Mineralium Deposita (v.50, #2)
Lithologically controlled invisible gold, Yukon, Canada by Doug MacKenzie; Dave Craw; Craig Finnigan (141-157).
The newly discovered Cretaceous Coffee orogenic gold deposit (>4 Moz resource) consists of an extensive oxidised zone developed on primary sulphidic rock. The primary mineralised rock is characterised by invisible gold in arsenian pyrite that has replaced biotite in selected host rocks. The deposit has a cryptic surface expression and is an example of an extremely subtle exploration target. Hydrothermal emplacement was controlled by extensional fractures, with breccias, but most mineralisation was focused on biotite-bearing granitic gneiss, metasedimentary gneisses, and younger biotite granite. Fine-grained (<0.1 mm) arsenian pyrite replaced biotite along mineral cleavage planes and followed biotite-rich metamorphic and post-metamorphic structural fabrics. Arsenian pyrite also formed overgrowths on earlier coarse-grained (up to 2 mm) barren hydrothermal pyrite. Arsenian pyrite is concentrically zoned on the 1–10-μm scale with respect to As, Sb, and Au contents and typically contains ∼5 wt% As, ∼500 mg/kg Sb, and ∼500 mg/kg Au, in solid solution. Biotite replacement was accompanied by sericitisation, silicification, and ankerite impregnation. Hydrothermal alteration involved dilution and localised depletion of K, Na, and Al in silicified host rocks, but most Ca, Mg, and Fe concentrations remained broadly constant. Magnesium-rich ultramafic host rocks were only weakly mineralised with auriferous arsenian pyrite and have fuchsite and magnesite alteration. Near-surface oxidation has liberated nanoparticulate and microparticulate supergene gold, which remains essentially invisible. Varying degrees of oxidation extend as deep as 250 m below the present subdued topographic surface, well beyond the present vadose zone, and this deep oxidation may have occurred during post-mineralisation uplift and erosion in the Cretaceous. Oxidation has leached some As from the surficial mineralised rocks, decreasing the geochemical signal, which is also obscured by the localised presence of high background As (up to 100 mg/kg) in metasedimentary quartzites in the region. Antimony provides more reliable soil anomalies than As, but most Sb anomalies are <100 mg/kg. The persistence of invisible gold into the extensive supergene zone, with little gold particle size enhancement, has ensured that no placer deposits have formed in nearby streams, further restricting the surface footprint and Au dispersal halo of this subtle exploration target.
Keywords: Gold; Arsenian pyrite; Arsenic; Nanoparticulate; Microparticulate; Supergene
Enrichment of U–Se–Mo–Re–V in coals preserved within marine carbonate successions: geochemical and mineralogical data from the Late Permian Guiding Coalfield, Guizhou, China by Shifeng Dai; Vladimir V. Seredin; Colin R. Ward; James C. Hower; Yunwei Xing; Weiguo Zhang; Weijiao Song; Peipei Wang (159-186).
We present multi-element data on the super-high-organic-sulfur (SHOS; 5.19 % on average) coals of Late Permian age from Guiding, in Guizhou Province, China. The coals, formed on restricted carbonate platforms, are all highly enriched in S, U, Se, Mo, Re, V, and Cr, and, to a lesser extent, Ni and Cd. Although the Guiding coals were subjected to seawater influence, boron is very low and mainly occurs in tourmaline and mixed-layer illite/smectite. Uranium, Mo, and V in the coal are mainly associated with the organic matter. In addition, a small proportion of the U occurs in coffinite and brannerite. The major carrier of Se is pyrite rather than marcasite. Rhenium probably occurs in secondary sulfate and carbonate minerals. The U-bearing coal deposits have the following characteristics: the formation age is limited to Late Permian; concentrations of sulfur and rare metals (U, Se, Mo, Re, V, and in some cases, rare earth elements and Y) are highly elevated; the U-bearing coal beds are intercalated with marine carbonate rocks; organic sulfur and rare metals are uniformly distributed within the coal seams; and the combustion products (e.g., fly and bottom ash) derived from the coal deposits may have potential economic significance for rare metals: U, Se, Mo, Re, V, rare earth elements, and Y.
Keywords: Super-high-organic-sulfur coal; U–Se–Mo–Re–V enrichment model; Late Permian coals; Marine carbonate succession; Hydrothermal fluids
Geology, mineralization, and fluid inclusion characteristics of the Kumbel oxidized W–Cu–Mo skarn and Au–W stockwork deposit in Kyrgyzstan, Tien Shan by Serguei G. Soloviev (187-220).
The Kumbel deposit is located within a metallogenic belt of W–Mo, Cu–Mo, Au–W, and Au deposits along the Late Paleozoic active continental margin of Tien Shan. The deposit is related to a Late Carboniferous multiphase pluton, with successive intrusive phases from early olivine monzogabbro through monzonite–quartz monzonite to granodiorite and granite, with the latest monzogabbro–porphyry dikes. The deposit represents an example of a complex W–Cu–Mo–Au magmatic–hydrothermal system related to magnetite-series high-K calc–alkaline to shoshonitic igneous suite. It contains large bodies of W–Cu–Mo oxidized prograde and retrograde skarns, with abundant andradite garnet, magnetite, and especially hematite, as well as K-feldspar, molybdoscheelite, chalcopyrite, and molybdenite, with transitions to zones of intense quartz–K-feldspar (with minor andradite and hematite) veining. The skarns are cut by quartz–carbonate ± adularia ± sericite veins (locally sheeted) and stockworks bearing scheelite and minor Cu, Zn, Pb sulfides, as well as Au, Bi, Te, and As mineralization. The association of these veins with the oxidized skarns and magnetite-series intrusion is consistent with the general oxidized, intrusion-related W–Mo–Cu–Au type of deposit, with an affinity to the alkalic (silica-saturated) Cu–Au ± Mo porphyry deposits. The fluid inclusion data show the predominance of magmatic–hydrothermal aqueous chloride fluid during the formation of skarns and quartz–carbonate–scheelite–sulfide veins. The high fluid pressures (∼1,750 bars), together with their high temperature (up to 600 °C) and high salinity (∼50–60 wt% NaCl-equiv.), suggest the formation of skarns and quartz–K-feldspar–andradite–hematite veins under conditions typical of magmatic–hydrothermal transition (depth of ≥4–5 km) of intrusion-related mineralized system, possibly by exsolution of the fluids from crystallizing magma. The auriferous quartz–carbonate–scheelite–sulfide veins formed from high to moderate salinity (from ∼40 to <20 wt% NaCl-equiv.) and high pressure (from ∼1,200 bars to 850–900 bars) aqueous chloride fluids under decreasing temperature (from ∼370 to 120 °C). The massive deposition of molybdoscheelite in retrograde skarn and scheelite in quartz–carbonate–scheelite–sulfide veins could correspond to enrichment of fluids in Ca (up to 18–25 wt% CaCl2), likely from crystallizing magma.
Keywords: Oxidized skarn; Tungsten; Gold; Copper; Fluid inclusions; Tien Shan; Central Asia
Structure, alteration, and geochemistry of the Charlotte quartz vein stockwork, Mt Charlotte gold mine, Kalgoorlie, Australia: time constraints, down-plunge zonation, and fluid source by Andreas G. Mueller (221-244).
The Kalgoorlie district in the Archean Yilgarn Craton, Western Australia, comprises two world-class gold deposits: Mt Charlotte (144 t Au produced to 2013) in the northwest and the Golden Mile (1,670 t Au) in the southeast. Both occur in a folded greenschist-facies gabbro sill adjacent to the Golden Mile Fault (D2) in propylitic alteration associated with porphyry dikes. At Mt Charlotte, a shear array of fault-fill veins within the Golden Mile Fault indicates sinistral strike-slip during Golden Mile-type pyrite–telluride mineralization. The pipe-shaped Charlotte quartz vein stockwork, mined in bulk more than 1 km down plunge, is separated in time by barren D3 thrusts from Golden Mile mineralization and alteration, and occurs between two dextral strike-slip faults (D4). Movement on these faults generated an organized network of extension and shear fractures opened during the subsequent infiltration of high-pressure H2S-rich fluid at 2,655 ± 13 Ma (U–Pb xenotime). Gold was deposited during wall rock sulphidation in overlapping vein selvages zoned from deep albite–pyrrhotite (3 g/t Au) to upper muscovite–pyrite assemblages (5 g/t Au bulk grade). Chlorite and fluid inclusion thermometry indicate that this kilometre-scale zonation is due to fluid cooling from 410–440 °C at the base to 350–360 °C at the top of the orebody, while the greenstone terrane remained at 250 °C ambient temperature and at 300 MPa lithostatic pressure. The opened fractures filled with barren quartz and scheelite during the retrograde stage (300 °C) of the hydrothermal event. During fracture sealing, fluid flux was periodically restricted at the lower D3 thrust. Cycles of high and low up-flow, represented by juvenile H2O–CO2 and evolved H2O–CO2–CH4 fluid, respectively, are recorded by the REE and Sr isotope compositions of scheelite oscillatory zones. The temperature gradient measured in the vein stockwork points to a hot (>600 °C) fluid source 2–4 km below the mine workings, and several kilometres above the base of the greenstone belt. Mass balance calculations involving bulk ore indicate enrichment of both felsic (K, Rb, Cs, Li, Ba, W) and mafic elements (Ca, Sr, Mg, Ni, V, Cr, Te), a source signature compatible with the local high-Mg porphyry suite but not with the meta-gabbro host rock. The initial 87Sr/86Sr ratios of the vein scheelites (0.7014–0.7016) are higher than the mantle ratio of the meta-gabbro (0.7009–0.7011) and overlap those of high-Mg monzodiorite intrusions (0.7016–0.7018) emplaced along the Golden Mile Fault at 2,662 ± 6 Ma to 2,658 ± 3 Ma.
Keywords: Kalgoorlie; Mt Charlotte; Gold; Ore zonation; Porphyry
The late Oligocene Cevizlidere Cu-Au-Mo deposit, Tunceli Province, eastern Turkey by Ali İmer; Jeremy P. Richards; Robert A. Creaser; Terry L. Spell (245-263).
The Cevizlidere deposit, located in the Tunceli Province of eastern Anatolia, is the largest porphyry Cu-Au-Mo system in Turkey. The deposit is spatially related to a composite stock, which was emplaced into Paleozoic limestones and Paleogene andesitic rocks to the southeast of the Munzur mountains, near the southwestern margin of the Ovacık pull-apart basin. The host plutonic rocks at Cevizlidere are porphyritic, medium-K calc-alkaline diorites and granodiorites. 40Ar/39Ar incremental step-heating analysis of two igneous biotite separates obtained from syn-mineral diorite porphyry yielded late Oligocene cooling ages of 25.49 ± 0.10 and 25.10 ± 0.14 Ma, whereas hydrothermal biotite yielded an age of 24.73 ± 0.08 Ma. Re-Os ages obtained from two molybdenite separates (24.90 ± 0.10 and 24.78 ± 0.10 Ma) indicate that porphyry-style alteration and mineralization developed shortly after magma emplacement. The whole-rock geochemical composition of the Cevizlidere porphyry intrusions is consistent with derivation from partial melting of the metasomatized supra-subduction zone mantle. However, based on regional tectonic reconstructions, Oligocene magmatic activity in this area appears to be related to a major kinematic reorganization that took place at around 25 Ma, during the switch from subduction to collisional tectonics in eastern Anatolia. This kinematic switch may be attributed to break-off of the Southern Neo-Tethys oceanic slab prior to the Arabia–Eurasia continent-continent collision (~12–10 Ma) following widespread middle Eocene (50–43 Ma) arc/back-arc magmatism. In this respect, the subduction-related tectonic setting of the late Oligocene Cevizlidere porphyry deposit is similar to that of the middle Eocene Çöpler epithermal Au deposit. The late timing of Cevizlidere with respect to the Southern Neo-Tethys subduction may be comparable to some early to late Miocene porphyry-epithermal systems that lie within the contiguous Urumieh-Dokhtar belt in central Iran. The later timing in Iran reflects the diachronous nature of the Eurasia-Afro-Arabia collision.
Keywords: Porphyry Cu-Au-Mo deposit; Tauride-Anatolide Block; Eastern Anatolia; Turkey; Late Oligocene; Pre-collision; Neo-Tethys