Geochemical Characteristics And Mineralization Of The Gaolong Gold Deposit In Guangxi

The Gaolong gold deposit is located in Youjiang basin of Southwest China.It is one of the typical Carlin-type Au deposits in NW Guangxi and located in the east edge of Gaolong dome.Gold mine is controlled by syn-faults include F₂ and F₃surrounding the platform margin.Ore bodies mainly distributed in tectonic fracture zones of the Middle Triassic Baifeng formation.For exploring the relationship between Au-bearing minerals and fluids and fluid sources,this paper,based on observation in field,use microscope,SEM,EDS to clarify the mineralization stage and carry out analysis of trace element and in situ sulfur isotope in each stage of mineral crystallization by LA-ICP-MS.Based on different textural pattern of pyrites revealed by BSE images,they are divided into four types:type-I diagenetic pyrite without core-rim structure,type-II pyrite with simple zoning,type-III pyrite with rhythmic zoning and type-IV pyrite developed on type-II or type-III pyrites with symbiotic arsenopyrite.Four paragenetic stages are recognized on the basis of textural observation:Py1,Py2,Py3 and Py4.Py1formed the type-I.Py2 formed the inner zone of type-II,III,IV,where fluid corrosion occurred,Py3 formed the type-II rim and type-III and IV middle zone that is enriched in As and Py4 formed the type-IV rim.LA-ICP-MS date shows that the concentrations of Au in Py2?less than 0.05ppm?,Py3?up to 2.27 ppm?,Py4?up to 22.8ppm?and arsenopyrite?up to 0.24ppm?.The S isotope compositions of Py2,Py3,Py4 and arsenopyrite are 8.3¹7.5‰,9¹3.3‰,12.3¹4.7‰,7.3¹0.5‰respectively.Significant difference in concentrations of As,Cu,Au,As/S ratios and value of?³⁴S support different origins and show that Py3 and Py4 are ore related.It is suggested that the ore-forming fluid is divided into two stages.Stage1 includes Py3+Py4 formation stage,characteristiced by high As,high Au,high Cu and ratios of As/S ranging from 0.01 to1,whereas Stage2 is arsenopyrite formation stage,characteristiced by high As,less Au,less Cu and ratios of As/S greater than 1.The formation of Py2 has nothing to do with ore-forming fluid,characteristiced by less As,no gold,no Cu and As/S ratio less than 0.01.The observed S isotope variations indicate that the source of ore-forming fluid is mixing deep basin brine?Stage1?with magmatic-related water later?Stage2?.Au precipitation in this deposit occurred in late Stage1.