Sedimentary rock-hosted gold mineralization at Zalaa Uul, Khentii Range, northeastern Mongolia

The recently discovered Zalaa Uul occurrence exhibits gold concentrations averaging about 1 ppm in silicified breccias as wide as 100 m. Most mineralization is hosted in brecciated siltstone, shale, and calcareous sandstone of the Permian Ulz Formation that exhibits multiple stages of silicification. Rock geochemistry indicates: (1) gold is strongly associated with arsenic and silver; (2) antimony, tellurium and thallium are locally anomalous but poorly correlated with gold; (3) mercury is spatially correlated with copper; and (4) Ag:Au ratios are low (≤3). A low-level Cu–(Hg + Sb, ±Au + As) anomaly occurs over an hypothesized feeder breccia. The feeder breccia occupies a major northwest-dipping reverse fault zone between dominantly greenschist-facies phyllite and schist of the Upper Proterozoic Toshint Formation and unmetamorphosed marine clastic rocks of the permian Ulz Formation. Ground magnetometer surveys identified a magnetic body, thought to represent part of an intrusive complex at depth, within the reverse fault zone, down-plunge from the ∼70° northwest-dipping feeder breccia. Altered rhyolite dikes crop out in the vicinity of the feeder breccia. The potentially economic gold grades are 2 to 3 km outboard of the feeder breccia and may represent the distal Au + As zone of an intrusion-related mineralizing system. Alteration, regional structural and geophysical setting, host rocks and trace element geochemistry, and finely disseminated nature of gold particles are similar to Carlin-type gold systems in the Great Basin of the western USA, but local geology, magnetically mapped intrusive bodies, and trace element zonation suggest affinity with some intrusion-related gold systems. Received: 28 February 1999 / Accepted: 3 March 2000