Font Size: a A A

The Genesis Of Makeng Fe Deposit And Mineral Prospectivity Mapping In Southwest Fujian, China

Posted on:2016-03-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z J ZhangFull Text:PDF
GTID:1220330473454969Subject:Mineral prospecting and exploration
Abstract/Summary:PDF Full Text Request
The Makeng deposit, an ore reserve of more than 350 Mt with an average grade of 37.85% total Fe, is one of the largest Fe deposits in Fujian Province. Since it was discovered in 1957 by the 403 Geophysical Prospecting Team of the former East China Bureau of Geology, the mineralization of the deposit has widely been studied. And there are different views on the mineralization process and genesis of this deposit, including (1) marine sedimentary; (2) marine volcanic sedimentary and hydrothermal re-working; and (3) skarn-type iron deposit. However, the widespread skarns in the deposit is uncontested. Recent studies have proved that the Makeng Fe deposit is a skarn type deposit, However, the skarns in Makeng, occurring primarily between limestone and sandstone, are not typical associated with limestone and plutons. Different periods of intrusions, e.g., Hercynian mafic intrusions and Yanshanian (i.e., early Cretaceous) Dayang-Juzhou granitic intrusion, occurred in Makeng deposit distrct. Whether the skarn alteration and Fe mineralization are raised by the Hercynian mafic intrusions or the Yanshanian Dayang-Juzhou granitic intrusion deposit is still a key factor to solve the origin of Makeng deposit, which also restricted the Fe deposit prospecting in Southwest Fujian, China.The origin of the iron and the mineralization process and age are the key factors to constain the origin of Makeng deposit. However, due to technical limitations, former scholars could not directly obtain the mineralization age and the origin of the iron. As the development of the MC-ICP-MS, now it is possible to constain the origin of the iron and its mineralization age.In this work, based on the genetic mineral model and a prospectivity model with five key factors, the Fe deposits prospectivity in the SFMB was carried on by the FWofE and RF method. The subsequent validation for the prospectivity shows a good performance with 20 m orebodies were found in a drill. The achievement of this work are listed by following:(1) Research on Makeng depositAccording to the detailed characteristics of geology and geochemistry on the Makeng deposit, the formation processes of the skarns and the Fe mineralization in the Makeng Fe deposit is reconstructed to a single process. The Sm-Nd isochron ages of seven samples of pure garnet and five of pure magnetite separates from the Makeng ores yielded an isochron age of 157±15 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the nearby exposed the Dayang-Juzhou (DJ) porphyritic biotite granite and fine-grained syenogranite yielded 206Pb/238U ages of 140.2±1.1 and 140.1±1.0 Ma, respectively. These results suggest that the intrusion of the DJ granite and the Makeng skarn alterations and Fe mineralization are contemporaneous.The results of the Sr, Nd, Pb and Fe isotopic geochemical characteristics of magnetite, the Fe contents in the different altered stratiform mafic intrusions, and the major element compositions of pyroxenes and garnets in the Makeng Fe deposit suggest that the initial mineralizing fluids have been mainly derived from granitic magmas but also involving materials originated from stratiform mafic intrusions and/or the intruded strata. LA-ICP-MS zircon U-Pb dating of the stratiform mafic intrusions raised that they intruded in about 302.0= 2.9 Ma and were strongly reformed by the DJ granite. In addition, the Fe in the stratiform mafic intrusions was also separated out and migrated to the ore-forming fluids and became the mineral materials (<15%). The initial mineralizing fluids might be the magmatic water derived from the gradually decreases process of the water solubility in DJ granite when it intruded in Makeng deposit district and was invloved many carbonate rocks. And some meteoric water may have been mixed in the later process of mineralization. The initial mineralizing fluids also separated out the Fe, Mg, Ca, and Si in the country rocks; and the metal was deposited and mineralization and skarn alteration occurred when the PH became higher resulted by more and more carbonate rocks invloved in the fluids.The geochronology and geochemistry characteristics of the DJ granite show that it intruded in 145-125 Ma; the average crystallizing temperature of the DJ granite is up to 850℃ and is typical A type granite. The DJ granite exhibits geochemical characteristics of A-type granites, including high Na2O+K2O, FeO./MgO, Ga/Al and HFSE values, and low Al2O3. Sr, Ti, Ba, and Eu values. On the Nb-Y-3Ga and Nb-Y-Ce triangular diagrams, the DJ A-type granite samples plot in the Al field. It is inferred that the DJ granite was derived from partial melting of the Paleoproterozoic metasedimentary rocks of the Cathaysia basement, and that some underplating of mafic magma in the lower tholeiitic crust and/or depleted mantle was also involved, which provided the heat source for the partial melting. Because there are some minor variations in the SiO2 contents and Rb/Sr, Rb/Ba, and δEu ratios, it is suggested that small amounts of fractional crystallization did occur after the high-silicate A-type granite magmas were generated. The intrusive age of the DJ granite is between the lithospheric extrusion in late Jurassic, which is represented by the crust sourced Caixi monzonitic granite (150 Ma) and widespread nappe structures, and the entire extensional tectonic environment, which is represented by the crust-mantle mixing sourced Sifang and Luoboling Granodiorite (111~102 Ma) and the large-scale continental volcanic eruption. And Makeng Fe mineralization and skarn alteration occurred in this background.(2) Research on genetic model for Makeng type iron depositsA genetic model for Makeng type iron deposits is devoloped in this work. Yanshanian granitoids provided heat and metal-bearing magmatic-hydrothermal fluids for the Fe mineralization. The metal-bearing magmatic-hydrothermal fluids migrated from the granites toward the contact zones within the late Paleozoic marine sedimentary rocks, especially between the sandstone and limestone, as well as upward along the NE-NNE-trending faults; they then penetrated planes of weaknesses in the carbonate rocks forming skarns. Metals, particularly Fe, and associated (i.e., Fe-scavenged) elements in the magmatic-hydrothermal fluids were deposited in skarnified carbonate rocks. Therefore, five key factors are considered as important criteria for mapping mineral prospectivity for the skarn-type Fe deposits in the SFMB:(1) the contact zones of Yanshanian granites (GRANITE); (2) the contact zones within the late Paleozoic marine sedimentary rocks and the carbonate formations (FORMATION); (3) the NE-NNE-trending faults (FAULT); (4) the zones of skarn alterations (SKARN); and (5) the aeromagnetic anomaly (AEROMAGNETIC).(3) Research on the prospectivity and its subsequent validationThe fuzzy weights of evidence (FwofE) (Cheng and Agterberg,1999) and random forests (RF) (Breiman,2001) method were performed for the mineral prospectivity. Based on the prediction-rate curves, both results show that the prediction model for the skarn-type Fe deposits in the SFMB is successful and efficient, because most of the known Fe deposits occur in or near areas of high posterior probabilities and both results outline only 10% of the total study area as prospective with a more than 75% prediction-rate. Futhermore, both methods suggested that the GRANITE and FORMATION, followed by SKARN, AEROMAGNETIC, and FAULT are the most valuable evidence maps, which is coincident with the skarn-type Fe deposit mineral model in the SFMB. The unstable performance experienced when FORMATION was omitted might indicate that the highest uncertainty and risk in follow-up exploration is related to the sequences. The FWofE and RF method for the skarn-type Fe deposits prospectivity in the SFMB both have a good performance, and could be used to guide further exploration of skarn-type Fe prospects in the SFMB. The successful prospectivity results also reflect that the genetic model and prediction model with five key factors are correct and efficient.Five prospect areas for Makeng type iron deposits were proposed in this work. And the subsequent validation for the prospectivity in the "Program of Integrated Prediction of Mineral Resources in Covered Areas" shows a good performance with 20 m orebodies were found in a drill.
Keywords/Search Tags:Mineral prospectivity mapping, minral model, ore deposit genesis, Makeng Fe deposit
PDF Full Text Request
Related items