Geological and Geochemical Characteristics of the Cherts in the Mojiang Gold Deposit and Their Implications

Geological and geochemical characteristics of cherts of the low\|middle parts of the Jinchang Formation indicate that cherts are associated with hydrothermal sedimentation, but the middle\|upper parts of the Jinchang Formation are mixed with normal sediments. The cherts are characterized by high Fe, As, Sb, Bi and Ga, low Al and total REE, negative Ce anomaly and HREE enrichment. Their \{δ\{\}\+\{18\}O\} values show that the forming temperatures of the cherts range from 128 ℃ to 146 ℃.

GeochemicalFeaturesandFormationoftheAuriferousChertsintheMojiangGoldDeposit,Yunnan

Auriferous cherts in the Middle Carboniferous Jinchang Formation are the dominant host rocks of auriferous quartz veins and mixed orebodies comprised of gold\|bearing quartz veins and cherts in the Mojiang gold deposit. The rocks exhibit sedimentary texture and structure and are composed of hot\|water deposited minerals. The FeO, Fe\-2O\-3, Au and Ag contents of the auriferous cherts are high; the Cr, Ni and Co contents are also high but significantly variable; MnO/TiO\-2 and TFe/TiO\-2 ratios are relatively high. As viewed from a few diagrams that distinguish different chert formations, the auriferous cherts are in or near the range of hot\|water deposited cherts. Because the correlation coefficients between Au contents and those of Cr, Ni of the rocks are negative, a great Au amount in the cherts might not be brought about by later hydrothermal alterations. The rare\|earth elements, O and Si isotopic compositions of the auriferous cherts demonstrate that the cherts belong to hot\|water deposited rocks. The later hydrothermal alterations made the petrochemical compositions of the cherts deviate from the characteristics of hot\|water deposition. In general, the geological and geochemical features of the auriferous cherts demonstrate that the rocks were formed by hot water deposition.

Geochemistry of Upper Devonian Hydrothermal Mammilated Chert Guangxi,Southwest China

The mammilated chert(MC) studied in this paper is a kind of silica-rich concretion(SiO2 93%) occurring in the Upper Devonian argillaceous silicolites(SiO2≈88%)in Mugui,Guangxi.Impurity components in the MC are relatively low and show systematic variationhs as compared with the country rocks.Abundances of Mg,Si,Ti,Fe,Mn,U and Th are characteristically similar to those of recent and fossil hot-water sediments,Moreover,the MC is also characterized by low REE baundance,LREE>HREE,apparent negative Ce anomaly and moderate positive Eu anomaly,OXygen isotope data show that the formation temperature of the MC is about 20℃,higher than that of the host rock.The results of geochemical,geological and lithological studies suggest that the MC may have been formed through metasomatism,fillinn and crystallization beneath the sedimentary basin during the Devonian period.

Petrologic and Geochemical Characteristics and Origin of Gusui Cherts, Guangdong Province, China

The characteristic structures of the Precambrian cherts from the Gusui section, Guangdong, Chi -na, include bedded structure, laminated structure, massive structure and pseudobrecciated structure.The chert is characterized by consistently low abundance of TiO2, Al2O3 and most trace elements.Howevver, it is enriched in Ba, As, Sb, Hg and Se. In Al-Fe-Mn temary diagrams, it falls into the "hydrothermal field". Correspondence analysis and factor analysis show that many elements show up in the factor that represents the leaching of country rocks by hydrothermal solutions, and are the very characteristic element association of the geochemically anomalous South China basement.Petrologic and geochemical evidence suggests a hydrothermal origin for the chert. The chert may have been formed in a Precambrian rift or an extension zone develoed within the Yunkai marginal geospyncline , with a fault system linking it to an unknown heat source at depth.