Mineralization Model For The Dapingzhang Cu Poly-metallic Deposits In Simao Basin

The Dapingzhang copper poly-metallic deposit is one of the most important deposits of Sanjiang tectonic metallogenic belt, and also is the main copper polymetallic deposit of Proto-Tethys VHMS type in western Yunnan of China. Based on the theory of plates-terrain, regional metallogeny, petrology, geology of mineral deposit and deposit geochemistry, and combined with the field investigation, indoor microscopic identification, collecting and analysis of predecessors research results, a systematic summary was performed on the ore deposit geology, ore deposit genesis, metallogenic conditions, metallogenic tectonic environment and exhalative ore-forming center; and the metallogenic regularity of Dapingzhang copper poly-metallic deposit was deeply studied. After the study, a mineralization space structure of "layered black for upper part ore body and veinlet yellow for the down part ore body" for this deposit was built by us. The results presented in the paper supplement and enrich the copper poly-metallic mineralization and its type in Tethys, and also have significant implications for promoting the research level for typical deposit in "sanjiang" metallogenic belt, the understanding the advantage of metallogenic types of simao basin in early palaeozoic basement, the potential and prospecting idea for prospecting and so on. The main research results have the following aspects.1. Combining the analysis of new chronology research and the contrast of lithology in regional stratigraphic, we presented that the epicontinental volcanic island arc activities exist in the west Lanping-Simao basin at early Paleozoic (the original Tethys). And the volcanic island arc was continued from late Paleozoic to Triassic, which is the important component of Paleozoic arc basin system. The Dapingzhang copper poly-metallic deposit belongs to the product of volcanic exhalation-sedimentary mineralization of back-arc rift zone. The result of the investigation shows that that the original tethys volcanic exhalative mineralization is an important metallogenic type in the red layer basement. It also provides a new enlightenment for the analysis of regional structure and metallogenic evolution.2. On the basis of analysis of the main geological body, trace elements, rare earth elements, nomerrous progresses in many aspects were achieved:â‘ We agree that the volcanic series of deposit could be diveded to4lithologic sections and three volcanic activity cycles. The top part of S2+3d1is the main ore-bearing unit. The V1 ore body group developed in the tuff of the top part of S2+3d1. It is a typical stratiform lenticular massive sulfide ore body. The V2ore body group developed in the sodic rhyolite and volcanic breccias of the upper part of S2+3d1. The V2ore body is below the V1ore body and is a typical veinlet-disseminated ore. The upper and lower volcanic rocks of deposit overall belong to mid-acidic and acidic volcanic rock type, encounter stronge altertion. And volcanic rocks below stratiform orebody have complete development of explosive phase, extrusive facies and sedimentary facies.â‘¡We think that the so-called indosinian period of rhyolitic porphyry(λπ) in the original geological map should belong to the Dawazhi group unit1(S2+3d1). It had a clearly the residual volcanic structure. However, the alteration dominanted by silicification is very strong due to the rhyolitic porphyry near the volcanic exhalative center. It has been converted to volcanic geologic body characterized of altered rock pipe. The volcanic rocks containing ore belong to mid-acidic-acidic volcanic rock type, but the stratiform orebody below volcanic rockâ‘¢Petrochemistry research shows that the volcanic rocks of Dapingzhang deposit belong to calc-alkaline series. Spilite, dacite, keratophyre, and quartz keratophyre belong to rock types of low K and high Na. The part of rhyolite are characterized by low K and Na. From not altered rhyolite of the periphery to weakly altered rhyolite of nearly orebody to strong altered rhyolite of orebody, SiO2is obviously increase,and Na2O+K2O is low. It explained to volcanic rock of below orebodies exist on the halo of low Na.â‘£According to the trace elements and rare earth elements in different volcanic rocks, it showed that the distribution curve of (alteration) spilite-keratophyre (alteration)-dacite, rhyolite-(alteration) quartz keratophyre, trace elements and rare earth elements are accordance, whhic have the homologous geochemical characteristics of the same period.⑤The consistency of petrochemistry discriminant shows that, volcanic rocks of ore district formed arc environment of active continental margin. Combining the analysis of regional geology, we think that volcanic rocks of the deposit should be formed in the back-arc basin. It belongs to the JingGu-JingHong island arc volcanic rocks zone, and the front of JingGu-JingHong island arc zone located on between Lancangjiang fracture and Jiufangfracture. The forearc was moved to the western near the lancang river fracture zone at late Paleozoic-Triassic.3. Combining the analysis of the geophysical characteristics and their interrelation of the two orebodies,we illuminated that the interrelation of a massive sulfide orebody V1and veinlet-disseminated orebody V2about in such aspects as material composition, structure and origin, we established a typical mineralization section structure, analysed the characteristics of ore deposit geochemistry, and confirmed the Dapingzhang deposit has a geology and geochemical signs of typical VMS deposit.4. Using uranium-Pb dating on zircon by LA-ICP-MS and rhenium-Os isotope data to chronology study of volcanic rocks, intrusive rocks and sulfide, we reclassify stratigraphic age of the Dawazhi group, the formed epoch of granite diorite porphyry and the metallogenic epoch of the orebody. The main progresses are listed as follows:â‘ Dawazhi group is the main ore-bearing volcanic unit. In the past, the stratigraphy of the Dawazhi Group was intensely debated. It think that formed in the late devonian-carboniferous period.This paper obtained a batch of main rock types of volcanic series data in the Dawazhi group (dacite428.28±6Ma and422Ma, rhyolite417±1.5Ma, rhyolitic tuff quality420.8±1.6Ma Ma), Combing with the comprehensive analysis of geological characters, we agreed to the Dawazhi group as a independent stratigraphic unit-Dawazhi group (S2+3d), and its formed in the Middle-Late Silurian.â‘¡LA-ICP-MS zircon U-Pb age of granite diorite porphyry is401.0±1.7Ma, it mean that plagiogranite porphyry is formed the late silurian, it is the rock mass in late Caledonian, Rock mass of Plagiogranite porphyry and volcanics rock of Dawazhi group constitute a typical arc magma eruption and intrusion activity sequence. And it is not formed in the Indosinian.â‘¢Rhenium-Os isotope data on chalcopyrite V1ore body define an isochron of437.35±11.2Ma. Rhenium-Os isotope data on chalcopyrite V2ore body define an isochron of410.9±6.0Ma. They are proximity with rhenium-Os isotope data of the predecessors of molybdenite (428.8±6.1Ma and442.4±5.6Ma). Combining the genetic relation between the orebody and volcanic rock, we confirm the Dapingzhang deposit and volcanic rocks were formed in the same period, during the the Middle-Late Silurian., but earlier than granite diorite porphyry.5. Combining the analysis of the geological and geophysical characteristics of the two ore bodies, we established the volcanic exhalative ore-forming center of the Dapingzhang copper-polymetallic deposit. The results of the investigation show that the volcanic exhalative center is located near the exploratory line No.10and the main hot-water exhalative ore-forming area is around the center.â‘ The data from the alteration zoning suggest that the surrounding rocks outside exploratory lines Nos.2-10are strongly altered in the following sequence:silicification and pyritization zone (breccia zone)â†'silicification and sericitization zone (pyritization belt)â†'sericitization and carbonatization zoneâ†'carbonatization, chloritization, and epidotization zone. The alteration of the surrounding rocks constitutes a complete mineralization-alteration pipe.The strongly altered central area is represented by strong silicification and pyritization of multiple generations, is an irregular ellipse as observed in exploratory lines Nos.7-16. In particularly, numerous exploding breccia and quartzite exist between exploratory lines Nos.2-10. The quartz veins containing minerals are relatively concentrated in exploratory lines Nos.2-10, which overall coincide with the center of the V2ore body.â‘¡The ore distribution exhibits irregularities in both the horizontal and vertical directions. In general, the central and lower parts of the ore body are relatively rich in copper, while the upper part and periphery are relatively rich in Pb and Zn. In the horizontal direction,the ore body forms an irregular spindle-like mineralization with the long axis in the NW direction, which is rich in Cu in the central part and rich in Pb and Zn in the periphery. In the vertical direction, the ore body forms sector mineralization zoning rich in Cu, Zn, and Pb in the upper part and rich in Cu in the lower part. Thus, the exploratory lines Nos.0-10in the central part of ore body are inferred to be the center of volcanic-exhalative ore-forming system and to represent the high-intensity mineralization area.â‘¢The highest and lowest homogenization temperatures of the fluid inclusions in the V1ore body are223℃and78℃, respectively (Tab.1). The ore-forming temperature range of the V1ore body is clearly narrow. Apparently, the V1ore body has typical low-temperature ore-forming characteristics. The highest homogenization temperature of the fluid inclusions in the V2ore body is298℃and most of the fluid inclusion homogenization temperatures are greater than200℃. Thus, the V2ore body is characteristic of mesothermal hydrothermal system. The area of high homogenization temperatures is located between the exploratory lines Nos.6-10, and the homogenization temperature isotherms extend to the NW direction having the shape of a long spindle. The general trend of the central ore-forming area is approximately consistent with the ore body and F5fault extending to the NW direction.The areas of high homogenization temperatures and strong mineralization zoning agree well in space. The thickest part of the ore body and its highest grade is located at and nearby the high-temperature isotherm. The mineralization intensity decreases with the distance away from the high-temperature isotherm. This phenomenon suggests that the ore-forming fluid permeates the central area between the exploratory lines Nos.6-10.â‘£The3D models of the orebodies suggest that the mineralization enrichment of V1type ore body and V2are consistent. The V2orebodies are mainly controlled by annular and radial structure. Ring structure zone is the distribution of the main ore body. The characteristic of a pipeline is obvious.⑤Geophysical EH4anomalies of ore-forming center indicate that the deep low-resistivity anomalies are speculated to be related to the mineralization of the ore-forming center of the exhalative pipe system, and may represent the mineralization belt in an exhalative pipe system. The outcrop of the orebody, area of high ore-forming temperatures, ring-radial cracks, and deep low-resistivity area of EH4are shown on the same map, and they overlap substantially.6. The paper systematic analysises of the basic ormative conditions of the Dapingzhang copper-polymetallic deposit, which is pointed out that the overall distribution of volcanic exhalation-sedimentary deposit of the Dapingzhang type were controlled regional fracture of Jiufang in the transition zone of volcanic island arc basin. And it proves that Dapingzhang deposit is mainly the result of submarine volcanic exhalation-sedimentary on the genesis. This deposit has typical characteristics of VMS type deposit. Based on the analysis of geology, ore deposit geochemistry and metallogenic mechanism, we established the metallogenic model and save pattern of the Dapingzhang deposit. Contrasting with the typical large-very large Marine volcanic massive sulfide deposit of domestic and foreign, we think that the Dapingzhang deposit is similar to the kuroko deposit.