Mineralogical Geochemistry And Prediction For Deep Gold Deposit In Shihu, Western Hebei Province, North China

The Shihu deposit, which is a well known quartz vein type gold deposit in thenorthern part of the Taihang orogen of the North China craton (NCC), is hosted byductile-brittle faults within Archean metamorphic core complex of the Fuping Group.The ore genesis, ore-controlling factores and metallogenic laws were discussed, andthe two new prospecting targets in the Shihu gold deposit were predicted based on thedata of rock-forming mieneralogy, ore mineralogy, inclusion mienralogy, isotopicgeochemistry and prospecting mineralogy .The Mapeng granitic stock is one of the most important plutons in the Shihu areawas emplaced into the Archean Fuping complex. Mafic enclaves are randomly foundthroughout the granitic plutons. The composition of total tocks and rock-formingmineralogical chemistry indicated that the pluton is adakitic rock with pronouncedmixture of crust and mantle. The pluton mainly contains magmatic zircons with theage at ca. 130Ma, and also includes minor inherited zircon (1.7².5Ga) probablycoming from the basement.40Ar/39Ar and Rb-Sr geochronology suggest themineralized quartz vein formed during Mesozoic era. Therefore, it is suggested thatthe age of 130Ma could be the upper limit of the metallogenic age of the Shihu golddeposit.Native gold is common minerals in the ore. The fineness of native Au is low,from 442 to 926. The ore minerals are mainly pyrite, sphalerite, galena, chalcopyrite,magnetite and pyrrhotite. The isotopic ratios (sulfer and lead of sulfides, carbon ofcarbonate, and He-Ar of pyrite) indicate that the metallogenic materials were mainlyderived from low crust, which were markedly interacted with mantle fluids. Moreover,the H-O isotopic ratios of quartz fluid inclusions suggested that the ore-forming fluidscould be mainly magmatic water, which were mixed by meteoric water of variousextents at different ore-stage.The fluid inclusions in quartz can be classified into the two types:CO₂-NaCl-H₂O and NaCl-H₂O types. The densities and pressures of the inclusionsthat were calculated according to the measured temperatures and salinities athomogenization of the inclusions, indicated the metallogenic depths could befrom5.93 to 8.03 km with average depth at 7.20 km. The vapor phase in these inclusionswas dominated by CO₂, H₂S, CH₄, C₂H₄, N₂and Ar except for H2O. The anion andcation of liquid phase were determined to be Cl^-, SO₄^2-, and Na⁺, K⁺with minor Mg²⁺ and Ca²⁺. The chondrite-normalized rare earth element (REE) patterns of fluidinclusions in quartz and pyrite apparently differentiated each other. The REEpatternsof quartz were charachterized by relatively enrichment of light rare earth elements andpositive Eu anomalies, but the REE patterns of pyrite have negtive Eu anomalies.Thermoelectricity, cell parameter and decrepitation temperature of pyrite, as wellas thermoluminescene and cell parameter of quartz were measured, which suggestedthat the Au ore bodies have stable occurrence, continous extension and weakdenudation. The two new prospecting target areas were predicted in the deeperregions (below 180 m level) at the 19-31 exploration line and 3-11 exploration line .Geodynamic background of the Shihu and other gold deposits in the northTaihang orogen was summarized. Plutonic rocks and the gold deposit were formedduring the same geological duration in whichlarge scale lithosphere thinning of NorthChina craton and crustal-mantle interaction occurred. Underplating of basaltic magmain the lower crust resulted in partial melting of TTG-dominated Archean basementrocks, producing granitic melts. Mixture between the basaltic and granitic magmas invariable proportions spawned hybrid magmas that evolved, coupled with assimilationof country crustal rocks during magma ascent, which formed the Mesozoic magmas inthe Taihang orogen. It was such interaction that caused intensive felsic magmatism,the "Mesozoic Metallogenic Explosion" in eastern China, and the consequentformation of most gold deposits in the North Chine Craton.