Short Wave Infrared And Illite’s Genetic Mineralogy Study Of Gold Deposit In Hezuo, Gansu Province

Gansu Gangcha gold deposit located in the west side of Xiahe-Lixian metalorganic belt which belong to the northwestern side of Luqu-Chengxian thrust nappe zone in Qinling orogenic belt.The deposit has the characters of epithermal deposit and the exploration work shows that the reserves for metal gold is up to medium scale.Now the mining area is in initial stage of deep prospecting work,and there are some urgent need for prospecting theory and method to support it.This paper attempts to give some suggestion about deep prospecting work by studying of alteration mineral with near infrared technology and the ore mineralogy theory.The analysis showed that, the region’s major alteration minerals(relative percentage greater than 5%) are muscovite(phengite), illite, kaolinite, dickite, montmorillonite; the secondary alteration minerals(relative percentage less than 5%) are chlorite, epidote, calcite, dolomite, and a small amount of gypsum. According to the distribution and relative content of the alteration minerals we think the alterative zone closing to ore body is phyllic alteration(alteration mineral assemblages: secondary muscovite+quartz+illite, etc.);the alterative zone far away from the ore body is mudding(alteration mineral assemblages:dickite+kaolinite+montmorillonite, etc.).Near infrared technology analysis of the same kind of core samples(tuff or tuffaceous sandstones) with same type of alteration(phyllic) all showed that, those close to the ore body have shallow absorption peak in its diagnostic segment and the spectrum has a lot of "serrated" like absorb peak, furthermore the whole spectrum seems relatively " flat "; however, those far away from the ore body have deep absorption peak in its diagnostic segment and the whole spectrum is more " smooth ".The further study of the phyllic alteration zone showed that the minerals composition are some illite with little muscovite. The composition analysis of illite from different occurrences(mineralized segment, alteration but no mineralized segment, non-alteration segment) with an energy operator showed that illite from mineralized segment has higher iron and magnesium content compared with those from no mineralized segment.The near infrared spectrum study of the area’s typomorphic mineral illite showed that, illite from mineralized segment has sharper Al-OH absorption peak, and illite’s near infrared crystallinities of the area have a positive correlation with the samples’ gold grades. In addition, illite from mineralized segment has bigger Al-OH absorption peak position(greater than 2200nm) compared with those samples from no mineralized segment. The possible cause of this phenomenon(Al-OH near-infrared absorption peak drift) of layered aluminum silicate is the mineral’s six coordinated Al ions substituted by other metal ions(such as Fe, Mg, etc.), leading to the mineral’s Fe, Mg content increased while Al content decreased.The laws referred to above might have some significance for deep prospecting work of the area.