西澳坎巴尔达镍矿床滴状硫化物的成因

THE GENESES OF BLEBBY ORE IN KAMBALDA,WESTERN AUSTRALIA

分布在坎巴尔达许多富矿体中的滴状矿石,是德坑和阿托矿区的主要矿石类型,目前对其成因争论较大。根据矿物共生组合及在相图中的位置、典型的矿石结构构造、硫化镍中钴元素及铂族元素的含量、滴状矿石中硅酸盐矿物成分与基质中相应矿物成分对比等推测,滴状硫化物是从硅酸盐溶浆中分凝出的磁黄铁矿-镍黄铁矿-黄铜矿溶浆在低温和富硫条件下,又重新组成针镍矿-紫硫镍矿-黄铁矿溶浆分凝出的。对几个钻孔剖面详细的岩性、橄榄石形状、粒度等系统研究发现,滴状硫化物富集在某些岩浆旋回的上部近鬣刺结构层,形成于岩浆流动过程中,以致可携带较大的(1～3cm)镍硫化物液滴在岩浆旋回的上部结晶而不沉入底部。

Blebby ore is located within the komatiites at a number of 'shoots' inkambalda and the most economic bodies occur in Durkin and Otter shoots.Their geneses are an open question. Based on the results of reseach work, there are four mineral assemblages within blebby ore. The sulfide mineralsin blebby ore are not associated with heazlewoodite or awaruite, whereaspyrite is very common occurring in several stages, even cross-cutting thehematite. Pyrite occurs mainly as vermicular intergrowths with pentlanditeor millerite and millerite is a dominant sulfide mineral in the blebby orein Otter shoot, even has found in Durkin shoot. These facts have been in-terpreted as resulting from sulfur-rich circumstances at low temperature.Fourmineral assemblages all can be found in the proposed phase relative diagramof the Fe-Ni-S system at temperature below 135℃. Olivilies were (nowserpentines) distributed disorderly within sulfides and formed annealingtexture. These indicate that some olivines mixed in the sulfide-rich magmaduring the exsolution processes. The blebby sulfides usually were replacedby alteration products (chlorite, carbonate and so on) which came frommatrix of rocks, the fine-grained magnetite which originated from olivinesurround the bleb. These indicate the blebby ore was formed earlier thanalterative processes. If the blebby ore was produced by filling the vesicles,both sulfides and most silicate minerals would be formed at same stage. According to many analytic results on electron microprobe especiallythe PGE-bearing minerals concentrated in blebby ore and the Ni sulfidescontain high Co, it is unquestionable that the blebby ore is a redepositedsulfide magma which was formed from an original stilfide magma duringthe metamorphic processes rather than amgdales. Based on the research of the systematic variation of mineral composi-tion, grain sizes and shapes of olivine phynocrystals within some drill holes,the sulfide blebs occur in the top of some definite generation of komatiiteand within a zone which contains elongate phynocrystals of olivine, relativelower MgO, high CaO, Al_2O_3 and Na_2O. These indicate that the komatiiticmagma was flowing rapidly when blebby ore was forming and preventedthe large sulfide immiscible globules in the komatiitic magma from setting,thus the sulfide blebs could be carried in it. In many places, the direction of blebby ore zone which is up to 10mthick and 100m long, is in agreement With the contact between the komati-ite and footwall basalt that indicate the blebby ore distributed in definitelayer of komatiite rather than arbitrary.