Epithermal gold mineralization, Acupan, Baguio District, Philippines; geology, mineralization, alteration, and the thermochemical environment of ore deposition

Abstract

A large gold-rich adularia-sericite-style epithermal vein system formed at 0.65 Ma at Acupan, Baguio district, Philippines, during a period of high K magmatism. Low-salinity (•0.5 wt % NaC1 equiv), gassy (up to 0.41 m CO.2) meteoric fluids with temperatures of up to 300øC or more xvere focused into a series of steeply dipping interconnected faults and joints in the Virac granodiorite, Balatoc diatreme, Lucbuban gabbro, and Zig-Zag Formation. Prolonged and varied geothermal activity resulted in the formation of a series of composite banded epithermal veins with an average width of 1 m that extend to depths of at least i km (relative to the present-day surface). The epithermal event was characteried by periods of hydrothermal brecciation interspersed with more quiescent periods when delicate to coarsely banded vein material was precipitated. Five distinct mineral assemblages have been defined in the epithermal veins: type A (characterized by fine-grained chalcedony), type B (fine-grained gray quartz), type C (coarse-grained white quartz), type D (coarse-grained calcite), and type E (coarse-grained anhydrite). Electrum and Au-Ag tellurides have been recognized in type B, C, and D bands. Interaction of ascending chloride brines with the wall rocks over the lifetime of the system produced intense, symmetrical, meterwide halos of altered rock characterized by sericitic, silicic, and sericite-chlorite mineral assemblages that grade out to a weak, diffuse widespread propylitically altered zone. Minor adularia alteration is restricted to the deeper mine levels.Sulfide-oxide-silicate-carbonate phase relationships in the Acupan vein system constrain the mineralizing fluids to values of logJ•o21 between -28 and -33, logJ•n21 between -1.5 and -3.7, and logJ•H2sl between -0.6 and -1.8 at 300øC. Abundant quartz-K mica-pyrite alteration and vein mineralization are consistent with silica-saturated, reduced, weakly acidic to weakly alkaline fluid compositions. Boiling caused the fluids to evolve to more alkaline conditions, leading to the sporadic precipitation of adularia in the veins and altered wall rocks. Dissolved COs concentrations were sufficiently high (up to 0.41 m) to stabilize calcite at the expense of calc-silicate minerals. Mg •+ concentrations were low in the mineralizing fluids but increased during dissolution of primary hornblende and biotite in the country rocks, resulting in the development of a sericitechlorite alteration assemblage in the deep mine levels. HsS and H.2COa were the predominant sulfur-and carbon-bearing species, and the low-salinity, gassy nature of the fluids favored gold and silver transport as bisulfide complexes. H.2Te and HTe-were the predominant aqueous tellurium species. Entrainment of minor amounts of magmatic volatiles during type C and D mineralization is responsible for Te enrichment in the dominantly meteoric fluids, and log fw•/ ranged between -7.8 and -8.6 at 300øC during periods of hessitc deposition. Magmatic contributions also account for the CO• and H•S-rich nature of the fluids ...

Calculation of the physicochemical parameters ΣS, ΣC, aH2S, I and of the metal ions content of the hydrothermall fluid that has precipitated Au-Ag-Te ores in Panormos - Liardades Bay area, Tinos Island, Cyclades

“…Barnes, Το επιθερμικό σύστημα της Τήνου στο σύνολο του χαρακτηρίζεται ως χαμηλής θείωσης, (Casadevall & Ohmoto, 1977, Barnes, 1979, Henley & Ellis, 1983, Shelton et al, 1990, Zhang & Spry, 1994, White & Hedenquist, 1995, Cooke et al, 1996, Spry et al, 1996, So et al, 1997, Choi et al, 1998. Οι παρατηρήσεις που οδήγησαν στο παραπάνω συμπέρασμα είναι οι ακόλουθες:…”

“…Σφαλερίτη-Βουρτσίτη-Γκρηνοκίτη, κυ ρίως στα αρχικά στάδια απόθεσης φανερώνει χαρακτήρες επιθερμικου συστήματος υψηλής θείωσης, το οποίο μεταβαίνει συνολικά προς χαμηλής θείωσης, (Zhang & Spry, 1994, White & Hedenquist, 1995, Cooke et al, 1996.…”

“…Χρησιμοποιώντας την υπόθεση ότι το επιθερμικό σύστημα στη Τήνο είναι χαμηλής θείωσης, τις σχετικές βιβλιογραφικές αναφορές για επιθερμικό φλεβικά συστήματα πολυτίμων μετάλλων και τελλουριδίων (Casadevall & Ohmoto, 1977, Barnes, 1979, Shelton et al, 1990, Zhang & Spry, 1994, Cooke et al, 1996, Spry et al, 1996, So et al, 1997, Choi et al, 1998 και τη μέθοδο εκτίμησης των Barton et al, (1977), συμπεραίνουμε ότι η συνολική συγκέντρωση LS στο ρευστό κυμαίνονταν από ΙΟ 3 m ως ΙΟ 2 m, δηλαδή από logES= -3 ως log£S= -2. Από την ισορροπία H 2 S<->2H + +S _ , (Casadevall & Omhotto, 1977, Henley et al, 1984 (Barnes, 1979).…”

“…Για τον προσδιορισμό της συγκέντρωσης του Si0 2 στο μεταλλοφόρο ρευστό που γεμίζει τα κενά στις χαλα-ζιακές φλέβες, χρησιμοποιήθηκε η ισορροπία Χαλαζίας<->5ίΟ , (Henley et al, 1984, Cooke et al 1996 Το επιθερμικό σύστημα στο νησί της Τήνου, είναι ένα σύστημα Υψηλού C, λόγω του γεγονότος ότι τα πετρώματα-ξενιστές είναι οι ασβεστοδολομίτες της ενότητας βάσης, και τα ασβεστιτικά μάρμαρα της ενότητας κυανοσχιστολίθων, (Shelton et al, 1990, Zhang & Spry, 1994, Choi et al, 1998. Η παρουσία στα ρευστά ε γκλείσματα υγρού ή αερίου C0 2 , δηλώνει μέγιστη τιμή LC= 10 m, (Zhang & Spry, 1994, Choi et al, 1998.…”

Later stages of evolution of an epithermal system: Au–Ag mineralizations at Apigania Bay, Tinos Island, Cyclades, Hellas, Greece

“…Experimental studies have showed that magnesium has the capacity to increase the rate of hydrogen dissociation, slow down its adsorption kinetics, and lead hydrogen-bearing gases to condense (Swartz et al 1999). This hypothesis is supported by the presence of magnesium minerals in the most studied Ag-Au-Te ore deposits such as: dolomite-talc-brucite (Panormos Bay; Tombros et al 2004Tombros et al , 2005Tombros et al , 2007, Mg-chlorite (Acupan; Cooke et al 1996;Cooke and McPhail (2001), dolomite (Cripple Creek; Thompson et al 1985;Kelley et al 1998), dolomite and Mg-chlorite (Emperor, Tuvatu; Ahmad et al 1987;Scherbarth and Spry 2006), roscoelite and dolomite (Porgera; Richards and Kerrich 1993;Ronacher et al 2004) and Mg-chlorite and manganese ores (Sunnyside mine; Casadevall and Ohmoto 1977).…”