Geochemical constraints on the tectonic setting of basaltic host rocks to the Windy Craggy Cu-Co-Au massive sulphide deposit, northwestern British Columbia

Windy Craggy is an approximately 300 Mt Cu-Co-Au volcanogenic massive sulphide (VMS) deposit in northwestern British Columbia, Canada. The Windy Craggy deposit is hosted by the Middle Tats Volcanics (MTV), a Late Triassic volcano-sedimentary sequence of intercalated mafic pillowed to massive volcanic flows and sills and calcareous argillite that are part of the Alexander terrane. The host footwall and hangingwall flows and sills are predominantly alkalic basalts (Nb/Y > 0.70). MTV alkali basalts at Windy Craggy are enriched in light rare earth elements (LREEs) >100X chondrite compared to chondrite, have steep REE patterns [(La/Yb)cn = 7.1–25.4], and generally lack the Ta and Nb depletions relative to primitive mantle (e.g. [Nb/Th]pm = 0.68–1.94) characteristic of arc environments, although most have [Nb/La]pm cn = 0.41–10.6], and show well-developed Ta and Nb depletions (arc signatures; [Nb/Th]pm = 0.20–0.79), consistent with formation in an oceanic arc environment. The co-occurrence of tholeiitic/calc-alkaline arc rocks with alkalic rocks indicates that the LTV (former) and MTV (latter) formed from melts that were influenced to varying degrees by subducted oceanic crust, and likely formed within a back-arc basin setting formed on a rifted oceanic arc. There is no geochemical or isotopic evidence for major involvement of continental crust. The LTV basalts likely were produced by progressive depletion in the source by partial melting of mantle overlying the subducting oceanic crust. The presence of the MTV alkalic Windy Craggy rocks overlying the LTV is consistent with the presence of a slab-window, perhaps related to subduction of a spreading centre, which allowed more enriched magmas to reach the surface with only minimal interaction with subduction-modified mantle. The presence of this slab-window might have provided the mechanism for the generation of anomalously high heat flow close to the seafloor, which initiated and sustained vigorous, long-lived hydrothermal activity necessary for the precipitation of large accumulations of massive sulphide. To our knowledge, this is the first example of a large VMS deposit associated with a slab-window.

温迪克拉吉（Windy Craggy）是一处位于加拿大不列颠哥伦比亚省西北部的规模约3亿吨的铜-钴-金火山成因块状硫化物（volcanogenic massive sulphide, VMS）矿床。该矿床赋存于中塔茨火山岩（Middle Tats Volcanics, MTV）中，后者属于晚三叠世火山-沉积序列，由互层的镁铁质枕状-块状火山岩流、岩床以及钙质泥灰岩组成，隶属于亚历山大地体。赋矿的下盘和上盘火山岩流与岩床以碱性玄武岩（Nb/Y > 0.70）为主。温迪克拉吉矿床的中塔茨火山岩碱性玄武岩相较于球粒陨石，轻稀土元素（light rare earth elements, LREEs）的富集程度超过球粒陨石标准值的100倍，具有陡倾斜的稀土元素配分模式[(La/Yb)cn = 7.1–25.4]，且整体缺乏弧环境典型的相对于原始地幔的Ta、Nb亏损特征（如[Nb/Th]pm = 0.68–1.94），不过多数样品的[Nb/La]pm cn = 0.41–10.6]，并表现出显著的Ta、Nb亏损（岛弧标识；[Nb/Th]pm = 0.20–0.79），与洋弧环境下的形成背景相符。拉斑玄武岩/钙碱性弧岩与碱性岩石的共生现象表明，LTV（前者）与中塔茨火山岩（后者）的形成源自受俯冲洋壳不同程度影响的熔体，且大概率形成于张裂洋弧之上的弧后盆地环境中。目前尚未发现大陆地壳大规模参与的地球化学与同位素证据。LTV玄武岩可能由俯冲洋壳之上的地幔源区经部分熔融、源区持续亏损而形成。温迪克拉吉矿床的中塔茨火山岩碱性岩体覆于LTV之上，这一特征与板片窗（slab-window）的存在相符，该板片窗可能与扩张中心的俯冲作用相关，使得富集程度更高的岩浆能够抵达地表，且仅与俯冲改造型地幔发生极微弱的相互作用。该板片窗的存在可能为海底附近异常高热流的形成提供了机制，进而启动并维持了强烈且持久的热液活动，这正是形成大规模块状硫化物堆积所需的必要条件。据我们所知，这是首个与板片窗相关的大型VMS矿床实例。