in covalent bonding, due to the similar radii of these elements. In pyrite,

528

particularly, the substituting As forms an anion pair (AsS)3-, allowing for charge balance by

529

substituting Au3+ for Fe2+ (Cook and Chryssoulis, 1990). The intake of gold into the

530

arsenopyrite is usually in the form of Au0 and Au1+ (Cabri et al., 2000). Fleet et al. (1993) and

531

Simon et al. (1999), suggested that As-rich sulphides (thus arsenopyrite) are good scavengers

532

of gold (see also Pokrovski et al, 2002).

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533

9.4 Remobilization of Gold

535

The invisible gold occurring in the arsenopyrite in the Bepkong deposit is primary, because it

536

precipitated at the same time as the host sulphide from the same hydrothermal fluid (e.g.,

537

Cook and Chryssoulis, 1990; Simon et al., 1999). Conversely, the micron-size visible gold

538

grains observed as inclusions, in factures in sulphides or at their interfaces, and the free

539

visible gold within fractures of the quartz veins and altered rocks, are, at least partly,

540

secondary. The most compelling evidence is the fact that, in arsenopyrite, visible gold occurs

541

exclusively in the altered rims and fractures, where invisible gold contents are clearly lower

542

than in unaltered zones (Table 2). This suggests that gold was scavenged from this sulphide

543

during alteration and precipitated as native metal. Consistent with this observation is the

544

presence of small grains of chalcopyrite and other metal and metalloid sulphides along with

545

most occurrences of native gold. Cu in the Apy correlates well with gold in LA-ICP-MS

546

analyses (r = 0.65), it is thus likely that this, and the other chalcophile elements that were also

547

detected in the arsenopyrite, were remobilised at the same time as gold. A similar scenario,

548

whereby during deformation and repeated hydrothermal fluid flow, the sulphides react with

549

the fluids causing gold dissolution and re-precipitation, has been described in several cases in

550

orogenic gold deposits (e.g., Putins, 2002; Morey et al., 2008; Velásquez et al., 2014) and is

551

considered by Mumin et al. (1994) and Fougerouse et al., 2016a as a major process for the

552

formation of gold in Southern Ghana.

553

Gold inclusions were not observed in unaltered arsenopyrite cores, whereas they are common

554

in pyrite. However, in many instances, these gold grains seem to be aligned along a trail in

555

pyrite (Fig. 9c) and in altered arsenopyrite (Fig. 11a), which may signify that they had

556

initially formed along a fracture (in arsenopyrite) and were subsequently incorporated in

557

pyrite during arsenopyrite alteration, rather than being trapped directly during pyrite

558

precipitation. In either case, the evidence suggests that the remobilisation took place during

559

fracturing of the arsenopyrite, just before or during pyrite precipitation. It has recently been

560

shown (Fougerouse et al., 2016b) that while arsenopyrite is very resistant to high-strain

561

deformation at elevated temperature, it is much more sensitive to low-strain microfolding at

562

lower temperatures. At these conditions, the sulphide deforms plastically, and undergoes

563

subgrain embrittlement and eventual fracturing, which enhances fluid circulation and

564

consequent gold (and other metals) release into the fluid. It is thus likely that gold

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565

remobilization took place during the late DB3 event, triggered by formation of the crenulation

566

cleavage at a high angle to SB1 foliation in the deposit area.

567

9.5 Temperature conditions during gold mineralization

569

In the Bepkong deposit, two main fluid inclusion types were recognized, Type 1, dominated

570

by CH4 and H2O, and Type 2, which contain similar amounts of CH4 and CO2 in addition to

571

water, and traces of SO2. Type-2 inclusions are the most abundant in the Bepkong deposit,

572

while Type-1 inclusions were mostly found in pressure shadows around arsenopyrite grains.

573

The particularly reduced composition of the carbonic phase in Type-1 inclusions probably

574

reflects circulation of this fluid in the graphitic shale within the deposit area. Similar CH4-

575

rich fluid inclusions in low-grade metamorphic organic-rich sediments are reported, for

576

instance, by Mullis (1987) and Fan et al. (2000). During arsenopyrite crystallization the rocks

577

were being ductilely deformed, and pressure shadows were forming during growth of this

578

sulphide (cf., Velasquez et al., 2014). Therefore, the CH4-H2O fluid that was trapped within

579

these pressure shadows represents the fluid responsible for the early stage of gold

580

mineralization, i.e., formation of invisible gold. Type-2 fluid inclusions are very abundant

581

throughout the Type-1 quartz veins, probably reflecting a high fluid flow, which is expected

582

in a scenario of vein opening. The presence of CO2 and SO2, more typical constituents of

583

metamorphic fluids than CH4, is consistent with a large input of fluid that did not completely

584

equilibrate with the shale host.

585

Microthermometric data indicate that the fluid represented by Type-1 inclusions circulated in

586

the rocks at temperatures of at least 310 °C to 370 °C, while the Type-2 fluid minimum

587

temperature is about 320 °C to 340 °C. These temperatures are consistent with widespread

588

arsenopyrite precipitation during early stages, although higher than those obtained by the

589

arsenopyrite geothermometer (270 °C to 290 °C). However, as discussed above, the

590

arsenopyrite had re-equilibrated with the fluid that caused the late alteration, and therefore the

591

conditions it recorded are more likely those prevailing during the lower-strain DB3

592

deformation regime.

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9.6 Genetic model for gold mineralization in Bepkong deposit

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The geological and geochemical characteristics described above have made it possible to

596

conceptualize a genetic model for the three forms of gold mineralization (invisible gold,

597

micron-sized gold and free gold grains) in the Bepkong deposit. The lithology also plays an

598

important role. A two-stage process is proposed for gold mineralization, controlled by the

599

formation of the regional D4 shear zones (Block et al., 2015b), known as the DB1 event within

600

the study area, and evolution to low-strain DB3 deformation.

601

9.6.1 Stage 1

602

In the Bepkong deposit, gold enrichment took place during formation of the anastomosing

603

DB1 shear zone, which resulted from strain partitioning owing to the presence of lithologies

604

with variable strength. The graphitic shale deformed in a ductile fashion, whilst the

605

volcaniclastic rock deformed in a brittle manner, thereby causing the rock to brecciate. The

606

anastomosing shear zones created dilatational sites, which acted as permeable sites for

607

hydrothermal fluid flow. Although shear zones can enhance hydrothermal fluid flow

608

(Kerrich, 1996), the overall ductile regime experienced by the graphitic shale does not really

609

permit large volumes of fluid circulation, as is the case of Bepkong which is dominated by

610

graphitic shale. The brecciated interlayered volcaniclastic sediments however, created

611

fractures and open space which favoured the circulation of hydrothermal fluids generating in-

612

fill veins, although veins are also present in the graphitic shale. The hydrothermal fluid

613

drained within the anastomosing shear zone altered the country rock and caused modification

614

of the original mineralogy of the rock, which has been superimposed by chlorite, calcite,

615

sericite and sulphides. Abundant arsenopyrite precipitated at this stage, incorporating gold in

616

its structure, from a CH4-bearing fluid of metamorphic origin that equilibrated with the

617

graphitic shale in the area and that circulated at temperatures higher than ~350 °C. During

618

this period, dykes of microdiorite are also emplaced within the shear zones. These dykes are

619

generally not mineralized, however, except in locations where rare quartz veinlets occur in

620

them.

621

9.6.2 Stage 2

622

Stage 2 is mostly responsible for the economic gold in the Bepkong deposit. With time,

623

probably as a response to temperature decrease during exhumation, the deformation regime

624

was limited to pervasive DB3 crenulation structures that developed at a high angle to the main

625

DB1 foliation. The alteration observed in the rims and around the fractures in arsenopyrite

626

indicate that the conditions of fluid circulation were also different than those prevailing

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during initial arsenopyrite formation. In particular, temperatures were lower, as recorded by

628

the arsenopyrite geothermometer and consistent with the abundant precipitation pyrite (e.g.,

629

Pokrovski et al., 2002). During this phase, microfolding may have weakened and fractured

630

the arsenopyrite grains (cf., Fougerouse et al., 2016b), enhancing dissolution of the invisible

631

gold. Remobilization took place to within a few microns, as small visible grains forming

632

inclusions or in cracks within the sulphides. However, it is likely that gold was also

633

transported to longer distances, where it formed larger free gold grains in fractures in the

634

quartz veins and in the altered country rock. Gold precipitation was probably induced by the

635

decrease in sulphur activity in the fluid that accompanied pyrite formation. A similar scenario

636

took place for at least part of the mineralization in the giant Obuasi deposit, in the south of

637

Ghana (e.g., Fougerouse et al., 2016a).

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10. Conclusions

640

From this study it can be concluded that the gold in the Bepkong deposit is associated with

641

graphitic shale and volcaniclastic rocks and controlled by the long lived DB1 anastomosing

642

shear zone in the Kunche-Atikpi shear zone. Regionally, this shear zone is part of the D4

643

regional transcurrent Jirapa shear zone in NW Ghana (Block et al., 2015b). The alteration

644

mineral assemblage overprinting the mineralization zone is chlorite + sericite + ankerite +

645

quartz + calcite + sulphides. Mineralization is mostly carried by Type-1 quartz veins and

646

surrounding alteration halos. A two-stage model has been proposed for the formation of gold

647

within the Bepkong deposit. The first stage is attributed to the initiation of the shear zones,

648

with early formation of arsenopyrite, which incorporated invisible gold in its structure. This

649

was mediated by metamorphic fluids that gained CH4 by circulating in the graphitic shale at

650

temperatures above 350 °C. Stage two is characterized by cooling of the hydrothermal system

651

and evolution to a more brittle deformation regime, accompanied by microfolding at high

652

angle to SB1 foliation, during DB3 deformation. This caused pervasive fracturing of

653

arsenopyrite, which in turn triggered its alteration by the enhanced fluid circulation in the

654

newly created space. Precipitation of a secondary arsenopyrite took place in the altered rims

655

and fractures, accompanied by pervasive pyrite precipitation. Gold in the arsenopyrite

656

structure was dissolved and re-deposited, as micron-size gold grains, into the healed fractures

657

of this sulphide, its altered rims, along grain boundaries, as well as larger free gold grains

658

within fractured quartz veins. Visible gold was accompanied by chalcopyrite, galena,

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659

sphalerite and pyrrhotite, which formed from Cu, Zn, Pb metals that were also remobilised

660

from the structure of first-stage arsenopyrite.

661

Acknowledgements

663

The authors would like to particularly thank Azumah Resources Limited, the owners of the

664

Bepkong deposit, for giving the permission to release the data and for financing the

665

fieldwork. AMIRA International and sponsors from industry, AusAid and the ARC Linkage

666

Project LP110100667 are gratefully acknowledged for their support through the WAXI

667

project (P934A). POA thanks Nick Franey and Stephen Stone for the continual support

668

throughout the program. Special thanks go to Richard Scriven Nyarko, Andrew Chubb,

669

Sylvain Block and Vitus Bomaansan for useful discussions on the geology of the Bepkong

670

area, and to Nick Franey for providing advice on an earlier version of this manuscript. We are

671

indebted to JAES reviewer German Velásquez for his thorough analysis of the manuscript,

672

which greatly contributed to enhance the quality of the resulting paper.

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after Milési et al., 2004), highlighting the positions of the Kenema-Man domain, Baoulé-

912

Mossi domain, Koudougou-Tumu granitoid domain (KTGD), and Diebougou-Bouna

913

granitoid domain (DBGD). The black box localises the Wa-Lawra belt. b) Geological map of

914

the Wa-Lawra belt (Modified after Block et al., 2015, Amponsah et al., 2015) locating the

915

Bepkong deposit (4) plus several other gold prospects: 1. Basabli 2. Duri 3. Yagha; 5.

916

Kunche 6. Butele. The black box shows the location of Figure 4.

917

Fig. 2. Photographs of drill core from the Bepkong deposit showing rock textures. (a)

918

Unaltered deformed shale. (b) Volcaniclastic rock exhibiting well-developed SB3 crenulation

919

cleavage. (c) Highly sheared microdiorite. (d) Volcaniclastic rock affected by brittle

920

deformation showing brecciation.

921

Fig. 3. Representative photomicrograph of rocks from the Bepkong deposit. (a) Shale

922

composed of quartz, sericites and graphite. Minerals in the thin section have a preferred shape

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ACCEPTED MANUSCRIPT

orientation along SB1 and are affected by SB3 crenulated. (b) Shale composed of plagioclase

924

and quartz also exhibiting a crenulation cleavage. (c) Volcaniclastic rock showing euhedral

925

plagioclase and quartz with secondary calcite. (d) Volcaniclastic rock showing strong

926

alteration by chlorite and newly-formed quartz. (e) Altered microdiorite exhibiting

927

phenocryst of plagioclase and amphibolite in a ground mass of plagioclase, micas and

928

accessory quartz. (f) Altered microdiorite showing poikilitic plagioclase. Amp-amphibole,

929

Cal-calcite, Chl-chlorite, Gr-graphite, Pl-plagioclase, Qz-quartz.

930

Fig. 4. (a) A simplified geological map of a portion of the Kunche-Atikpi shear zone, west of

931

the Jirapa shear zone (see Fig. 1 for location). In the black box is the Bepkong deposit

932

(subpanel b). (b) Map of the deposit, showing the N to NNW anastomosing shear zones and

933

the mineralized corridors. The diagrams in the bottom half illustrates equal-area lower-

934

hemisphere stereo plots and rose diagrams of structural orientation.

935

Fig. 5. Field and drill core photographs of representative structures in the Bepkong deposit. a)

936

SB1 foliation observed within a drill core with a boudinaged quartz vein which is flanked by

937

SB1 foliation related to the DB1 deformation event. (b) N-S penetrative SB1 foliation in the syn-

938

tectonic plutons. (c) North trending subhorizontal stretching lineation formed on an SB1

939

foliation plane in the magmatic rocks. (d) Volcaniclastic rock cropping out within the

940

Kunche-Atikpi shear zone exhibiting N striking SB1 foliation and north verging FB1 isoclinal

941

fold which is axial planar to SB1. (e) An asymmetric boudinaged vein exhibiting a sinistral

942

sense of shear along the shear fabric in the DB1 deformation event. (f) DB2 E-W tension

943

gashes cross-cutting the SB1 foliation. (g) Zoom at the drill core showing the DB2 E-W

944

trending tension gashes. (h) a drill core exhibiting E-W trending FB3 fold axis with the axial

945

plane acting as the SB3 foliation plane. E-W spaced cleavages can be observed and this is

946

characteristic of DB2 deformation. (i) Drill core showing the DB3 NW brittle fault.

947

Fig. 6. A cross-section (looking north) of the Bepkong deposit along 1,152,175 m N, showing

948

mineralized shoots within the main ore body, along subvertical DB1 shear zones (cf., Fig. 4

949

for location). High-grade mineralization is confined to a 100 to 150 m-thick alteration zone

950

defined by an assemblage consisting of chlorite + calcite + sericite + quartz + sulphides.

951

Graphitic layers within the shale and the volcaniclastic rocks limit the mineralized

952

hydrothermal conduits (modified after Amponsah et al., 2015).

953

Fig. 7. Photographs of drill core showing the various characteristic of Type-1 quartz veins

954

observed within the Bepkong deposit. (a) Drill core showing details of a 3-m thick interval

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31

ACCEPTED MANUSCRIPT

containing several Type-1 veins made up of crystalline quartz, within the dilational zone of

956

the anastomosing DB1 shear zone. Within these veins are ankerite (Ank) and quartz (Qz). (b)

957

Boudinaged Type-1 quartz veins reworked by the FB3 fold. Pyrite (Py), calcite (Cal), chlorite

958

(Chl) and arsenopyrite (Apy) are in the selvages of the veins. (c) Veins exhibiting brecciation.

959

(d) Type-1 Quartz veins showing fractures filled with arsenopyrite and secondary minerals

960

such as chlorite, calcite, and sericite.

961

Fig. 8. Photomicrographs of a Type-1 quartz vein in the Bepkong deposit. (a) Type-1 quartz

962

vein exhibiting elongate blocky textures. (b) Type-1 quartz vein showing sheared

963

recrystallised and ribbon-like textures with preferential orientation, and interstitial calcite.

964

Fine-grained recrystallized quartz develops along the shear planes, affected by grain

965

boundary migration and subgrain rotation. Cal-calcite, Qtz-quartz.

966

Fig. 9. Photomicrograph under reflected light showing the textural relationship of sulphides

967

in mineralized rocks from the Bepkong deposit. (a) A collage of two fields of view showing

968

pyrite overgrowing fragmented arsenopyrite. (b) An inclusion of chalcopyrite, galena and

969

sphalerite in pyrite (c) Photomicrograph showing aligned micron-size visible gold grains.

970

Apy-arsenopyrite, Ccp-chalcopyrite, Gn-galena, Py-pyrite, Sp-sphalerite.

971

Fig. 10. BSE SEM images of sulphides in the Bepkong deposit. (a) Subhedral to euhedral

972

pyrite (Py) overgrowing anhedral arsenopyrite (Apy). Note the fragmented shape of the

973

arsenopyrite crystals. (b) A close-up view of the white box in (a), using highly contrasted

974

BSE settings in order to highlight the primary crystallographic zoning and alteration rims in

975

arsenopyrite. (c) BSE SEM images of mineralized sulphides in the Bepkong deposit. (a)

976

Pyrite (Py) showing euhedral texture and uniform composition. (d) Fractured arsenopyrite

977

(Apy) crystals overgrown by subhedral pyrite. Note inclusions of Au, Ag, galena (Gn) and

978

Sphalerite (Sp) in pyrite.

979

Fig. 11. (a) Highly contrasted BSE image showing micron-size visible gold grains and other

980

sulphides in an alteration rim of arsenopyrite. (b) Broken-up arsenopyrite crystal containing

981

visible micron-size gold grains within the fractures; chalcopyrite is also present in the

982

fractures. Ag-silver, Apy-arsenopyrite, Au-gold, Ccp-chalcopyrite, Gn-galena, Ms-

983

muscovite, Po-pyrrhotite, Py-pyrite, Rt-rutile.

984

Fig. 12. Images of drill core showing free gold occurrences in Type-1 quartz veins (a) within

985

a fracture and (b) in the host rock next to a vein

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Fig. 13. Logarithm plot showing the variation in trace element (Au, Ag, Sb, Te, Bi, Se, Ni,

987

Co) distribution in pyrite and arsenopyrite from the Bepkong deposit (LA-ICP-MS analyses).

988

Fig. 14. (a) BSE SEM image of arsenopyrite with micron-size visible gold occurring as

989

inclusion together with rutile and albite. (b) BSE SEM image showing micron-size visible

990

gold at the grain boundary between arsenopyrite and pyrite. Ab-albite, Apy-arsenopyrite, Py-

991

pyrite, Rt-rutile.

992

Fig. 15. The stability and phase relationships of arsenopyrite in temperature-fS2 space, after

993

Kretschmar and Scott (1976) and Sharp et al. (1985). The average compositions of

994

arsenopyrite (Table 2) were used to constrain the temperature conditions during arsenopyrite

995

alteration (see text). Apy = arsenopyrite, Lö = löellingite, Py = pyrite.

996

Fig. 16. Photomicrograph showing fluid inclusions from the Bepkong deposit, in quartz

997

crystals found in Type-1 mineralized veins and pressure shadows. (a) A cluster of primary

998

fluid inclusions ((FI) (in the white box) and isolated primary fluid inclusions. (b) Close-up

999

view of the white box in (a) showing Type-2 fluid inclusions. (c) Intragranular

1000

pseudosecondary fluid inclusion trail. (d) A close-up view of a fluid inclusion trail showing a

1001

Type-2 fluid inclusion. (e) Pressure shadow around arsenopyrite showing pseudosecondary

1002

trails of Type-1 fluid inclusions. The black box shows a site of isolated fluid inclusions,

1003

detailed in (f). L-liquid, V-vapour.

1004

Fig. 17. Raman spectra showing the signals of Type-1 and Type-2 fluid inclusions in the

1005

Bepkong deposit. (a) Spectrum of the liquid phase of a Type-1 fluid inclusion, showing only

1006

the presence of H2O. (b) Spectrum of the vapour phase of a Type-1 fluid inclusion showing

1007

the presence of CH4. (c) Spectrum of the liquid phase of a Type-2 fluid inclusion showing the

1008

presence of SO2 and H2O (d) Spectrum of the vapour phase of a Type-2 fluid inclusion

1009

showing the presence of CO2, SO2 and CH4.

SC

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1010

RI PT

986

1011

Table captions

1012

Table 1. Electron microprobe analyses of arsenopyrite, reported in atomic %, from Type-1

1013

quartz veins in the Bepkong deposit.

1014

Table 2. LA-ICP-MS analyses showing trace element concentrations in pyrite and

1015

arsenopyrite from Type-1 quartz veins in the Bepkong deposit (values are in ppm). 33

ACCEPTED MANUSCRIPT

Table 3. Summary of fluid inclusion microthermometric data from mineralized Type-1 quartz

1017

veins in the Bepkong deposit.

AC C

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TE D

M AN U

SC

RI PT

1016

34

S

Fe

As

Au

Sample 1

core

38.0

33.9

28.0

0.0

Sample 2

core

37.1

33.9

29.0

0.0

core

37.2

33.9

28.8

0.0

rim

38.2

33.8

28.0

0.0

rim

38.1

33.6

28.3

0.1

core

37.6

33.9

28.5

0.0

core

37.1

33.8

29.1

0.0

Sample 4

core

37.6

33.8

28.6

0.0

Sample 5

core

37.4

33.8

28.8

0.0

core

37.3

33.9

28.7

0.0

Sample 6

core

38.0

34.0

28.0

0.0

core

37.4

33.9

28.7

0.0

Sample 7

core

37.1

33.8

29.0

0.0

core

36.7

33.9

29.4

Sample 8

Sample 9

0.1

core

36.8

33.9

29.2

0.0

rim

38.3

33.8

28.8

0.0

core

37.0

33.8

29.2

0.0

core

37.3

33.7

29.0

0.0

core

37.3

33.7

29.0

0.0

core

37.5

34.1

28.4

0.0

core

37.3

34.0

28.7

0.0

core

36.9

33.9

29.1

0.0

rim

37.2

33.7

29.1

0.0

rim

36.3

33.8

29.9

0.0

TE D

Sample 3

36.8

33.7

29.5

0.0

core

38.2

33.6

28.2

0.0

Sample 11

core

37.3

33.9

28.7

0.0

37.3

33.7

28.9

0.0

37.4

33.7

28.9

0.0

37.4

33.9

28.7

0.0

core

37.9

28.5

0.0

core

37.3

33.5

29.2

0.0

core

36.7

34.0

29.3

0.0

core

36.9

33.8

29.3

0.0

core

37.4

33.8

28.7

0.0

core core

AC C

Sample 12

EP

rim Sample 10

core

33.6

core

36.9

34.0

28.9

0.0

core

37.2

33.8

28.9

0.0

core

37.6

33.6

28.8

0.0

rim

37.0

33.9

29.1

0.0

rim

36.9

33.8

29.3

0.0

rim

37.4

33.6

29.0

0.0

Sample 13

Sample 14 Sample 15

Analytical results are recalculated to At. %

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Spot location

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Sample name

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Sample type

Au

Ag

Sb

Te

Pb

Bi

As

Se

Cd

Ni

Co

Cu

Zn

LOD

0.01

0.01

0.1

0.12

0.1

0.1

0.8

1.4

2.5

0.6

0.2

0.09

0.07

Py

BD

BD

BD

BD

BD

BD

2106.5

29.4

BD

BD

BD

BD

0.41

Py

BD

BD

BD

0.28

BD

BD

3355.9

44.6

BD

7.0

BD

0.2

0.84

Py

0.2

0.13

0.6

0.4

BD

3.0

4601.2

1.12

Py

0.04

BD

BD

0.25

BD

BD

4040.4

Py

0.7

0.1

1.0

0.2

BD

4.6

1492.4

Apy_rim

1.56

1.35

872.5

904.05

64.2

35.7

N/A

Apy_rim

1.62

3.07

911.6

310.96

336.4

141.4

N/A

1147.6

Apy

5.67

0.98

477.2

464.48

46.0

24.0

N/A

770.1

Apy

4.79

2.09

471.9

101.54

312.5

44.4

N/A

763.2

Apy_rim

0.82

1.35

1225.3

146.45

80.7

39.8

N/A

Apy

1.42

1.08

775.4

178.89

18.7

17.4

N/A

Apy_rim

2.72

1.53

790.5

280.88

52.9

Apy_rim

0.88

0.38

547.0

80.07

4.4

Apy

0.36

0.42

541.2

151.99

9.1

Apy_rim

1.56

0.51

484.8

50.42

9.3

Apy_rim

0.88

0.38

547.0

80.07

4.4

Apy

5.58

0.46

385.3

11.63

Apy

16.33

1.02

580.7

18.46

Apy

19.05

6.77

330.8

35.86

Apy

14.40

0.61

557.2

74.77

Apy

36.64

2.03

373.3

417.07

Apy

24.57

1.67

414.7

101.16

BD

1010.7

373.2

0.67

BD

90.5

1.7

0.15

0.8

25.3

BD

3.0

BD

1.62

1.84

1147.2

BD

99.4

16.6

7.34

2.29

BD

14.2

6.4

11.37

<2.95

BD

44.2

2.1

<2.68

<1.48

BD

51.5

7.0

<4.79

4.16

964.5

BD

382.7

3.1

<5.43

4.75

933.7

BD

19.5

1.9

<3.63

<2.24 <2.71

M AN U

SC

RI PT

57.1

54.9

N/A

882.8

BD

420.4

52.9

12.09

4.3

N/A

593.5

BD

23.7

0.8

<2.05

2.09

4.5

N/A

709.5

BD

29.9

3.1

<1.99

<1.13

5.0

N/A

504.9

1.12

72.9

1.8

8.94

<1.26

4.3

N/A

593.5

1.07

23.7

0.8

<2.05

2.09

0.2

6.3

N/A

286.3

BD

175.4

9.0

10.56

0.69

BD

88.2

N/A

194.0

BD

173.2

27.5

4.48

4.26

BD

97.1

N/A

301.5

BD

712.1

128.7

13.57

2.54

BD

16.9

N/A

405.9

BD

201.5

18.0

2.24

1.79

BD

67.5

N/A

510.9

BD

262.6

68.5

56.32

<0.90

EP

TE D

39.6

30.0

N/A

425.9

BD

198.5

66.7

2.27

<0.95

BD

8.75

0.52

306.6

26.28

BD

55.0

N/A

281.8

BD

323.0

48.6

14.69

2.07

Apy

23.78

1.39

429.5

137.34

0.3

41.6

N/A

412.2

BD

154.0

55.8

3.92

5.02

AC C

Apy

Apy= arsenopyrite; Apy_rim = alteration rim; Py = pyrite; LOD = limit of detection; BD = below detection; N/A = not analysed. Apy analyses are from cores except where indicated.

ACCEPTED MANUSCRIPT Quartz vein type

Host mineral

Environment

Inclusion type

Size (µm)

Degree of filling at room temperature

Tm carb phase

Thclath

Th carb phase

Thtotal

(°C) quartz

Type 1

quartz

Type 1

quartz

quartz

Type 1

quartz

Type 1

quartz

Isolated primary inclusions in pressure shadows Primary and pseudosecondary inclusions as clusters or short trails Isolated primary inclusions Isolated primary inclusions

-184 to -180 mean = -182.6 -183 to -181 mean = -182.5

14 to 17 mean = 16 15 to 17 mean = 16

-84.2 to -80 mean = -82.6 -83 to -80 mean = -82.6

95-100

-183 to -181 mean = -182.5

14 to 17 mean = 16

-84.2 to -80 mean = -82.6

30

40

-56.8 to -74

Type 2 n=15

10

10

-56.8 to -70

Type 2 n=10

20

90

-56.8 to -74

Type 1 n=15

2-5

100

Type 1 n=25

2-10

100

Type-1 n=15

2-5

Type 2 n=25

8 to 16 mean = 16

18 to 25 mean = 22

8 to 14 mean = 10 8 to 16 mean = 14

19 to 25 mean = 22.5 20 to 24 mean = 22

AC C

EP

TE D

M AN U

SC

Type 1

Primary inclusion forming clusters Isolated primary inclusions

RI PT

Type 1

310 to 360 310 to 370

310 to 370

320 to 342

321 to 324 320 to 340

AC C EP TE D

SC

M AN U

RI PT

PT ED

S

M AN U

CE ED

PT

SC

M AN U

RI

CC EP TE D

SC

M AN U

RI P

PT ED

S

M AN U

PT ED

S

M AN U

PT ED

S

M AN U

D

M A

D

M A

PT ED

S

M AN U

D

M A

D

M A

ED

M AN

D

M A

EP TE D

SC

M AN U

CE ED

PT

SC

M AN U

RI

ED

M AN

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Highlights One of the few published work on gold in NW Ghana Using a multiscale approach to understand a gold deposit Relative rare case of gold in arsenopyrite with no gold associated with pyrite Using fluid inclusion and arsenopyrite geothermobarometry to understand the trapping conditions for gold

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• • • •