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2001 Vol. 12, No. 1

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Fischer Plots of Wumishan Cyclothems, Precambrian Records of Third-Order Sea-Level Changes
Mingxiang Mei, Yongsheng Ma, Hongrui Zhou, Benming Du, Zhiqing Luo, Qingyin Guo
2001, 12(1): 1-10.

The Mesoproterozoic Wumishan Formation ((1310 ± 20) Ma - (l 207 ± 10) Ma) in Jixian, Tianjin represents a succession of 3 300 meter thick carbonate strata where are the best developed stromatolite biostromes and thrombolite bioherms. These biostromes and bioherms, together with tidal-flat dolomites and muddy dolostones, form Wumishan cyclothems with approximately symmetrical faciessuccession fabrics, which belong to carbonate meter-scale cyclic sequences of peritidal type. These cyclothems are overlain by lagoon dolomitic mudstones and shales and by frequently developed paleosol layers, indicating that the boundary of Wumishan cyclothems is marked by an instantaneously-exposed punctuated surface. Viewed form the regularly vertical stacking patterns of Wumishan cyclothems in long-term sequences, 26 third-order sequences may be discerned in Wumishan Formation. These thirdorder sequences may also be incorporated into six second-order sequences. In order to study the variation range of third-order sea-level changes and the formation period of third-order sequence, 626 cyclothems are discerned in the middle and upper parts of Wumishan Formation where the outcrops are well developed. In terms of the total thickness and the formation period of Mesoproterozoic Wumishan Formation, the crust subsidence rate in the sedimentary period of Wumishan Formation is supposed to be 3 meters at each interval of 100 thousands years. Based on this assumption, Fischer plot might be designed to simulate the third-order sea-level changing pattern.

Study on Sequence Stratigraphy of Zhujiang and Zhuhai Formations, Zhu Ⅲ Subbasin, Pearl River Mouth Basin, South China Sea
Fengjun Nie, Sitian Li, Hua Wang, Xinong Xie, Keqiang Wu, Meizhu Jiang
2001, 12(1): 11-21.

The early Miocene in the Zhu III subbasin, the Pearl River Mouth basin, includes two formations-Zhujiang and Zhuhai. There are 8 third-order sequences, S1, S2, S3, S4, S5, S6, S7 and S8 from the bottom of Zhuhai to the top of Zhujiang in these two formations. There are only one transgressive systems tract (TST) and one highstand systems tract (HST) in each sequence because the whole Zhu III subbasin was located updip the shelf break during sequence deposition. The boundaries and maximum flooding surfaces (mfs) are in good response to both gamma and acoustic log curves in the study area. In seismic profile 1249, sediments obviously onlap over the unconformity (SB0., the bottom of Zhuhai Fm), SB1 and SB2, but obviously over only SB2 in seismic profile 1283 since the well-developed faults in the subbasin. The sand bodies with high porosity and permeability for petroleum migration and accumulation had been reworked by tidal currents before their burial. Hence, the tidal influenced parasequence sets occur both in TST and HST. Through detailed analysis, the sand bodies in TST are more favorable for petroleum to migrate and accumulate than those in HST.

Griesbachian Cyclostratigraphy at Majiashan Section, Anhui Province, China
Yuanqiao Peng, Jinnan Cong, Yongqun Gao
2001, 12(1): 22-26.

The Lower Triassic at Majiashan Section, deposited in the deep part of the Lower Yangtze carbonate ramp, is the only outcrop of the deep Lower Triassic preserved in the present Lower Yangtze region. Even so, the alternations of mudrock of mudrock and marl (or limestone) are still very distinct and recognizableable, though usually more or less condensed. The lithological and geochemical cycles at Majiashan Section are discussed in terms of the sedimentary characters and the contents of some elements analyzed at a 2-centimeter-interval with no hiatus of the Lower Triassic. The division resolution of the Griesbachian stratigraphy may increase to the scale of ten thousand years by the mudrock-marl (or limestone) bundles and the geochemical cycles. The analysis of the relationship between the sedimentary cycles/the geochemical cycles and the Milankovitch cycles may suggest the time limit and the sedimentary rates.

Phytolith Records in Vermicular Red Earth: Implications for Bioclimatic Variations during Pleistocene
Yansheng Gu, Ying Zhuo, Zejun Zhang
2001, 12(1): 27-30.

Phytolith analysis is employed in bioclimatic research into vermicular red earth, especially into its form, assemblage and zonation. The phytolith assemblage is divided into 10 zones. The statistics and main factor analysis of phytolith show that the obtained main factor load curves could suggest a climate change. Combined with phytolith assemblage feature, the bioclimatic variation of vermicular red earth during its formation stage, consisting of 5 arid-cold stages, 4 warm-humid stages and 2 mild stages, is reconstructed in detail. The research results indicate that phytolith records are ideal paleoclimatic signals in vermicular red earth, and that abundant information on environmental evolution can be located.

Multifractal Simulation of Geochemical Map Patterns
Frederik P Agterberg
2001, 12(1): 31-39.

Using a simple multifractal model based on the model De Wijs, various geochemical map patterns for element concentration values are being simulated. Each pattern is self-similar on the average in that a similar pattern can be derived by application of the multiplicative cascade model used to any small subarea on the pattern. In other experiments, the original, self-similar pattern is distorted by superimposing a 2-dimensional trend pattern and by mixing it with a constant concentration value model. It is investigated how such distortions change the multifractal spectrum estimated by means of the 3-step method of moments. Discrete and continuous frequency distribution models are derived for patterns that satisfy the model of De Wijs. These simulated patterns satisfy a discrete frequency distribution model that as upper bound has a continuous frequency distribution to which it approaches in form when the subdivisions of the multiplicative cascade model are repeated indefinitely. This limiting distribution is lognormal in the center and has Pareto tails. Potentially, this approach has important implications in mineral and oil resource evaluation.

Some Suggested Future Directions of Quantitative Resource Assessments
Donald A Singer
2001, 12(1): 40-44.

Future quantitative assessments will be expected to estimate quantities, values, and locations of undiscovered mineral resources in a form that conveys both economic viability and uncertainty associated with the resources. Historically, declining metal prices point to the need for larger deposits over time. Sensitivity analysis demonstrates that the greatest opportunity for reducing uncertainty in assessments lies in lowering uncertainty associated with tonnage estimates. Of all errors possible in assessments, these affecting tonnage estimates are by far the most important. Selecting the correct deposit model is the most important way of controlling errors because of the dominance of tonnage-deposit models are the best known predictors of tonnage. Much of the surface is covered with apparently barren rocks and sediments in many large regions. Because many exposed mineral deposits are believed to have been found, a prime concern is the presence of possible mineralized rock under cover. Assessments of areas with resources under cover must rely on extrapolation from surrounding areas, new geologic maps of rocks under cover, or analogy with other well-explored areas that can be considered training tracts. Cover has a profound effect on uncertainty and on methods and procedures of assessments because geology is seldom known and geophysical methods typically have attenuated responses. Many earlier assessment methods were based on relationships of geochemical and geophysical variables to deposits learned from deposits exposed on the surface-these will need to be relearned based on covered deposits. Mineral-deposit models are important in quantitative resource assessments for two reasons: (1) grades and tonnages of most deposit types are significantly different, and (2) deposit types are present in different geologic settings that can be identified from geologic maps. Mineral-deposit models are the keystone in combining the diverse geoscience information on geology, mineral occurrences, geophysics, and geochemistry used in resource assessments and mineral exploration. Grade and tonnage models and development of quantitative descriptive, economic, and deposit density models will help reduce the uncertainty of these new assessments.

Grade-Tonnage, Ore Value-Tonnage, and Enrichment Ratio-Tonnage Models for Resource Assessment
Tetsuya Shoji
2001, 12(1): 45-53.

According to grade-tonnage diagrams of nickel and zinc deposits, their critical grades are 0.4% and 3.4%, respectively, and hence the former resources can be considered optimistic and the latter pessimistic. The grade-tonnage diagram of gold deposits is convex downwards suggesting that the critical grade is 1×10-6 in the low-grade part. The ore value (OV)-tonnage diagram of all deposits In the world consists of three parts: high, middle and low vain f classes. The enrichment ratio (ER)-tonnage diagram of all deposits in the world ho consists of three parts: high, middle and low ratio classes.Nine quality categories defined by ER and OV are characterized by some keywords indicating deposit types as follows: category RH (high ER-high OV: 0.7%) by "unconformity" and "Mississippi Val-ley", category HM (high ER-middle: OV: 0.7%) by "vein", category ML (middle ER-low OV: 0%) by "sandstone", "stockwork" and" dissemination", category LM by "orthomagmatic", " laterite", komatiite and " chemical", and category LL by "porphyry", "dissemination" and "placer". Category MM is not characterized by any keyword. If the commodities of a deposit are defined by both the enrichment ratio and the ore value, the defined commodities are relatively coincident for gold and nickel, but different for copper, silver and zinc, and greatly different for molybdenum and lead. Deposits containing lead and/or zinc are complimentary. If the commodity Ph+Zn is applied, most lead or zinc deposits are classified as Ph+Zu by both definitions. Accessory metals are commonly expected for deposits of kuroko-type zinc, epithermal silver, massive sulfide-type zinc and volcanogenic zinc, but uncommon for deposits of orthomagmatic chromium, chemically precipitated copper and sandstone-type uranium.

Selection of Multifractal Scaling Breaks and Separation of Geochemical and Geophysical Anomaly
Qiuming Cheng
2001, 12(1): 54-59.

Spatially superimposed multiple processes such as multiplicative cascade processes often generate multifractal measures possessing so-called self-similarity or self-affinity that can be described by power-law type of functions within certain scale ranges The multifractalities can be estimated by applying multifractal modeling to the measures reflecting the characteristics of the physical processes such as the element concentration values analyzed in rock and soil samples and caused by the underlying mineralization processes and the other geological processes. The local and regional geological processes may result in geochemical patterns with distinct multifractalities as wall as variable scaling ranges. Separation of these multifractal measures on the basis of both the distinct multifractalities and the scaling ranges will be significant for both theoretical studies of multifractal modeling and its applications. Multifractal scaling breaks have been observed from various multifractal patterns. This paper introduces a technique for separating multifractal measures on the basis of scaling breaks. It has been demonstrated that the method is effective for decomposing geochemical and geophysical anomalies required for mineral exploration. A dataset containing the element concentration values of potassium and phosphorus in soil samples was employed for demonstrating the application of the method for studying the fertilizer and yield optimization in agriculture.

Weathered Mantle Hosted Gold Deposit at Shewushan, Hubei, Central China
Hanlie Hong, Qinyan Wang, Qiujuan Bian
2001, 12(1): 60-67.

A supergene gold deposit is located at Shewushan, south Hubei, Central China. The Au mass fraction of the supergene deposits ranges from 1.0×10-6 to 19.5×10-6, with an average of about 2.2×10-6. Primary gold mineralization is closely associated with the well developed faults and fractures mainly on the crest of the reverse anticline with gold mass fraction exceeding 1.0×10-6(average 0.6×10-6), the gold occurs as small Ag-bearing particles in association with illite and kaolinite, and partly incorporated within the lattice of pyrite. Fluid inclusion studies on the gold mineralization yield trapping temperatures and pressures from 110 to 290 and 410×105 to 460×105 pa, respectively. The salinities of mineralization fluids range from 2.6% to 8.4% w(NaCl, equivalent). Calculated δ(18O) values and measured δ(D) data of the hypogene fluids indicate a meteoric origin. These fluids underwent extreme 18O enrichment by reacting with country carbonate rock. Supergene gold minerali-zation is confined largely to the lower portion at the weathered mantle; the gold occurs as small native gold particles accompanied by clay minerals, kaolinite and illite and Mn and Fe oxides. Geochemical investigation shows that Au, together with As and Sb, decreases progressively from the top of ore body to some 2 kilometers away and define a lateral dispersion halo. It is proposed that the gold is mobilized by surficial leaching and concentrated at the lower portion of the weathering profile, and thio-sulfate may play an active role in gold dissolution and re-precipitation.

Tectonic Dynamic Metallization of Silver-Gold Hydrothermal Fluids in Proterozoic Gneiss Terrene Shear Zones, Suichang, Zhejiang, China
Siyuan Wang, Qibing Xiao, Jianguo Zeng
2001, 12(1): 68-74.

The Suichang mine is the largest silified vein-type silver-gold mineralization system in Southeast China, whose ore bodies are controlled by shear zones developing in Lower Proterozoic gneiss terrene with initial migmatization, which is covered by Upper Jurassic and Lower Cretaceous volcanic rock system and cut by acidic igneous veins of Jurassic and Cretaceous. The conclusions are as follows: (1) The ore-forming fluid is defined as superhigh tectonic-metamorphic fluid on the base of: ① δ (D)-δ(18O) values; ②fluid inclusions; ③trace elements of pyrite from ores. (2) The shear zone silicifed orebodies occurred in Proterozoic, Jurassic and Cretaceous, which have been transformed in part by ore-bearing comb quartz vein of volcanism.

Chemical Characteristics of Stream Draining from Dudu Glacier: An Alpine Meltwater Stream in Ganga Headwater, Garhwal Himalaya
Ahmad Sarfaraz, Hasnain Syed Iqbal
2001, 12(1): 75-83.

The EC (electrical conductivity), pH and concentrations of major anions, cations and dissolved silica were determined in the stream meltwater draining from the glacier. Stream meltwater samples were sampled during June and October 1997 from Dudu glacier, Bhngirathi valley, Garhwal Himalaya. This study is an attempt to reveal the hydrochemical processes operating in the glacialized regime of Garhwal Himalaya. The results show that the abundance order of cations and anions in the meltwater is c(Ca2+) > c(Na+) > c(Mg2+) > c(K+) and c(SO42-) > c(HCO3-) > c(NO3-) > c(CI-). The rock weathering is the most important mechanism controlling the water chemistry in the basin. Pyrite oxidation and carbonation are the main hydrogen ion supply reactions contributing to the chemical weathering in the basin. There is a distinct difference between the solute concentrations in samples collected during June and October.

Fluid Dynamic Field in Bozhong Depression, Bohai Bay Basin
Jiaren Ye, Honghan Chen, Xianghua Yang
2001, 12(1): 84-89.

The data from regional geology, boreholes, geophysics and tests are integrated to analyze the fluid dynamic field in the Bozhong depression, Bohai Bay basin. The current geothermal gradient is determined to be about 2.95 ℃/100 m by integrating 266 drill-stem test (DST) measurements and comparing with the global average value. The paleogeothermal gradients are calculated from the homogenization temperatures of saline inclusions, which vary both laterally and vertically. The data from sonic logs, well tests and seismic velocities are used to investigate the pressure variations in the study area. The mudstone compaction is classified as three major types: normal compaction and normal pressure, under-compaction and overpressure, and past-compaction and under- overpressure. The current pressure profile is characterized by normal pressure, slight pressure and intense overpressure from top to bottom. The faults, unconformity surfaces and interconnecting pores constitute a complex network of vertical and horizontal fluid flows within the depression. The fluid potential energy profiles present a"doubledeck"structure. The depocenters are the area of fluids supply, whereas the slopes and uplifts are the main areas of fluids charge.

Seismic Response of Base-Isolated Structures under Multi-component Ground Motion Excitation
Yicheng Jiang, Jiaxiang Tang
2001, 12(1): 90-94.

An analysis of a base isolated structure for multi-component random ground motion is presented. The mean square response of the system is obtained under different parametric variations. The effectiveness of main parameters and the torsional component during an earthquake is quantfied with the help of the response ratio and the root mean square response with and without base isolation. It is observed that the base isolation has considerable influence on the response and the effect of the torsional component is not ignored.