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.

风崖（Windy Craggy）是位于加拿大不列颠哥伦比亚省西北部的一处规模约300百万吨的铜-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系（Middle Tats Volcanics, MTV）中，该岩系属于晚三叠世火山-沉积序列，由互层的镁铁质枕状至块状火山岩流、岩床以及钙质泥板岩组成，隶属于亚历山大地体（Alexander terrane）。赋矿的下盘与上盘火山岩流及岩床以碱性玄武岩为主（Nb/Y > 0.70）。风崖矿床的中塔茨火山岩系碱性玄武岩的轻稀土元素（light rare earth elements, LREEs）富集程度较球粒陨石高出100倍以上，稀土元素配分模式陡峭[(La/Yb)_{cn} = 7.1–25.4]；整体上并不具备弧环境典型的、相对于原始地幔的Ta和Nb亏损特征（例如[Nb/Th]_{pm}值为0.68–1.94），不过多数样品的[Nb/La]_{pm}为0.41–10.6，并呈现出显著的Ta、Nb亏损（弧环境地球化学标识；[Nb/Th]_{pm} = 0.20–0.79），这与洋弧环境下的成矿背景相符。拉斑玄武岩/钙碱性弧岩与碱性岩石的共生现象表明，LTV与MTV的岩浆均受到俯冲洋壳不同程度的影响，二者大概率形成于裂谷洋弧之上的弧后盆地环境中。未发现地球化学或同位素证据显示大陆地壳有显著参与。LTV玄武岩可能源自俯冲洋壳上覆地幔的部分熔融，且源区经历了逐步亏损的过程。中塔茨火山岩系的碱性风崖岩体覆于LTV之上，这与板片窗（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，使得更富集的岩浆能够抵达地表，且仅与俯冲改造的地幔发生极少量的相互作用。该板片窗可能为海底附近异常高热流的产生提供了机制，进而启动并维持了强烈且持久的热液活动——这是形成大规模块状硫化物堆积所必需的条件。据我们所知，这是首个与板片窗相关的大型VMS矿床的实例。