RESUMEN
SignificanceThere is a common consensus that lode gold deposits mostly precipitated from metamorphic fluids via fluid boiling and/or fluid-rock interaction, but whether magmatic hydrothermal fluids and the mixing of such fluids with an external component have played a vital role in the formation of lode gold deposits remains elusive. We use garnet secondary ion mass spectrometry oxygen isotope analysis to demonstrate that the world-class Dongping lode gold deposit has been formed by multiple pulses of magmatic hydrothermal fluids and their mixing with large volumes of meteoric water. This study opens an opportunity to tightly constrain the origin of lode gold deposits worldwide and other hydrothermal systems that may have generated giant ore deposits in the Earth's crust.
RESUMEN
To investigate the factors controlling the mineralization in ophiolites we systematically compared the petrology and mineral compositions of the harzburgites/lherzolites, dunites and chromitites in the Luobusa and Purang ophiolites. Generally, the petrological features and trace element compositions of chromite and olivine in peridotite and chromitite are distinctly different between the two ophiolites. In Luobusa, boninitic melts are inferred to have interacted with the harzburgites and modified the distributions of some trace elements (e.g., Ni, Mn and V) in chromite and olivine. The subsequently formed dunites and chromitites experienced significant elemental exchange. In contrast, the Purang ophiolite contains a wider range of chromitite compositions and records diverse melt activities, such as the growth of relatively abundant secondary clinopyroxene. The metasomatic melts were enriched in Al and depleted in Si, Na and highly incompatible trace elements (e.g., Nb, Zr). Such melts resemble MORB-like melts proposed in the literature but are assumed to be more hydrous than typical MORB because of presence of hydrous minerals. The parental magmas of the Purang dunites and intermediate chromitites are inferred to be compositionally intermediate between boninitic and MORB-like melts. In addition, the more refractory nature of the Luobusa harzburgites facilitated a high Cr concentration gradient with the interacting melts, making it easier to increase Cr in the melts. Crystallization of clinopyroxene and amphibole in the Purang ophiolite accommodated significant amounts of Cr and water, respectively, and negatively affected Cr concentration and chromite crystallization. The concentration of chromite to form chromitites requires the presence of focused melt channels.
RESUMEN
We report Li isotopic compositions of olivine from the mantle sequence of the Luobusa ophiolite, southern Tibet. The olivine in the Luobusa ophiolite has Li concentrations from ~0.1 to 0.9 ppm and a broad range of δ(7)Li (+14 to -20). An inverse correlation of Li concentration and δ(7)Li in olivine from harzburgite suggests recent diffusive ingress of Li into the rock. Olivine from dunite enveloping podiform chromitites shows positive δ(7)Li values higher than those of MORB, whereas olivine from the chromitite has negative δ(7)Li values. Such variations are difficult to reconcile by diffusive fractionation and are thought to record the nature of the magma sources. Our results clearly indicate that the Luobusa chromitites formed from magmas with light Li isotopic compositions and that the dunites are products of melt-rock interaction. The isotopically light magmas originated by partial melting of a subducted slab after high degrees of dehydration and then penetrated the overlying mantle wedge. This study provides evidence for Li isotope heterogeneity in the mantle that resulted from subduction of a recycled oceanic component.