Fredrik Sahlström, Zhaoshan Chang, Paul Dirks, Antonio Arribas, Isaac Corral GSQ seminar Townsville, 7 December 2017 What is Mt Carlton? An Early Permian high sulfidation epithermal (HS) deposit in NE Queensland, Australia Discovered in 2006 Estimated resource: 10.42 Mt averaging 2.92 g/t Au, 30.60 g/t Ag and 0.47 % Cu A medium grade, unoxidized deposit Mined since 2013 by Evolution Mining in an open pit operation
Regional setting Mt Carlton occurs in the northern segment of the Bowen Basin The Bowen Basin is a NNWtrending sedimentary basin formed as part of the New England Orogen during the Early Permian to Middle Triassic Complex and polyphase evolution backarc extension, thermal relaxation and foreland basin phases A variety of epithermal and porphyry deposits and prospects that are hosted in an Early Permian backarc rift sequence (chiefly the Lizzie Creek Volcanic Group) Stratigraphy A granite basement (297 294 Ma) is overlain by a sequence of volcanic and sedimentary rocks belonging to the Lizzie Creek Volcanic Group (288 275 Ma) Mineralization at Mt Carlton is hosted partly the massive porphyritic rhyodacite (Unit 3), and partly in the overlying rhyodacite tuffs and interbedded lacustrine sediments (Unit 4A)
Deformation sequence D 1 Hydrothermal alteration and epithermal mineralization occurred during a stage of rifting, in response to both E W and N S extension (D 1 ) Mineralization occurred partly contemporaneously with the deposition of volcanic sediments into local graben and half graben basins Ar Ar dating of hydrothermal alunite yielded an age range of 284 277 Ma, which links the formation of Mt Carlton to the Early Permian backarc rifting stage in the Bowen Basin Deformation sequence D 1 N S Flowbanded rhyolite (Unit 8) x Andesite dacite volcano sedimentary rocks (Units 5, 6 and 7) Rhyodacite volcano sedimentary rocks (Units 3 and 4) Andesite lava (Unit 2) Basement granite (Unit 1) Porphyry 3 Porphyry 2
Deformation sequence D 2 Continued E W extension produced 1 5 m wide, low angle (and locally layer parallel) fault zones and associated, high angle antithetic normal faults at Mt Carlton (D 2 ) within the pit area, the D 2 low angle faults accommodated a top to the E displacement along a broadly E W axis D 2 low angle faults have truncated the stratigraphy, the hydrothermal alteration halo and the ore zones. Major displacements (e.g., on the order of kilometers) are limited to younger stratigraphic units (Unit 6 and above) E W Deformation sequence D 3 and D 4 D 3 High angle normal faulting in response to N S extension, and partial re activation of D 1 and D 2 faults the overall sense of movement related to D 3 faults is probably S down, resulting in an overall shallow, southerly tilt of the layering D 4 Large scale block rotation of kilometer scale lithological domains across steep NNWtrending normal faults and ENE trending cross faults
Deformation sequence D 5,D 6 and D 7 D 5 Emplacement of basaltic dykes along high angle D 1, D 3 and D 4 faults, and to a lesser degree, along D 2 faults D 6 Predominantly dextral strike slip faulting along the margins of D 5 dykes D 7 Emplacement of WNWtrending basaltic dykes E trending D 5 basaltic dyke in the V2 pit Pit geology The stratigraphy in the open pits is separated into a number of tectono sedimentary domains, bounded by D 2 layer parallel faults A NNW trending D 4 normal fault cuts across between the open pits Bedding planes in the V2 pit have remained near horizontal after the D 4 event In contrast, bedding planes in the A39 pit have been rotated in a WSW direction by ~ 32
Ore zones Proximal Au Cu mineralization in the V2 pit occurs in steeply dipping fracture networks in Unit 3 mineralized fractures have a predominantly NE to NNE trend Three ore zones (Western, Eastern and Link) are aligned en echelon along an E W corridor Metal zonation along the Western ore zone (proximal to distal): Au Cu Cu+Zn+Pb+Ag Ag+Pb+(Cu) Ag Distal Ag mineralization in the A39 pit occurs in Unit 4A, in a stratiform position parallel to sedimentary layering Long section (parallel to Western ore zone)
Cross section (perpendicular to ore zones) Mineral paragenesis
Interesting alteration features magmatic steam alunite Veins and void fill of banded, plumose alunite of magmatic steam origin occur within the silicic alteration zones at Mt Carlton Forms from very rapidly ascending magmatic vapor columns Magmatic steam alunite has been documented in a limited number of HS deposits worldwide (Rye et al. 2005) At Mt Carlton, magmatic steam alunite is predominantly confined to the high grade feeder structures alu Magmatic steam alunite can, if present, be a good exploration indicator for proximity to potential high grade HS mineralization Mineral paragenesis
Hydrothermal mineralization Stage 2A (Cu Au Ag) enargite + luzonite + pyrite + Tetrahedrite Group minerals + electrum + chalcopyrite + bornite + argyrodite + polybasite + pearceite + Ag 4 TeSe + sphalerite + galena + barite eng py Stage 2B (Zn Pb Au Ag) sphalerite + galena, pyrite, electrum, Tetrahedrite Group minerals, bornite, chalcopyrite and barite sph eng Stage 2C (Cu Au Ag) tennantite + luzonite, chalcopyrite, galena, electrum, hessite and petzite luz el gal tn Mineral paragenesis
Overprint Stage 3 (hydrothermal veins and void fill) massive dickite and pyrite that has overprinted mineralization 1 cm Dickite pyrite crosscutting disseminated enargite Stage 5 (supergene oxidation) secondary copper minerals (malachite, covellite, chalcocite) are locally developed on minerals such as enargite and bornite in the upper ca. 50 m of the deposit 1 cm Malachite overprinting enargite Ore textures V2 pit Proximal Au Cu mineralization in the V2 pit occur in structurally controlled veins and hydrothermal breccias Interpreted as the deeper feeder zones of the Mt Carlton deposit Enargite pyrite cemented hydrothermal breccia from V2 pit
Ore textures A39 pit Syn sedimentary ore textures in the A39 pit are interpreted to have formed in a near surface environment, in and beneath small lakes developed within localized rift basins Critical elements (Ge, Ga, In) Association 1: Ga enrichment in Al bearing hydrothermal sulfates and silicates Up to 339 ppm Ga in Stage 1 alunite Up to 150 ppm Ga in Stage 3 dickite Association 2: Ge ±In Ga enrichment in Stage 2A enargite ores argyrodite (Ag 8 GeS 6 ; up to 6.95 wt% Ge) up to 2189 ppm Ge in enargite up to 143 ppm Ge, 1181 ppm Ga and 571 ppm In in sphalerite Ge Association 3: In Ga ±Ge enrichment in Stage 2B sphalerite ores Zn In mineral [CuZn 2 InS 4 ;up to 19.6 wt% In and 1.5 wt% Ga) up to 611 ppm Ge, 2839 ppm Ga and 2169 ppm In in sphalerite Sp
Alunite chemistry Alunite at Mt Carlton shows K dominated compositions, indicating relatively low formation temperatures Suggests that Mt Carlton formed in a distal position relative to its causative intrusion The Na content is increasing to the NE, indicating that formation temperatures were higher in the V2 pit compared to the A39 pit Sulfur isotopes Sulfur isotopes in alunitepyrite pairs indicate a bulk δ 34 Svalue of 1.3 (i.e. magmatic source) The fluid was oxidized (H 2 S/SO 4 = 0.18) Alunite pyrite sulfur isotope geothermometry suggest fluid temperatures between 250 130 C
Oxygen and hydrogen isotopes Calculated isotopic compositions of fluids that formed early alunite and late dickite suggest a dominantly (ca. 50 95 %) meteoric component The gradual depletion in δd with time is consistent with continuous degassing of the mineralizing magma The low temperature, oxidized and diluted character of fluids at Mt Carlton is consistent with a near surface genetic environment Implications for exploration Due to high erosion rates in most volcanic arcs, the preservation potential of HS deposits in older terranes is generally poor The study of Mt Carlton highlights that shallow level high sulfidation epithermal mineralization can be preserved in Paleozoic settings, under appropriate geodynamic conditions Extensional segments of volcanic arcs, such as backarc rifts, are particularly prospective for this type of mineralization
Implications for exploration The feeders at Mt Carlton should theoretically extend to the causative intrusion, which may be mineralized with porphyry style Cu Au Mineralization at Mt Carlton occurs in predominantly NE to NNE trending fractures Garcia, 1991 Vectors defined by metal zonation, alunite composition and ore textures within the Mt Carlton deposit indicate that more proximal mineralization should occur to the NE of the current V2 pit however, no extensions of the high grade ore zones to the immediate NE of V2 have been found Implications for exploration The low angle normal faults developed during D 2 should have cut the mineralized feeders at deeper levels the deeper parts of the feeder system and any linked porphyry mineralization should, therefore, be displaced relative to the currently mined Mt Carlton deposit Based on the kinematics of D 2 faults observed in the open pits, this displacement is expected to be to the W of Mt Carlton Mineralization occurs in the deeper parts of the stratigraphy, where D 2 faults are poorly developed, and the amount of such displacement is most likely small (e.g., on the order of tens to hundreds of meters) Additional displacements across NNW trending D 4 normal faults (e.g., on the order of tens to hundreds of meters)
Conclusions The Mt Carlton deposit represents shallow level high sulfidation epithermal mineralization that formed during active rifting, coinciding with the opening of the Bowen Basin in the Early Permian The extensional tectonic setting at Mt Carlton facilitated a rapid burial beneath post mineralization cover, which helped preserving this deposit Post mineralization extensional deformation has displaced the porphyry epithermal system, with important implications for exploration