Facies architecture of the felsic lava‐dominated host sequence to the Thalanga massive sulfide deposit, Lower Ordovician, northern Queensland

Abstract

The Thalanga volcanic-hosted massive sulfide deposit occurs in the Cambro-Ordovician Mt Windsor Subprovince in northern Queensland. The orebody comprises steeply dipping, stratiform, sheet-like, polymetallic massive sulfide lenses. Overall, the volcanic facies architecture at Thalanga is dominated by quartz-and/or feldspar-phyric lavas and synvolcanic intrusions that comprise coherent facies and in situ and resedimented autoclastic facies. Systematic phenocryst logging (mineralogy, abundance, size) has been used to discriminate separate emplacement units of rhyolite in the footwall and dacite in the hangingwall. Some of the petrographically different rhyolite and dacite types can also be distinguished using immobile-element geochemistry. Rhyolitic lavas and intrusions in the footwall are weakly to strongly altered. Apparent clastic textures resulting from hydrothermal alteration and metamorphism are widely developed in the coherent facies. Genuine clastic textures are characterised by clasts with randomly oriented internal laminar or banded fabric (e.g. rotated, flow-laminated clasts), marked and consistent differences in quartz phenocryst abundance and/or size range between clasts and matrix, and normal grading. Mass-flow-emplaced, rhyolitic breccia units delineate palaeo-sea-floor positions in the footwall that are potentially prospective for exhalative massive sulfide mineralisation. A comparison of the distribution of clastic and coherent facies with the geometry of strongly altered zones in the footwall indicates that intense hydrothermal fluid flow was independent of the facies arrangement. The massive sulfide lenses conformably overly altered footwall rhyolite and occur in a distinctive facies association which includes coarse quartz-phenocrystrich rhyolitic sills with peperitic contacts and crystal-rich polymictic breccia. The hangingwall to the orebody consists of largely unaltered dacitic lavas and synvolcanic intrusions and minor dacitic pumice breccia, dacitic breccia and polymictic volcanic breccia. The facies architecture shows that the Thalanga massive sulfide deposit formed in a below-storm-wave-base depositional environment on top of an elevated, lava-dominated, rhyolitic volcanic centre. A modern analogue for the setting of the Thalanga massive sulfide is the PACMANUS hydrothermal field on the crest of the dacite lava-dominated Pual Ridge in the eastern Manus backarc basin (Papua New Guinea).

The Origin of Chlorite-Tremolite-Carbonate Rocks Associated with the Thalanga Volcanic-Hosted Massive Sulfide Deposit, North Queensland, Australia

“…This represents several eruptive units (Paulick and McPhie, 1999). Nevertheless, the high positive correlations shown by the rhyolite data in immobile element plots suggest that the footwall rocks of the mine area originally were compositionally relatively uniform.…”

“…Even so, on the basis of a detailed volcanic facies analysis, Paulick and McPhie (1999) found that zones of intense alteration cut across coherent emplacement units with little evidence of control by volcaniclastic permeability, except at West Thalanga. They considered that the pyritic stringer zones may originally have been normal to stratigraphy and were rotated into the present oblique orientation by shearing associated with regional folding.…”

“…Underlying the sulfide lenses is an extensive semistratiform zone of quartz + sericite ± chlorite + pyrite representing pervasive feldspar and glass-destructive hydrothermal alteration. The pseudoclastic textural modifications produced by domainal alteration of formerly glassy volcanics (described by Paulick and McPhie, 1999) significantly obscure the detailed interpretation of volcanic facies in the footwall rocks.…”

“…It is here referred to as the "favorable unit." That is equivalent to the terms "favorable horizon" and "mineralized horizon," respectively applied by Paulick and McPhie (1999) and Berry et al (1992), and includes the "Thalanga ore horizon" of Gregory et al (1990). It is comprised of lenses or layers of massive to semimassive sulfides, barite, magnetite-quartzite, and chlorite-tremolite-carbonate rocks in and below a distinctive association of coherent sills and flows of quartz-feldspar porphyry and quartz crystal-rich volcaniclastics.…”

Facies architecture of the Palaeoproterozoic VMS-bearing Maurliden volcanic centre, Skellefte district, Sweden

“…Allen 1992;McPhie & Allen 1992;Allen et al 1996aAllen et al , 1996bPaulick & McPhie 1999;Soriano & Marti 1999;Gibson et al 1999;Doyle & McPhie 2000;Lafrance et al 2000;Paulick et al 2004;Montelius 2005). Ancient volcanic successions are an important source of information on the character, product and architecture of submarine volcanism, since direct observation of silicic submarine volcanic centres is very limited, and in any case their sub-seafloor structure and facies are concealed.…”