Application of Multi-Fractal Filtering to Extraction of Geochemical Anomalies from Multi-Geochemical Backgrounds: A Case Study of the Southern Section of "Sanjiang Ore-Forming Zone", Southwestern China
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摘要: 西南“三江”南段位于印度板块与扬子板块的结合带, 是特提斯-喜马拉雅成矿域的重要组成部分.在多旋回的构造岩浆活动过程中, 形成了复杂的成矿地质地球化学背景和丰富的有色金属和贵金属矿产.正是这种复杂的地质地球化学背景, 使得应用地质统计学方法提取致矿地球化学异常遇到了挑战.为此, 多重分形滤波技术被引进, 试图从复杂的多重地球化学背景中有效地分离与成矿有关的Cu、Zn异常.研究表明: (1) 由于研究区存在多重地球化学背景, 克里格方法揭示的Cu、Zn剩余异常通常是受区域线形构造控制的区域异常, 多重分形滤波技术则揭示了与矿化有关的Cu、Zn局部致矿异常; (2) Cu致矿异常的提取表明, 多重分形滤波技术能够克服某些岩性(譬如玄武岩) 引起的Cu高背景的影响, 并从多重地球化学背景中有效提取致矿异常, 包括低背景中的弱异常和隐蔽异常; (3) 运用多重分形滤波技术清晰揭示了一系列Zn原始数据图上没有显示的、规模不等的NNE向Zn矿化异常带, 其中那条穿越整个研究区的Zn矿化异常带含有金顶巨型Pb-Zn矿床和一系列矿床、矿点、矿化点.整个研究工作是基于MORPAS3.0软件系统实现的.Abstract: The southern section of "Sanjiang ore-forming zone" (the western Yunnan terrain), southwestern China, located at the joined belt between the Indian plate and Yangtze plate, is an important area of eastern Tethyan tectonic ore-forming domain. The polycyclic tectono-magmatic activities resulted both in the diversity of ore-forming geochemical background, and in the numerous base metal deposits and precious metal deposits, including a few such world-class deposits as the Jinding Pb-Zn deposit and the Laowangzhai Au deposit within the section. Such a complex geochemical background challenges us with the geostatistical extraction of geochemical anomalies for the mineralization purpose. For this, the multi- fractal filtering (S-A method) technique is introduced to isolate effectively the Cu and Zn anomalies related to mineralization in the complex multi-geochemical background. The following three research results are obtained in this study: (1) In the complicated geochemical background, the residual anomalies of Cu and Zn uncovered by Kriging are typical of the regional anomalies controlled by the regional linear fractural pattern; but the S-A method reveals the local anomalies of Cu and Zn associated with mineralization; (2) The separation of the Cu anomaly from its background demonstrates that the S-A method could effectively identify the high geochemical background of Cu from basalts and extract the Cu anomalies from multi-geochemical background, including weak and hidden Cu anomalies from their low background; (3) A few NNE-trending Zn mineralized and anomalous belts containing the super-large Jinding Pb-Zn deposit and a series of some other Pb-Zn deposits, that are absent at the Zn original data map, are clearly shown at the S-A anomalous map. This discovery presents us with not only new areas for prospecting Cu-Pb-Zn deposits but also new perspectives for the research into new ore-controlling factors, for the regional distribution of Pb-Zn deposits, which may include NNE-trending hidden fractures. All the research work is completed with the help of MORPAS 3.0.
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图 1 西南“三江”南段地质矿产简图(据云南省地质调查院 1∶50 万数字地质图编绘, 2003)
Kz.新生代碎屑岩、局部夹火山岩; Mz.中生代碎屑岩、碳酸盐岩、局部夹火山岩; Pz.古生代碳酸盐岩、碎屑岩、夹火山岩; Pt.元古宇变质岩; 1.新近纪-古近纪安山岩; 2.二叠纪玄武岩; 3.中酸性岩体; 4.基性岩体; 5.断层; 6.地质界线; 7.国界; 8.省界; 9.湖泊
Fig. 1. Geological map and the distribution of the known mineral deposits in southern region of "Sanjiang ore-forming zone", southwestern China
图 2 (a) Cu原始含量; (b) IDW方法获取的Cu剩余异常
Fig. 2. (a) Cu original concentration; (b) Cu residual anomaly using the IDW method
图 3 Cu的lnA (> S) -lnS图
使用最小二乘法模拟4条具有不同斜率的线段, 并分别获取3个临界点: lnS0=9.05, lnS1=7.3, lnS2=6.5
Fig. 3. ln-ln plot showing the relationships between power spectrum value S and area A (> S) on Cu
图 5 (a) Zn原始含量; (b) IDW方法获取的Zn剩余异常
Fig. 5. (a) Zn original concentration; (b) Zn residual anomaly using the IDW method
图 6 Zn的lnA (> S) -lnS图
使用最小二乘法模拟3条具有不同斜率的线段, 并分别获得2个临界点: lnS0=12.52, lnS1=11.49
Fig. 6. ln-ln plot showing the relationships between power spectrum value S and area A (> S) on Zn
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