A Study On The Formation And Evolution Of The Minerals In The Jinchuan Ni-Cu Deposit

The Jinchuan Ni—Cu Sulfide Deposit occurs in the Jinchuan Ultramafic Mass which is lo- cated in the middle part of Gansu Province. The Jinchuan Ultramafic Mass mainly consists of pargasite- bearing lherzolite, pargasite iherzolite, plagioelase lherzolite, olivine websterite, sulfide dunite etc.. The mass was formed through intrusion of three stages. The third stage formed the Sulfide dunite, and the first and second stage formed the other rock types men- tioned above. The Ni—Cu ore bodies of the Jinehuan Deposit mainly occur in the pargasite lherzolite and sulfide dunite.The major minerals in the mass and ore bodies are chrysolite, bronzite, endiopside, parg- asite, plagioclase. Ti—bearing phlogopite, phlogopite, clinochlore, pyrrhotite, pentlandite. chalcopyrite, cubanite, mackinawite magnetite, chromite etc.. The chrysolite, pyroxene is of- ten replaced by serpentine, magnesio—cummingtonite and tremolite.The results of detailed investigations on mineralogy, ore petrography, petrology and geo- chemistry have clarified that:1. The original magma of the Jinchuan Mass is komatiitic;2. The three intrusives have similar evolution history. The evolution history of each in- trusive can be divided into two stages: The first stage is the main solidifying stage. During this stage.chrysolite, pyroxene, plagioclase, euhedral pargasite etc. were formed.and about 70—80% of the volume of the intrusive was solidified. The second stage is the ore forming and au- tometamorphism stage. In this stage, pyrrhotite, pentlandite, chalcopyrite, chromite(Chr²), magnetite etc. and some hydrous silicate minerals such as Ti—bearing phlogopite, phlogopite, fibrous pargasite, tremolite, clinoehlore, serpentine etc. were formed. The Ni—Cu ore bodies also were formed in this stage.Based on the studies of mineral paragenesis, the author gets to the conclusion that there was a late stage liquation of Fe—Ni—Cu—Cr—S—O melt and silicate melt which was rich in water and alkalis in the residual magma after the main solidifying stage. The ore forming pro- cess of the Jinchuan Deposit is recognized as " crystallization differentiation + late stage liqua- tion". The evidences for this conclusion are: 1. rock textures show that the chrysolite, pyrox- ene. plagioclase crystallized in the original magma in the early stage; 2. the metal sulfide miner- als . some oxides such as chromite (Chr²), magnetite and the hydrous silicate minerals such as fibrous pargasite. Ti—bearing phlogopite, phlogopite, clinochlore, tremolite are in close inter- growth; 3. there often are a few spheric, flaky or irregular metal sulfide or oxide inclusions in those hydrous silicates, and the inclusions consist of one or two to three minerals; 4. pent- landite and irregular small interlacing veiny magnetite(Mt²) are in close intergrowth;5. ore minerals and the intergrown silicates occur in the intergranular space of chrysolite, pyroxene,plagioclasse etc. ; . . . .The results of mineral composition analyses show that the Mg/Fe ratio of the Mg—Fe sil- icates which formed by the crystallization in magma is about 5. 6; the variation range of the Mg/Fe ratio is very narrow, generally, less than 1. The author call this phenomenon as "Mg/ Fe ratio certainty". Based on the "Mg/Fe ratio certainty", and using several special programs designed by the author, the compositions of residual magma of different rock types were calcu- lated. The calculation results show that the residual magma increased the content of the minerogenic elements such as Cu, Ni. S, . . . to 3—5 times as compared with the original mag- ma. The results also confirm that the main compositions of ore forming melt formed by the late stage liquation are Fe.Ni.Cu, Cr, S. O etc..Researches on paragenesis and relative experimental results show that the ore minerals of the Jinchuan deposit had a very special crystallization process. Simply, the process can be de- scribed as : chromite ( Chr²)→pyrrhotite→"pentlandite + magnetite solid solution" (PMSS) + iss: when PMSS exsolved. pentlandite and magnetite (Mt²) were formed ; chalcopyrite and cubanite were formed when iss exsolved. This crystallization sequence is quite different from that of the Fe—Ni—Cu—S system described by Craig(1969).More than ten geothermometers, geobarometers.oxygen barometers and heating experi- ments were used to study the forming condition about temperature, pressure, fo₂, fs₂ of the mass. ore bodies and minerals such as chrysolite, pyroxene, pargasite, pyrrhotite, pentlandite etc. and those results are summarized as following:the main solidifying stage:T=1215C±; P=12Kb±; logfo₂=—4.7～—7.7 (fo₂ in bars);the ore forming and autometamorphism stage:T=780—1080C±; P=7.8Kb±; logfs₂ =—2.4～0.5,logfo₂=—7.35～—19.2 (fo₂, fs₂ in bars).The statistic data of the compositions of 26 ore—bearing ultramafic masses in China and other countries show that, for these ultramafic masses having a composition of CaO<7%, MgO<41%. there is a tendency that when the Fe³⁺/Fe²⁺ of the rock of an ultramafic mass is greater than 1, the mass may produce Cr deposit; when Fe³⁺/Fe(2+)<1, the mass may produce Cu—Ni deposit.