Abstract: Northwest Guangxi underwent a crustal extension-depression environment during Variscan-Indosinian epoches, when magmatic activities took place frequently and intensely, and multiphase basic volcanic rocks were formed broadly.In Yanshanian epoch, the area was situated in a continental marginal mobile belt, where hiden granitoid was developed,but only a few small acid rock bodies and dykes outcropped.
The ultralmicro-grained gold deposits in Northwest Guangxi experienced depositional diagenetic primary enrichment stage, main hydrothermal mineralization stage, and supergene oxidation leaching and enrichment stage.The ore-forming materials were derived from late paleozoic carbonate formation in the upwelling area within depression region, from outer Triassic sandy mudstone, and partly from magmatic hydrothermal.Study on H and O istope-tracer and mineral inclusion composition suggests that the ore-forming fluid be mainly from infiltrated mereoric water, and secondly from magmatic water.The heat sources are geothem, concealed magmatic activities and structural stress heat.
The tectonic evolution, especially magmatic activities,of the area shows an obvious correlation to the mineralization of ultramicro-grained gold deposits.The submarine basic magmatic intrusion and eruption during Variscan-Indosinian period is corresponding to the depositional-diagenetic primary enrichment stage,in which the source bed formed.That means, by etching and transporting of sea-water,the Au from basic magma precipitates and concentrates in the sedimental environment transformation belt on the margin of upwelling area,forming relatively enriched derivative source bed.The acid magmatic activities developed in continental marginal mobile belt during late Indosinian-Yanshanian epoches corresponds to the main hydrothermal mineralization stage.The multiphase inherited reactivation of basement faluts at the margin of upwelling area results in upwelling and mineralizing of ore fluid.The acid magmatic activities not only contribute to part heat source for the ore fluid, but the magmatic hydrothermal may also be involed in mineralization by supplying a part of ore-forming material and fluid.