Lala Iron Oxide Copper Gold Deposit: Metallogenic Epoch And Metal Sources

Lala copper deposit is located in the mid-west of Kangdian copper belt, tectonicly on the western margin of Yangtze Block, and is important Cu-Au producing area in South-West China. This deposit is hosted by Paleoproterozoic greenschist metamorphosed volcanic-sedimentary sequences of Luodang Formation of Hekou Group and is a typical Iron Oxide copper-gold (IOCG) deposit in China. Based on study on regional geology, mining area geology and ore deposit geology, the electron microprobe, element geochemistry (including major elements, trace elements, REEs and PGEs), LA-MC-ICP-MS U-Pb zircon geochronology and Re-Os isotope geochemistry were applied to study the nature of the host rocks, ores and intrusive rocks, the sources of ore-forming metals, and mineralization age. Finally, the ore genesis is demonstrated. In conclusion, main understandings and progresses are as following:(1) Sichuan Lala copper deposit is one of important large deposits in SW China not only for Cu but also for considerable economic Au-Mo-Co-REE-Fe. Ore minerals are mainly composed of hydrothermal magnetite, chalcopyrite and molybdenite. The wall-rock alterations include biotitization, silicification, carbonatation, albitization, potash feldspathization, apatitation, actinolitation and fluoritation. The ores have euhedral-subhedral-allotriomorphic crystalline grained texture, metasomatic relict texture, poikilitic texture, and dendritic texture, and have disseminated, banded, vein-stockwork and breccia structure. Copper is mainly in chalcopyrite, Cobalt is mainly in pyrite and marcasite, molybdenum is mainly in molybdenite and gold is mainly in chalcopyrite, pyrite and pilsenite as native or silver-bering native gold. The main mineralization stages include magnetite-apatite stage, chalcopyrite-pyrite stage and molybdenite stage. The content of Ti in magnetite is low, the content of S is low in ore with a little Pb-Zn sulfide minerals. So these evidences show that Lala copper deposit is a typical Iron Oxide-Cu-Au (IOCG) deposit.(2) A weighted mean LA-MC-ICP-MS 206Pb/207Pb age of 1667±11Ma for a tuffaceous schist unit confirms the Paleoproterozoic formation age of the host rock of Hekou group. These volcanic rocks mainly consist of andesites and basalts. Combination with element geochemistry, it suggests that these volcanic rocks were formed in continental back-arc environment.(3) The diabase-gabbro intrusions formed later than mineralization age, so these intrusive rocks show no genetic relation to Cu-Au mineralization and only modified or destroyed the orebodies locally.(4) The Re-Os isotope data of chalcopyrite yield a isochron age of 1262±19Ma as direct Cu-Au mineralization age. The high initial 187Os/188Os ratio of the chalcopyrite may suggest a crustal-dominated source with pre-enriched Re and Os. In addition, the sources of sulfur mainly derived from sulfate reduction of seawater but not exclude from magma.(5) In conclusion, the Lala deposit experienced three stages as following: the first is Paleoproterozoic (ca. 1670Ma) volcanogenic mineralization stage. In this stage, the ore-forming metals (e.g., PGE, LREE, Cu and Au) were brought about from submarine volcanic eruption and precipitated and pre-enriched in volcanic-sedimentary sequences of Luodang Formation of Hekou Group under suitable physicochemical conditions. The second is mesoproterozoic (ca. 1260Ma) hydrothermal mineralization stage and is intial stage of collision between Yangtze and Cathaysia in South China Block. In this stage, the basin brine infiltrated along with the faults and leached ore-forming metals in volcanic rocks of Hekou Group to form metal-bearing hydrothermal fluid. This fluid ascended for its higher temperature and lower density, and mixed with seawater to precipitate sulfide such as the chalcopyrite and pyrite. The third is Neoproterozoic (ca. 1000~850Ma) metamorphic hydrothermal alteration of concluding stage of collision between Yangtze and Cathaysia Block. In this stage, the orebodies were tectonicly uplifted and molybdenite might precipitate.