Geology and lithogeochemistry at the Hidden Creek deposit, Anyox, west-central British Columbia.

Macdonald, R.J.W., Barrett, T.J. and Sherlock, R.L., 1996. Exploration and Mining Geology, v. 5, p. 339-368.
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 average Zr/Y and Zr/Ti ratios of 2.4 and 9.9, respectively. Although they are mainly normal mid-ocean ridge basalts (N-MORB), it is possible to distinguish enriched (E-MORB) and transitional (T-MORB) groups. N-MORBs have P/Ti ratios <0.075 and are depleted in the LREEs, whereas E-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-epidote-quartz 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 + Na2O 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. TiO2/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 at Middle Valley on the Juan de Fuca Ridge, and along the East Pacific Rise from 11o-13oN. Alteration and mineralization in the sedimentary sequence at Hidden Creek are similar to sediment-hosted alteration and mineralization at the Broken Hill upflow zone adjacent to Middle Valley sulfide deposits. Fluid inclusion homogenization temperatures for feeder veins in the Hidden Creek footwall volcanics range from 194 to 295oC, with salinities of 4.6-8.9 wt % NaCl eq., similar to fluid inclusion data from sediment-covered hydrothermal systems at Windy Craggy in B.C. (Triassic)., 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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