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Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines

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Cooke, DR and Deyell, CL and Waters, PJ and Gonzales, RI and Zaw, K (2011) Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines. Economic Geology, 106 (8). pp. 1399-1424. ISSN 978-1-887483-01-8

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Abstract

The Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large,
broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in
response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon over
the past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Au
deposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, the
two largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfide
veins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly
includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillic
alteration.
Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct temperatures
and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into each
deposit type. The epithermal mineralizing fluids were dilute (generally, <2 wt % NaCl equiv) and had moderate
temperatures (<300°C). Porphyry-style mineralization was associated with high temperature (300° to
>600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositions
of sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and central
Baguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineralizing
fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermediate
sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (–6.9
to +0.8‰), consistent with oxidizing (SO42–-predominant) mineralizing fluids.
Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stockwork
at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations
(Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping of
the two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measured
δ34Ssulfide (+1.1 to +6.6‰), δ34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite
(0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components for
the Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantlederived
components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon isotopes
provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric
waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relative
to meteoric water and precious metal grades both decreased with time during epithermal vein formation at
Acupan. The common observation of cross-cutting relationships between porphyry and epithermal veins
observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidationstyle
mineralization was a common phenomenon in this region, and contributed significantly to its rich metal
endowment.

Item Type: Article
Journal or Publication Title: Economic Geology
Page Range: pp. 1399-1424
ISSN: 978-1-887483-01-8
Additional Information:

©2011 Society of Economic Geologists, Inc.

Date Deposited: 30 Jan 2012 05:50
Last Modified: 18 Nov 2014 04:27
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