Study On Metallogenic Mechanism And Regularities Of VMS Deposit From Shire Region In Ethiopia

Abstract:North of Ethiopia, located in the northern margin of west Gondwana, is an important part of Arabia-Nubian shield southwest Neoproterozoic arc accretionary zones. Shire is in the southwest extension area of VMS copper polymetallic mineralization belt in Asmara, Eritrea-Ethiopia, with superiority of regional geology and metallogenic conditions and prospecting potentials. In recent years, the metallogenic belt found a batch of Adi Nefas, Debarwa and Emba Derho medium VMS type copper zinc ore bed, was the greatest achievements in VMS deposit prospecting exploration in nearly three decades in the world. However, the research degree of the basic geology and mineral resources geology in the research area is low. The Neoproterozoic stratum with VMS type deposit has widely experienced the greenschist regional metamorphism and intense tectonic deformation. In addition, the cranite magma intrusion activities were relatively intensive during preorogenic, orogenic and postorogenic periods, making regional geological and mineralization features complex and deposit genesis model, deposit location mechanism and location law hard to grasp, seriously affecting copper polymetallic ore of the exploration deployment and the peripheral metallogenic prediction. Therefore, there is an urgent need to strengthen the research on massive sulfide copper polymetallic deposit ore-forming geological conditions, mineralization mechanism and ore-controlling factors and mineralization locating mechanism in Shire.Through the research work in this project, the main achievements were obtained as follows:(1) It was found out that there were three kinds of granitoids of periods of preorogenic, orogenic and postorogenic in Shire in northern Ethiopia, granitoids and copper polymetallic deposit were not genetic relationship, there is no superimposition of ore-forming elements. For different granitoids, the differences of petrology characteristics, rare earth microelements and major elements were obvious. And the genetic types, tectonic environment and the background of crust evolution were completely understood. These findings provided the necessary basic data for the metallogenic environment of copper polymetallic metallogenic belt and offered a further basis for exploring tectonic characteristics and the evolution mechanism of subduction zone in the northeast of west Gondwana.(2) Sm-Nd isochron age (824.4Ma±15.5Ma) of total rock of the granitoids of the preorogenic period in Shire in northern Ethiopia, was gained for the first time, which belonged to Tonian of the Neoproterozoic; the I type granite in source area mainly consisted of mantle-derived materials, and its tectonic environment was the passive continental margin-volcanic island arc environment in the trench-arc-basin evolution stage of early Neoproterozoic.(3) Methods of geochemical element frequency histogram, alteration index and tectonic environment were used in the research, and it was found out that Pvai metavolcanic rock did not have obvious characteristics of bimodal volcanic rock, indicating that it was non-rift valley origin volcanic rock. Sm-Nd isochron age (830.7Ma±15.4Ma) of total rock of the metavolcanic rocks and granitoids of the preorogenic period belonged to Tonian of the Neoproterozoic, and were formed on the site of back-arc spreading basin with strong tensile stress in island arc tectonic domain, where oceanic crust was the bottom.(4) It was the first time to use the geochemical data to make wavelet analysis and processing. Not only the volcanic cycle was effectively divided, but also the rhythm of the cycle was discriminated effectively, which provided a new way for studying volcanic cycle rhythm with the geochemical data. According to comprehensive analysis of the geologic features of volcanic cycle, geochemistry and remote sensing interpretation, metavolcanic series in the study area were divided into6cycles and15rhythms.(5) It was the first time that the profile geochemical data was used and calculated conversional to get the chloritization index, and sericitization index, kaolinization and silicification index to study the relationship between alteration and mineralization. The results showed that the orebody locality was always right in the crest of the alteration index, the comprehensive alteration index was significantly greater than that of non-enriched ore surrounding rock, and the feature of symmetry of wall rock alteration was obvious.(6) The copper polymetallic mineralization in Shire was found out. Mineralization was divided into subvolcanic hydrothermal period, regional metamorphism reform period and supergene oxidation period, among which the subvolcanic hydrothermal period was the main metallogenic period in the study. Superposition of regional metamorphism is limited, there were not the new ore-forming elements.(7) We found out the copper polymetallic metallogenic material source. The main part was from subvolcanic hydrothermal solution with rich volatile copper polymetallic ore-forming elements, with a small part from the flowing formed basic lava, and some from the ore-forming elements infiltrated and extracted by sea water from overlying intermediate-acid volcanic clastic rock.(8) We found out copper polymetallic ore deposit genesis and ore-controlling factors and metallogenic regularity in Shire in Ethiopia in detail. The genesis of ore deposit of copper polymetallic in Shire was a subvolcanic hydrothermal filling and metasomatic type copper polymetallic massive sulfide deposit related to the epicontinental island arc marine volcanic activities. The main orebody was near the basic-(mid) acidic lithology transition interface, intersection parts of the structure and the3rd rhythm in the II volcanic cycle, the2nd rhythm in the III cycle, the3rd rhythm in the IV cycle, etc.(9) Metallogenic mechanism of the subvolcanic hydrothermal filling and metasomatic type massive sulfide deposits and the study of metallogenic theories were proposed and deeply discussed, massive sulfide copper polymetallic deposit metallogenic pattern in Shire was established, and the VMS deposit metallogenic theory and model research was enriched. It was considered that massive sulfide orebody was a kind of subvolcanic hydrothermal filling metasomatic type deposit formed in the intermittent period of volcanic eruption due to the change of physical and chemical conditions, when the mid-acidic subvolcanic hydrothermal (ore-bearing hydrothermal) invaded into the lithologic fabric interface of the basic lava and mid-acidic tuff, tuffite or the internal space of mid-acidic tuff and tuffite, then met with the infiltrated seawater (ore-bearing) from hanging wall.(10) Based on the modeling guiding ideology of "metallogenic model, mineralization locating mechanism and locating rule", the model of copper polymetallic multivariate information mineralization locating prediction in the study area was established; and six prospecting target areas were framed in Pvai metavolcanic series in Shire.