Recycling of metal-fertilized lower continental crust: Origin of non-arc Au-rich porphyry deposits at cratonic edges

Abstract

Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383-445 ppm) and Au (7-12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6-16 ppb) with higher Au/Cu ratios (2 × 10 −4 to 8 × 10 −4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au-fertilized low-crustal cumulates at 40-30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits. LOWER CRUSTAL XENOLITHS AND THEIR ORIGINAbundant xenoliths have been found with Eocene stocks and associated volcanic rocks at six locations exposed along the cratonic edge (Fig. 1B). Amphibolites and garnet amphibolites are the primary types, the former hosted by Liuhe syenites and Beiya monzogranite porphyries ( Fig. 2A), and the latter widely occurring in the Liuhe stock (Fig. 2B). Their mineralogical and whole-rock compositions, and zircon, oxide, and sulfide geochemical *

Hydrothermal ore deposits in collisional orogens

“…For porphyry Cu deposits in the Gangdese orogen, Hou et al [30,31] link metallogenesis to pre-enrichment of the ore-forming element in the mantle wedge above the Neo-Tethyan oceanic subduction zone. Similar arguments are also shared for the genesis of REE-rich carbonatite at Mianning-Dechang in Southwest China [32] and Au-rich porphyry deposits at Beiya along the western margin of the South China Block [33]. In this regard, the temporal and spatial distributions of hydrothermal ore deposits at convergent plate boundaries are the result of spectacular metallogenic processes.…”

Two‐Stage Sulfide Mineral Assemblages in the Mineralized Ultramafic Rocks of the Laowangzhai Gold Deposit (Yunnan, SW China): Implications for Metallogenic Evolution

“…Cenozoic high‐potassium alkaline rocks with age ranging from 41 to 32 Ma intruded the low‐grade metamorphosed Paleozoic‐Mesozoic unit and the Ailaoshan suture zone (Liang et al, ; Sun et al, ; Zhang et al, ). These potassic magmatisms triggered the formation of many porphyry ore deposits, for example, Beiya Au‐Cu deposit and Habo Cu deposit (Hou et al, ). The Cenozoic lamprophyres with a phlogopite Ar‐Ar age of 29.6–36.8 Ma were considered to have intimately temporal and spatial relationships with hydrothermal gold deposits in the Ailaoshan gold belt (Chen et al, ).…”

Sample Digestion and Combined Preconcentration Methods for the Determination of Ultra-Low Gold Levels in Rocks

“…Gold is one of the rare elements and precious metals present in the Earth crust with average concentrations in the igneous, sedimentary, and metamorphic rocks varying between 0.5 and 5 ppb [1]. Proper quantification of the gold abundances in basic rocks is critical for many leading-edge areas of geoscience, such as planetary differentiation [2,3,4,5,6,7,8,9,10], redistribution of elements during crustal processes [11,12,13,14,15,16,17], and ore genesis [18,19,20,21,22,23]. According to the latest results reported by Brenan and McDonough [10], the metal–silicate partition coefficient of Au is approximately 300, whereas its minimum values measured for Os and Ir in the same experiments are ~10 7 , which differs from the former parameter by at least a factor of 10 4 .…”