| Abstract: | The Jinchuan Ni-Cu-PGE deposit(500 Mt @1.2%Ni,0.7%Cu,~0.4 g/t PGE),one of the largest magmatic sulphide deposits in the world,is located within the westernmost terrane of the North China Craton.It is hosted within the 6.5 km long,Neoproterozoic(~0.83 Ga) Jinchuan ultramafic intrusion,emplaced as a sill-like body into a Palaeoproterozoic suite of gneisses,migmatites,marbles and amphibolites,below an active intracratonic rift.The parental magma was high-Mg basalt,generated through melting of subcrustal lithospheric mantle by a mantle plume during the initiation of Rodinia supercontinent breakup.The lower Palaeozoic collision of the exotic Qilian Block with the breakup-related southern margin of the craton accreted a subduction complex,and emplaced voluminous granitic intrusions and foreland basin sequences within the craton,to as far north as Jinchuan.During the Cainozoic,allochthonous lower Palaeozoic rocks were thrust up to 300 km to the northeast over cratonic basement,to within 25 km of the Jinchuan deposit.The Jinchuan ultramafic intrusion was injected into three interconnected sub-chambers,each containing a separate orebody.It essentially comprises an olivine-orthopyroxene-chromite cumulate,with interstitial orthopyroxene,clinopyroxene,plagioclase and phlogopite,and is predominantly composed of lherzolite(~80%),with an outer rim of olivine pyroxenite and cores of mineralised dunite.Mineralisation occurs as disseminated and net-textured sulphides,predominantly within the dunite,with lesser,PGE rich lenses,late massive sulphide accumulations,small copper rich pods and limited mineralised diopside skarn in wall rock marbles.The principal ore minerals are pyrrhotite(the dominant sulphide),pentlandite,chalcopyrite,cubanite,mackinawite and pyrite,with a variety of platinum group minerals and minor gold.The deposit underwent significant post-magmatic tremolite-actinolite,chlorite,serpentine and magnetite alteration.The volume of thejinchuan intrusion accounts for 3% of the total parental magma required to generate the contained olivine and sulphide.It is postulated that mafic melt,intruded into the lower crust,hydraulically supported by density contrast buoyancy from below the Moho,ponded in a large staging chamber,where crystallisation and settling formed a lower sulphide rich mush.This mush was subsequently injected into nearby shallow dipping faults to form the Jinchuan intrusion. |
