Geochemistry, Zircon Genetic Mineralogy, U-Pb Geochronology Of The Dykes Around Shihu Gold Mine, Hebei Province And Its Implication On Gold Mineralization

Intermediate-felsic dykes temporally and spatially associated with gold quartz vein are widely distributed in Shihu gold mine, Hebei Province. The Shihu gold mine is hosted in Archean Fuping Formation metamorphic rocks and Mapeng granitiod is located northwest of Shihu mine. In order to distinct the formation mechanism and role for gold mineralization, we prepare to use the analysis methods of petrology, geochemistry, geochronology and zircon genetic mineralogy. Europium anomalyThe lithology of dykes mainly composed of dioritic porphyrite, quartz dioritic porphyrite, granite-porphyry and so on. These dykes are highly enriched in LIL elements such as Rb, Sr, Ba and K as well as LREE, while they are highly depleted HFS elements such as Nb, Ti and HREE. The dykes show a high ratio of (La/Yb)N(15.6-73.3) and no pronounced europium anomaly but have very weak positive Eu anomaly(δEu* =0.98～1.06). The intense fractionation of REE and the obvious depletion of HREE suggest that there are garnet in magma resource region and they are derived from thicken sub-continental crust with depth more than 60km. In addition, phenomena of field investigation and petrography show features of magma mixing in dykes. Combined with previously results, we put forward the most possible reason of formation mechanism of dykes: during the orogeny process of Taihang Mountain, felsic magma derived from enriched mantle formed thicken sub-continental crust, then the mantle-derived magmas ascend later made the thicken sub-continental crust partial melting to form intermediate-felsic magma chamber. In this condition, double-layer chambers, the upper was intermediate-felsic magma and lower was mafic magma, were formed. Injection of mafic magma from lower to upper could produce mixing magma, when the crust structural changed to extensional environment, faults could let the mixing magma ascend fast to surface to form dykes with different composition.The statistical data from external morphology of zircons show the developed degree of cylindrical {110} and conical {211} are better than {100} and {101} respectively. Based on the results, we suggest the magma formed intermediate-felsic is rich water and other volatile. The CL images show clear internal morphology of zircons, and from core to edge, the morphology of zircons had changed in the process of evolution, suggesting the decreased of alkalinity of hosted magma and indicating the occurance of magma mixing. In this paper, we introduced an efficient method, using ratio of Ce(Ⅳ)/Ce(Ⅲ) in zircon to distinct oxygen fugacity of magma. The results of ratio of Ce(Ⅳ) /Ce(Ⅲ)from dykes in research field show they were belong to high oxygen fugacity magma.Zircon from two samples obtained 129±2Ma and 131±2Ma LA-ICP-MS U-Pb age respectively, showing the similar time of intrusion of intermediate-felsic dykes and Mapeng granitic batholiths. Based on the geochronology and charachetristic of geochemistry of dykes and Mapeng granitic batholiths, we concluded that they have the same source region.The intermediate-felsic dykes derived from earth depth can provide part of gold for Shihu mine. The features such as high volatile content, high oxygen fugacity and rapid ascending replacement of the dykes indicate they can provide mineralizer and in favor of migration and deposit of gold. Integrated the above mentioned study results, we proposed some prospecting indicators:large distribution density of high K calc-alkaline intermediate-fesic dykes, low content of HREE, high volatile content and oxygen fugacity.