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Geology and lithogeochemistry at the Hidden Creek massive sulphide deposit, Anyox, west-central British Columbia MacDonald, Robert W.J.

Abstract

The Hidden Creek deposit constitutes the largest accumulation of massive sulphides in the Anyox Pendant, a volcanic-sedimentary succession preserved as a roof pendant along the eastern margin of the Coast Plutonic Complex, about 160 kilometres north of Prince Rupert, B.C. The deposit produced 21 Mt of ore grading 1.57% Cu, 9.26 g/t Ag and 0.17 g/t Au. It consists of 8 ore zones that occur near the volcanic-sedimentary contact. Each ore zone includes a number of lenticular to sheet-like, massive sulphide bodies consisting of pyrite, pyrrhotite and lesser chalcopyrite, with minor sphalerite and magnetite. Stockwork veins in the upper volcanic and lower sedimentary sequence are interpreted as footwall feeders to the massive lenses. The Anyox volcanic rocks are tholeiitic basalts and basaltic andesites with Zr/Y[sub (avg)]=2.4 and Zr/Ti*1000[sub (avg)]=9.9. Although they are mainly normal mid-ocean ridge basalts (NMORB), it is possible to distinguish enriched (E-MORB) and transitional (T-MORB) groups. N-MORBs have P/Ti ratios 0.15 and are enriched in the LREE. T-MORBs are transitional between these two end-members. Hydrothermal alteration increases in proximity to mineralized zones. Chlorite-epidotequartz alteration is prevalent in the footwall volcanic rocks. Alteration in the sedimentary sequence is zoned outward from a quartz-chlorite core to a quartz-sericite-pyrite margin. In the sediment-hosted ores, quartz and calcite are the common gangue minerals, whereas in volcanic-hosted ores, Mg-Ca-Al silicates are common. There is a strong association between chalcopyrite and pyrrhotite in the sulphide lenses and in the underlying vein networks. Mass change calculations for the volcanic rocks indicates a progressive loss of CaO + Na₂0 and gain in MgO + FeO corresponding to breakdown of plagioclase and formation of chlorite during hydrothermal alteration. K has been added (now biotite) to upper volcanic rocks. Ti0₂/Zr ratios indicate that the detrital component in the sediments cannot be related to the volcanic rocks and must have been derived from a more evolved source. Chemical changes in the altered sediments are similar to those in the volcanic rocks, although they probably had higher initial K values. In the eastern Pacific Ocean, N-MORB is common but E-MORB and T-MORB are reported from Middle Valley on the Juan de Fuca Ridge, and along the East Pacific Rise from 11°-13°N. Alteration and mineralization in the sedimentary sequence at Hidden Creek are similar to sediment-hosted alteration and mineralization adjacent to sulfide deposits at Middle Valley and at Windy Craggy deposit (Triassic) in northern BC. Fluid inclusion data from the Hidden Creek deposit are similar to sediment-covered hydrothermal systems at Windy Craggy, and in the Guaymas Basin in the Gulf of California. The modern examples provide partial analogs for the seafloor setting and styles of mineralization at Anyox.

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