卡林型金矿床中自然砷的特征与成矿物理化学条件示踪

在"滇黔桂"、"川甘陕"两个金三角密集区内的某些卡林型金矿床中,自然砷矿物的存在并非个别现象。自然砷呈致密块状、致密凝胶体状(肾状)、脉状或微细脉状产出。自然砷呈他形粒状,颗粒大小变化较大,一般为0.05～0.50mm,最大可达2mm。反光显微镜下为白色,显微硬度为114.21～150.60kg/mm2,相当于摩氏硬度3.27～3.59。矿物主要化学成分As的质量分数为92.74%～99.74%,并含有S0.18%～5.25%,Sb0.04%～3.65%。矿物为三方晶系,晶胞参数值a=0.3759nm,c=1.0527nm。利用矿床中含砷矿物的共生组合特点及热力学资料,探讨了卡林型金矿床形成的温度、成矿流体的f(O2)、f(S2)变化范围及金以金砷络合物形式迁移的可能性。

Mineralogical characteristics of native arsenic and tracing the metallogenic physicochemical condition in Carlin-type gold deposits

Native arsenic is known as a rare mineral phase,but it is found in the Carlin-type gold deposits of the Yunnan-Guizhou-Guangxi and Sichuan-Gansu-Shanxi areas.Native arsenic closely associated with arsenopyrite,pyrite,stibnite,and realgar was discovered in Carlin-type gold deposits at Yata,Dongbeizhai and Jinya of the Triassic age in China.Samples were collected from Au-As or Au-As-Sb ores;native arsenic occurs in these ores in the form of compact massive,reniform shapes and as veins and veinlets.It is normally of allotriomorphic granular form and its grain sizes are commonly 0.05-0.50 mm with the largest being 2 mm.The native arsenic is liable to oxidation as shown by the fact that the tin-white colour of a fresh fracture with metallic luster may gradually change to dark-grey when exposed to air.The mineral is white in colour with strong metallic luster,and brassy;its microhardness is 114.21-150.60 kg/mm2(for 100 g load),equivalent to 3.27-3.59 of Mohs' hardness.Representative reflectivities of the mineral are(470 nm)Rg'=52.87%,Rp'=48.07%;(550 nm)Rg'=50.65%,Rp'=45.32%;(590 nm)Rg'=49.32%,Rp'=43.89%;(650 nm)Rg'=46.67%,Rp'=41.16%.Electron microprobe analyses of native arsenic gave the following results(in %):As 92.74-99.74,S 0.18-5.25,Sb 0.04-3.65.The mineral is of the trigonal system,a=0.375 9 nm,c=1.052 7 nm.Based on mineral associations and thermodynamic data,we propose that the ore-forming temperature was lower than 281℃;that the range of oxygen fugacity(f(O2))of the system was from 10-44.85-10-43.60 Pa in the main stage of metallogenesis,and 10-50.32-10-48.73 Pa in the late stage of metallogenesis.The range of sulfur fugacity(f(S2))was from 10-16.96-10-13.22 Pa in the main stage of metallogenesis,and 10-19.62-10-11.74 Pa in the late stage of metallogenesis.The gold was possibly transported by Au-As complexes.