Geochemical signatures of waste rocks around Sarcheshmeh porphyry copper mine dumps, southeastern Iran: Implications for exploration, economic by-products and the environment

摘要

The Sarcheshmeh porphyry copper deposit (1.7 Gt @ 0.65% Cu, 0.02% Mo, 1.22 ppm Ag, 0.06 ppm Au) is one of the giant Miocene continental arc porphyry ore systems in the Tethyan metallogenic copper belt. The supergene sulfide ore zone (6 Mt. @ 2.43% Cu, 0.022% Mo, 1.55 ppm Ag and 0.114 ppm Au) is mainly capped by a hematitic gossan. This hematitic gossan is a possible surficial exploration target for unexposed porphyry copper ores. The total tonnage of waste rocks is about 2.6 G tonnes of gossanized as well as sulfide-bearing waste rocks with an average modal content of 10% pyrite-chalcopyrite. The gossanized waste rocks show boxwork texture and are composed of hematite, goethite, jarosite, malachite, chalcanthite, brochantite and delafossite. In order to assess the exploration, economic and environmental aspects of chalcophile and precious elements in the waste rocks, a total of 86 waste rocks and 10 residual gossan samples were collected by a systematic random sampling method, analysed by XRF and ICP-MS methods for major oxides and 50 minor and trace elements at ALS Minerals Lab, Ireland. The highly anomalous elements in immature residual gossans are Au (0.775 ppm), Ag (13.4 ppm) and Mo (122 ppm). The gossanized waste rocks are also enriched in Au (0.608 ppm), Ag (87 ppm) and Mo (2960 ppm). These geochemical signatures could be used as possible exploration guides for concealed porphyry copper deposits. The enrichment of Au, Ag and Mo is caused by the lower mobility of these elements in the immature gossans and gossanized waste rocks. The elemental enrichment factors normalized to the Sarcheshmeh ore zone are Au (2.50), Pd (1.05), Ag (4.67), Pb (4.6), Zn (17.34), Sn (2.64), W (1.47), Re (22.22), Se (5.38), Te (5.75), Tl (4.1), Bi (3.29), Ga (1.15), In (11.5), La (1.2), Ce (1.2) and Y (4.1), respectively. The mean contents of the most valuable by-product elements in the gossanized waste rocks include Cu (3168.2 ppm), Mo (180.22 ppm), Au (0.085 ppm), Ag (5.7 ppm), W (42.76 ppm), Re (0.01 ppm), Bi (10.87 ppm) and Se (13.45 ppm). The S-NNP diagram highlights that the sulfide-bearing waste rocks have also a negative impact on the environment, are prone to generate acid mine drainage (AMD) and release the potentially harmful metal (loids): Pb (107 ppm), Zn (365.3 ppm), As (83.53 ppm), Sb (10.24 ppm), In (0.44 ppm), and Se (13.45 ppm) into the surrounding area. These elemental values occurred at concentrations about 4-269 times greater than the crustal abundances and natural background of fresh granodiorite. As such, the heavy metal-rich gossans and sulfide-bearing waste rocks around the Sarcheshmeh mining areas may be considered as an exploration guide for valuable by-product elements as well as environmental concern.