Liang Huaying

Ridge subduction and porphyry copper-gold mineralization: An overview

Many large porphyry Cu-Au deposits are connected to adakitic rocks known to be closely associated with ridge subduction. For example, there are several subducting ridges along the east Pacific margin, e.g., in Chile, Peru, and South America, most of which are associated with large porphyry Cu-Au deposits. In contrast, there are much fewer ridge subductions on the west Pacific margin and porphyry Cu-Au deposits are much less there, both in terms of tonnage and the number of deposits. Given that Cu and Au are moderately incompatible elements, oceanic crust has much higher Cu-Au concentrations than the mantle and the continental crust, and thus slab melts with their diagnostic adakitic chemistry have systematically higher Cu and Au, which is favorable for mineralization. Considering the geotherm of subducting slabs in the Phanerozoic, ridge subduction is the most favorable tectonic setting for this. Therefore, slab melting is the likely link in the spatial association between ridge subduction and Cu-Au deposits. Geochemical signatures of slab melting and hence maybe ridge subduction in less eroded regions in eastern China, the central Asian orogenic belt etc. may indicate important exploration targets for large porphyry Cu-Au deposits.

Zircon LA-ICP-MS U-Pb age, Ce4+ /Ce3+ ratios and the geochemical features of the Machangqing complex associated with copper deposit

The Machangqing complex is composed of early syenitic-monzonitic porphyry and late granite-porphyry. Geochemically, they are similar in major and trace element characteristics, and are characterized by relatively low TiO2 ( < 0.5) and P2O5 (< 0.4) and high alkali contents (K2O + Na2O > 8% ), with K2O/Na2O > 1, the absence of significant Eu anomaly and pronounced negative anomalies in Nb, Ta, Ti, and enrichment in large-ion lithophile elements and light rare-earth elements. Zircon U-Th-Pb ELA-ICP-MS age determination of the Machangqing complex has revealed two discrete igneous events. The syenitic-monzonitic porphyry has an age of 35. 6 ±3 Ma, MSWD = 2. 80 ; the granite-porphyry, 35. 0 ±0.2 Ma, MSWD =2.91. The lifespan of magmatic activities of the Machangqing complex associated with Cu + Mo mineralization is about 0.6 Ma. The late-stage ore-bearing granite-porphyry has higher Ce4+ /Ce3+ ratios (264) than the early-stage barren syenitic-monzonitic porphyry (102). The increase of zircon Ce4+ /Ce3+ ratios from the early-to late-stage porphyries indicates that the oxidation state of the magmas associated with copper mineralization increases with time.

Metallogenic age and genesis of Jinshan gold deposit, Jiangxi Province, China

The Jinshan gold deposit consists of gold-bearing ultramylonite and gold-bearing quartz vein ores. The Rb-Sr isochron age of fluid inclusions in quartz from quartz veins is the same as that of the gold-bearing ultramylonite, suggesting that both the types of orebodies were formed simultaneously in the Caledonian period, in the range 406–409 Ma. REE patterns and sulfur, lead, carbon, hydrogen and oxygen isotope data, as well as the composition of fluid inclusions, have shown that the ore-forming fluids were derived from formation water, and the ore-forming materials came from the gold-hosted rocks. The Jinshan gold deposit occurring in a Caledonian brittle-ductile zone in metamorphosed microclastic rocks owes its origin to Caledonian reworking processes.The Jinshan gold deposit consists of gold-bearing ultramylonite and gold-bearing quartz vein ores. The Rb-Sr isochron age of fluid inclusions in quartz from quartz veins is the same as that of the gold-bearing ultramylonite, suggesting that both the types of orebodies were formed simultaneously in the Caledonian period, in the range 406–409 Ma. REE patterns and sulfur, lead, carbon, hydrogen and oxygen isotope data, as well as the composition of fluid inclusions, have shown that the ore-forming fluids were derived from formation water, and the ore-forming materials came from the gold-hosted rocks. The Jinshan gold deposit occurring in a Caledonian brittle-ductile zone in metamorphosed microclastic rocks owes its origin to Caledonian reworking processes.

Geology and Geochemistry of Reworking Gold Deposits in Intrusive Rocks of China──II.Gold Deposits and Their Genesis

Gold deposits in intrusive masses include the veinlet dissemination, quartz vein and veinlet dissemination + vein types. They are distributed in fracture zones along the endocontact zone of a batholith or in the centre and edge of a stock. The metallogenic epochs are Yenshanian, Hercynian, Archean, Proterozoic and Himalayan. The gold deposits are characterized by a big difference in time span between gold mineralization and the formation of host masses. Ore-forming materials were derived from the masses and auriferous strata and ore-forming fluids came from meteoric and formation waters. When circulating water was heated by ascending heat flow, gold would be extracted, concentrated and transported from auriferous rocks and then precipitated in the masses during the late tectonic movement. Finally gold deposits were formed in the intrusive masses.

Single Zircon Pb-Evaporation Age of Shuiquangou Gold-Bearing Alkali-Intrusion in Zhangjiakou Area,Hebei Province

This paper deals with the forming age of the Shuiquangou gold-bearing alkaline rock with the help of the single zircon Pb-evaporation method. The apparent age (1607–1667 Ma) got from crystalline zircon grain domains, where there exists an almost enclosed system, is in good agreement with zircon U-Pb conventional upper intercept age (1718 Ma), suggesting that the Shuiquangou gold-bearing alkali rock intruded at 1600–1700 Ma ago.

Geology and geochemistry of reworking gold deposits in intrusive rocks of China—I. Features of the intrusive rocks

Most gold deposits in intrusive rocks were formed as a result of reworking processes. The intrusive rocks containing gold deposits and consisting of ultramafic-mafic, intermediateacid and alkaline rocks of the Archean, Proterozoic, Caledonian, Hercynian and Yenshanian periods occur in cratons, activated zones of cratons and fold belts. Among them, ultramaficmafic rocks, diorite, alkaline rocks, and anorthosite are products of remelting in the mantle or mantle-crust or mantle with crustal contamination. However, auriferous intermediate-acid rocks are products of metasomatic-remelting in auriferous volcanic rocks or auriferous volcanosedimentary rocks in the deep crust.

A three-stage metallogenic model for gold deposits in metamorphosed microclastic rocks

Gold deposits occurring in metamorphosed microclastic rocks are distributed extensively at home and abroad. Some deposits of this type are of superlarge tonnage. The formation of gold deposits in metamorphosed microclastic rocks involves three stages : the sedimentary stage, the regionally metamorphic stage, and the ore-forming stage. At the first stage, microclastic sedimentary source rocks were developed in a relatively semi-enclosed reducing sea basin and were enriched in carbon, sulfur and gold. At the second stage, the gold adsorbed on organic matter and clay minerals was released and poorly concentrated during the destruction of organic matter and the depletion of clay minerals by regional metamorphism with increase temperature and pressure. At the third stage, a tectono-hydrothermal event took place. As a result, gold was leached from metamorphosed microclastic rocks, transported to ore depositional locus and/or mixed with gold of other sources in the course of migration, and finally precipitated as ores. Gold deposits of this type were eventually formed at the third stage, and they also can be classified as the orogenic belt type and the activation zone type. The gold deposits occurring in metamorphosed microclastic rocks are the products of reworking processes and the influence of magmatism should be taken into consideration in some cases.

Isotopic Characteristics and Key Factors Favoring the Formation of Fuwan Super-large Silver Deposit in Guangdong Province

Rb−Sr isochron age of fluid inclusions in quartz from the Fuwan super-large silver deposit is 68±6 Ma, the silver deposit is characterized by high μ values (10.67–10.95), which are much higher than those of the ore-hosted Paleozoic strata and are close to those of ores hosted in the Proterozoic metamorphic basement in western Guangdong Province. Based on the Pb isotopic characteristics, coupled with much high background silver contents (200–1000 ng/g) in the Proterozoic basement and relatively low silver contents in the Paleozoic strata in the region of the Sanshui Basin, it is concluded that the ore-forming material of the super-large silver deposit came mainly from the old basement. The super-large silver deposit related genetically to the intense volcanic activities during the Upper Cretaceous to Eogene. The formation of the Fuwan super-large silver deposit is controlled by the following favorable geological conditions: (1) The intersection of deep faults and contemporaneous faults at the margin of the Sanshui Basin led to the formation of an excellent structure as passageway for ore fluids; (2) The special ore-hosted rock association forms a ore gathering-trap structure that favors the precipitation of ore; (3) The silver-rich old basement, multi-stage mineralization and multi-episode volcanic activities which constitute a geothermal convection system.