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A silica-deficient, shallow-marine zeolite assemblage in the Foveaux Formation, Bluff Peninsula, New Zealand

D. S. Coombs^*, C. A. Bosel, Y. Kawachi and L. A. Paterson

Geology Department, University of Otago, Dunedin, New Zealand

^* E-mail: doug.coombs{at}stonebow.otago.ac.nz

A gonnardite-thomsonite-chabazite-calcite assemblage forms a cement in the Foveaux Formation, a fossiliferous gabbroic boulder bed that accumulated at the base of a sea cliff cut in a Permian igneous complex during late Oligocene–early Miocene time. Gonnardite was the earliest zeolite to form, locally following minor calcite. It was followed epitaxially by thomsonite, co-precipitating with chabazite. Crystal habits indicate a low-temperature origin. The maximum temperature to which the deposit may have been subjected is estimated as not more than ~30°C. The chabazites are Ca-poor chabazite-K and chabazite-Na. Representative electron microprobe analyses are as follows, all + nH₂O:

thomsonite: Na_3.77Ca_7.73(Al_19.39Si_20.65)O₈₀ and Na_3.78K_0.04Ca_7.25Mg_0.05(Al_19.13Si_21.05)O₈₀;

gonnardite: Na_6.95K_0.03Ca_4.73(Al_16.99Si_23.15)O₈₀ and Na_8.56K_0.03Ca_4.05(Al_17.32Si_22.84)O₈₀;

chabazite-K: Na_1.18K_1.72Ca_0.08Mg_0.23(Al_3.51Si_8.49)O₂₄ and Na_1.67K_1.92Ca_0.18Mg_0.17(Al_4.11Si_7.85)O₂₄;

chabazite-Na: Na_2.51K_1.13Ca_0.17Mg_0.02(Al_4.08Si_7.93)O₂₄.

Such a Si-poor zeolite assemblage is unusual for marine sediments and is attributed to precipitation from marine water impoverished in silica in the gabbroic boulder bed and interacting with shell material and calcic plagioclase. In contrast, a dioritic clast in the boulder bed provides an example of less silica-poor zeolites originally formed in the parent igneous complex. Veinlets in the clast contain scolecite averaging Na_1.19Ca_7.36(Al_15.84Si_24.14)O₈₀.nH₂O, and mesolite averaging Na_5.13K_0.03Ca_5.24 (Al_15.93Si_24.13)O₈₀.nH₂O, in part as sub-microscopic intergrowths. The composition of scolecite closely corresponds to the most Na-rich scolecite reported hitherto.

KEYWORDS: zeolites, chabazite, gonnardite, mesolite, scolecite, thomsonite, Foveaux Formation, Bluff Gabbronorite, Chatton Formation, New Zealand