Petrogenesis And Associated Enrichment Mechanism Of Key Metals In Granite Intrusions In North Central Nigeria:Geological And Geochemical Constraints

The Precambrian granite plutons along the western margin of the anorogenic Ropp complex at Gindi Akwati represent a classic site with imprints of multiple orogenies(Eburnean,Kibaran and Pan African events)that could define the geodynamic regime during the formation of the I-type granitic rocks in the west-eastern Nigeria terrane.Zircon U-Pb LA-ICP-MS dating of the granitic gneiss yield ages 2157,1427,and 631 Ma while the Older Granite yield 620 Ma,suggesting that metamorphism occurred at 1.43 Ga,between three magmatic episodes:2.16 Ga(Eburnean orogeny)and 0.63-0.62 Ga(Pan-African orogeny).Negative to positive zircon ?Hf(t)values(Granitic gneiss=-16.25 to+6.09 and Older Granite(OG)=-3.96 to+3.86)for these plutons indicate that partial melting of older crust with mantle input formed the parent magma.The low Mg#(<40)in the OG and the span of the ratios of some trace elements,including low Nb/La(average=0.33),Nb/U(average=10.60)is typical of magma sourced from the crust.Conversely,the low Y/Nb(average?1.02),and Yb/Ta(average=1.34)in the OG indicates mantle input signatures.The OG is moderately evolved calc-alkaline granites that are peraluminous,show ferrous and high-K characteristics.The weak negative Eu anomaly,high Ba,Sr,and K/Rb,low U,Th,Nb,and Ta,relatively low Rb,and very low HREE and Y of the OG resemble those of typical high Ba-Sr granitoid.The magma's physicochemical conditions from biotite and whole-rock analyses suggest they are ilmenite series granites formed under low fO2 conditions(?FMQ=-14.87 to-17.18)condition.The OG's crystallization temperature and pressure are moderate(T=706-770?and P=100 to 295 MPa),and the depth of emplacement was determined as 5.2 to 8.6 km using the newly improved Qtz-geobarometer.Metamorphism(the syn-collisional 631 Ma granitic gneiss)and magmatism(the post-collisional 620 Ma OG)in the area are contemporaneous with other granitoids in the ENT.The slab-melt from sinking oceanic crust subduction convects and underplates the lower crust(reworked older crust)to generate the granitic gneiss magma.Slab break-off during the waning stage of Pan-African orogeny induced the upwelling the asthenosphere that partially melted the sub-continental lithospheric mantle(SCLM and the lower crust to generate the OG.After that,the OG was uplifted into a vertical tectonic extensional fault.The mineralizations along greisenized portions of the Older Granites in Gindi Akwati have links to the Jurassic biotite granite.The forceful intrusion of the granite porphyry sheared the Older Granites along its rim and created pathways for the influx of hydrothermal fluids at late-stage consolidation of biotite granite to form cassiterite-sulphide ores in greisenized granites and quartz veins.The low Zr/Nb ratio(-5)in Jurassic Younger Granites reveals a magmatic-hydrothermal transition that probably triggered the formation of ore-bearing veins in the greisenized granites.The high Sn content(88 to 189 ppm)in biotite granites specifically suggests that chloride ions take the form of significant complex-forming ligands and efficiently sequestrate,transport and deposit ore metals(Sn,Zn,Fe and Cu)within the greisenized portions of Older Granites.The high stability of Sn at high redox potential(Eh>1 V)compared to sulphide that becomes unstable at Eh>0.2 V probably gave a false impression of metal zoning with a relatively higher abundance of the oxide ores than the sulphides at the surface.Zn and Cu are more dominant at depth(>10 to 40m).The alteration mineralogy(sericite-topaz-lepidolite-fluorite-bearing assemblages),re-equilibrated annite temperature-500? in Younger Granites,coupled with stable isotope and fluid inclusion systematics suggest the hydrothermal history of f'greisens' and veins started with hot(homogenization temperature?>300?),low to moderate salinity(mean=6.7 wt%NaCl eq.),low density(?0.7 g/cm3)fluids and>20 MPa trapping pressure.Its isotopic composition(?34S=-1.30 to+0.87‰)is very similar to typical magmatic fluids.It favours a model of late-stage hydrothermal fluids from anorogenic granites mineralized the greisenized parts of the host rock(OG).The Dutsen Wai and Ropp complexes are parts of the over fifty within-plate alkaline ring complexes in north-central Nigeria.These complexes consist mainly of A-type granites that were emplaced between 175 Ma(in Dutsen Wai)and 149 Ma(in Ropp).They are characterized by moderately negative zircon ?Hf(t)values(-9.8 to-5.86)with Paleoproterozoic(1597-1884 Ma)crustal model ages(TDMC),suggesting the mantle-derived parental magmas have been substantially modified by assimilation of crustal materials.The granites exhibit elevated HFSEs(e.g.,Nb and Zr)and LILE(e.g.,Rb and REE)concentrations,with higher values in the peralkaline than in the aluminous granites,but are depleted in compatible elements(e.g CaO,MgO,Ba and Sr).This is typical of highly differentiated granites associated with rare metal mineralization.The peralkaline plutons show REE patterns with strong negative Eu anomalies,significant M-type lanthanide tetrad effect(TE1,3=1.16-1.32),and non-CHarge and Radius Controlled(CHARAC)behavior of some HFSEs.The positive correlation of Na2O vs.Nb(r2=0.92)and Nb vs.TE1,3(r2=0.7),increasing degree of albitization and shift in eutectics of granite minimum towards Ab-Qtz sideline indicate that Na-rich fluid plays an important role in the complexation between F and other incompatible elements in open system processes.Fluorine also favoured the retention of these crucial metals in the peralkaline melt until the late magmatic stage,during which the breakdown of the F-HFSEs and F-REE complexes was accompanied by crystallization of pyrochlore and REE-bearing accessory minerals.We proposed that the emplacement of these A-type granites was probably linked to the prevailing transtension regime before the fragmentation of Gondwana during the Mesozoic period,which reactivated some deep-seated Pan-African transcurrent faults.This,in turn,caused pressure release and channelled fluid,which triggered partial melting of the lower part of the lithospheric mantle.