Characteristics and genesis of Zn-Pb-Fe-bearing minerals in the Grieves Siding peat, western Tasmania

Many minerals that form in surface and near-surface environments are emerging to be mineralogically and geochemically complex. The characteristics of such minerals are observed to diverge from those that are predicted by equilibrium thermodynamics, and in some instances are found to originate through metastable precursor phases. The base metal-rich Grieves Siding peat, western Tasmania, Australia, provides an insight into the evolution of low-temperature mineralising systems in the groundwater environment, and allows the investigation of the early stages of mineral formation at ambient conditions. Mechanisms of a stepwise mineral formation pathway, and the characteristics of precursor and intermediate phases, which have been established experimentally, are examined here. The occurrence and characteristics of colloids and microorganisms, two of the recognised contributors in mineral formation processes at surface conditions, are also probed. At the Quaternary Grieves Siding peat, a significant concentration of base metals also occurs within the underlying Ordovician Gordon Group carbonates (Grieves Siding carbonate prospect), and the spatial relationship strongly implies a link between these two metal-rich zones. Despite nearly four decades of geologic evaluations at the site, the genesis of the two differing mineral concentrations remains ambiguous. Adding to the enigma of the deposit are the complex mineralogical characteristics and assemblages of the ore minerals in the peat (i.e., sphalerite, galena, and Zn-Pb-bearing silicates, oxides, and carbonates) that pose difficulties for future metallurgical extraction. The genetic relationship between the Zn-Pb mineralisation in the peat, and its carbonate counterpart, is established in this study. The geochemical characteristics of the Grieves Siding peat profile overlying the mineralised metasediments were determined by major and trace element geochemistry, and C and S isotope analyses. The mineralogical characteristics, mineral associations, phase evolution, mineral paragenesis, and mechanisms of formation of Zn-Pb-Fe bearing minerals within the peat were evaluated by quantitative X-ray diffraction, scanning electron microscopy, electron backscatter diffraction, electron probe microanalysis, cathodoluminescence, Raman and Fourier transform infrared spectroscopy, and laser ablation-inductively coupled plasma mass spectroscopy. The metal-rich peat possesses exceptionally high base metal contents reaching up to 27.2 wt. % Zn and up to 3.8 wt. % Pb, making it an effective repository for metals. The extreme enrichment in Zn and Pb in Grieves Siding peat registered as the highest among reported peat-hosted mineralisations in the world. The enrichment of metals in the studied peat is hypothesised to be a collective result of the weathering and leaching of metals from the underlying mineralised carbonate sequence, with some acquisition from surrounding volcanic rocks. Metal leaching is proposed to have occurred under oxidising conditions and produced a shift in groundwater pH from pH 6.9 (tannic acid-controlled regions) to pH 4.3. The detected sulfur in the surface and groundwaters posited to occur as H\\(_2\\)SO\\(_4\\) and dissociated SO\\(_4\\)\\(^{2-}\\) ligands, which supports transport of Zn mainly as ZnSO\\(_4\\). The accumulation of metals in the peat is shown to be influenced by (i) availability of metals derived from mineralised underlying and regional rocks; (ii) continuous availability of oxygen in the waters to liberate bedrock sulfur; (iii) efficiency of the metal transport by sulfate and organic ligands in ground and surface waters; (iv) a mechanism for the reduction of aqueous sulfate to H2S in the peat, probably as a by-product of the metabolic activity of sulfate-reducing bacterial communities thriving in a biofilm along the sub-surface flow path at the base of the living peat mass; (iv) the trapping of metals as metal sulfide minerals (i.e., Zn, Pb, and Fe) following interaction of H\\(_2\\)S and aqueous metals and other dissolved components; adsorption to clay minerals of metals (i.e., Zn, Pb, Cd, Cu, Fe, Ag, Ti, Ni, and Co); complexation and fixation of selected metals (i.e., As, Sb, Mo, U, and Au) to organic matter; and association of selected metals (i.e., Zn, Pb, Cd, Cu, Ag, and Fe) to nonstoichiometric (gel) phases, bacterial cells, and biofilms. The mineral assemblages in the peat consist of sulfides, silicates, sulfates, oxides, carbonates, and phosphates that are here classified as (i) detrital minerals, which are derived from the surrounding rocks (i.e., Cambrian volcanic rocks and sediments, Ordovician carbonates), and (ii) authigenic phases, including the predominant Zn-Pb-Fe bearing phases. Zinc occurs in sphalerite, baileychlore, Fe-Zn-Pb carbonate and nonstoichiometric (i.e., clay and colloform) phases that also contain S, O, Al, Pb and C. Authigenic Pb- and Fe-bearing phases such as galena, anglesite, plumbojarosite, magnetite, and pyrite are also detected. Pyrite shows textural evidence of stepwise transformations from disseminated and framboidal textures to a massive polycrystalline configuration. The mineral chemical and microcrystallographic features of the key Zn-bearing phases such as clay and colloform phases, and sphalerite were further examined. The clay comprises O (up to 44.1 wt. %), Al (up to 25.5 wt. %), Si (up to 11.3 wt. %), S (up to 3.1 wt. %), and Zn (up to 1.5 wt. %) with smaller amounts of Pb, Fe, and Cd. The absence of electron backscatter patterns (EBSPs) in the clay phase indicate that it exhibits low crystallinity and could potentially be amorphous and in some cases, semi-crystalline. Mineral chemistry of the colloform phase includes Zn, S as well as O (up to 18.5 wt. %), Al (up to 8.5 wt. %), Pb (up to 2.9 wt. %), Si (1.7 wt. %), smaller amounts Fe, Cd, Cu, Ag, Sb, As, and also lighter elements such as H and C. Brighter bands in backscattered electron images correspond to highest concentration of Zn and S, whereas the darker bands contain more O, Al, Si, and C. Lead and other trace metals display no correlation with either the band types. Similar to the clay, the colloform phase also lacks any EBSPs, suggesting that it has low crystallinity and may be amorphous. Infrared spectra of the clay and colloform phases have distinctive signatures matching that of water, Al-rich phases and in some cases, organic molecules. Sphalerite (~1 to 5 ˜í¬¿m size) is crystalline and commonly occurs adjacent to the Zn-bearing clay and colloform phases. The coexistence of crystalline sphalerite and amorphous, nonstoichiometric Zn-rich phases is inferred to represent a continuing evolution from metastable precursors to sphalerite. In more detail, this study has identified a multi-stage crystallisation route for sphalerite. It commences with heterogeneous nucleation of precursor phases on and within a Zn-rich clay substrate; it progresses to recurring growth and maturation by aggregation, coalescence, and impurity expulsion. It culminates in the transformation to sphalerite. Each step is observed to be a progression towards increased crystallinity and homogeneity, tending to pure ZnS. The entire formation processes of the colloform phase occur within bacterial cells, biofilms and on inorganic substrates. Hence, a profound microbiological influence and control are implicated in the mineralisation process. Morphological, textural, and chemical features of the colloform ZnS phases also point to a colloidal (gel) state and origin. Genetic associations of authigenic Zn-Pb-Fe phases in the peat, typically in a nonlinear progressions are, Zn-rich clay‚Äövúv¿ disseminated pyrite‚Äövúv¿framboidal pyrite‚Äövúv¿anhedral pyrite‚Äövúv¿ polycrystalline pyrite‚Äövúv¿ galena‚Äövúv¿anglesite‚Äövúv¿Zn-rich colloform ‚Äövúv¿sphalerite. The assemblages of authigenic minerals in the peat reflect dynamic physical and chemical conditions that are not necessarily in equilibrium with one another. The tendency of nonequilibrium or metastable precursor phases to naturally progress into more stable configurations, in a low-temperature setting, is demonstrated in this research. The results of this study demonstrate the capability of peat to not only preserve metal sulfides but also to create significant concentrations that could be a feasible mineral deposit. This research also marks the potential of peat to provide exploration vector information about for underlying hypogene ores, which could be harnessed by systematically spatially determining peat metal content. The proposed stepwise pathway could apply to other low- to intermediate-temperature base metal forming systems that conceivably also have organic accumulations, such as carbonaceous, sediment-hosted base metal deposits, and Mississippi Valley-type Pb-Zn-Ag deposits. Such a pathway is of most interest to the study of ore genesis because it expands and redefines our knowledge, in a way that it opens our eyes to the possibility that the very coolest early stages of sulfide formation in these types of mineral deposits may in some circumstances have developed by a stepwise progression, the effects of which would be annealed at higher temperature. This study also provides comprehensive mineral chemical and phase characterisation of known minerals as well as nonstoichiometric phases in the peat that could aid in establishing an effective means of extracting metals from peat, and probably from other organic-rich mineral deposits. Furthermore, this research demonstrates Grieves Siding peat is a natural and modern analogue for sequestering metals from metalliferous surface and groundwaters, and therefore, raises the possibility that peat domains should be further investigated as a strategy for remediation of metal loads in acid mine drainage waters, particularly in western Tasmania and other wet temperate mining districts.