Abstract
AbstractThe Hera Au–Pb–Zn–Ag deposit in the southeastern Cobar Basin of central New South Wales preserves calc‐silicate veins and remnant sandstone/carbonate‐hosted skarn within a reduced anchizonal Siluro‐Devonian turbidite sequence. The skarn orebody distribution is controlled by a long‐lived, basin margin fault system, that has intersected a sedimentary horizon dominated by siliciclastic turbidite, with lesser gritstone and thick sandstone intervals, and rare carbonate‐bearing stratigraphy. Foliation (S1) envelopes the orebody and is crosscut by a series of late‐stage east–west and north–south trending faults. Skarn at Hera displays mineralogical zonation along strike, from southern spessartine–grossular–biotite–actinolite‐rich associations, to central diopside‐rich–zoisite–actinolite/tremolite–grossular‐bearing associations, through to the northern most tremolite–anorthite‐rich (garnet‐absent) association in remnant carbonate‐bearing lithologies and sandstone horizons; the northern lodes also display zonation down dip to garnet present associations. High‐T, prograde skarn assemblages rich in pyroxene and garnet are pervasively replaced by actinolite/tremolite–biotite‐rich retrograde skarn which coincides with the main pulse of sulfide mineralization. The dominant sulfides are high‐Fe–Mn sphalerite–galena–non‐magnetic high‐Fe pyrrhotite–chalcopyrite; pyrite, arsenopyrite; scheelite (low Mo) is locally abundant. The distribution of metals in part mimics the changing gangue mineralogy, with Au concentrated in the southern and lower northern lode systems and broadly inverse concentrations for Ag–Pb–Zn. Stable isotope data (O–H–S) from skarn amphiboles and associated sulfides are consistent with magmatic (or metamorphic) water and sulfur input during the retrograde skarn phase, while hydrosilicates and sulfides from the wall rocks display comparatively elevated δD and mixed δ34S consistent with progressive mixing or dilution of original magmatic (or metamorphic) waters within the Hera deposit by unexchanged waters typical of low latitude (tropical) meteoritic waters. High precision titanite (U–Pb) and biotite (Ar–Ar) geochronology reveals a manifold orebody commencing with high‐T skarn and retrograde Pb–Zn‐rich skarn formation at ≥403 Ma, Au–low‐Fe sphalerite mineralization at 403.4 ± 1.1 Ma, foliation development remobilization or new mineralization at 390 ± 0.2 Ma followed by thrusting, orebody dismemberment at 384.8 ± 1.1 Ma and remobilization or new mineralization at 381.0 ± 2.2 Ma. The polymetallic nature of the Hera orebody is a result of multiple mineralization events during extension and compression and involving both magmatic and likely formational metal sources.
Highlights
The Siluro-Devonian Cobar Basin is a major mining province in central New South Wales, Australia
Sensitive high-resolution ion microprobe (SHRIMP), isotope dilution thermal ionization mass spectrometry (IDTIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating was carried out on titanite from three different settings at Hera; (1) quartz–ilmenite–titanite vein that cross-cuts prograde skarn in the Far West lode (Sample JAF01), (2) remnant low-T retrograde, K-feldspar-altered skarn fragments associated with low-Fe sphalerite–titanite–Au-rich breccia (Sample JAF02) and, (3) thrust fault-related late-stage quartz–chlorite–titanite veins that crosscut the orebody (Samples JAF03 and JAF04)
The features of skarn mineralization, including the preservation of prograde, high-T skarn assemblages hosted within sub-greenschist facies basin sedimentary rocks (Fitzherbert et al, 2017a, b) and thermal changes reflected by changing skarn mineralogy at the Hera orebody are inconsistent with the current model for formation of Cobar-type deposits through deep burial and regional-scale ingress of basement and basin derived fluids during basin inversion (Stegman, 2001)
Summary
The Siluro-Devonian Cobar Basin is a major mining province in central New South Wales, Australia. Mineralization in the main (northern) Cobar mineral field is associated with greenschist facies high-strain zones and is linked with fluids that exploited major fault systems and regional lithological contacts Correlative mineralization along these same fault systems and lithological contacts occurs on the southeastern margin of the basin, hosted within siliciclastic-dominant turbidite sequences of the lower Amphitheater Group and to a lesser extent shallow-water carbonate-bearing sequences of the Mouramba Group (Figure 1). The orebody and overprinting foliation are cut by late-stage, west-verging reverse faults defined by quartzrich veins with titanite, clinozoisite, chlorite, albite, adularia, tourmaline and zeolite (Figures 5 and 6k) These are generally barren but minor high-Fe sphalerite can be present, a single fibrous tourmaline-rich vein was observed to contain abundant native gold.
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