Genetic significance of fluid inclusions in the CSA Cu-Pb-Zn deposit, Cobar, Australia

Abstract

CSA mine exploits a ‘Cobar-type’ Cu–Pb–ZnFAuFAg deposit within a cleaved and metamorphosed portion of the Cobar Supergroup, central New South Wales. The deposit comprises systems of ‘lenses’ that encompass veins, disseminations and semi-massive to massive Cu–Pb–Zn ores. The systems and contained lenses truncate bedding, are approximately coplanar with regional cleavage and similarly oriented shear zones and plunge parallel to the elongation lineation. Systems have extreme vertical continuity (>1000 m), short strike length (f400 m) and narrow width (f100 m), exhibit vertical and lateral ore-type variation and have alteration haloes. Models of ore formation include classical hydrothermalism, structurally controlled remobilisation and polymodal concepts; syntectonic emplacement now holds sway. Fluid inclusions were examined from quartzFsulphide veins adjacent to now-extracted ore, from coexisting quartz– sulphide within ore, and from vughs in barren quartz veins. Lack of early primary inclusions precluded direct determination of fluids associated with D2–D3 ore and vein emplacement. Similarly, decrepitation (by near-isobaric heating) of the two oldest secondary populations precluded direct determination of fluid phases immediately following D2–D3 ore and vein emplacement. Post-decrepitation outflow (late D3 to early post-D3) is recorded by monophase CH4 inclusions. Entrained outflow of deeply circulated meteoric fluid modified the CH4 system; modification is recorded by H2O+CH4 and H2O+(trace CH4) secondary populations and by an H2O+(trace CH4) primary population. The contractional tectonics (D2–D3) of ore emplacement was superseded by relaxational tectonics (D4P) that facilitated meteoric water penetration and return flow. Under D2 prograde metamorphism, entrapment temperatures (Tt) and pressures (Pt) for pre-decrepitation secondary inclusions are estimated as Ttf300–330 jC and Ptf1.5–2 kbarcPlith (the lithostatic pressure). Decrepitation accompanied peak metamorphism (Tf350–380 jC) in mid- to late-D3, while in late-D3 to early post-D3, essentially monophase CH4 secondary inclusions were entrapped at Ttf350 jC and Pt = 1.5–2 kbarcPlith. Subsequently, abundant CH4 and entrained meteoric water were entrapped as H2O+CH4 secondaries under slowly decreasing temperature (Ttf330– 350 jC) and constant pressure (Ptf1.5–2 kbar). Finally, with increasingly dominant meteoric outflow, H2O+(trace CH4) populations record decreasing temperatures (Tt>300 to < 350 down to 275–300 jC) at pressures of Phydrostatic < Pt (f1 kbar) < Plith (f1.5 kbar). The populations of inclusions provide insight into fluid types, flow regimes and P–T conditions during parts of the deposit’s evolution. They indirectly support the role of basin-derived CH4 fluids in ore formation, but provide no insight into a basement-sourced ore-forming fluid. They fully support post-ore involvement of meteoric water. The poorly constrained entrapment history is believed to span 10 Ma from f395 to f385 Ma.