P-T-T-DEFORMATION-FLUID CHARACTERISTICS OF LODE GOLD DEPOSITS - EVIDENCE FROM ALTERATION SYSTEMATICS

Structurally hosted lode gold-bearing quartz vein systems in metamorph ic terranes possess many characteristics in common, spatially and thro ugh time; they constitute a single class of epigenetic precious metal deposit, formed during accretionary tectonics or delamination. The ore and alteration paragenesis encode numerous intensive and extensive va riables that constrain the pressure-temperature-time-deformation-fluid (P-T-t-d-f) evolution of the host terrane and hence the origin of the deposits. The majority of lode gold deposits formed proximal to regio nal translithospheric terrane-boundary structures that acted as vertic ally extensive hydrothermal plumbing systems; the structures record va riably thrust, and transpressional-transtensional displacements. Major mining camps are sited near deflections, strike slip or thrust duplex es, or dilational jogs on the major structures. In detail most deposit s are sited in second or third order splays, or fault intersections, t hat define domains of low mean stress and correspondingly high fluid f luxes. Accordingly, the mineralization and associated alteration is mo st intense in these flanking domains. The largest lode gold mining cam ps are in terranes at greenschist facies; they possess greenschist fac ies hydrothermal alteration assemblages developed in cyclic ductile to brittle deformation that reflects interseismic-coseismic cycles. Inte rseismic episodes involve the development of ductile S-C shear zone fa brics that lead to strain softening. Pressure solution and dislocation glide microstructures signify low differential stress, slow strain ra tes of less than or equal to 10(-13) s(-1), relatively high confining stress, and suprahydrostatic fluid pressures. Seismic episodes are ind uced by buildup of fluid pressures to supralithostatic levels that ind uce hydraulic fracturing with enhanced hydraulic conductivity, accompa nied by massive fluid flow that in turn generates mineralized quartz v eins. Hydrothermal cementing of ductile fabrics creates 'hardening', l owers hydraulic conductivity, and hence promotes fault valve behaviour . Repeated interseismic (fault valve closed), coseismic (valve open) c ycles results in banded and/or progressively deformed veins. Alteratio n during both interseismic and coseismic episodes typically involves t he hydrolysis of metamorphic feldspars and Fe, Mg, Ca-silicates to a m uscovite/paragonite-chlorite + albite/K-feldspars assemblage; carboniz ation of the metamorphic minerals to Ca, Fe, Mg-carbonates; and sulphi dation of Fe-silicates and oxides to sulphides. Geochemically this is expressed as additions of K, Rb, Ba, Cs, and the volatiles H2O, CO2, C H4, H2S in envelopes of meter to kilometer scale. K/Rb and K/Ba ratios are close to average crustal values, potentially ruling out late stag e magmatic fluids where K/Rb and K/Ba are respectively lower and highe r than crustal values. Smaller deposits are present in subgreenschist, and amphibolite to granulite facies terranes. The former are characte rized by subgreenschist facies alteration assemblages, vein stockworks , brittle fracturing and cataclastic microstructures, whereas the latt er feature amphibolite to granulite facies alteration assemblages, duc tile shear zones, ductilely deformed veins, and microstructures indica tive of dislocation climb during interseismic episodes. Hence the lode gold deposits constitute a crustal continuum of deposits from subgree nschist to granulite facies, that all formed synkinematically in broad thermal and rheological equilibrium with their host terranes. These c haracteristics, combined with the low variance of alteration assemblag es in the higher temperature deposits, rules out those being metamorph osed counterparts of greenschist facies deposits. Deposits at all grad es have a comparable metal inventory with high concentrations of Au an d Ag, where Au/Ag averages 5, with enrichments of a suite of rare meta ls and semi-metals (As, Sb, +/- Se, Te, Pi, W, Mo and B), but low enri chments of the base (Cu, Pb, Zn, Cd) and other transition (Cr, Ni, Co, V, PGE, Sc) metals relative to average crust. The hydrothermal ore-fo rming fluids were dilute, aqueous, carbonic fluids, with salinities ge nerally less than or equal to 3 wt.% NaCl equivalent, and X(CO2+/-CH4) 10-24 Wt.%. They possess low Cl but relatively high S, possibly refle cting the fact that metamorphic fluids are generated in crust with sim ilar to 200 ppm Cl, but similar to 1 wt.%S. Primary fluid inclusions a re: (1) H2O-CO2, (2) CO2-rich with variable CH4 and small amounts of H 2O, and (3) 2-phase H2O (liquid-vapor) inclusions. Inclusion types 2 a nd 3 represent immiscibility of the type 1 original ore fluid. Immisci bility was triggered by fluid pressure drop during the coseismic event s and possibly by shock nucleation, leading to highly variable homogen ization temperatures in an isothermal system. A thermodynamic evaluati on of alteration assemblages constrains the ore fluid pH to 5-6; redox controlled by the HSO4/H2S and CO2/CH4 buffers; and XCO2 that varies. The higher temperature deposits formed under marginally more oxidizin g conditions. Stable isotope systematics of the ore and gangue mineral s yields temperatures of 200-420 degrees C, consistent with the crusta l spectrum of the deposits, very high fluid rock ratios, and disequili brium of the externally derived ore fluids with wall rocks. The ore fl uid delta D and delta(18)O overlap the metamorphic and magmatic ranges , but the total dataset for all deposits is consistent only with domin antly metamorphic fluids. Carbon isotope compositions of carbonates sp an -11 to +2% and show provinciality: this is consistent with variable proportions of reduced C (low delta(13)C) and oxidized C (higher delt a(13)C) in the source regions contributing CO2 and CH4 to the ore flui ds. In some instances, C appears to have been derived dominantly from proximal to the deposits, as in the Meguma terrane (delta(13)C similar to -22%). Sulphur isotope compositions range from 0 to +9 parts per t housand, and are consistent with magmatic S, dissolution or desulphida tion of magmatic sulphides, or average crustal sulphides. S-34-deplete d sulphides occur in ore bodies such as Hemlo where fluid immiscibilit y led to loss of H2S and consequent fluid oxidation. Gold is probably transported as an Au(HS); complex. Relatively high S but low Cl in the hydrothermal fluid may explain the high Au slow base metal characteri stic of the deposits. Gold precipitated in ore bodies due to