Abstract From integrated textural and compositional studies of auriferous and barren pyrite/marcasite in the epithermal Axi gold deposit, China, we have identified a relationship between multiple gold mineralizing events, mafic magma recharge, and fluid-rock reactions. Three generations of pyrite (Py1–3) and four generations of marcasite (Mar1–4) record episodic gold mineralizing events, followed by silver-copper-lead-zinc-cadmium enrichment. The gold mineralizing events are recorded by high concentrations of subnanometer-sized gold in Py1, Py3, and Mar3 (max. = 147, 129, and 34 ppm, med. = 39, 34, and 12 ppm). Based on previous Re-Os age determinations of pyrite and U-Pb zircon ages of the andesitic wallrock, these gold events slightly postdate pulsed mafic magma recharge and represent the incursion of Au-As-S-rich magmatic volatiles into circulating meteoric water. Silver-Cu-Pb-Zn-Cd enrichment in Py2, Mar2, and Mar4 are consistent with quiescent degassing and gradual Ag-Cu-Pb-Zn-Cd enrichment in an evolved felsic magma. Barren Mar1 records the dominance of meteoric water and a limited magmatic fluid contribution. High-Co-Ni-V-Cr-Ti contents in porous cores of Py1 and Mar2 are attributed to wall rock alteration and dissolution-reprecipitation. The results provide convincing evidence that the metal budget (especially for Au, Ag, Cu, Pb, Zn, Sb) of the hydrothermal fluids and sulfides in epithermal systems are controlled by the influx of magmatic fluids and associated magma, whereas the enrichment of certain fluid-immobile elements, such as Co, Ni, V, Cr, and Ti, is caused in part by fluid-rock interaction.
Western Tianshan along the southwestern Central Asian Orogenic Belt is well-endowed with epithermal and porphyry Au-Cu deposits. The Kuruer Cu-Au deposit, recently discovered in the eastern part of Western Tianshan, represents a transition from a high-sulfidation epithermal to a porphyry system. In this contribution, we explored the capacity of chalcopyrite to record direct mineralization time and fluids evolution in the Kuruer deposit, by presenting Re-Os isotopic data and the concentration range of 16 trace elements obtained through LA-ICP-MS. The chalcopyrite Re-Os isochron age of 256 ± 12 Ma (MSWD = 2.6) is comparable within error with the U–Pb age of the ore-causative albite porphyry which demonstrates a Permian porphyry-epithermal system instead of the previously proposed Carboniferous epithermal mineralization. The initial 187Os/188Os ratio of 0.81 ± 0.17 for chalcopyrite implies mixing between crust and mantle. Both pyrite and chalcopyrite can host trace elements in considerable amounts, where pyrite primarily hosted Co, As, Au, V, and Bi, but chalcopyrite concentrated more Zn, Cd, In, Sn, Sb, Mo. The study also highlighted systematic variation of trace element concentrations and Cd/Zn ratios in chalcopyrite and pyrite that can be powerful indicators of fluid phase separation and fluid mixing with meteoric water.
The Aktyuz Terrane in Kyrgyz North Tianshan is of particular interest due to the occurrence of high and ultrahigh pressure (HP–UHP) rocks and it containing the third largest gold deposit in Kyrgyz North Tianshan, i.e., Taldybulak Levoberezhny (abbreviated to Taldybulak Lev.). To constrain the ages of the host Kemin Complex and its auriferous monzogranite porphyry, detailed zircon U–Pb dating [by laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS) and secondary ion mass spectroscopy (SIMS)] and Lu–Hf isotopic analyses were carried out. The intensively altered auriferous monzogranite porphyry yielded two weighted mean ages of 444 ± 3 Ma ( n = 14, mean squared weighted deviation (MSWD) = 0.49, by LA-ICPMS) and 440 ± 5 Ma ( n = 8, MSWD = 0.82, by SIMS) that are indistinguishable within error ranges. Such ages are consistent with a previously reported sulfide Re–Os isochron age of 434 ± 18 Ma, supporting a Silurian porphyry gold mineralization. The granitic gneiss yielded a protolith age of 773 ± 7 Ma ( n = 7, MSWD = 0.04) and two metamorphic ages of 514 ± 4 Ma ( n = 8, MSWD = 0.09) and 483 ± 3 Ma ( n = 11, MSWD = 0.04). Detrital zircons from one fuchsite schist sample yielded highly variable ages from 729 ± 13 Ma to 2,463 ± 30 Ma, with 12 data points weighted at 740 ± 5 Ma (MSWD = 0.95). The metamorphic overgrowth yielded a weighted mean age of 460 ± 4 Ma ( n = 4, MSWD = 0.15). Detrital zircons in the migmatitic amphibolite are aged from 788 ± 7 Ma to 3,447 ± 32 Ma, with two major concentrations at 941 ± 7 Ma ( n = 13, MSWD = 0.95) and 794 ± 5 Ma ( n = 8, MSWD = 0.19). The metamorphic overgrowth yielded an average age of 555 ± 4 Ma ( n = 8, MSWD = 0.65). The detrital and xenocryst zircons, and evolved εHf(t) values (−20.9 to −7.8) and old two-stage Hf model ages (1,367–3,159 Ma), revealed the presence of a Precambrian basement that may be dated back to the Archean Eon. The two metamorphic ages may correlate with oceanic subduction and continental collision, respectively.
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