The Dugald River-type, shear zone hosted, Zn-Pb-Ag mineralisation, Mount Isa Inlier, Australia

The Dugald River Zn-Pb-Ag mine is situated in the Mount Isa Inlier, a globally significant base metal province. Zn-Pb deposits in the Mount Isa Inlier are stratabound with four main genetic models, including SEDEX-style, remobilised SEDEX, epigenetic and Broken Hill-type mineralisation applied to interpret their formation. We propose that the Zn-Pb-Ag mineralisation at Dugald River represents a unique, shear zone hosted deposit type that formed through a series of successive deformation events during the Paleoproterozoic Isan Orogeny that concentrated the mineralisation within the Dugald River Shear Zone during two main mineralising phases. The first phase of mineralisation occurred during regional D2 shortening, which is associated with the formation of large-scale F2 folds and a regionally penetrative S2 fabric. During this phase, progressive tightening of upright F2 folds resulted in several sets of secondary space accommodating quartz-carbonate veins that were progressively rotated into parallelism with the pervasive, steep, W-dipping S2 cleavage. The quartz-carbonate veins were coevally replaced by sulphides, which migrated to extensional sites (boudin necks and fold hinges) in tight folds. Thereby creating a sulphide-rich horizon within a developing high strain zone, which during D4 developed into the Dugald River Shear Zone. The second phase of mineralisation occurred during the regional D4 transpressional deformation event and resulted in significant metal enrichment and the current geometry of the ore bodies. The significant enrichment of the mineralisation during D4 resulted from further fold tightening within the high strain zone, which resulted in the attenuation and dismembering of folds and produced a transposed fabric (S4). The sulphide veins were transposed into parallelism with S4 forming sulphide-rich planar ore textures. Strain partitioning at the contact between the ductile deforming sulphide horizon and the brittle deforming slates resulted in the development of an anastomosing shear zone, known as the Dugald River Shear Zone. A right-handed releasing bend in the shear zone produced a dilational jog and a thick, high-grade ore body. The mobilisation of sulphides within the dilational jog involved fragmentation of sulphides and wall rock, brecciation, rotation and rolling of fragments, and the formation of durchbewegung texture

Source of sulphur and the alteration footprint of the George Fisher Zn-Pb-Ag massive sulphide deposit, Australia

The George Fisher deposit is located in the northern Australian Carpentaria province, which is host to several of the world’s largest Zn and Pb mineral deposits. The annual metal production from George Fisher is crucial in order to meet the global demand for Zn and Pb. The main ore minerals at George Fisher are sphalerite (ZnS) and galena (PbS), which occur in stratabound, and in discordant, massive sulphide ore bodies together with pyrite (FeS2) and pyrrhotite (FeS). These massive sulphide ore bodies are hosted within carbonaceous, pyritic, calcareous, dolomitic siltstones and mudstones of the Paleoproterozoic Urquhart Shale Formation (ca. 1654 Ma), which is also host to two other world class base metal deposits (Mount Isa and Hilton). All three deposits are highly deformed, the textural relationships are complex, and there is a lack of indicator minerals to constrain the metamorphic grade of the Urquhart Shale. As a result, there has been considerable debate over (1) the processes that led to the accumulation of such huge amounts of base metal sulphides in the deposits of the area, and (2) the alteration footprint these processes have produced beyond the massive sulphide zones. In this project, petrographic observations across several scales (drill core logging down to backscatter-electron microscopy) were combined with whole rock, and in situ, mineralogical, geochemical, and isotopic analyses of representative samples from (1) four drill holes that intersected the main ore bodies at George Fisher, and also from (2) a drill hole that intersected the barren Urquhart Shale Formation. These data were collected in order to constrain the background heterogeneity that is inherent to rocks of the Urquhart Shale Formation (e.g., from background diagenetic processes). Using this framework, the nature of some of the hydrothermal processes responsible for ore formation were constrained. Particular emphasis was put on constraining (1) the accumulation processes of reduced sulphur, (2) the ore forming processes, and (3) the mineralogical and geochemical alteration footprint of the George Fisher deposit. In brief, the data from this study suggest that the Urquhart Shale Formation in the location we studied has not undergone regional greenschist metamorphism. The mineralogy and the paleoredox proxies (S-isotope data, Mo concentrations, rare earth elements in carbonate minerals) from the barren Urquhart Shale sequence can, therefore, be interpreted in a sedimentary and diagenetic context; and those data concur with deposition of the Urquhart Shale Formation in a ferruginous, marine environment, which is consistent with the current understanding for the Paleoproterozoic oceans. Using this background composition as base line the petrographic, mineralogical, geochemical, and isotopic data from George Fisher suggest that (1) ore formation occurred in multiple events during diagenesis and later deformation, that (2) reduced sulphur in the deposit was likely derived via thermochemical sulphate reduction and the recycling of sulphur from pre-ore diagenetic pyrite, and that (3) fluid-rock interaction of hot (>200-250 °C), saline (Cl-rich), metal-bearing hydrothermal fluids with the Urquhart Shale Formation led to the dolomitization and replacement of pre-ore carbonate and the precipitation of base metal sulphides. Besides ore formation at George Fisher, these hydrothermal processes have resulted in mineralogical and bulk geochemical changes that include (1) albite, chlorite, and calcite depletion, (2) dolomite, phyllosilicate, and sulphide formation, (3) Na and Sr depletion, and (4) Tl and Mn enrichment relative to the barren host rocks. Furthermore, the fluid-rock interaction has led to light rare earth element (LREE) depletion in hydrothermal and hydrothermally altered carbonate minerals relative to whole rock and pre-ore carbonate LREE compositions. Overall, this project has provided new constraints on background diagenetic and hydrothermal processes, and footprints, in the Urquhart Shale Formation at George Fisher. Moreover, the findings from this study can help to further refine exploration models for Zn-Pb deposits in one of the world’s most important base metal provinces

3D structural controls of the shear zone hosted Dugald River zinc-lead-silver deposit, Mount Isa Inlier, Australia

Pieter Creus undertook a detailed 3D structural geological study of the Dugald River Zn-Pb-Ag deposit. In the study he found that the deposit formed during two successive mineralisation events. The mineralisation model is a new style of shear-zone hosted Zinc mineralisation in the region

Paleomagnetism and Rock Magnetism of SEDEX Zn-Pb-Cu Ores in Black Shales in Australia and Yukon and of Fluorite Veins in Granite in Newfoundland.

The paleomagnetic age dating technique is used to study the genesis of a fluorite and four sedimentary exhalative (SEDEX) deposits which are hard to date by conventional radiometric methods because they lack suitable minerals, which causes controversy about their ore genesis. Paleomagnetic analysis of 359 specimens from the Devonian St. Lawrence Granite and fluorite veins in Newfoundland shows that the granite and fluorite veins are coeval and retain a primary remanence carried by pyrrhotite and/or magnetite. The difference between the measured and expected paleopoles is attributed to postemplacement counterclockwise rotation of ~17░ of the area about a vertical axis during the Late Devonian Acadian Orogeny. Paleomagnetic analysis of 333 specimens from the Century Zn-Pb-Ag deposit in Australia isolates a stable characteristic remanent magnetization (ChRM) for ore sites only that resides mainly in single (SD) or pseudosingle domain (PSD) titanomagnetite and pyrrhotite. A positive fold test shows that the ores ChRM predates D2 deformation in the Mesoproterozoic Isan Orogeny. The paleomagnetic age of 1558▒4 Ma supports a late diagenetic replacement model for ore genesis. Paleomagnetic and rock magnetic analyses of 333 specimens from the Mount Isa Zn-Pb-Cu-Ag and George Fisher Zn-Pb-Ag deposits in Australia isolate a stable ChRM carried mainly by SD or PSD pyrrhotite and/or titanomagnetite. The negative fold test shows that the ChRM postdates the ~1510 Ma D3 deformation in the Isan Orogeny. Thus the ~1505 Ma paleomagnetic age provides a minimum age for the ores and an age for the greenschist metamorphism during the orogeny. Paleomagnetic and rock magnetic analyses of 339 specimens from six mineralized panels in the Howards Pass Zn-Pb deposits in Yukon isolate a stable ChRM that mainly resides in SD or PSD pyrrhotite and titanomagnetite. The paleopole falls on Middle Jurassic portion of the North American apparent polar wander path (APWP) or corrected APWP for the Intermontane terranes, giving ~170 Ma and ~162 Ma, respectively. The results indicate that the Selwyn Basin had undergone ~35░ clockwise rotation with ~10░ northward translation between Middle Jurassic and mid-Cretaceous

Zinc on the edge—isotopic and geophysical evidence that cratonic edges control world-class shale-hosted zinc-lead deposits

The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of these basins

Recognising cryptic alteration surrounding the Mount Isa Copper Deposits: Implications for controls on fluid flow, and mineral exploration

This study investigated cryptic alteration haloes associated with copper mineralisation at Mount Isa, Northwest Queensland, Australia. New insights from hydrothermal alteration were used to constrain aspects of hydrothermal fluid flow and fluid-rock interactions that occurred as part of the mineralising system responsible for forming this world-class orebody. This study demonstrates the utility of several different approaches to identify and map cryptic alteration haloes associated with copper mineralisation at Mount Isa. Bulk geochemical techniques, such as carbon and oxygen stable isotope analysis and four-acid digest ICP-AES/MS analysis of assay pulps represent methods that can be readily applied during exploration to gain representative information about the mineralising system. Portable X-Ray Fluorescence (pXRF) represents a tool that can be used in a more targeted approach during exploration activities. With rigorous quality assurance and control procedures, pXRF can quickly and cost-effectively collect large datasets to identify broad trends across mineralising systems while minimising issues arising from sample heterogeneity at the scale of analysis. Integration of observations from previous studies and exploration data, with newly acquired petrographic and geochemical data collected across a range of scales, identified an extensive zoned alteration system that manifests as a series of interpreted reaction fronts, which extend at least 1500 m beyond mineralisation. Copper mineralisation is contained within a zone of visible mineral alteration, hydrothermal brecciation, and veining, locally known as the ‘silica-dolomite’. The silica-dolomite is characterised by silicification and brecciation of shales, recrystallisation of dolomite, and intense ¹⁸O-depletion, from δ¹⁸O ≈ 22‰ VSMOW in the least altered rocks to δ¹⁸O ≈ 10‰ in the most altered zone. These zones are spatially associated with structural dislocations on the contact between the Eastern Creek Volcanics and overlying metasediments of the Mount Isa Group that host copper mineralisation. These dislocations are interpreted to represent the fluid input zones to the mineralising system. A cryptic halo of K- and Ca-depletion extends from the inferred fluid input zones to include the region outboard of the visible mineral alteration envelope. This element depletion is interpreted to result from chloritisation and talc formation during silicification of white mica- and dolomite-bearing shale. Beyond the region of Ca-depletion and K-depletion, a large halo of cryptic potassic alteration is identified by whole-rock geochemical analysis. Potassium responsible for this alteration is interpreted to have been remobilised from zones of silicification and K-depletion at the core of the hydrothermal system. Although the ultimate sink of calcium mobilised from this core zone of element depletion remains unclear, a spatially extensive network of ore stage quartz-dolomite-calcite-pyrite veins has been documented. The change from dolomite- to calcite-dominated vein cement within individual veins is interpreted to be driven by increases in the relative activity of calcium during the evolution of the hydrothermal fluid as it moved away from the fluid input zone. Consequently, ore-stage veins with mixed dolomite-calcite vein cement potentially represent a distal expression of silicification and decalcification within the core of the mineralising system. The alteration haloes described above, both visible and cryptic, are all contained with the broad zone of ¹⁸O-depletion, representing the most spatially extensive alteration halo to copper mineralisation at Mount Isa. Copper mineralisation and associated hydrothermal alteration at Mount Isa developed due to fluid-rock interaction between ¹⁸O-depleted, low pH, silica-rich, cupriferous hydrothermal brines and variably carbonaceous and pyritic, carbonate-rich metasediments. Fluid flow responsible for developing the zoned hydrothermal alteration system was predominantly upward-directed and focused by structurally controlled permeability pathways. This fluid flow was sufficiently slow and warm enough to allow only moderate oxygen isotope disequilibrium between rock and the infiltrating hydrothermal fluids. In hydrothermal systems where fluids are undersaturated in metals, the size of the ore deposit will be limited by the time-integrated fluid flux. Not only can large hydrothermal systems with high time-integrated fluid fluxes form large ore deposits, but they will also be associated with large alteration haloes with greater distance between reaction fronts than smaller hydrothermal systems. Consequently, the zoned alteration system presented in this study assists in vectoring towards fluid input zones. At the same time, the spatial extent and spacing between reaction fronts provide an indicator of the prospectivity of the mineralising system at relatively early stages of exploration

The mineralisation, alteration paragenesis and hydrothermal fluids at the Geita Hill gold deposits, NW Tanzania

Archean greenstones represent a large percentage of worlds total gold endowment and are actively mined on every continent barring Antarctica. Greenstone-hosted gold deposits often have complex deformation and alteration histories, and a general deposit model remains controversial. The aim of this thesis is to improve our understanding of gold mineralisation in Archean greenstone belts, based on a comprehensive case study of the world-class Geita Hill deposit in Tanzania. Geita Hill is one of the largest gold deposits within the Geita Greenstone Belt in north-western Tanzania and has been mined as an open pit since 2002. The deposit is hosted within a greenschist facies metamorphosed and complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately west dipping deformation zone of discontinuous shear fractures. Detailed structural studies have defined a deformation history for the deposit, providing an opportunity for an in-depth study of the hydrothermal alteration and fluids associated with gold mineralisation. The first component of this thesis builds a paragenetic framework for the Geita Hill deposit. The regional metamorphism is characterised by biotite + chlorite + actinolite + K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies metamorphism. The gold-related alteration overprints the regional metamorphism and is characterised by silicification and sulfidation fronts that end within one meter of the mineralised zone. Locally, the silicification and sulfidation of the wall rock occurs along a series of mineralised quartz veins which have a sub-vertical dip and tend E-W. Paleostress analysis of the mineralised shear fractures of the Geita Hill Shear Zone suggests vertical maximum compressive stress (σ₁) and northerly extensional stress (σ₃) consistent with the orientation of the mineralised quartz veins and indicating N-S extension. The composition of the mineralised quartz veins is characterised by quartz + biotite + K-feldspar + pyrite, which also overprints the metamorphic mineral assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but gold inclusions in biotite and along quartz grain boundaries are also present. The gold-bearing pyrite is associated with secondary biotite and K-feldspar. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite and inclusion free euhedral pyrite. It is common for the inclusion rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multi-phase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite + siderite + chlorite ± quartz ± pyrite ± barite. The thesis then builds on the paragenetic framework through silicate and sulfide geochemistry. Biotite was identified as a primary mineral both in the metamorphic assemblage and gold-related hydrothermal alteration assemblage. The study of silicates was conducted through detailed core logging, petrography, SEM mineral identification of alteration assemblages, SWIR measurements and microprobe analyses of biotite in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite + magnetite) compared to the metamorphic background (pyrrhotite + magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite composition can be detected using short wavelength infrared spectra, though a shift in the Fe-OH 2250 nm absorption feature to lower wavelengths. This shift is also correlated with an increase in gold grade within the mineralised zone. Halogen chemistry of the biotite implied the presence of multiple hydrothermal fluids during mineralisation, suggesting that within the ore zone a metamorphic fluid in equilibrium with the host rock was overprinted by and mixed with an infiltrating fluid that was enriched in fluorine. Study of the sulfides in the deposit was conducted through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of pyrite and magnetite grains. The gold distribution correlates well with Te, Ag, Bi and Pb consistent with its occurrence as micro- and nano-inclusions of gold bearing telluride minerals. As, Co and Ni distribution in pyrite grains hosted in quartz veins is patchy, whereas in pyrite grains from ironstone and diorite these elements show zoning characteristic of growth pattern. Elements including As, Ni, Co, Cu and Zn appear to be dominantly locally derived, and remobilised into the pyrite during sulfidation. The concentrations of these elements are highly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Sn, Bi and Pb appear to be dominantly externally derived, and closely correlate in all varieties of pyrite. The Se content is typical of pyrite from Archean gold deposits (~30ppm) and reflects to an average temperature of ~340°C for the mineralising fluid on the basis of temperature dependent incorporation into pyrite. Lastly, the gold-bearing hydrothermal fluids were studied directly through detailed microthermometry and raman microspectroscopy, and three principle fluids were identified: (1) A low salinity, carbonic-rich (XCO₂ > 0.8) fluid with minor N₂ (XN₂/(XCO₂ + XN₂) 20 wt.%; NaCl/(NaCl+CaCl₂) mass ratio > 0.45), aqueous brine that was interpreted to be magmatic in origin; and (3) A low salinity (NaCl < 5 wt.%) aqueous fluid that was interpreted to be meteoric in origin. Preserved fluid assemblages imply mineralisation occurred at pressures of less than 2 kbar, likely from 1.4 to 1.7 kbar, at temperatures of approximately 350 °C. C-O-H fluid modelling of the carbonicrich fluid has constrained ƒO₂ fluid to 1.5-1.8 log10 units above ƒO₂ FMQ corresponding to absolute values of 10- 30.5 bar. The gold was likely transported in the high salinity brine as Au-bisulfide complexes with tellurium, potentially introduced as a vapour. Deposition of Au was triggered via interaction of gold-bearing fluids with the relatively reduced Fe-rich host rocks and the low salinity CO₂-rich fluid

Vent-proximal sub-seafloor replacement clastic-carbonate hosted SEDEX-type mineralization in the Mehdiabad world-class Zn-Pb-Ba-(Cu-Ag) deposit, southern Yazd Basin, Iran

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The Mehdiabad Zn-Pb-Ba (Cu-Ag) deposit in the southern Yazd Basin, central Iran, is the largest sediment-hosted Zn-Pb deposit in Iran. This deposit is hosted by organic carbon matter-rich shale, fine-grained black siltstone, and dolomite interlayered with sandstone of the Taft Formation. Sedimentological and geochemical studies of the Taft Formation have shown that these organic carbon matter-rich shales formed during a period of basin deepening and under anoxic conditions. Based on the orebody structure, mineralogy, and ore fabrics, we recognize five different ore facies types in the Mehdiabad deposit: (1) a stockwork/feeder zone, consisting of a discordant sulfide mineralization, forming a stockwork of sulfide-bearing dolomite veins, cutting the sedimentary rocks of the footwall; (2) massive-replacement ore, including pervasive replacement carbonate by pyrite, chalcopyrite, galena, and sphalerite with minor barite; (3) a bedded ore, with laminated to disseminated pyrite, sphalerite, and galena; (4) barite ore, which contains accessory sulfide minerals and rare calcite at the top of the deposit, and (5) a distal facies, with minor disseminated and laminated pyrite, banded cherts, and disseminated barite. Two stages of base metal sulfide replacement have separate origins. Fine-grained sulfide bands (stage I), intricately interlayered with organic carbon-rich beds and thin turbidite beds, exhibit lamina and bedding textures, supporting a syn-sedimentary (onto the sea floor) origin. Coarse-grained base metal sulfides (stage II), occurring within breccias and veins to veinlets are considered to have formed by replacement during post sedimentation sub-seafloor fluid flow. d34S values of pyrite, sphalerite chalcopyrite and galena range from -22 to +4.6.8‰. The highest d34S values correspond to the feeder zone (+4.6 and -10.5‰), whereas massive-replacement (+3.7 to -13.7‰) and bedded (-22 to -17‰) ore facies display depleted compositions. The overall range of d34S values is remarkably higher than typical magmatic values, suggesting that sulfides formed from the reduction of seawater sulfate by bacteriogenic sulfate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulfate reduction likely played an important role in the feeder zone. The formation of the Mehdiabad deposit follows the evolution of the sedimentary basin. Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccias and debris flows within the ore sequence, suggest the proximity to synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable for the formation of the ore deposit. The deposit formed from a combination of hydrothermal and syn-sedimentary processes. Sulfur isotopes, together with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal sub-seafloor replacement SEDEX ore deposit.Peer ReviewedPostprint (author's final draft