Genesis of the E1 group of iron oxide-copper-gold deposits, Cloncurry district, North West Queensland

Case, George N.D. (2016) Genesis of the E1 group of iron oxide-copper-gold deposits, Cloncurry district, North West Queensland. PhD thesis, James Cook University.

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Abstract

The E1 Group of iron oxide-copper-gold (IOCG) deposits is located in the metal-rich Cloncurry District of northwest Queensland. The E1 Group contains a total resource of 47 Mt averaging 0.72% Cu and 0.21 g/t Au, and has not been previously investigated in detail. This study aims to understand the genesis of the E1 Group by characterising its geology, alteration paragenesis, ore chemistry, structural controls, and mineralising fluid properties. These features are investigated using drill core logging, petrography, whole-rock geochemistry, microprobe analysis, LA-ICP-MS U-Pb dating, 3-D implicit geological modeling, fluid inclusion studies, and SHRIMP and IRMS oxygen and sulfur stable isotope analyses.

The E1 Group comprises three distinct orebodies: E1 North, E1 East and E1 South. The orebodies are hosted mainly in marble and carbonaceous metasiltstone of the Corella Formation (1750–1720 Ma). The metasedimentary rocks are intercalated with mineralised clastic metavolcanic rocks and barren meta-andesite of the Mount Fort Constantine Volcanics (~1750 Ma). These rocks were intruded by Ernest Henry Diorite, cut across by a discordant, polymictic, breccia, and then intruded by dolerite.

Drill core logging, petrography and Mineral Liberation Analysis were used to study the E1 Group alteration textures and styles. They show that E1 Group mineralisation is typified mainly by fine- to medium-grained (<500 μm) stratabound and shear zonehosted replacement bodies. The ores are typically layer-controlled; some ores are also in veins. The discordant breccia is barren, and pre-dates mineralisation.

The alteration paragenesis was constrained with drill core logging, petrography, and Energy-Dispersive and Wavelength-Dispersive analyses. The E1 Group paragenetic sequence is characterised by three major stages. Stage 1 is dominated by albite (- hematite), with lesser quartz, actinolite, scapolite, and titanite. The second stage is broken into three sub-stages. Stage 2a is dominated by magnetite, fluorophlogopite and fluorannite, fluorapatite, K (-Ba)-feldspar and lesser quartz and pyrite. Stage 2b is a minor phase of albite (-hematite)-rutile-ilmenite alteration. Stage 2c is composed of ankeritic carbonate, magnetite, pyrite, and minor chalcopyrite. Stage 3 is the main mineralising event, and is dominated by carbonate (-Fe-Mn), chalcopyrite, barite, fluorite, pyrite, chlorite, sericite; trace amounts of monazite, bastnäsite, uraninite and coffinite are also present.

Whole-rock geochemical analysis indicates that the ores are highly enriched in Fe, Ba, F, P, and locally Mn, and are less enriched in U, LREE, Co, Mo, As, Sn, Ag while depleted in Si, Na and K. Delineation of the deposit zonation patterns shows a transition from the E1 North orebody into a barren magnetite-apatite ± pyrite zone to the southwest. Barium and fluorine are elevated over 200 m from mineralisation.

The relatively new technique of three-dimensional implicit geological and geochemical modeling was used to study the structural history and controls of the E1 Group. The deposit is hosted within a series of northwest-plunging folds that formed during regional D2 deformation event and peak metamorphism. The E1 North and E1 South orebodies are hosted in the hinges of the E1 North Antiform and E1 South Synform, respectively, while E1 East occurs in the limb of the E1 East Antiform. The E1 North Antiform is cut by the northeast-southwest-trending brittle-ductile E1 North Shear Zone that dips ~70° northwest. The shear zone is an R-shear of a dextral Riedel structure caused by transpressional movement on the regional Mount Margaret Fault during local D3 / regional D4. Implicit geochemical modeling suggests that the spatial distributions of Cu, Au, Fe, U, Co, Mo, and La are controlled by the fold hinges and E1 North Shear Zone, and the highest-grade orebody occurs at their intersection. Drill core and petrographic observations indicate that ore formation took place around local D3 / regional D4. A later local D4 / regional D5 event caused brittle reactivation of the E1 North Shear Zone and formed northeast-southwest-trending reverse-oblique faults at E1 South that offset mineralisation.

Fluid inclusion analyses were conducted on Stage 2a quartz and Stage 3 barite to determine the composition of the mineralising fluids. Stage 2a quartz hosts a primary fluid inclusion assemblage, 1A, that is characterised by halite-rich, aqueous liquid-solidvapour fluid inclusions with >50 wt% NaCl(eq); the assemblage was heterogeneously trapped. Stage 3 barite hosts two major fluid inclusion assemblages (2A and 2B). Assemblage 2A comprises primary, moderate to low salinity (<15 wt% NaCl(eq)) aqueous liquid-vapour, inclusions that homogenise between 160° and 190°C. Assemblage 2B is composed of secondary, moderately saline (<9 wt% NaCl; <18 wt% CaCl₂), liquidvapour inclusions.

In order to constrain fluid and metal sources and precipitation mechanisms, the δ¹⁸O(VMSOW) values of Stage 2a quartz-magnetite pairs, along with the δ³⁴S(CDT) values of and Stage 3 barite-chalcopyrite pairs, were studied; Stage 2 pyrite was also measured. For fine-grained ores, the in-situ Sensitive High Resolution Ion Microprobe method was used, while conventional ex-situ Isotope Ratio Mass Spectrometry was used for vein samples. Stage 2 quartz δ¹⁸O values at E1 North have a narrow range of +12.7 to +14.8‰. In contrast, magnetite δ¹⁸O values are characterised by a wider range from 0 to +8‰. Calculated isotopic equilibrium temperatures from quartz and magnetite range from 350° to 540°C, and the calculated δ¹⁸O range of the fluid at these temperatures is +8.4 to +10.9‰.

Stage 3 chalcopyrite δ³⁴S values are distinct between E1 North (–5.8 to +2.7‰), E1 South (+1.7 to +17.3‰), and E1 East (+12.5 to +16.9‰). Stage 3 barite δ³⁴S values vary from +6.7 to +21.2‰ in E1 North, +19.1 to +29.5‰ in E1 South, and +5.6‰ to +26.5‰ in E1 East. Vein-hosted barite δ³⁴S values are 5 to 10‰ lower than those in fine-grained samples. Stage 2 pyrite is typically 1 to 2‰ higher than Stage 3 chalcopyrite. Calculated equilibrium temperatures for Stage 3 barite and chalcopyrite in fine-grained samples range from 230° to 340°C; vein-hosted samples did not reach equilibrium. Estimated trapping pressures of barite 2A fluid inclusions, based on these temperatures, range from 2.2 to 3.3 (±0.5) kbar. This corresponds to a depth range of 8– 12 km. The estimated values of δ³⁴S(Σ)ₛ range from +4.9 ± 5.3‰ at E1 North, to +15.9 ± 3.6‰ at E1 South.

The δ¹⁸O(fluid) and δ³⁴S(Σ)ₛ values at E1 North, coupled with the high salinity of 1A fluid inclusions, are consistent with those from a magmatic-hydrothermal fluid. The F-UREE enrichment of the paragenesis suggests that the magma was an evolved, alkaline, granite. It is speculated that this granite was related to the (1550–1490 Ma) Williams- Naraku Batholith; it may have supplied some of the Cu and Au. The shifts in the values δ³⁴S(mineral) and δ³⁴S(Σ)ₛ at E1 South can be explained by mixing of the magmatic fluid with a shallower fluid that had equilibrated with the Corella Formation host rocks. Covariance of the δ³⁴S of barite and chalcopyrite between E1 North and South suggests that both fluids supplied SO²⁻₄. Ore precipitation was likely caused by salinity decrease as a result of fluid-fluid mixing. Dilation in the E1 North Shear Zone and fold hinges during local D3 / regional D4 provided the main conduits for mixing of the mineralising fluids.

Item ID: 49998
Item Type: Thesis (PhD)
Keywords: Cloncurry district, drill core logging, E1 group, IOCG, iron-oxide-copper-gold deposits, mineral exploration, mineral liberation analysis, mineralizing fluids, ore deposit petrology
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Case, George, Blenkinsop, Thomas, Chang, Zhaoshan, Huizenga, Jan Marten, Lilly, Richard, and McLellan, John (2017) Delineating the structural controls on the genesis of iron oxide–Cu–Au deposits through implicit modelling: a case study from the E1 Group, Cloncurry District, Australia. Geological Society, London, Special Publication, 453. (In Press).

Date Deposited: 31 Aug 2017 04:03
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040307 Ore Deposit Petrology @ 50%
04 EARTH SCIENCES > 0402 Geochemistry > 040299 Geochemistry not elsewhere classified @ 50%
SEO Codes: 84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840199 Mineral Exploration not elsewhere classified @ 100%
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