Hydrothermal alteration of magnetic fabrics of rocks in the Xiaoban gold-bearing shear belt, Fujian Province, China

La zona de deformación de Xiaoban experimentó alteración hidrotermal a temperaturas moderadas en el Mesozoico (180 Ma). Las muestras estudiadas corresponden a unidades alteradas e inalteradas provenientes de tres niveles en la mina. Después de un estudio sistemático de las fábricas de rocas magnéticas encontramos que: 1) La susceptibilidad se debe principal- mente a la presencia de minerales paramagnéticos como la biotita y la pirita. La reducción de la susceptibilidad promedio se debe a la disminución de hierro y al cambio del tamaño del mineral magnético, así como al incremento de minerales diamagnéticos. 2) Decrecimiento de la anisotropía (P) después de la alteración hidrotermal. La tendencia al cambio es la misma que la de la susceptibilidad promedio. 3) El elipsoide de AMS no cambia sustancialmente su forma después de la alteración hidrotermal, y se mantiene una esfera oblada. El eje menor de susceptibilidad axial principal (K3) cae cerca del plano perpendicular al plano de equistosidad de todas las muestras. Sin embargo, el eje mayor de susceptibilidad axial principal está sujeto a grandes cambios. Los resultados indican que los nuevos minerales magnéticos están distribuidos en el plano existente de equistosidad, pero no tienen una orientación preferencial. doi: https://doi.org/10.22201/igeof.00167169p.2003.42.1.36

A Study of Distribution and Change of Regional Tectonic-Fluid in Goldfields of Jiaodong Peninsula

Jiaodong is the biggest gold mineralization concentration area in China. The mineralization has direct relationship with Mesozoic granite. Most of deposits exist in regional shear zone where has the broken altered rock zone, which is the result of magmatic hydrothermal metasomatism. The main research problem of structure-fluid is how to guarantee the chosen rock sample can be used to do chemical fluid composition analysis and also used to observe tectonic deformation. We suggested that the potash feldspathized altered rock and the quartz-sericitized altered rock can be chosen as the research samples to solve this problem. This research method overcame the research difficulty in the regional structure-fluid area. The research result showed that the strain value decreases in potash feldspathized altered rock zone but increases in quartz-sericitized altered rock zone from west to east in the Jiaodong area; that the fluid homogenization temperatures increase in potash feldspathized altered rock zone but decrease in quartz-sericitized altered rock zone from west to east in the Jiaodong area. The chemical fluid composition has an obvious variation tendency. The Na+/K+ variation tendency in quartz-sericitized altered rock zone shows that the west of Jiaodong has more K+ than east and K+ is beneficial for mineralization; that the east of Jiaodong has more Na+ than west and Na+ is beneficial for fluid migration. The contents of CH4 and CO2 decrease from west to east, which is positively correlated with the enrichment of Au element. The characteristics of structure-fluid in fractured altered rock zone revealed the trend that the metallogenic hydrothermal fluid migrated from west to east in the Jiaodong region. This trend had its impact on the gold deposit scale, type and distribution

Multiple and prolonged porphyry Cu–Au mineralization and alteration events in the Halasu deposit, Chinese Altai, Xinjiang, northwestern China

The Halasu area is located in the southeastern margin of the Chinese Altai in Xinjiang, China. It is part of the Altaid orogenic collage where a number of porphyry-type Cu–Mo–Au deposits have been discovered in recent years. Geological mapping and drilling indicate the presence of various mineralized porphyritic intrusions in the Halasu Cu–Au deposit, which is currently under exploration. U–Pb dating of zircon crystals from four different mineralized porphyries reveals three significantly different ages of magmatic intrusion, i.e., ca. 372–382 Ma granodioritic porphyry and porphyritic granite, ca. 266 Ma quartz monzonitic porphyry, and ca. 216 Ma quartz dioritic porphyry. Re–Os dating of molybdenite from veinlet-dissemination ores in the granodioritic porphyry yields an age of mineralization of ca. 377 Ma, and Ar–Ar dating of K-feldspar from K-feldspar–quartz–chalcopyrite veins produces ages of ca. 269 and ca. 198 Ma. The mineralization (and alteration) ages correspond broadly to the three episodes of magmatic intrusion, suggesting three overprinting porphyry mineralization events that are significantly separated in time. The first episode of porphyry intrusion and mineralization may be related to the magmatic arc being above a plate subduction zone, and the second was formed in a late-collisional environment during the closing of the Junggar Ocean, whereas the third episode of mineralization took place in the post-collisional stage. This case study suggests that in orogens where major porphyry deposits have been found in magmatic arc environments, the potential of discovering late- to post-collisional porphyry deposits cannot be neglected; conversely, in orogens where most porphyry deposits have late- to post-collisional ages, more attention should be paid to porphyries that were formed earlier in magmatic arc environments

Late Paleozoic to Mesozoic extension in southwestern Fujian Province, South China: Geochemical, geochronological and Hf isotopic constraints from basic-intermediate dykes

The tectonic evolution of SE China block since late Paleozoic remains debated. Here we present a new set of zircon U-Pb geochronological, Lu-Hf isotopic data and whole-rock geochemistry for two stages of basic-intermediate dykes from the southwestern Fujian. The samples were collected from the NE-trending (mainly diabases) and NW-trending (mainly diabasic diorites) dykes and yielded zircon U-Pb ages of 315 and 141 Ma, with εHf (t) values of −8.90 to 7.49 and −23.39 to −7.15 (corresponding to TDM2 values of 850 to 1890 Ma and 737 to 2670 Ma), respectively. Geochemically these rocks are characterized by low TiO2 (0.91–1.73 wt.%) and MgO (3.04–7.96 wt.%), and high Al2O3 (12.5–16.60 wt.%) and K2O (0.60–3.63 wt.%). Further they are enriched in LREEs and LILEs (Rb, Ba, Th and K), but depleted in HFSEs (Nb, Ta and Zr). The tectonic discrimination analysis revealed that the dykes were formed in an intraplate extensional environment. However, the NW trending dykes show crust-mantle mixed composition, which indicate an extensional tectonic setting with evidence for crustal contamination. The SE China block experienced two main stages of extensional tectonics from late Carboniferous to early Cretaceous. The tectonic evolution of the SE China block from late Devonian to Cretaceous is also evaluated

Zircon U–Pb geochronology and geochemistry of rhyolitic tuff, granite porphyry and syenogranite in the Lengshuikeng ore district, SE China: implications for a continental arc to intra-arc rift setting

SE China is well known for its Mesozoic large-scale granitoid plutons and associated ore deposits. Here, zircon U–Pb geochronological and geochemical data have been used to better constrain the petrogenesis of the igneous rocks associated with porphyry Ag–Pb–Zn deposits in the Lengshuikeng ore district, SE China. The Lengshuikeng rhyolitic tuff, granite porphyry and syenogranite yielded zircon U–Pb ages of 161, 155 and 138 Ma, respectively. The Lengshuikeng granite porphyries belong to calc-alkaline series and show fractionated I-type affinities. The rhyolitic tuffs show almost similar characteristics as the granite porphyries. The Lengshuikeng syenogranites are all alkali-rich and show A-type affinities. The syenogranites have high contents of high field strength elements such as Nb, Ta, Zr, Hf; with Zr + Nb + Ce + Y contents of >350 ppm. Chondrite-normalized REE patterns show relative enrichment of LREEs and strong negative Eu anomalies. The Lengshuikeng granite porphyries, syenogranites and tuffs were probably derived from partial melting of underlying Proterozoic metasedimentary rocks with minor addition of mantle-derived magmas, accompanied by fractional crystallization. Detailed petrologic and geochemical data for the Jurassic igneous rocks from the Lengshuikeng ore district imply that during the Late Jurassic, SE China on the southeast of the Shi-Hang zone was a continental arc associated with the subduction of the Palaeo-Pacific plate and that since the beginning of the Early Cretaceous an intra-arc rift has been formed along the Shi-Hang zone