The Archean Cu-Zn magnetite-rich Gossan HilludVHMS deposit, Western Australia: Evidence of audstructurally-focussed, exhalative and sub-seafloorudreplacement mineralising system

摘要

The Archean Cu-Zn Gossan Hill volcanic-hosted massive sulphide deposit is situated onudthe northeast flank of the Warriedar Fold Belt in the Yilgarn Craton, Western Australia.udThe deposit is hosted within re-deposited rhyodacitic tuffaceous volcaniclastics of theudGolden Grove Formation and is overlain by rhyodacite-dacite lavas and intrusive domesudof the Scuddles Formation. The Gossan Hill deposit consists of two discrete subverticaludore zones situated stratigraphically 150 m apart in the middle and upper Golden GroveudFormation. The stratigraphically lower Cu-rich ore zone (7.0 Mt @ 3.4% Cu) consists ofudstratabound, podiform to discordant massive pyrite-chalcopyrite-pyrrhotite-magnetite. Inudaddition to massive sulphides, the lower ore zone also contains discordant to sheet-W,eudzones of massive magnetite-carbonate-chlorite-talc (-12 }·ft). The upper Zn-Cu ore zoneud(2.2 Mt @ 11.3% Zn, 0.3% Cu, 15 glt Au and 102 glt Ag) is mound-shaped with sheetW,ude, stratabound, massive sphalerite-pyrite-chalcopyrite overlying discordant massiveudpyrite-pyrrhotite-chalcopyrite-magnetite. A sulphide-rich vein stockwork connects theudupper and lower ore zones. Metal zonation grades from Cu-Fe (±Au) in the lower oreudzone to Zn-Cu-nch sulphides at the base of the upper ore zone. The upper ore zoneudgrades upwardS and laterally from Zn-Cu to Zn-Ag-Au (tCu, tPb)-rich sulphides.udRegional preservation of primary tuffaceous volcanic textures within the Golden GroveudFormation is attributed to an early syndepositional, quartz-chlorite alteration. Indurationudand differential permeabilityI porosity reduction of the succession during the earlyudalteration W,ely promoted more-focussed pad1ays for successive hydrothermal fluids.udSubsequent hydrothermal alteration related to mineralisation at Gossan Hill has a limitedudlateral extent, and forms a narrow Fe-chlorite-ankerite-siderite envelope to the massiveudmagnetite and sulphide of the lower ore zone, and an intense siliceous envelopeudsurrounding d,e stockwork and upper ore position. Pervasive calcite-muscovite alterationudis recognised Ul d,e hangingwall volcanics of the ScuddIes Formation.udThe nature of deformation and metamorphism (greenschist facies: 454 t 4°C at I kbarudbased on andalusite-chloritoid-quartz equilibrium) is uniform throughout d,e massiveudmagnetite, massive sulphide and host succession. Sedllnent-sulphide-magnetiteudrelationships at Gossan Hill suggest d,e formation of magnetite and sulphide duringuddeposition of the upper Golden Grove Formation. Massive magnetite formed entirely byudsub-seafloor replacement processes as inferred from gradational upper and lower contactsudand interdigitating volcaniclastics. Replacement occurred along permeable tuffaceousudstrata outward from a discordant feeder. Massive magnetite was later veuled, replacedand cut by massive sulphide. The synchronous formation of both upper and lowerudsulphide ore zones is indicated by the connecting sulphide stockwork. Both sulphide oreudzones formed by sub-seafloor replacement, although stratiform hydrothermal chertsulphide-udsediment layers in, and adjacent to, the upper sulphide zone attest to someudexhalation of fluids onto the seafloor.udThe thickest occurrence of massive magnetite, massive sulphide and stringer stocb.-workudspatially coincide and support a common feeder conduit during massive magnetite andudsulphide mineralisation. The asymmetry of hydrothermal alteration envelopes, massiveudmagnetite and massive and veins sulphide zones are consistent with synvolcanic structuraludcontrols, with a growth structure occupied and obscured by a younger dacite dome fromudthe Scuddles Formation.udA systematic increase in sulphide 8;4S values (range of -4.0 to 7.8%0, average 2.1 ± 1.7%0)udstratigraphically upwards through massive and vein sulphide is suggestive of progressiveudmixing of upwelling ore fluids with entrained seawater. Homogeneous 8"s values ofud-1.5%0 in the lower ore zone have a consistent homogeneous rock sulphur source withudpossible magmatic contributions.udThe 8180 H20 values of ore fluids responsible for deposition of magnetite in massiveudmagnetite and disseminated magnetite in the sulphide zones range from 6%0 to 13%0.udThis data is inconsistent with the direct input of Archean seawater, and favoursudderivation of hydrothermal fluids by rock buffering of circulating fluids, or by directudmagmatic contribution.udThermodynamic considerations suggest massive magnetite and sulphide formed from highudtemperature (300° to 350°C), reduced (low f 0,), slightly acidic hydrothermal fluids. HzSdeficientudfluids formed massive magnetite, whilst HzS-rich fluids formed massiveudsulphides. Fluid chemistry differences are attributed to magmatic sulphur contributionsudduring sulphide mineralisation. Precipitation of sub-seafloor sulphide in the lower oreudzone resulted from chemical entrapment by the interaction of upwelling HzS-rich fluidsudwith pre-existing massive magnetite. It is suggested that shallow parental magmaudchambers to the Scuddles Formation drove hydrothermal convection of seawater and mayudhave supplied volatiles and HzS to the ascending hydrothermal fluids.udThe Gossan Hill sulphide-magnetite deposit represents an evolving hydrothermal systemudin an environment characterised by rapid volcaniclastic sedimentation and changingudstructural and magmatic processes. An important influence on this hydrothermal systemudwas the creation and destruction of porosity and permeability in the host succession. Theudhydrothermal system initiated as part of a regional seawater convection-alteration systemudthat led to VHMS mineralisation at Gossan Hill by (1) synsedimentary metasomatism andudprogressive heating of convecting fluids, (2) formation of massive magnetite by host rockudreplacement above a buried synvolcanic conduit, and (3) structural re-activation andudtapping of deeper HzS-rich and metal-bearing fluids, leading to the sub-seafloor sulphideudreplacement and local exhalation of hydrothermal fluids forming sulphide and chert.udBurial by proximal felsic volcanism led to preservation of the deposit.