DETECTING HYDROTHERMAL ALTERATION OF COARSE-GRAINED IGNEOUS ROCK IN DRILL CORE WITH PORTABLE XRF: A CASE STUDY IN THE ERTSBERG PORPHYRY DISTRICT, INDONESIA

Single-point chemical analysis of coarse-grained igneous rocks using a hand-held XRF spectrometer with a 2-mm x 4-mm analytical window does not produce bulk chemical composition data representative of the whole rock because the scale of the analytical window is similar to individual mineral grains in the sample. We evaluated three procedures for analyzing drill core samples with the goal of determining representative bulk compositions of hydrothermally-altered Ertsberg monzodiorite exclusive of vein material in a practical amount of time: 1) physical homogenization, 2) point count analysis, and 3) sliding analysis.

Physical homogenization (pulverizing the sample into a pressed-powder pellet) required inefficient sample preparation time (15 minutes/sample), and gave inaccurate results for low-Z elements due to matrix factors.

Point count method involved analyzing multiple spots, then calculating an average. After each of 30 successive points analyzed, we calculated a cumulative average of the data to determine where the average converged to an acceptable precision. After 10 analyses, the cumulative average stabilized to < 1% variation. Ten 30-second analyses would require 300 seconds per sample, potentially impractical in a mineral exploration application.

Actively sliding cut drill core samples over the analytical window during 60-second XRF analyses produced a consistently repeatable result in a time period short enough to be practical when analyzing multiple samples in long stretches of drill core.

We applied the active sliding technique to Ertsberg porphyry deposit core samples intercepting variably K-altered monzodiorite and an unaltered control sample, analyzing matrix rock rather than veins to avoid nugget effect. CuO ranged from 0.01-0.03 wt%, with local peaks due to accidental analysis of vein material. K₂O concentrations in altered core were 0.1-1.25 wt% higher than the unaltered control sample, but did not vary systematically. K₂O/(K₂O+Na₂O+CaO) ratios in altered core samples (0.4-0.6) were consistently higher than the control sample (0.3), with local 10 m zones of low ratios due to less intense alteration.

We conclude that the most efficient method of detecting and distinguishing hydrothermal alteration types is visual inspection using a hand lens by a trained geologist.