Oxygen Isotope Fine Structure and Fluid Throughput of the Tongonan Geothermal Field, Philippines - a Preliminary Report

Abstract

Oxygen isotope ratios for 40 reservoir rocks from the plutonic basement and overlying andesitic rocks, and 14 separated geothermal quartz samples from the volcanics, range from 2.5 t o 9.9 per mil. The lowest 6îO values (average 2.9 per mil ) in diorite cores from wells 401, 407 and 410 a relocated in the most productive northwest (Mahiao) sector of the field. for basement rocks are higher (c. 4.6 per mil). Plutonic rock samples from the Mamban (we.ll MN1) sector, located outside the present-day field margin, are only slightly altered ( 6 per mil ) except possibly near the contact zone between the basement and overl y i n g volcanics. ratios are calculated for the Mahiao sector, whereas Malitbog is possibly a relatively recent extension of the field. quartz has 6 í 8 0 values suggesting past tectonic uplift.