Iron-polymetallic Mineralization Of The Maizi Volcano-Sedimentary Basin And Loca Ting Prediction Of Orebody, Xinjiang, China

The Maizi Devonian volcano-sedimentary basin is located in the south margin of the Altay orogenic belt. Most iron-polymetallic mineralization in the area is connected with marine volcanic-sedimentary rocks and is controlled by tectonic evolution of the basin. Study on the typical mineral deposit, volcanic-sedimentary facies and geochemistry of ore-forming fluid shows that there exist five ore deposit types in this basin as follows: volcanic (exhalative)-sedimentary massive magnetite deposit, magnetite-sulfide type lead-zinc deposit, massive sulfide type lead-zinc deposit, fluorite (-barite)-bearing lead deposit and structural-altered rock-type gold-silver deposit.Gas- and liquid-phase composition, REE and trace elements of ore-forming fluid have been analyzed on the QMS (Quadrupole Mass Spectrograph), and isotopic trace has been studied as well. Considering the relationship of spatio-temporal distribution of deposits and evolution of basin, mechanism for large-scale polymetallic metallogenic system of the volcano-sedimentary basin was discussed in this thesis. Several understand about the metallogenic system of the Maizi basin has been concluded as follows:â… . The lead-zinc deposit (Keketale deposit) in the Maizi volcano-sedimentary basin occurs in stratiform-oid and lentiform. The ore bodies are closely hosted in volcanic-sedimentary rocks. Ore structure is predominantly banded, massive and taxitic with relatively simple ore mineral composition. The δ³⁴S value of sulfur isotope in the ores ranges from -20.6%o to -10‰. The water of quartz-hosted fluid inclusion has 8D value of -89.1‰^-49.48‰ and δ¹⁸O_SMOW value of 4.7‰¹.2‰. Calcite in mineralized horizons yields δ¹³C value of -6.7‰^-14.3‰ and δ¹⁸O value of 10.3‰¹2.9‰, and correspondingly, the ore-barren or slightly lead-mineralized calcite produces δ¹³C value of 0.6‰^-1.7‰ and δ¹⁸O value of 13.5‰¹8.0‰. It is suggested that the deposit belongs to a volcanic-sedimentary hosted massive sulfide deposit, i.e., a new-type massive sulfide deposit or an intermediate type betweentypical volcanic-hosted massive sulfide deposit (VHMS type) and typical sedimentary exhalative deposit (SEDEX type). Their mineralization characteristics are similar to that of "Iberian-type" deposits.The iron deposit (Mengku deposit) in the Maizi volcano-sedimentary basin occurs in stratiform, stratiform-oid and lentiform in conformable and clear contact with wallrock (stratum). Ore structure is mainly ribbon-banded, massive and taxitic with relatively simple ore mineral composition as well. Fluid inclusion in quartz has 5Dh2o value of-94.38%o^-76.82%o and 518Oh2O value of-4.3%o—+2.48%O. Measured 834S value of pyrite in the ore ranges from -3.2 to +10.7%o with "tower-form" in histogram. Calcite in the ore-bearing bed yields 513C value of -2.4%o^-5.4%o and 518O value of 5.4%o⁶.1%o, and there exists distinct difference on 813C value in orebody types. It is concluded that this iron deposit is formed by volcanogenic exhalative sedimentation and paulopost metamorphism.II. Fluid inclusion of the Keketale lead-zinc deposit in the Maizi basin has high CO2 and H2O contents, and the sequence of contents is: H2O>CO2>N2>CH4-C2H6 >Ar>H2S. There exists negative correlation between CO2/ H2O ratio, CO2 and CH4 contents with H2O content, while positive correlation between C2H6 content with H2O content. The lead-zinc ore-forming fluid is characterized by high contents of Ca2+, Na+, SO42", Cl" and light REE (LREE), with the range of 0.14-8.15 F/Cl" ratio and of 0.75—7.9 Na+/K+ratio. The REE content of quartz-hosted fluid inclusion ranges from 7.74x10"9 to 40.1 xlO'9 with LREE enrichment. The REE pattern exhibits a characteristic of distinct right-incline curve, intensive REE fractionation, negative Eu anomaly and Gd enrichment. Temperature and salinity of ore-forming fluid of the basin tends to decrease from northwest to southeast, i.e., the temperature and salinity of iron deposit is higher than that of lead-zinc deposit. The characteristics of high Na+, Ca2+, SO42" and F" contents of iron ore-forming fluid are in accord with that of volcanic hydrothermal deposit.III. Polymetallic metallogenic series of the Maizi volcano-sedimentary basin have been systematically studied and classified. Those iron and polymetallic deposits in the basin are composed of a iron-polymetallic (Fe—Fe-Pb-Zn—Pb-Zn-Ag—Pb-Ag — Au-Ag) metallogenic series that controlled by structure-magma hydrothermal system. From early to late along with basin evolvement, i.e., from Dikj2, D^1, Dik22 to Djk23, and finally to fold-orogeny stage, the mineralization sequence is from hematite-magnetite (Mengku type), to magnetite-sulfide lead-zinc (Ashilesayi type),massive sulfide lead-zinc (Keketale type), fluorite(-barite)-galena (Akeharen type), and structural-altered rock-type gold-silver (Daqiao type) mineralization, while the element assemblage is Fe->Fe-Pb-Zn->Pb-Zn-Ag->Pb-Ag-(F, Ba)^Au-Ag, which is corresponding to the evolution stage of preliminary extension (sea-water encroachment) —? spreading depression —* pulsation inversion (fold-orogeny and metamorphism) of the basin. Iron mineralization mainly occurs in late stage of the preliminary extension and lead-zinc mineralization predominantly occurs in the spreading depression stage.IV. Iron-polymetallic metallogenic regularity of the Maizi Devonian volcano-sedimentary basin was summarized. Mineralized type in the basin is controlled by stratigraphic formation, and four ore-bearing formation types have been identified as follows: iron, lead-zinc-iron, lead-zinc sulfide and lead-zinc-gold-silver ore-forming formation. Hosting volcanic rocks of each mineralization type exists distinct difference, and mineralization is restricted by the volcano magma of different stages. Volcanic rocks related to iron deposit is sodic type, and the mafic volcanic rock is alkali series with large c value and has higher TiO2 content. However, the volcanic rocks related to lead-zinc massive sulfide deposit is sodic and /or sodic-potassic type with obviously various K2O content, and the felsic rocks has higher Th and U contents.V. Although both iorn and lead-zinc deposits in the basin are connected with volcanism and sedimentation, their ore-forming mechanisms are distinctive: the lead-zinc massive sulfides was deposited in sedimentary rocks of medium-late volcanic eruption interval, and the leaching mechanism of volcanic rocks by seawater plays important part in lead-zinc mineralization. Hence, it is consideration that coupling between submarine epithermal circulation system and magmatic katathermal system is very important for forming large-scale lead-zinc deposit and/or cluster of mineral deposit. Magnetite (oxide type) deposit formed in volcanic-sedimentary rocks of early volcanic eruption interval, and the mineralization is bound up with migma of mafic and felsic magma.VI. The mineralization is characterized by multiple overprint and reformation. Ore texture and structure shows that there exists post-mineralized hydrothermal overprint after the master metallogenic epoch. Ore-forming fluid also exhibits a feature of multiple source and multi-stage mineralization: the early ore-forming fluid shows "bi-leaching" model that possibly related to volcanic exhalation. During thelater compression of orogeny, the mine area has suffered metamorphism and deformatiom and finally positioned on the north-east limb of the Maizi overturned syncline. The deposit belongs to volcanogenic exhalative-refomatiom type.VII. The author has summarized the prospecting indicators, established a prospecting model and predicted potential area. More than 100 million metric tons of Pb+Zn was blocked out reserves (333+334i) evidenced by engineering demonstration based on prospecting prediction.