Geology, host rock succession, and hydrothermal alteration of the Waterloo volcanic-hosted massive sulphide deposit (Northern Queensland, Australia)

摘要

The Waterloo volcanic-hosted massive sulphide (VHMS) deposit is located in the ChartersudTowers Region in northern Queensland, Australia. The deposit forms part of the CambroOrdovicianudSeventy Mile Range Group that represents a major belt of E-W striking and subverticaluddipping volcanic-sedimentary rocks. The volcanic host rocks and the base metal mineralisationudof the Waterloo deposit are not exposed in surface outcrops because of a thickudcover by Pliocene fluvial sedimentary rocks. Exploration diamond drilling (total -10 km ofudcore) led to the delineation of a relatively small high-grade base metal resource of 243,500udtonnes ore grading 3.8 % Cu, 13.8 % Zn, 3.0 % Pb, 74 glt Ag, and 1.2 glt Au. The mineralisationudcomprises several small semiconnected stratiform, blanket-like, pyrite-chalcopyritesphalerite-udgalena massive sulphide lenses.udThe structural style of the Early Ordovician Waterloo sequence has been constrained usingudmacroscopic structural techniques. The massive sulphides at Waterloo are interpreted to beudsyn-volcanic in origin because they have been overprinted by the same generations of tectonicudstructures as the host stratigraphy. The Waterloo sequence was tilted into a subvertical positionudduring north-south compression that is possibly Mid- to Late Ordovician in age. Thisudregional folding event also resulted in the development of an axial plane cleavage that is particularlyudwell developed in a high strain zone surrounding the massive sulphides. The spatialudrelationship between the folded bedding plane and the axial plane cleavage as well as the consistentudsouth facing of the bedding of volcaniclastic sediments indicate that the deposit is locatedudat the southern limb of a major east-west trending antiform. This antiform has a shallowudplunge to the west. The Waterloo sequence was affected by two faulting events that areudyounger than the regional folding. Early steeply dipping ENE striking faults interpreted to beudSilurian or Devonian were accompanied by significant dip-slip normal movement, whereasudyounger strike-slip faults have no affect to the geometry of the Waterloo sequence.udBased on the improved understanding of the structural style of the Waterloo sequence, theudvolcanic facies architecture of the host sequence was investigated to unravel the temporal andudspatial relationships between volcanism and massive sulphide formation. The massive sulphidesudformed in a below storm wave base depositional enviromnent on top of a nonexplosive,udnear-vent, andesite-dominated facies association containing coherent volcanic unitsudand related juvenile volcaniclastic rocks. The massive sulphide lenses are overlain, and partiallyudhosted in, a coarse quartz-feldspar crystal-rich sandstone and breccia facies. These rocksudare interpreted to be mass flows that record contemporaneous probably explosive dacitic toudrhyolitic volcanism outside the Waterloo area. The still wet and unconsolidated coarse sedimentudin the immediate hanging wall of the massive sulphides was intruded by a feldspar porphyriticuddacite cryptodome that was partly emergent at the ancient seafloor. The emplacementudof the cryptodome indicates that the magmatic source feeding the volcanism within the Waterlooudarea shifted towards an acidic composition at the time of massive sulphide formation.udDacite cryptodome volcanism at Waterloo was followed by the waning of the hydrothermaludactivities. The subsequent period of relatively quite sedimentation was occasionally interruptedudby the emplacement of syn-sedimentary basaltic to andesitic sills and was followed byudthe mass flow deposition of a coarse feldspar-quartz sandstone and breccia facies. Finallyudthere was a period of intense non-explosive, near-vent basalt to andesite-dominated volcanism.udPetrochemical investigations demonstrated that the coherent volcanic rocks of the Waterlooudsequence belong to a subalkaline volcanic suite. The basalt, andesite, and dacite of the Waterlooudsequence are cogenetic. The petrographic and petrochemical characteristics of the reworkedudvolcaniclastic facies suggest that the material was derived from a petrogeneticallyudsimilar volcanic source of dacitic to rhyolitic composition. The geochemical signatures of theudmost primitive volcanic rocks from the Waterloo sequence are similar to modern subductionrelatedudvolcanics, such as back-arc basin basalts forming during the early stages of back-arcudbasin evolution. Based on these findings and the results of previous regional studies, it is suggestedudthat volcanism in the Waterloo area occurred in a bac - arc basin that developed onudthinned Precambrian continental lithosphere flanking a continental margin volcanic arc.udMineralogical investigations on the volcanic rocks hosting the massive sulphides revealed thatudtwo types of alteration can be distinguished. Least altered rocks were affected by weak regionaludalteration that was caused by the combined effects of devitrification, hydration, burialuddiagenesis, seawater interaction, regional metamorphism of the lower greenschist facies, anduddefOlmation. In contrast, volcanic rocks located in the footwall and the immediate hangingudwall of the massive sulphides were subject to a combination of hydrothermal and regionaludalteration.udThe spatial distribution of alteration mineral associations as well as the mineralogical andudgeochemical attributes of the hydrothermal altered rocks constrain the environment of hydrothermaludalteration. The massive sulphide lenses at Waterloo are underlain by an extensiveudfootwall alteration halo that is typified by a semiconformable zonation defined by an innerudzone of silicic-altered volcanics (pyrite-quartz-muscovite) that laterally passes into a zone ofudphyllic alteration (pyrite-muscovite-chlorite-quartz, pyrite-paragonite-muscovite-chloriteintermediateudNaIK mica-quartz, and pyrite-muscovite-albite-chlorite-paragonite-intermediateudNaIK mica-quartz-calcite) and a zone consisting of propylitic-altered volcanics (albitechlorite-udepidote-muscovite-paragonite-quartz-calcite-pyrite and albite-chlorite-epidote-quartzcalcite).udIt is demonstrated that the development of the zonation of the alteration halo can beuddirectly linked to the nature and evolution of the fluids interacting with the volcanic rocks inudthe different parts of the hydrothermal alteration halo. Hydrothermal alteration in the upflowudzones of the mineralising fluids resulted in the formation of large amounts of muscovite onudthe expense of primary rock-forming silicates by the combined effects of potassium and hydrogenudmetasomatism. This type of alteration was principally linked to the acidity of the mineralisingudhydrothermal fluids. The alteration in the upflow zones also involved a sulphidisationudof the rocks due to the reaction of ferrous iron contained in rock-forming silicates and theudvolcanic glass matrix with HzS supplied by the hydrothermal fluids. Silicification was pronouncedudin the upflow zones because the mineralising fluids cooled by moving down a temperatureudgradient. Outward percolation of the hydrothermal fluids into zones surrounding theudthermal upflow was accompanied by a rapid neutralisation of the strong acids and, therefore,udan increasing reactivity of CO2 with respect to hydrogen metasomatism. The percolation ofudseawater into the zones surrounding the high temperature upflow zones was intrinsically involvedudin the development of the alteration zonation. Heating of seawater, by moving up audtemperature gradient, resulted in a pronounced sodium metasomatism in the outer parts of theudalteration halo that caused the formation of sodium silicates (albite, intermediate NaIK mica,udand paragonite) at the expense of primary rock-forming silicates such as feldspars and earlierudformed products of hydrothermal alteration, such as muscovite. In contrast to the footwalludalteration halo, alteration of the volcanic facies overlying the ore horizon is limited in extentudand rapidly fades in intensity with increasing distance from the sulphides. The zonation of theudhanging wall alteration is defined by an inner zone of phyllic alteration (pyrite-muscovitequartz,udpyrite-muscovite-paragonite-intermediate NaIK mica-chlorite-quartz, muscovitechlorite-udquartz, and muscovite-paragonite-intermediate NaIK mica-chlorite-quartz) and anudouter zone comprising propylitic-altered volcanics (albite-muscovite-chlorite-paragoniteintermediateudNaIK mica-quartz-calcite and albite-muscovite-chlorite-epidote-quartz-calcite).udPhyllic alteration in the immediate hanging wall of the massive sulphides can be accountedudfor by the ongoing intense alteration following the burial of the ores by the mass flow derivedudcoarse quartz-feldspar sandstone and breccia facies and the emplacement of the dacitic cryptodome,udwhereas the outer zone of propylitic alteration records the waning of the hydrothermaludactivities where alteration occurred at successively decreasing temperatures in a moreudoxidising environment.udBased on the results of this study it is suggested that the genetic relationship between volcanismudand massive sulphide formation can be constrained by integrating volcanological studiesudon the host rock sequence with detailed mineralogical and geochemical investigations of theudhydrothermally altered rocks. The volcanological investigations demonstrate that the mineralisationudevent occurred in close temporal and spatial relationship to felsic volcanism culminatingudin the emplacement of a dacite cryptodome in the immediate hanging wall to the massiveudsulphides. The findings of the alteration halo study are consistent with this observationudbecause the mantle-derived volcanism in the Waterloo area may not only have provided theudheat to drive the hydrothermal system, but may also have acted as a source of chemical components,udsuch as volatile species that controlled the acidity of the mineralising hydrothermaludfluids.