GEOCHEMICAL CHARACTERISTICS OF GOLD-BEARING GRANITOIDS AT AYANFURI IN THE KUMASI BASIN, SOUTHWESTERN GHANA: IMPLICATIONS FOR THE OROGENIC RELATED GOLD SYSTEMS

Received 01 March 2020 Accepted 30 May 2020 Available online 23 June 2020 This study investigates auriferous granitoids from the Esuajah and Fobinso pits within the Ayanfuri environment in the Paleoproterozoic Kumasi basin. The aim is to establish the geochemical characteristics of the granitoid gold ores and the possible deposit type which may influence mineral project development. 13 major and 51 trace elements were analyzed using XRF and ICP-MS devices, respectively. The granitoids are mainly classified as granodiorite that crystallized from a calc-alkaline magma series. The Fobinso granodiorite derived from the partial melting of the Birimian metasedimentary rocks, while the Esuajah granitoid derived from igneous rock melts. The granitoid are linked to magma source depleted in mantle material that contains crustal components through subduction processes. Major oxides of the granitoid vary lowly from the average background values derived for basin type granitoid in such terrains. Generally, the granitoid are enriched in Large Ion Lithophile Elements (LILE), while High Field Strength Elements (HFSE) and base metals are within background values when compared to Primitive Mantle (PM) values. Gold mineralisation is associated with Ag, As, Bi, Sb, Te, Pb and S in the peraluminous granitoids. Geochemical characteristics and field observations identify the deposit style as an orogenic related gold deposit type.


INTRODUCTION
Until the revival of gold exploration in Ghana in the late 1980s, only two major types of gold deposit types were extensively studied within the Birimian metallogenic province in Ghana (Yao and Robb, 2000). These deposit types are the shear zone hosted lode-quartz veins and/or disseminated sulphides and the auriferous quartz-pebble conglomerates (Kesse, 1985;Hirdes and Leube, 1989;Leube et al., 1990 andSchmidt Mumm et al., 1997).
Extensive exploration works in the Birimian terrain of Ghana have led to the discovery of gold-bearing granitoid, which constitute a third and recent style of gold mineralization in the Birimian (Yao et al., 2001). The earliest notable occurrence of gold-bearing granitoid in the Birimian of Ghana was at Ayanfuri, where several of the deposits are hosted by intermediate to felsic granitoid that have significant mineralized quartz stockwork systems (Griffis et al., 2002).
For several decades, gold exploration has been mainly focused on sediment-hosted shear zones, where significant deposits have in time past formed the basis of gold exploitation. Currently, however, Gold deposits hosted within the Birimian granitoid are increasingly becoming potential targets for exploration to maximise the gold ore resource base to aid increased productivity. Examples of the granitoid bodies identified for study include the Ayanfuri, Nhyiaso, and Ayankyerim, gold deposits in the western part of the Ashanti Belt, the Chirano gold deposits in the Sefwi Belt (Fougerouse et al., 2017;Allibone et al., 2004). Also, of interest is the Dynamite Hill and Abore gold deposits along a 10-20 km wide shear zone within the Kumasi Basin and equidistant from the Sefwi and Ashanti Belts (Chudasama et al., 2016).
Although the Ashanti style mineralization occurs with gold-bearing granitoid in the study area, mineralized intrusions have been the main attraction at the Ayanfuri and its environs (Calderwood and Thompson, 2007). There is, however, only very little in terms of geological characterisation of these deposits (Griffis et al., 2002). Emplacement of granitic plutons in the sedimentary basins is coeval with the tectonothermal orogeny that is constrained to the time range of 2120 -2080 MY (Oberthur et al., 1998). Orogenic deposits are formed during crustal shortening in compressional deformation processes within metamorphosed terranes with consequent generation of large volumes of granitic melts (Grooves et al., 2003;Grooves et al., 1998).
Differences between orogenic gold-bearing granitoids and the intrusionrelated gold deposits remain unresolved (Sillitoe, 1991). This is because the two deposit types have many similarities in terms of element associations, ore fluids wall-rock alteration and structural controls (Goldfarb, 2001;Grooves et al., 2003). Consequently, some gold-bearing granitoids have been classified as both orogenic and intrusion-related deposits that includes the Muruntau gold deposits of Uzbekistan and the True North deposit in Canada (Kempe et al., 2001;Hart et al., 2000). With the different approaches to gold exploration associated with the intrusionrelated and orogenic gold classifications, it is pertinent to suitably identify each model (Hart and Goldfarb, 2005).
The Birimian Supergroup in the Kumasi basin comprises metasedimentary rocks, metavolcanic rocks, and basin type granitoid that intruded the basin sequences from c. 2,116 to 2,088 Ma (Adadey et al., 2009). The metasedimentary units consist of greywackes, graphitic shale, and argillaceous sediments, while the metavolcanic rocks generally include basalt, dacite and rhyolite (Hirdes and Davis, 2002). The metasedimentary country rocks are intruded by dykes and small irregular plugs of granitoid (Tourigny et al., 2018). The rocks are intensely folded, faulted and metamorphosed to upper greenschist facies during regional metamorphism ascribed to the Eburnean orogeny (Hirdes and Davis, 2002). The gold deposition is associated with granitoid bodies, and shear zones within the Birimian. In this study, major elements classification and trace elements geochemistry are used to constrain the deposit type of the granitoids, which have only been studied sparingly, either on an individual basis or regional basis (Yao et al., 2001). For an enhanced modeling and exploration approach in the study area, a more detailed understanding of the nature and origin of the granitoid and their associated gold mineralization is required. The study assessed auriferous granitoid suites from the Esuajah granitic plug and Fobinso dyke in Ayanfuri of the Kumasi Basin.

Study area
The study area is situated within the Kumasi basin, near the western flank of the Ashanti Greenstone Belt, at the extreme south-eastern portion of the West African Craton and underlain by the tectonostratigraphic Leo Man shield (Jessell et al., 2016). Rocks are dominated by the Paleoproterozoic Birimian rocks of Eburnean age (Leube et al., 1990) (Figure 1). A sketched local geological map is presented in Figure 2. The Esuajah gold deposit is hosted in cylindrical granitoid plug whereas the Fobinso gold deposit is contained in a single, continuous dyke (Tourigny et al., 2018). The granitoid hosted gold deposits are associated with lesser arsenopyrite and traces of sphalerite, chalcopyrite, galena and rutile. Very fine-grained gold occurs along sulphide grain boundaries and in fractures in sulphide crystals (CAGL, 2011). The study area is well noted for historic mining activities (Griffis et al., 2002). Esuajah and Fobinso open pits of metasediment hosting auriferous intrusive were the main source of rock samples for the study (Figure 2). The pits are barren of saprolite as old mine workings focused much of their activities on oxide material leaving behind competent in-situ rock exposures with strongly preserved primary and secondary structures.

Sampling strategy and procedure
Pre-sampling mapping of pit walls was carried out to identify prevailing lithology and structures in the various pits as they are major factors controlling mineralization. Representative and fresh samples were carefully taking from intrusives at Fobinso and Esuajah pits. Samples were taken from chloritic and sericite alterations zones, veins crosscutting granitoid intrusions (Figure 3), as well as altered granitoids with disseminated sulphides. All the samples are coarse-grained and partially altered. Sample locations and field descriptions are presented in Table 1. A total of 10 samples were selected for determination for major elements and selected trace elements including gold. Samples were packaged and dispatched to the ALS Geochemical Laboratory for chemical analyses. Analytical methods used are inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry ICP-AES and X-Ray Fluorescence. Chemical analysis was done on about 5.0 g of sample portions each, using the X-ray fluorescence (XRF) as described (Wirth and Barth, 2016). Also, the volatile content of each sample was determined by loss on ignition (LOI), as summarized (Santisteban et al., 2004). The concentrations of the elements and the LOI values were reported in percentages and part per million for major oxides and trace elements respectively. A model-based ordinary and robust Expectation-Maximisation algorithms outlined was used to generate element concentrations below the detection limit of analytical equipment (Palarea-Albaladejo et al., 2007;Martin-Fernandez et al., 2012). Data replacement was done using the R package zCompositions. The Centered log-ratio (clr) transformation proposed was used to open the constrained compositional data to reveal inherent patterns in the data structure using the expression (Aitchison, 1982): where x represents the composition vector, g(x) is the geometric mean of the composition x, and x1, …., xN are the concentrations of the individual elements. The R Package Geochemical Data Toolkit (GCDkit) was used to plot classification diagrams for the granitoids (Janoušek et al., 2016). Table 2 presents the analytical data for each rock type from Esuajah and Fobinso intrusions.

Major Elements
The majority of oxides analyzed show little variations from the average background values of the basin type granitoids except for Na2O which in both Esuajah intrusive and Fobinso granitoids are higher than background values (Table 3) (Leube et al., 1990).    (Wilson, 1989 On the Total Alkalis Silica (TAS) diagram (Figure 5a), The Esuajah and Fobinso granitoid samples plot as granodiorite while some straddle the granodiorite and granite boundary with a few plotting within the granite region. The high content of silica is evident in the TAS diagram as all samples plot in the acid region. The Esuajah intrusive and the Fobinso intrusive are identified to originate from calc-alkaline magma series (Figure 5b) which is a sub-division of the sub-alkaline series (Figure 5a). A subdivision of the calc-alkaline magmas plots the intrusive bodies as a calc-alkaline series member (Figure 5c) (Peccerillo and Talor, 1976).
The Al2O3/ (Na2O + K2O) versus Al2O3/ (CaO + Na2O + K2O) binary diagram (Figure 5e) of Shand deals with discriminating peraluminous, metaluminous and peralkaline magma series (Shand, 1943). This plot shows that all granitoid samples are peraluminous (i.e. Alkalinity index (AI) and Aluminuim Saturation Index (ASI) are greater than one). The peraluminous imprint on samples is indicative of excess aluminium required to form feldspars and can be accommodated in other aluminous phases such as Al2SiO5 polymorph (Frost et al., 2001;. The Esuajah granitoid samples plot within the I-type region with a boundary division of ASI >1.1 (Chappel et al., 1974). All samples of Esuajah granitoid have ASI values below 1.1. Four out of five of the Fobinso granitoid samples plot within the S-type region with their ASI values ranging from 1.1 to 1.7, suggesting their S-type affinity. A group of researchers classified the I-type and S-type granites as rocks originating from melts of meta-igneous and metasedimentary material respectively (Chappel et al., 1974). Considering the magnesian nature on the granitic classification diagram (Figure 5d), the anatexis of metasedimentary rocks may have been promoted by hydrous conditions (Frost et al., 2001;Nude et al., 2012).
The peraluminous nature of the samples is again substantiated from the major cation parameters (Figure 5f). The bulk of Ayanfuri intrusive bodies plot in fields II and III which mostly consist of biotite and some amphiboles as the mafic minerals. Biotite is identified to be the main ferromagnesian mineral within the Birimian sedimentary basin type granitoid (Yao, 2000;Nyarko et al., 2012). Granitoid suites plotting within the peraluminous domain are known to be the product of anatexis of continental crust, contrary to the mantle-derived metaluminous domain (Debon and Le Fort, 1983).

Trace elements assessment
To assess trace element behavior of the Fobinso and Esuajah granitoids, the raw multi-element data of the granitic rocks were normalized against Primitive Mantle/Silicate Earth (McDonough and Sun, 1995) and presented in Figure 6. Esuajah and Fobinso granitoids show significant enrichment in LILE such as Rb, Ba, Th and U and depletion in HFSE such as Zr, Hf, Nb and Ta. Trace elements pattern of the granitoid samples resembles the upper continental crust pattern (Rudnick and Gao, 2003). Th/U ratio of more than 2.5 of the samples further suggests the continental crust affinity as proposed for the upper continental composition (Taylor and McLennan, 1985).
A common magma source can be inferred for the Esuajah granitoids, since the Zr:Hf ratios (computed from Table 2) shows very little variations. On the other hand, Zr: Hf ratio within the Fobinso granitoids show significant variation which implies an interplay of magma fluids of different source or interaction of magma with crustal materials by the process of crustal assimilation (Yang et al., 2008). Ferromagnesian elements such as Co, Cr, Ni, Sc and V also showed marked depletion within the samples.

Geochemical constraints on tectonic regimes
The millication R1 and R2 plot (Figure 7a) identifies the Ayanfuri intrusive bodies as emplaced during an orogenic event. A group of researchers showed that granitic plutonism within the sedimentary basin are coeval with the Eburnean orogeny that affected much of the Birimian and Tarkwain (Oberthur et al., 1998). The plot of granitoid in the syn-collision region also reflects restricted anatectic two-mica granite with S-type affinity as a result of the partial melting of metasedimentary crustal material.
The sedimentary components of the magma may be carried by the descending slab during subduction (Saleh et al., 2002). This is consistent with field observation as the granitoid are hosted by metasedimentary country rocks. The trace element discrimination diagram (Figure 7b) plot samples as volcanic arc granites. Volcanic arc granites are linked to magma source depleted in mantle material that contains crustal components through the subduction process (Pearce, 1996).

Gold Mineralization
Gold values in all granitoids are anomalous when compared with the highly felsic average granitoid gold value of 0.004 ppm (Turekian et al., 1961). The average values of gold in Fobinso and Esuajah samples are 0.15 ppm and 0.8 ppm respectively. The mineralized granodiorite are calcalkaline, peraluminous intrusions of felsic composition emplaced during the Eburnean tectono-thermal orogeny that is constrained to time range of 2120 -2080 Ma (Oberthur et al., 1997). The study area is identified to have undergone a complex polyphase deformation ( Figure 2) (Tourigny et al., 2018). The intrusives deformed in a brittle manner during the later stage penetrative deformational event that affected the area. This generated a network of brittle faults with contrasting kinematics under a hybrid compression stress regime (Tourigny et al., 2018). The fractures that resulted from the hybrid compression tectonic regime host auriferous quartz veins and stockworks as gold-bearing fluids infiltrate them as shown in figure (3a).
Large ion lithophile elements (LILE) (e.g., Rb, Cs, Sr and Ba) are enriched, while base metals such as Cu, Mo, Pb, Sn and Zn are within background values when compared to PM values. Enrichment of LILE can be correlated to the regional Eburnean greenschist facie metamorphism of the Kumasi supergroup country rocks (Chudasama et al., 2016). Significant amounts of LILE occur in wall rock alteration in metamorphic terranes (Grooves et al.,1998). The element suite of Au-Ag-Te-Pb-Bi-Sc-Mo-W-As-Sb which is of interest in gold exploration (Nichol, 1983), is enriched in the granitoids as compared to the primitive mantle (PM) values ( Figure 6). From Table 4a and 4b, gold shows good correlation (in bold) with Ag-As-Bi-Sb-Te-Pb-S and Ag-As-Te-Sb-S suites, at Esuajah and Fobinso respectively. The metal associations are typical of orogenic gold deposit suite of Au-Ag ± As ± B ± Bi ± Sb ± Te ± W (Grooves et al., 2003;Robert et al., 2007;Grooves et al.,1998). From field observations, alteration minerals such as sericite and the chlorites show the pervasive alteration of the plagioclase and amphiboles respectively. According to a study, these alterations are indicators to orogenic gold deposits (Grooves et al., 1998).

CONCLUSION
The granitoids are predominantly granodiorite that has crystallized from a calc-alkaline magma series. The Esuajah and Fobinso peraluminous granodiorite are products of the anatexis of continental crustal material of igneous and metasedimentary compositions and were emplaced during a regional tectonic activity. Geochemical data is consistent with a classic orogenic model for the granitoid-hosted gold deposits. Gold is associated with As, Ag, Te, Sb, S, Bi and Pb for mineralized Esuajah rocks and As, Ag, Sb and S in the mineralized rocks of Fobinso. These elements that form an association with gold are ubiquitous in the environment and usually occur in quantities that can be recovered easily making them useful pathfinders for the gold. According to Goldfarb and Groves (2015), carbonaceous or shale-rich sequence are preferred sources of ore-forming components before remobilization into other structures for the orogenic gold deposit. Hence further research should focus on the sedimentary country-rock of the Fobinso and Esuajah intrusive as a potential source of ore-forming components.