Chemistry of Hydrothermal Solutions in Drill Hole GPK-1, Palaea Kameni, Santorini, Greece

The large consumption of sea water, used as drill fluid, indicated a highly permeable rock. The rocks are dacites of the types described previously and show abundant thin rusty coatings and bleached sections, primarily in the interval 7-55 m, which suggest hydrothermal alterations.

The system was sampled in September-October 1988, using a submersible pump system. The temperature was measured every 20 m and two water samples were taken, one at 111 m and the other at 171 m. Measurements of temperature, oxygen, pH and alkalinity were made immediately after recovery of these samples. Waters for chemical analyses were filtered through 0.45 μm Millipore^® filters. The waters show considerable resemblance to hot-spring waters from Nea Kameni.

INTRODUCTION

Drilling in a metalliferous hydrothermal system was carried out at Palaea Kameni in a joint Greek-Swedish project during 1987-88 (Arvanitides et al. 1988). The drill hole was set some 40 m north-north-east of the most intense hot springs on Palaea Kameni and stopped at a depth of 201.5 metres. To support the hole 170 m of iron casing was inserted. The large consumption of sea water, which was used as drill-fluid, indicated a highly permeable rock. Geochemical and petrological studies (Arvanitides et al. 1990; Boström et al. 1990) show that the rocks are dacites of the types described previously (Pichler and Kussmaul 1972). The rocks have abundant thin rusty coatings and bleached sections, primarily in the interval 7-55 m, which suggest hydrothermal alterations.

Drilling stopped in January 1988 and the system was sampled in September-October 1988. The 30 m section below the casing had collapsed when the sampling started and maximum depth in the hole was therefore approximately 171 m. Sampling of the water in the drill hole was carried out with a submersible - pump system. The tubing was made of Tecalan^® (inner diameter 10 mm) and was lowered into the hole by two cylindrical teflon^® weights. Two submersible pumps were placed some 5 m below the water surface in the hole. The upper end of the tube was placed in a 10 litre plastic measurement bottle. The tubing was composed of 20-metre sections, temperature being measured every 20 m, when a new section was added. Two water samples were taken, one at 111 m and the other 0.3 m below the casing. Measurements of temperature, oxygen, pH and alkalinity were made immediately after recovery of these samples. Waters for chemical analyses were filtered through 0.45 μm Millipore^® filters. For analytical methods and preparation steps in the field see Boström et al. 1990.

RESULTS AND DISCUSSION

Pumping of water started from the surface of the drill hole and continued down with 20 m intervals. The removal of water did not affect the water level in the hole, which means that new water entered the casing from the region below the 170 m level. Before the samples at 170.3 m and 111.3 m were taken, approximately 180 litres of water had been removed from the drill hole. This corresponded to the volume in the lowermost 60 m of the hole.

The temperature measurements are plotted in Fig. 1 and analytical results from water analyses are given in Table 1. The temperature profile indicates that most of the hydrothermal flow is lateral; a source directly underneath the hole would show the opposite temperature gradient. Another interpretation, however, is that the original hydrothermal system has not returned completely to the pre-drilling state. Water from 111.3 m in the bore-hole is significantly lower in carbon dioxide, alkalinity, silica, iron, copper and zinc compared with the 170.3 sample, whereas the pH and manganese concentrations are higher (Table 1). The 111.3 m sample probably represents a mixture of stagnant drill-fluid (sea water) and hydrothermal water, and thus is not representative of the hydrothermal solutions at Palaea Kameni. However, filtered water from 170.3 m resembles filtered waters from hot springs at Palaea Kameni and Nea Kameni (Table 1) and probably represents the composition of hydrothermal solutions at depth.

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