EARLY CEMENTATION OF SILICEOUS SINTERS IN HOT-SPRING SYSTEMS OF NEW ZEALAND

Siliceous sinters found on the discharge aprons of acidic and alkaline hot-springs in the Taupo Volcanic Zone on North Island, New Zealand, are characterized by highly variable and complex fabrics that typically reflect the balance between microbe growth and mineral precipitation. Opal-A, the dominant precipitate in most of these settings, is commonly precipitated very rapidly. The growth attitude of microbes commonly dictates the fabrics of many sinters because rapid opal-A precipitation commonly preserves them in growth position. Most of the opal-A that is precipitated around the microbes seems to be featureless, irrespective of the scale at which it is viewed. As a result, such precipitation commonly appears to be the result of a one-stage process that took place while surface waters were flowing over the discharge apron. Etching of opal-A by natural acidic steams commonly reveals micro-laminations (typically < 1 µm thick) in the opal-A that appear to reflect slight compositional differences, possibly with respect to the OH content. The laminations clearly show that the opal-A was incrementally precipitated on substrates and did not form as a result of a single phase of precipitation. Cavities in sinters are commonly lined and/or filled with isopachous cements that must have formed in a shallow phreatic setting. The processes involved in the development of siliceous sinters can be illustrated by considering a setting in which winter conditions led to slow growth of flat lying microbes whereas summer growth promoted growth of upright filaments and/or columns formed of intertwined filaments. Potentially, this cyclic alternation of growth forms would produce a layered fabric with open cavities bounded by the horizontal winter layers and the vertical pillars that developed during the summer. That fabric is maintained because the filaments and/or sheaths are commonly replaced and encrusted with opal-A while the mats are still in direct contact with surface waters. With the development of new mats, the older mineralized mats become progressively buried. Although no longer in direct contact with surface waters, they will be affected by subsurface spring water. Precipitation of opal-A from the subsurface waters will produce the isopachous cements that will progressively fill the open cavities.