Mineralogy Of The Zijinshan Porphyry-epithermal System,Fujian Province

The Zijinshan orefield is a Cu-Au-Mo-Ag porphyry-epithermal mineralization system, comprising the Zijinshan high-sulfidation (HS) Au-Cu deposit, the Luoboling porphyry Cu-Mo deposit, the Yueyang low-sulfidation (LS) Au-Ag-Cu deposit, and the Wuziqilong and the Longjiangting intermediate-sulfidation (IS) Cu deposit. A number of ore minerals occur in the Zijinshan orefield, including principal copper minerals of digenite, covellite, bornite, and chalcopyrite; Au-Ag containing minerals of calaverite, hessite; Sn-containing minerals of kiddcreekite, hemusite, colusite, vinciennite etc; fahlores of tennantite, tetrahedrite, goldfieldite minerals; Bi, Pb-containing sulfosalts of wittichite, emplectite and miharaite. Of all, hemusite, vinciennite, colusite, emplectite, miharaite are first found in Zijinshan orefield, especially, hemusite and vinciennite are first reported in China.The major Cu-mineral paragenetic sequence is as follows:chalcopyrite+tennantiteâ†'borniteâ†'digenite+enargiteâ†'covellite. The sequence of copper minerals indicates a temporal evolution from intermediate-sulfidation to extreme HS mineralization conditions. In the Zijinshan HS Au-Cu deposit, the main Cu-mineral assemblage varies from bornite-rich to digenite-rich to covellite-rich with increasing depth. Correspondingly, tin mineralogy varies from ke’sterite at upper levels to stannoidite-dominant at mid-to-upper levels of the copper orebody, to vinciennite-and mawsonite-dominant at depth, implying an increase in sulfidation and oxidation state downwards. Kiddcreekite occurs widely in the Zijinshan orefield with mineral association of porphyry-style mineralization, implying the presence of the mineral has potential as an indicator of porphyry systems. Whereas, the occurrence of hemusite represents crystallization under high-fS2and low-pH conditions. Vinciennite-bearing Cu-Sn-As mineral associations occur in a specific position in the Zijinshan HS Au-Cu, Wuqiqilong and Longjiangting Cu deposits, indicative of a transitional zone between porphyry and HS epithermal-style mineralization. The assemblage thus carries implications in prospecting for underlying porphyry Cu mineralization.EPMA results reveals the heterogeneous compositions of enargite and colusite in the Zijinshan Cu-Au deposit with zoning expressed by W, Sn, suggesting continuous growth up to the HS stage. The volcanic hydrothermal covellite of is characterized by composition zoning of Se. Sphalerite and galena in Zijinshan Cu-Au deposit has the compositions of higher Cu and low Fe, implying crystallization during the high-sulfidation conditions. Hydrothermal evolution from the IS style Cu minerlization to HS style Au mineralization, the composition sequence of fahlores is as follows:Fe-rich tennantiteâ†'Bi-rich tennantiteâ†'Zn-rich tennantite+Zn-rich tetrahedriteâ†'enargiteâ†'Te-rich tennantite (goldfieldite), implying an enrichment of As, Sb and Te during the high-sulfidation stage. In situ laser ablation inductively-coupled mass spectroscopy (LA-ICP-MS) has been used to acquire the minor elements contents in major copper mineral in Zijinshan orefield. The results show bornite contains significant concentrations of Bi and Ag, digenite is the major Au and Ag-carrier, chalcopyrite contains significant concentrations of Ag and In, part of covellite contain higher Se and most the ore minerals contains significant concentrations of Ge.The crystal structure of the kiddcreekite was solved using the direct-space method (EPCryst) from laboratory X-ray powder micro-diffraction data and refined by the Rietveld method. The R-values of the final Rietveld refinement were Rp=9.06%, Rwp=8.31%, RB=3.16, and RF=2.17%. These results suggest that kiddcreekite has a cubic structure with space group F-43m and lattice parameter a=10.8178A (Z=4, V=1265.95A3). In the unit cell, W, Sn, and Cu atoms occupy the4a,4c, and24f Wyckoff positions, respectively, and S atoms occupy two sets of16e Wyckoff positions. The cubic structure of kiddcreekite is visualized as the stacking of double MS4layers (W-Sn-Cu tier and Cu-vacancy tier) according to the sphalerite principle.Multi-collector inductively-coupled mass spectroscopy (MC-ICP-MS) has been used to acquire the copper isotope ratios of copper mineral in Zijinshan orefield. The overall range of Cu isotopic ratios (865Cu) of copper mineral varies from-11.84%o to1.52%o. The major copper mineral of digenite-covellite-bornite in Zijinshan Cu-Au deposit have the Cu isotope ratios show a typical hypogene mineralization value of approximately O%。and the copper minerals in the leach cap have measured Cu isotope ratios significantly depleted in comparison to hypogene. Whereas, the nonexistence of an enriched565Cu supergene reservoir suggest the enriched65Cu downward fluid was bring away to the Tingjiang river along the fault structure during weathering or presence of undiscovered lateral Cu occurrence.According to the systematic studies of mineralogy and mineral geochemistry, the Zijinshan orefield is a giant porphyry-epithermal system and there is considerable potential for discovery of deep porphyry Cu mineralization at depth within the orefield, especially in the ore district of Ermiaogou and Dayanli.