The Occurrence and Origin of Pentlandite-Chalcopyrite-Pyrrhotite Loop Textures in Magmatic Ni-Cu Sulfide Ores
The Occurrence and Origin of Pentlandite-Chalcopyrite-Pyrrhotite Loop Textures in Magmatic Ni-Cu Sulfide Ores
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DOI：
10.5382/econgeo.4757
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发表时间：
2020-12
期刊：
影响因子：
5.8
通讯作者：
S. Barnes;V. Taranovic;L. Schoneveld;E. Mansur;M. Vaillant;S. Dare;S. Staude;N. Evans;D. E. Blanks
中科院分区：
文献类型：
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作者：
S. Barnes;V. Taranovic;L. Schoneveld;E. Mansur;M. Vaillant;S. Dare;S. Staude;N. Evans;D. E. Blanks
镍黄铁矿是大多数岩浆硫化物矿床中主要的含镍矿石矿物，通常被解释为完全由高温单硫化物固溶体 (MSS) (Fe,Ni)1-xS 中的固态溶出产生。该过程导致磁黄铁矿晶粒周围形成镍黄铁矿环。最近人们认识到并非所有镍黄铁矿都是通过溶出形成的。有些可能是由于早期形成的 MSS 和残余的富镍铜硫化物液体在硫化物熔体分异过程中发生包晶反应而形成的，因此至少一些环状结构可能是真正的岩浆起源。测试这一假设涉及微束 X 射线荧光映射，以对来自一系列不同矿床的镍黄铁矿-磁黄铁矿-黄铜矿共生体进行成像。这些矿床代表了缓慢冷却的岩浆环境（西澳大利亚诺瓦；加拿大萨德伯里）、来自浅层侵入体的球状矿石（西伯利亚诺里尔斯克）、来自低和高变质级地体的喷出科马提岩矿石，以及许多其他矿床。我们的方法得到了对这些矿床内不同结构类型的镍黄铁矿中钯的激光烧蚀-电感耦合等离子体质谱分析的补充。与明显以片状形式从 MSS 或磁黄铁矿中溶出的镍黄铁矿相比，形成粗粒状聚集体的镍黄铁矿以及环结构镍黄铁矿（其中黄铜矿也形成环骨架的一部分）始终具有最高的 Pd 含量。这与许多粒状和环状镍黄铁矿是通过富 Pd 残余硫化物液体和早期结晶的 MSS 之间的包晶反应形成的，而不是像迄今为止所假设的那样，完全是通过 MSS 的亚固相线晶界溶出形成的。各个样品中镍黄铁矿中 Pd 含量的大范围反映了包晶反应和晶界溶出之间的连续过程。变质重结晶矿石的结构与环结构矿石明显不同，这意味着环结构不能通过原始岩浆结构矿石的变质重结晶来再生（特殊情况除外）。因此，环形结构的存在可以被视为在岩浆下温度下缺乏穿透变形和再活动的证据，这一结论对于解释 Nova 矿床具有特别重要的意义，因为它是与硫化物熔化范围内的温度下的区域变形峰值同步形成的。
Pentlandite is the dominant Ni-hosting ore mineral in most magmatic sulfide deposits and has conventionally been interpreted as being entirely generated by solid-state exsolution from the high-temperature monosulfide solid solution (MSS) (Fe,Ni)1–xS. This process gives rise to the development of loops of pentlandite surrounding pyrrhotite grains. Recently it has been recognized that not all pentlandite forms by exsolution. Some may form as the result of peritectic reaction between early formed MSS and residual Ni-Cu–rich sulfide liquid during differentiation of the sulfide melt, such that at least some loop textures may be genuinely magmatic in origin. Testing this hypothesis involved microbeam X-ray fluorescence mapping to image pentlandite-pyrrhotite-chalcopyrite intergrowths from a range of different deposits. These deposits exemplify slowly cooled magmatic environments (Nova, Western Australia; Sudbury, Canada), globular ores from shallow-level intrusions (Norilsk, Siberia), extrusive komatiite-hosted ores from low and high metamorphic-grade terranes, and a number of other deposits. Our approach was complemented by laser ablation-inductively coupled plasma-mass spectrometry analysis of palladium in varying textural types of pentlandite within these deposits. Pentlandite forming coarse granular aggregates, together with loop-textured pentlandite where chalcopyrite also forms part of the loop framework, consistently has the highest Pd content compared with pentlandite clearly exsolved as lamellae from MSS or pyrrhotite. This is consistent with much of granular and loop pentlandite being formed by peritectic reaction between Pd-rich residual sulfide liquid and early crystallized MSS, rather than forming entirely by subsolidus grain boundary exsolution from MSS, as has hitherto been assumed. The wide range of Pd contents in pentlandite in individual samples reflects a continuum of processes between peritectic reaction and grain boundary exsolution. Textures in metamorphically recrystallized ores are distinctly different from loop-textured ores, implying that loop textures cannot be regenerated (except in special circumstances) by metamorphic recrystallization of original magmatic-textured ores. The presence of loop textures can therefore be taken as evidence of a lack of penetrative deformation and remobilization at submagmatic temperatures, a conclusion of particular significance to the interpretation of the Nova deposit as having formed synchronously with the peak of regional deformation at temperatures within the sulfide melting range.