The Paleoproterozoic Copper-Gold Deposits of the Gaoua District, Burkina Faso: Superposition of Orogenic Gold on a Porphyry Copper Occurrence?
Élodie Le MignotLuc SiebenallerDidier BéziatAnne‐Sylvie André‐MayerLaurie ReisbergStefano SalviGermán VelásquezCatherine ZimmermannAthanase NaréGuy Franceschi
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Abstract:
The Gaoua mining district, underlain by Paleoproterozoic volcanic and intrusive rocks of the Boromo-Goren greenstone belt in southern Burkina Faso, is a rare case where economic copper and gold resources occur together in the Birimian terrane of West Africa. The main country rock lithologies consist of flows and volcaniclastic sediments of basaltic to andesitic compositions. These are intruded by subvolcanic diorite and quartz diorite stocks and dikes, granophyre dikes and, less commonly, gabbroic dikes. The stocks are porphyritic in texture, locally brecciated, and characterized by development of a carbonate, anhydrite, and quartz stockwork system. Structural observations, detailed petrography, and sulfide geochemistry reveal that copper and gold in the stockworks and surrounding altered rocks cannot be associated with the same mineralizing event. Copper mineralization, mainly in the form of chalcopyrite, is typically observed in hydrothermal breccia and is associated with early arsenic-poor and gold-free pyrite. The geochemistry of the plutonic rocks and associated basalts and andesites implies formation in an arc setting. Textural evidence indicates that gold mineralization is closely tied to a second generation of sulfides, with the later pyrite hosting visible gold and minor chalcopyrite that clearly postdates the copper ore. Analyses of these pyrites by LA-ICP-MS reveal compositions rich in Ag, As, Au, Bi, Cu, and Pb. Pyrite with this signature was only observed in local shear and fracture zones that crosscut the host rocks and copper-rich breccia. Therefore, our data indicate the strongly deformed and metamorphosed porphyry copper mineralization in the Gaoua district is overprinted by orogenic gold mineralization. Formation of the early porphyry copper deposit took place at 2165 ± 24 Ma based upon Re-Os dating of the early pyrite; the age of the younger gold event, which is similar in style to that described throughout the West African craton, is not known but may be related to one of the lode gold episodes that are widespread through the West African craton at ~2150 and ~2100 to 2040 Ma. The Gaoua copper deposit, one of very few Precambrian porphyry copper deposits known in West Africa, formed in a volcanic-arc setting that predated the Eburnean orogeny in the Boromo-Goren greenstone belt.Keywords:
Stockwork
Breccia
Porphyritic
Diorite
Greenstone belt
Dike
Argillic alteration
Quartz monzonite
Hypogene
Porphyritic
Stockwork
Argillic alteration
Molybdenite
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Τhe Vathi porphyry Cu–Au±U±Mo mineralization occurs in the Serbo-Macedonian metallogenic province, 18.5 km NW of Kilkis city at N. Greece. Α trachydacite porphyry and quartz monzonite dikes (18-17 Ma), which emplacement was structurally controlled by the Tertiary evolution of the Serbo-Macedonian Massif, host the mineralization. Genetically the mineralization is linked to the quartz monzonite dikes which intrude the trachydacite porphyry and the crystalline basement rocks of the Vertiskos Unit. The trachydacite porphyry is affected by propylitic alteration, which is overprinted by an extended sericitic alteration. The quartz monzonite is affected by potassic alteration which is also overprinted by sericitic alteration. Stockwork, sheeted veins and oxidized pyrite, chalcopyrite and quartz D-type veins, outcrop in the trachydacite porphyry. Locally, a strongly potassically altered structural trend controls the qtz-monzonite emplacement, while elsewhere the quartz monzonite appears venting the trachydacite porphyry and creates a hydrothermally altered massive phreato-magmatic breccia. Disseminated and vein-type mineralization consisting of pyrite and chalcopyrite is found in the quartz monzonite and along the potassically altered area of the trachydacite porphyry. At depth, the most common mineralization consists of disseminated chalcopyrite – bornite – molybdenite – magnetite – native gold. The microscopic study of drill core samples revealed additionally the presence in depth of disseminated pyrite – chalcopyrite – stibnite, and of vein-type sphalerite – galena – arsenopyrite and pyrrhotite – pyrite – chalcopyrite mineralization. Along and near the surface a characteristic and extent supergene oxidation zone is developed dominated by iron and copper oxides and meta-torbernite, while deeper it is followed by a restricted secondary enrichment zone which consists mainly of chalcocite and covellite. The Vathi porphyry style mineralization bears exceptional enrichment in U and REEs which is unusual for porphyry systems. It shares several features in common to the Skouries porphyry Cu-Au deposit, in the adjacent Chalkidiki district and it could be a promising future exploration target for gold and REEs. Στη Βάθη 18.5 χλμ ΒΔ του Κιλκίς στα όρια της Σeρβομακeδονικής μeταλλογeνeτικής eπαρχίας eντοπίζeται μeταλλοφορία Cu–Au±U±Mo πορφυριτικού τύπου. Ένας τραχeιδακιτικός πορφύρης και διeισδύσeις χαλαζιακού μονζονίτη (18-17 Ma), η τοποθέτηση των οποίων eλέγχeται από την Τριτογeνή τeκτονική της Σeρβομακeδονικής Μάζας, φιλοξeνούν την μeταλλοφορία. Γeνeτικά η μeταλλοφορία συνδέeται μe τις διeισδύσeις του χαλαζιακού μονζονίτη, οι οποίeς τέμνουν τον τραχeιδακιτικό πορφύρη και τα πeριβάλλοντα κρυσταλλοσχιστώδη πeτρώματα του υποβάθρου της eνότητας Βeρτίσκου. Ο τραχeιδακιτικός πορφύρης χαρακτηρίζeται από μια προπυλιτική ζώνη eξαλλοίωσης, η οποία eπικαλύπτeται από μια eκτeταμένη σeρικιτική eξαλλοίωση. Ο χαλαζιακός μονζονίτης χαρακτηρίζeται από μια ποτασσική ζώνη eξαλλοίωσης, η οποία eπίσης eπικαλύπτeται από μια σeρικιτίωση. Στον τραχeιδακιτικό πορφύρη παρατηρούνται πλέγματα φλeβιδίων μe οξeιδωμένη μeταλλοφορία, παράλληλeς φλέβeς και D-τύπου φλέβeς οξeιδωμένου σιδηροπυρίτη, χαλκοπυρίτη και χαλαζία. Τοπικά μια έντονα ποτασσικά eξαλλοιωμένη τeκτονική ζώνη eλέγχeι την παρουσία του χαλαζιακού μονζονίτη, eνώ αλλού συνδeόμeνο μe αυτόν eντοπίζeται ένα μαζώδeς φρeατομαγματικό λατυποπαγές το οποίο διeισδύeι στον τραχeιδακιτικό πορφύρη. Διάσπαρτη και φλeβική μeταλλοφορία σιδηροπυρίτη – χαλκοπυρίτη eντοπίζeται στο χαλαζιακό μονζονίτη και σe βάθος στον τραχeιδακιτικό πορφύρη. Σe μeγαλύτeρα βάθη διασπορές σιδηροπυρίτη – χαλκοπυρίτη – βορνίτη – μολυβδαινίτη – μαγνητίτη – αυτοφυούς χρυσού πeριγράφουν την μeταλλοφορία πορφυριτικού τύπου. Η μικροσκοπική μeλέτη δeιγμάτων προeρχόμeνων από γeώτρηση eντόπισe την παρουσία τριών ακόμη μeταλλοφοριών, μιας διάσπαρτης σιδηροπυρίτη – χαλκοπυρίτη – αντιμονίτη, και δυο φλeβικού τύπου, μία σφαλeρίτη – γαληνίτη – αρσeνοπυρίτη και μία μαγνητοπυρίτη – σιδηροπυρίτη – χαλκοπυρίτη. Τέλος, χαρακτηριστική eίναι η eπιφανeιακή αλλά και σe βάθος παρουσία σιδηρούχων και χαλκούχων οξeιδίων και μeτα-τορβeρνίτη τα οποία σχηματίζουν μια eκτeταμένη ζώνη οξeίδωσης η οποία ακολουθeίται βαθύτeρα από μια μικρής έκτασης ζώνη δeυτeρογeνή eμπλουτισμού αποτeλούμeνη κυρίως από χαλκοσίνη και κοβeλλίνη. Η πορφυριτικού τύπου μeταλλοφορία της Βάθης χαρακτηρίζeται από υψηλές συγκeντρώσeις Au, αλλά και U και ορισμένων REEs το οποίο δeν eίναι σύνηθeς για τα πορφυριτικά συστήματα, και θα μπορούσe να αποτeλέσeι μeλλοντικό eρeυνητικό στόχο για χρυσό και REEs.
Quartz monzonite
Hypogene
Argillic alteration
Stockwork
Breccia
Porphyritic
Molybdenite
Sericite
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Abstract Copper and gold mineralization in the Maher-Abad area, eastern Iran, is closely related to multiple episodes of emplacement of a late Eocene granodiorite into a quartz-monzonitic stock and andesitic volcaniclastic rocks. Hypogene and supergene porphyry Cu–Au mineralization occurred within the porphyritic granodiorite and quartz-monzonite host rocks extensively altered into dominantly potassic, propylitic, phyllic, and argillic assemblages. Temperature and pressure estimates using the plagioclase–hornblende thermometer and Al-in-hornblende barometer indicate that the granodiorite intruded at 758 ± 10°C and 1.4 ± 0.2 kbar. Biotites from the alteration zones have more variable AlIV than those in the fresh granodiorite, but nearly all are close to the ideal phlogopite composition. Biotite compositions display an increase in Al2O3, FeO, TiO2, and Cl, but a decrease in SiO2 and F, from the porphyritic granodiorite and potassic to the transitional phyllic alteration zones. Biotite from the potassic zone (X phl = 0.63–0.67) possesses a moderate F content (0.53 to 0.82 wt.%) that is significantly higher than that in the phyllic zone (0.22 to 0.38 wt.%), exhibiting a positive correlation with X Mg and negative correlation with Cl. With a decrease in the temperature, log (fH2O/fHF) and log (fH2O/fHCl) values calculated for fluids equilibrated with biotite increase progressively from the granodiorite through the potassic to the phyllic zones, whereas log (fHF/fHCl) shifts towards more negative values. Fugacity ratio trends in the Maher-Abad porphyry copper deposit are quite similar to those of other porphyry copper systems. The decrease in halogen content of hydrothermal fluids towards outer parts of the deposits reflects an increase in the degree of mixing between magmatic fluid and meteoric water. Keywords: biotitehalogen fugacityhydrothermal alterationCu–Au porphyryMaher-AbadIran Acknowledgements This contribution is a part of the first author's PhD thesis with laboratory studies carried out at the Institute of Mineralogy and Economic Geology, RWTH Aachen University, Germany. Funding for this project was provided by the Ministry of Science, Research and Technology, Iran. My grateful appreciation is extended to Professor F. M. Meyer for all his constructive scientific criticisms and R. Klinghardt for his assistance in electron microprobe analysis. Deserved thanks go to B. Ahin for his field assistance.
Porphyritic
Argillic alteration
Quartz monzonite
Hypogene
Hornblende
Sericite
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The Chahfiruzeh deposit is a newly discovered porphyry-style copper deposit in the southern part of the Cenozoic Urumieh-Dokhtar magmatic arc of Iran. Mineralization is associated with a Miocene quartz-diorite to quartz-monzodiorite porphyritic intrusion (Chahfiruzeh porphyry) intruded into older basaltic and andesitic lava flows and pyroclastic rocks. Alteration assemblages and alteration zoning, typical of porphyry copper deposits, are well developed. Mineralization occurs in quartz-sulfide stockworks and as sulfide disseminations in the porphyritic intrusion and the immediate wall rocks. Pyrite and chalcopyrite are the main hypogene sulfide minerals; bornite and molybdenite are rare. Representative magmatic and alteration minerals, including plagioclase, amphibole, biotite, sericite, and chlorite, are analyzed, and the data are used to constrain the crystallization conditions of the magmas and the nature and evolution of the hydrothermal fluids. The fluorine-chlorine fugacity in the magma during crystallization of the Chahfiruzeh porphyry, represented by log (fH_2O)/(fHF) - log (fH_2O)/(fHCl) and determined from the chemical composition of magmatic biotite, ranges between 5.23 and 6.80 and between 5.05 and 5.13, respectively. A comparison to several other intrusions associated with porphyry-style mineralization suggests that the Chahfiruzeh intrusion crystallized at relatively high fH_2O/fHCl ratios. The intercept value (F/Cl) of the magmatic biotite in the Chahfiruzeh porphyry ranges from 5.5 to 7.02. The calculated F/Cl intercept values for the biotite are consistent with those reported from many other porphyry copper systems. The secondary reequilibrated biotite in the intrusion is distinguished from the primary magmatic biotite by a slightly higher Mg component (X_{Mg} = 0.53-0.68 compared to 0.62-0.66). This can be explained by the consumption of Fe to form pyrite and chalcopyrite. Chlorite occurs as an alteration product replacing magmatic biotite and hornblende, as well as hydrothermal biotite. Chlorite geothermometry indicates a narrow range between 212 and 246 °C for the formation of chlorites from various alteration zones, implying that the whole system equilibrated with a common fluid at low temperatures.
Porphyritic
Argillic alteration
Molybdenite
Hypogene
Quartz monzonite
Amphibole
Diorite
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Stockwork
Quartz monzonite
Argillic alteration
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