Cretaceous metamorphism, magmatism and shearing in the Waipuna Valley, directly south of the Reefton Goldfield

Rocks in the Waipuna Valley in Westland show that the area immediately south of the Reefton Goldfield has been dramatically affected by Cretaceous tectonism. The main lithology in the Waipuna Valley is metasedimentary Greenland Group that is here subdivided into four types (I, II, III and IV). Type I contains abundant detrital grains with a weak Palaeozoic greenschist facies overprint and is compositionally indistinguishable from unmineralised Greenland Group within the goldfield. Type II contains detrital grains, but also a weak amphibolite facies foliation or, where proximal to granitoid intrusions, a hornfelsic texture. Type III contains no detrital components and records polyphase recrystallisation. U–Th–Pb monazite dating indicates that metamorphism in the Type III rocks occurred, at least in part, at 108.1 ± 1.2 Ma. Type IV Greenland Group has partially melted. Zircon from granitoids intruding Greenland Group in the Waipuna Valley indicate Cretaceous (c. 111 Ma) emplacement. Greenland Group Types III and IV, and the granitic plutons, have been variably sheared with quartz textures indicating progressive cooling during deformation. A mylonite zone occurs just above outcrop of the Type IV rocks. Field and age relationships are interpreted to show that the Waipuna Valley exposes portions of an upper plate and lower plate to a Cretaceous ductile shear zone that is here named the Waipuna Shear Zone. The overall structure is similar to the nearby Cretaceous extensional Paparoa Metamorphic Core Complex. The Reefton Goldfield lies in the upper plate to the Waipuna Shear Zone, not far above the mylonite zone. Cretaceous metamorphism and shearing of Greenland Group could have aided formation and then transport of gold-bearing fluids into structurally higher rocks.

新西兰韦斯特兰地区（Westland）怀普纳谷（Waipuna Valley）的岩石露头显示，里夫顿金矿区（Reefton Goldfield）南侧紧邻区域受到白垩纪构造活动的强烈改造。怀普纳谷的主要岩性为变质沉积成因的格林兰群（Greenland Group），本次研究将其划分为四种类别（I、II、III及IV型）。I型岩石富含碎屑颗粒，仅表现出微弱的古生代绿片岩相叠加改造，其成分与金矿区内未矿化的格林兰群无法区分。II型岩石同样含有碎屑颗粒，但发育微弱的角闪岩相面理；若靠近花岗岩类侵入体，则可见角岩结构。III型岩石不含碎屑组分，记录了多期重结晶作用。U-Th-Pb独居石定年结果表明，III型岩石的变质作用至少有一期发生于108.1 ± 1.2 Ma。IV型格林兰群发生了部分熔融。侵入怀普纳谷内格林兰群的花岗岩类岩体中的锆石定年结果显示，其侵位时代为白垩纪（约111 Ma）。III、IV型格林兰群以及花岗岩岩体均遭受了不同程度的剪切变形，石英结构特征指示变形过程中岩体持续冷却。IV型岩石露头上方紧邻一条糜棱岩带。通过对野外地质关系与年代学数据的综合解读，可知怀普纳谷出露了白垩纪韧性剪切带的上盘与下盘部分区段，本次将该剪切带命名为怀普纳剪切带（Waipuna Shear Zone）。其整体构造样式与附近的伸展型白垩纪帕帕罗阿变质核杂岩（Paparoa Metamorphic Core Complex）相似。里夫顿金矿区位于怀普纳剪切带的上盘，距离糜棱岩带不远。格林兰群在白垩纪发生的变质作用与剪切变形，可能促进了含金流体的形成，并将其搬运至构造位置更高的岩层中。