Recognition of two contrasting structural- and mineralogical-gold mineral systems in the Youjiang basin, China-Vietnam: Orogenic gold in the south and Carlin-type in the north

The gold deposits in the Youjiang basin, totaling > 25 Moz gold, have traditionally been thought to be of Carlintype, particularly those with extensional structural geometries in the northern basin dominated by platform sedimentary sequences. However, the structural geometries, mineralization styles and alteration types for the Jinya, Gaolong and Nakuang gold deposits in the south-central part of the basin are remarkably similar to those of unequivocal orogenic gold deposits. Structural studies show that gold mineralization in the three gold deposits was controlled by tight “locked-up” anticlines with NW
SE- to E
W-trending and/or concomitant thrusts and/or shear zones, which resulted from NE
SW- to N–S-trending compression or transpression following the Early Triassic closure of the Paleo-Tethyan Ocean. Alteration zones in these deposits are dominated by silicification (quartz), sericitization, sulfidation and carbonation. Zoned pyrites in these deposits comprise Au-poor cores and invisible Au-bearing rims with minor external free gold. Euhedral to subhedral auriferous arsenopyrites also contribute to the gold budget. These features indicate that the three gold deposits are sediment-hosted orogenic gold deposits that contrast markedly with the Carlin-type gold deposits in the northern part of the Youjiang basin in terms of structural geometry and timing, mineralization style and nature of associated alteration. Although additional reliable ages using robust methodologies are still required, the older isotopic ages of the gold deposits in the south-central Youjiang basin are also consistent with earlier formation during transpression that predated extension during orogenic collapse, the period of formation of the Carlin-type gold deposits in the northern Youjiang basin.