Genetic Mineralogy And Ore-forming Fluid Study Of Gangcha Gold Deposit In Hezuo, Gansu Province

The Gangcha gold deposit, which is in Hezuo City of Gansu Province, is located in the Luqu-Chengxian thrust and nappe tectonic belt of northern rim of Western Qinling Orogenic Belt. It belongs to the low sulfur type epithermal deposit strictly controlled by fracture structures. In this thesis, we apply theories and methods of genetic mineralogy and prospecting mineralogy to study the genetic mineralogy and oreforming fluid characteristics based on the analysis of the geological background, the geological conditions of mineralization and geochemical characteristics of Gangcha gold deposit. The main research results are as follows:a. First we collated and stipulated the diorite extrusion-facies rock and discovered the immiscibility liquid drips at the first time. The main mineral compositions of the immiscibility liquid drips were quartz, potash feldspar, apatite, calcite and magnetite. The major metallic minerals were pyrite, arsenopyrite, marcasite, galena, sphalerite, iron-cobaltite etc. The metallic minerals developed zoisitization, epidotization, carbonation, talcization and clay mineralization. Ppreliminarily, we confirmed that the formation of immiscibility liquid drips had relationships with strong interaction of the melt-fluid. They were the products of magma rapid emplacement. This suggested that the deep ore fluid was quite active.b. As the diorite had close spatial relationships with mineralization in Gangcha gold deposit, we applied the Al-in-Hornblende Barometry to calculate the formation depth of rock. The results indicated that the diorites formed at the depth of 3.85~8.61 km and the average was 5.18 km. We first discovered rutiles in the rocks and ores. They might belong to magmatic origin according to the crystallization temperature of 800~900℃. The rutiles producted with pyrites and arsenopyrite in the ores were capture crystals and residual crystals.c. The test results of fluid inclusions in quartzs showed that the homogenization temperature was 190 ~ 250℃, the salinity was 3.17 ~4.91%, density was 0.80 ~0.87g/cm3, calculated pressure range was 156.0 ~ 213.3bar, the depth was 1.56 ~2.13 km. The gas phase composition of the inclusions was mainly carbon dioxide with little CH4 and H2 O. Thus it was low salinity and density epithermal system.d. The mottled arsenic pyrite, whose formation had relationships with volcanic edifices, were produced in the ores. This indicated that there was preliminary enrichment of ore-forming elements during the period of magmatic activities.e. The gold occurrence in primary ores was “invisible gold”, which was produced in the volcanic rocks and volcanic clasitic rocks. The invisible gold mainly existed in the form of nanoscale microgranular gold, while we still could not rule out the possibility that a small amount of gold existed in the form of solid solution or lattice gold.Based on the results above, we confirm that the Gangcha gold deposit is an epithermal deposit controlled by fracture structures. There was preliminary enrichment of ore-forming elements during the period of magmatic activities.The gold deposit was nearly intact with low denudation rate. Therefore the deep ore deposit had great prospecting potential.