Giant Mineral Deposits and Their Geodynamic Setting in the Lanping Basin, Yunnan, China

There are giant mineral deposits, including the Jinding Zn-Pb and Baiyangping Ag-Co-Cu, and otherimportant mineral deposits (e.g., Baiyangchang Ag-Cu, Jinman Cu deposits, etc.) in the Lanping Mesozoic-Cenozoic Basin, Yunnan Province, China. The tabular ore-bodies and some veins hosted in terrestrial clastic rocks of the Mesozoic-Cenozoic age and no outcropping of igneous rocks in the giant deposits lead to the proposal of syngenetic origin, but the giant mineral deposits are not stratabound (e.g. MVT, sandstone- and Sedex-type). They formed in a continental red basin with intense crust movement. The mineralization is controlled by structures and lithology and occurs in different strata, and no sedimentary nature and no exhalative sediments are identified in the deposits. The deposits show some relations with organic matter (now asphalt and petroleum) and evaporates (gypsum). The middle-low-temperature (mainly 110℃ to 280℃) mineralization took place at a depth of about 0.9 km to 3.1 km during the early Himalayan (58 to 67 Ma). The salinity of ore-forming fluids is surprisingly low (1.6% to 18.0 wt% (NaCl)eq). Affected by the collision of the Indian and Eurasian plates, the mantle is disturbed under the Lanping Basin. The large-scale mineralization is closely linked with the geodynamics of the crust movement, the mantle and mantle-flux upwelling and igneous activity. Giant mineral deposits and their geodynamic setting are unique in the Lanping Basin.

Basin Fluid Mineralization during Multistage Evolution of the Lanping Sedimentary Basin,Southwestern China

The Lanping sedimentary basin has experienced a five-stage evolution since the late Paleozoic： ocean-continent transformation （late Paleozoic to early mid-Triassic）; intracontinental rift basin （late mid-Triassic to early Jurassic）; down-warped basin （middle to late Jurassic）; foreland basin （Cretaceous）; and strike-slip basin （Cenozoic）. Three major genetic types of Ag-Cu polymetallic ore deposits, including the reworked hydrothermal sedimentary, sedimentary-hydrothermally reworked and hydrothermal vein types, are considered to be the products of basin fluid activity at specific sedimentary-tectonic evolutionary stages. Tectonic differences of the different evolutionary stages resulted in considerable discrepancy in the mechanisms of formation-transportation, migration direction and emplacement processes of the basin fluids, thus causing differences in mineralization styles as well as in genetic types of ore deposit.

EVOLUTION OF ORE-FORMING FLUIDS AND Ag-Cu POLYMETAL MINERALIZATION IN THE LANPING BASIN, YUNNAN

The Lanping Mesozoic—Cenozoic sedimentary basin, situated in the Middle section of the “Sanjiang" (Nujiang—Lancangjiang—Jinshajiang) area in the east margin of the Tibet plateau, is well known for its large production of base\|metal sulphide deposits. The worldwide famous super\|large Jinding Pb\|Zn deposit is located in the middle of the basin. The evolution history of the Lanping basin since Mesozoic can be divided into six stages, i.e., ①active continental marginal basin (T 1—T 2); ②back\|arc rift basin (T 3—J 1); ③intracontinental depressive basin (J 2—J 3); ④foreland basin (K); ⑤strike\|slipping and mutual thrusting (E 1—E 3); and ⑥strike\|slipping and pull\|apart basin (N 1\|present). Three main types of Ag\|Cu polymetal deposits are recognized in the basin. Deposits of sedimentary exhalation\|hydrothermal reworking origin (type Ⅰ) are hosted chiefly in limestones, dolomitic limestones, and siliceous rocks of the Upper Triassic Sanhedong Formation (T 3 s ) in Sanshan area. Deposits formed through normal chemical sedimentation in closed to semi\|closed environments (type Ⅱ; e.g., Jinman and Baiyangchang) during the depressive and foreland basin stages occur in various horizons of Jurassic and Cretaceous ages. Hydrothermal reworking on deposits of this type during the Himalayan period are locally pronounced, especially in the west margin of the basin near the Lancangjiang thrust fault. The third deposit type in the basin (type Ⅲ; e.g., Baiyangping and Fulongchang) is the Ag\|bearing tetrahedrite vein deposits occurring almost in all Mesozoic—Cenozoic strata, especially in the Cretaceous. Ore minerals formed during synsedimentary periods of types Ⅰ and Ⅱ are relatively simple and dominated by chalcopyrite and bornite, though sphalerite, galena, pyrite, tetrahedrite and pyrite are also present. In the deposits of type Ⅲ as well as in the ores formed during the hydrothermal reworking period in deposits of type Ⅰ and Ⅱ, ore minerals are extremely complicated and characterized by predominant Ag\|bearing tetrahedrite and other complex sulfosalts of Cu\|Ni\|Co\|Fe\|As\|S and Cu\|Bi\|S series. The associated gangue minerals are mainly quartz, siderite, Fe\|dolomite, barite, and celestite.

The Electrical Conductivity Structure of the Lanping–Simao Basin and its Implications for Mineralization

Objective The Lanping-Simao Basin in western Yunnan, located in the southeastern margin of the Tibetan Plateau, is tectonically in the transition zone between the Gondwana and Eurasia tectonic domains. It is also the frontier zone of northeastern extrusion of the Indochina Plate towards the Eurasia Plate as well as the escape zone for the deep material. The middle axial tectonic zone, also known as the Lanping-Simao Fault （LSF） in previous study, is a giant intraplate tectonic belt composed of a series of narrow uplift belt, rupture depression zone, metamorphic belt, alteration belt and marginal fracture system, which were formed by the compressional uplift of the central depression of the Lanping-Simao Basin. This tectonic unit controls the geological evolution, seismic activity, hot spring distribution and ore formation of the LanpingSimao Basin since the Mesozoic and Cenozoic.

Tectonic Setting and Nature of the Provenance of Sedimentary Rocks in Lanping Mesozoic-Cenozoic Basin: Evidence from Geochemistry of Sandstones

The geochemical composition of sandstones in the sedimentary basin is controlled mainly by the tectonic setting of the provenance, and it is therefore possible to reveal the tectonic setting of the provenance and the nature of source rocks in terms of the geochemical composition of sandstones. The major elements, rare\|earth elements and trace elements of the Mesozoic\|Cenozoic sandstones in the Lanping Basin are studied in this paper, revealing that the tectonic settings of the provenance for Mesozoic\|Cenozoic sedimentary rocks in the Lanping Basin belong to a passive continental margin and a continental island arc. Combined with the data on sedimentary facies and palaeogeography, it is referred that the eastern part of the basin is located mainly at the tectonic setting of the passive continental margin before Mesozoic, whereas the western part may be represented by a continental island arc. This is compatible with the regional geology data. The protoliths of sedimentary rocks should be derived from the upper continental crust, and are composed mainly of felsic rocks, mixed with some andesitic rocks and old sediment components. Therefore, the Lanping Mesozoic\|Cenozoic Basin is a typical continental\|type basin. This provides strong geochemical evidence for the evolution of the paleo\|Tethys and the basin\|range transition.

Metallogeny of the Baiyangping Lead-Zinc Polymetallic Ore Concentration Area, Northern Lanping Basin of Yunnan Province, China

The Lanping Basin in the Nujiang-Lancangjiang-Jinshajiang （the Sanjiang） area of northeastern margin of the Tibetan Plateau is an important part of eastern Tethyan metallogenic domain. This basin hosts a number of large unique sediment-hosted Pb-Zn polymetallic deposits or ore districts, such as the Baiyangping ore concentration area which is one of the representative ore district. The Baiyangping ore concentration area can be divided into the east and west ore belts, which were formed in a folded tectogene of the India-Asia continental coUisional setting and was controlled by a large reverse fault. Field observations reveal that the Mesozoic and Cenozoic sedimentary strata were outcropped in the mining area, and that the orebodies are obviously controlled by faults and hosted in sandstone and carbonate rocks. However, the oreforming elements in the east ore belt are mainly Pb-Zn -Sr-Ag, while Pb-Zn-Ag-Cu-Co elements are dominant in the west ore belt. Comparative analysis of the C-O-Sr-S-Pb isotopic compositions suggest that both ore belts had a homogeneous carbon source, and the carbon in hydrothermal calcite is derived from the dissolution of carbonate rock strata; the ore- forming fluids were originated from formation water and precipitate water, which belonged to basin brine fluid system; sulfur was from organic thermal chemical sulfate reduction and biological sulfate reduction; the metal mineralization material was from sedimentary strata and basement, but the difference of the material source of the basement and the strata and the superimposed mineralization of the west ore belt resulted in the difference of metallogenic elements between the eastern and western metallogenic belts. The Pb-Zn mineralization age of both ore belts was contemporary and formed in the same metaliogenetic event. Both thrust formed at the same time and occurred at the Early Oligocene, which is consistent with the age constrained by field geological relationship.

Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin,Southwest China

Large-scale gypsum rocks associated with world-class Pb-Zn ore formations are widely distributed in the Lanping Basin,Sowthwest China.Geochemical studies alongside field investigations were conducted in this study to determine the source and evolutionary processes of the gypsum rocks in this area.The gypsum sequences in the Lanping Basin developed in two formations:the Triassic Sanhedong Formation and the Paleogene Yunlong Formation.The gypsum hosted in the former displays a primary thick-banded structure withδ34SV-CDT values in the range of 14.5‰−14.8‰.Combined with the 87Sr/86Sr values(0.707737−0.707783)of limestone,it can be suggested that the Sanhedong Formation is of marine origin.In contrast,the gypsum from the Paleogene Yunlong Formation is characterized by the dome,bead and diapiric salt structures,wider range of both 87Sr/86Sr(0.707695−0.708629)andδ34SV-CDT values(9.6‰−17‰),thus indicating a marine source but with the input of continental materials.The initial layered salt formations were formed by chemical deposition in a basin and were later intensely deformed by collisional orogeny during the Himalaya period.As a result,variable salt structures were formed.We hereby propose an evolutionary model to elucidate the genesis of the gypsum formations in the Lanping Basin.

Zinc, copper, and strontium isotopic variability in the Baiyangping Cu–Pb–Zn–Ag polymetallic ore field, Lanping Basin, Southwest China

The Baiyangping Cu-Ag polymetallic ore district is located in the northern part of the Lanping-Simao foreland fold belt,between the Jinshajiang-Ailaoshan and Lancangjiang faults,and the deposit can be divided into eastern and western ore zones.Based upon microscope observation of ore minerals and analysis of zinc,copper,and strontium isotope composition,we conclude that:(1)the zinc isotopic compositions of sphalerite from the eastern and western ore belt of the Baiyangping polymetallic ore deposits are enriched in both the heavy(-0.09‰ to+0.15‰) and light(-0.19‰ to-0.01‰)zinc isotopes.Rayleigh fractionation is likely the additional factor controlling the observed temporal and spatial variations in zinc isotopes in the two studied ore zones.The zinc isotopic composition in the Baiyangping polymetallic Pb-Zn deposits may have the same fractionation as that of magmatic-hydrothermal,VHMS,SEDEX,and MVT deposits,as demonstrated by geological and other geochemical evidence;(2) the range of δ^(65)Cu in massive tetrahedrite is from-0.06‰ to+0.12 ‰ that relates to the early stages of ore-formation,which are higher than that of venial chalcopyrite(from-0.72‰ to-0.07‰)formed at a late ore-forming stage in the western ore belt.Different ore-forming stages and alteration or leaching processes are likely the main factors controlling the observed variations in copper isotopes in the western ore zone;(3) the ^(87)Sr/^(86)Sr value of hydrothermal calcite in eastern(0.7080-0.7093) and western(0.7085-0.7113) ore belt suggested that mineralization of early calcite,with^(87)Sr/^(86)Sr values much higher than in ancient Late Triassic seawater,may be related to recrystallization from a radiogenic Sr-rich or silicifying fluid,either from the strata that the ore-forming fluid flows through or from other fluids.

THE EVOLUTION OF LANPING RIFT BASIN FROM LADINIAN IN MIDDLE TRIASSIC EPOCH TO EARLY JURASSIC EPOCH

Lanping basin was a massif (land massif) in late Palaeozoic Era. The ocean of Jinshajiang separated it from Yangtze plate in east. Lancangjiang ocean separated it from Yunnan—Tibet plate in west. From late Permian Epoch, the oceanic crust of Jinshajiang subduced the west from east, the one of Lancangjiang down went the east from west, and then the Yunnan—Tibet ancient land gradually closed to the Yangtze. In the end of the Permian Period, two continents and Lanping plate touched together, and the evolution history of the Paleotethys was end. Hercynian orogenic belt in the east and west sides of Lanping had volcanic rock colliding in early—middle Triassic Epoch. In Ladinian in middle Triassic and Carnian in late Triassic, the north side of Lanping basin formed the serial volcanic rock of spilite—quartz keratophyre because mantle\|derived magma causing by delamination rose and mixed with the constituent of continental crust. The volcanic rock overlapped the middle Triassic and late Palaeozoic stratum in angular unconformity. It was the feature of double peak or evolution from the basic to the acid. The race element distribution of volcanic rock was same as the one of tholeiite in island and inter\|arc basin. The rate of lead isotope of the volcanic rock was much higher. These points all distributed above the NHRL in Pb\|Pb. This indicated that the Pb of volcanic rock was the mantle\|derived magma mixed with crust one. The large\|area progression in Lanping rift basin begun in late Carnian.. The east side in Lanping basin developed the sedimentary system that was granule gravel (grit) rock in border facies—limestone in beach facies—black shale, and the middle had black shale, banded siliceous rock, brecciform limestone in late Carnian to Norian. The geochemistry research of siliceous rock showed that the genesis of the chert was hot water. The development of brecciform limestone was related with action of central\|axis rift. The Lanping rift basin went into consuming stage in Rhaetian Epoch of the late Triassic. The basin developed clastic rock bearing coal of continental\|oceanic alternation facies. In early Jurassic, the sedimentary area atrophied further, and the fine lacustrine sediment whose thickness was not great developed in the east of central\|axis. The west stratum of the basin in late Triassic Epoch touched directly with the one in middle Jurassic. Lanping basin was going into another evolution stage that was down\|warped basin.

A Predictive GIS Model for Potential Mapping of Copper, Lead, and Zinc in Langping Area, China

Mineral resource potential mapping is a complex analytical process,which requires the consideration and the inte-gration of a number of spatial evidences like geological,geomorphological,and wall rock alteration.The aim of this paper is to establish mineral exploration model for copper,lead,and zinc in Lanping basin area using the capability of analytical tools of Geographic Information System(GIS) and remote sensing data to generate maps showing favorable mineralized area.The geo-exploration dataset used for the research includes copper,lead,and zinc deposits,geological maps,topographic maps,structural maps,and ETM+ imagery.Geological features indicative of potential copper,lead,and zinc were extracted from the datasets input in the predictive model.The method of weights of evidence modeling is a probability-based technique for generating mineral potential maps using the spatial distribution of indicative features with respect to the known mineral occur-rences.The method of weights of evidence probabilistic modeling provides a quantitative method for delineating areas with potential of copper,lead,and zinc mineral deposits in the Lanping Basin area.weights(W+,W-) and contrast(C=(W+) -(W-) ) calculations guide the data-driven modeling.The four most important spatial features for exploration guide for copper,lead,and zinc mineralization hosted in the Lanping Basin area are alteration zones,faults,host rocks,and lineaments.The host rocks and deep faults have the strongest spatial association with the known copper,lead,and zinc deposits.The hydrothermal alteration zones have the moderate spatial association with the copper,lead,and zinc deposits.The predicted high-favorability zones do not show the strong affinity with lineaments.The distribution of 22(copper,lead,and zinc) occur-rences in the Lanping Basin was examined in terms of spatial association with various geological phenomena.The analysis of these relationships using GIS and weights of evidence modeling has predicted areas of high and moderate mineral potential,where a little or no mining activities exist.