Using electrogeochemical approach to explore buried gold deposits in an alpine meadow-covered area

Exploration for buried gold ores and other deeply buried ores, especially in high altitude localities, is one of the tough challenges facing the geological world today.Fast and efficient ore prospecting methods are badly needed to deal with the situation. This paper documents a test that, for the first time, uses an electrogeochemical approach to prospect ores in the alpine meadow-covered Bangzhuoma area and its periphery in Qinghai–Tibet Plateau.The results were compared with conventional soil measurements from a 2 D prospection, and an ideal model of electrogeochemical anomaly formation in the area was established based on the comparison in order to provide theoretical guidance to buried ore prospecting in areas with similar conditions. The research shows that:(1) For exploration of deeply-buried mineral deposits, an electrogeochemical approach is better than soil measurements in terms of correspondence between element content values and anomaly forms and spatial distribution of known deposits in sections. Anomalies of high to low temperature element associations(Bi–Mo; Au–Ag–As–Bi and Au–Ag)and clear zonation were also observed along vertical vein runs in the sections. Based on integration of the observation with geological characteristics of the sections, we propose to use Au, Ag and As as the electrogeochemical indicators and Bi and Mo as the electrogeochemical tracing elementsto guide further analysis.(2) Judging from element statistics and the scale, intensity, and range of anomalies in plan maps, we found that an electrogeochemical approach is less affected by topography and secondary actions. The plan maps also show that elemental differentiation coefficients of the study area are in an ascending order of Ag(0.67) \ Mo(0.85) \ Bi(0.97) \ Au(1.51) \ As(2.35),better representing the element distribution in the area and yielding more striking and concentrated anomalies for known deposits than that of the soil measurements. Apart from that, electrogeochemical anomalies were observed in the south of lines 002 and 003 and the central of lines 008 and 009. We suggest the existence of deeply-buried mineral deposits based on analyses of element combinations and gold grade variations in samples taken from exploratory trenches in the area.(3) A three-stage(referring to the ore body dissolution, the mineralogenetic particle migration, and the mineralogenetic particle unloading) electrogeochemical ideal model was established for the study area, which takes into account moderate rainfall, high altitude, low air pressure, well-developed vegetation and roots, and an Upper Triassic Nieru Formation carbonaceous sandy slate as overburden.

A geochronological and geochemical study on the granodiorite porphyry and its implication for the mineralization in the Dayaoshan metallogenic belt,Southeastern China

A systematic study combining U-Pb zircon dating,lithogeochemical and Sr-Nd isotopic analyses was carried out upon the Xinping granodiorite porphyry in the Dayaoshan metallogenic belt to understand its petrogenesis and tectonic significance.LA-ICP-MS U-Pb zircon dating yielded a 442.7±5.8 Ma age,indicating that the granodiorite porphyry was emplaced during the Llandovery Silurian of the Early Paleozoic.The granodiorite porphyry shares the same geochemical characteristics such as Eu negative anomaly as other syn-tectonic granite plutons in the region,including the granodiorite porphyry in Dawangding and granite porphyries in the Dali Cu-Mo deposit and Longtoushang old deposit,indicating a similar magma evolution process.The Xinping granodiorite porphyry has high contents of SiO2(67.871.8%)and K2O(1.78-3.42%)and is metaluminous-peraluminous with A/CNK ratios ranging from 0.97 to 1.06,indicative of high-potassium calc-alkaline to calc-alkaline affinity.It is a I-type granite enriched in large ion lithophile elements Rb,Sr,while depleted in Ba and high field-strength element Nb.Tectonically,a collision between the Yunkai Block from the south and the Guangxi Yunnan-North Vietnam Block from the north during the Early Paleozoic was followed by uplifting of the Dayaoshan terrane.The Xinping granodiorite porphyry was likely emplaced during the collision.Sr-Nd isotopic analyses show that the granodiorite porphyry has initial 87Sr/86Sr ratios(Isr)of 0.7080-0.7104,εNd(t)range from-0.08 to-4.09,and t2DM between 1.19 and 1.51 Ga,well within the north-east low-value zone of the Cathaysia block,indicating a Paleoproterozoic Cathaysia basement source and an involvement of under plating mantle magma.Field observations,geochronological data,and 3D spatial distribution all lead to the conclusion that the Early Paleozoic Xinping granodiorite porphyry does not have any metallogenic and temporal relationships with the Xinping gold deposit(which has a Jurassic-Early Cretaceous age based on previous studies)but a close metallogenic relation to W-Mo mineralization.