Structure, volcanic setting, hydrothermal alteration and genesis of the Thalanga massive sulphide deposit
Creators: name: Hill, Anthea P
Publisher: name: University Of Tasmania
Year: 2023

description: The Thalanga Zn-Pb-Cu-Ag deposit is located at the contact between the rhyolitic volcanics of the Mount Windsor Volcanics and the overlying dacitic and andesitic volcanic units of the Trooper Creek Formation in the Cambro-Ordovician Mount Windsor subprovince, northern Queensland. The sheet-like massive sulphide lenses and enclosing volcanic units are locally strongly deformed and metamorphosed to upper greenschist facies (T = 485 ¬¨¬± 23¬¨‚àûC, P = 2.5 ¬¨¬± 1.5 kbars). Bedding is now subvertical (D2) and the ore horizon has been offset by two generations of normal faults. Normal faults at a high angle to the stratigraphy separate West and Central Thalanga from the Vomacka Zone and East Thalanga, whereas younger normal faults (D3 ), at a low angle to stratigraphy, have repeated and locally structurally thickened the ore lenses. The pervasive S2 cleavage has been locally weakly crenulated by S3. Decussate biotite has overgrown the crenulations, indicating that peak metamorphism at Thalanga was post-S 3 . The presence of rare kinked chlorite pseudomorphs of biotite is interpreted to indicate that there was an earlier metamorphic event, which may correlate with regional metamorphism. The massive sulphides are interpreted to be syn-volcanic in origin because (1) they have been overprinted by the same generations of tectonic structures as the host stratigraphy, (2) the rhyolitic volcanics stratigraphically underlying the massive sulphide lenses contain abundant quartz ¬¨¬± muscovite ¬¨¬± chlorite ¬¨¬± phlogopite ¬¨¬± pyrite assemblages (inferred to be metamorphosed quartz-sericite ¬¨¬± chlorite ¬¨¬± pyrite alteration), whereas the overlying dacite lavas contain metamorphic biotite and local epidote-quartz ¬¨¬± albite-rich assemblages, (3) subvertical pyrite stringer zones (5-40 % pyrite) within the footwall rhyolitic volcanics intersect the ore horizon at the thickest lenses of massive sulphides, and (4) there is a strong stratigraphic control on the location of massive sulphides. The ore lenses at Thalanga occur within and stratigraphically below a poorly-sorted, coarse quartz-bearing, polymict breccia. The internal organisation of this breccia is consistent with transportation to the site of final deposition by subaqueous mass-flows. The breccia has a similar distribution to the massive sulphide lenses and is interpreted to have filled local seafloor depressions. The presence of massive sulphide clasts within the polymict breccia indicates synchronous volcanism and mineralisation. Comagmatic, non-vesicular quartz-feldspar porphyry, with peperitic margins, has locally intruded the polymict breccia. The main ore minerals are sphalerite, pyrite, chalcopyrite and lesser galena, with minor magnetite, arsenopyrite and tetrahedrite-tennantite. In most places, the sulphides are coarsely recrystallised and preserve no evidence of the deformation history. However, banding in the polymetallic sulphide lenses is interpreted to be tectonic in origin because the alternating pyrite- and sphalerite-rich bands are subparallel to S2. Chalcopyrite, sphalerite and galena have been remobilised during deformation and now occupy faults and sites of dilation within the ore horizon, including subhorizontal boudin necks, shallowly-dipping extension veins and piercement structures at the contact between massive sulphides and the overlying dacite. Metal zonation at Thalanga is poorly developed due to widespread sulphide remobilisation, although there is a general zonation from pyrite-chalcopyrite-rich sulphides at the stratigraphic base of ore lenses, to sphalerite-galena ¬¨¬± barite-rich sulphides at the top. The ore lenses at West and parts of Central Thalanga are composed of massive sulphide veins and disseminations that have cross-cut carbonate (dolomite-calcite) and Mg-rich chlorite assemblages. The Ti/Zr values of the carbonate- and chlorite-rich assemblage are similar to those of the underlying footwall rhyolitic volcanics (Ti/Zr = 2-5), and locally to those of the overlying coarse quartz-bearing polymict breccia (Ti/Zr = 3-10). Carbonate- and chlorite-rich units are therefore interpreted to have formed by the replacement of formerly glassy rhyolitic volcanics and the base of the coarse quartz-bearing polymict breccia by cooler Mgand HCO3- -bearing hydrothermal solutions prior to sulphide deposition. The coarse quartzbearing polymict breccia may have acted as an impermeable barrier during carbonatechlorite alteration and trapped the ascending hydrothermal solutions, restricting alteration to a narrow zone at the top of the footwall rhyolitic volcanics. The lack of extensive carbonate-chlorite alteration in the Vomacka Zone and East Thalanga may be due to higher temperature hydrothermal solutions or lack of a capping unit. The high Mg content of chlorite within the carbonate-chlorite assemblages is consistent with mixing between cold seawater and warm hydrothermal solutions in porous perlitic rhyolite lavas or rhyolitic breccia units in the footwall. HCO3- -bearing hydrothermal solutions are interpreted to have been derived from seawater that had circulated through the rhyolitic volcanics of the Mount Windsor Volcanics and possibly some magmatic solutions containing HCO3-. The 634S values of sulphides (5.6-17.3 %co) are consistent with formation from solutions containing dissolved igneous sulphur and Cambro-Ordovician seawater that had been inorganically reduced during hydrothermal convection. The increasing ‚Äö534S of pyrite towards the stratigraphic top of footwall is interpreted to be a consequence of progressive mixing between the hydrothermal solutions and cold seawater. The 6M5 values of barite (27.6-32.4 700) are consistent with a Cambro-Ordovician seawater source of sulphur, and support the interpretation that barite was deposited on the seafloor. The abundance of proximal coherent lavas, domes and syn-volcanic intrusions and associated volcaniclastic facies underlying and overlying Thalartga, and absence of coherent units in thehangingwall along strike (closest is ‚ÄövÑvÆ2 km along strike), is interpreted to indicate that the massive sulphide lenses formed within a volcanic centre. Local formation of massive sulphides on the seafloor is inferred to have been interrupted by deposition of the polymict breccia units. The sulphide lenses are interpreted to have formed predominantly by subseafloor replacement of the coarse grained bases of polymict breccia units, and local carbonate- and chlorite-rich alteration assemblages, prior to emplacement of the overlying dacite lavas. The presence of coarse quartz-bearing polymict breccia and porphyry units, is interpreted to indicate a long-lived hydrous magma at depth that probably drove hydrothermal convection and may have released volatiles (including CO2 and potentially metals) to the ascending hydrothermal solutions.