Bikpela: A Large Siliceous Chimney from the PACMANUS Hydrothermal Field, Manus Basin, Papua New Guinea

The 2.7 m Bikpela chimney differs from sulfide-rich chimneys at PACMANUS, eastern Manus basin, by virtue of high silica content, a stromatolite-like layered structure, and the presence of a large internal collapse cavity. Its interior is dominated by 5- to 10-mm-thick primary laminae of opal-A-dominated fronds clouded by submicroscopic sphalerite inclusions, set between thin subhorizontal growth borders. Larger sphalerites with sulfosalt-galena intergrowths at frond ends project into discoid cavities under the growth borders. A steeper, second-order lamination reflects arrested expansion of the 50-cm-high internal collapse cavity. Underlying the cavity floor an axial breccia pipe contains distorted fragments of laminated interior modified by dissolution and then redeposition of opal and further growth of pyrite and sphalerite. Beneath a thin Mn oxide crust, a hard outer sphalerite-barite carapace ~5 cm thick and devoid of opal is banded parallel to the chimney exterior. At least three stages of chimney growth are evident, beginning with repetitive deposition of opaline frond layers probably developed from microbial mat precursors by overgrowth of hydrothermal opal and fine sphalerite. Next, with rising fluid temperatures, hydrothermal Fe-poor sphalerite with Pb sulfosalts and galena deposited at frond ends and under growth borders and, via lateral diffusion and mixing with seawater, formed abundantly in the outer carapace of the chimney. Passage of hot, more acid fluids into the chimney axial zone then caused dissolution of primary opal and consequent formation of the collapse cavity and underlying breccia pipe, together with deposition of Fe-rich sphalerite and pyrite in second-order fractures and cavities related to the collapse. Finally, conductive cooling during or after this latter stage produced films and spherules of clear opal coating pores and grain margins throughout the structure, apart from in the carapace. Bikpela contains significant gold and silver (avg: 8.7 ppm Au, 250 ppm Ag; n = 9), but it is low overall in Cu (up to 0.5 wt % in the pipe) and Pb (up to 0.7 wt % in the carapace). Its carapace is relatively rich in Zn (up to 54 wt %), Ag, As, and Sb, with one sample containing 0.12 wt % silver.