A Preliminary Assessment on
the Geologic Setting And Classification of Gold-Silver Deposits in
the Philippines
JOSE S.
PORTACIO, Jr., P. Geo., FSEG, SME
International Explorationist
11544 25th Avenue NE, Seattle, Washington 98125, U. S. A.
Abstract
Gold and
silver are accounted for a very significant value to the Philippine
metals production in the minerals industry. Shallow and large
tonnage low-grade orebodies are extracted from the earth’s crust by
bulk open pit mining. For deep and narrow rich veins, selective
underground mining is the most likely extraction method applied. The
gold is recovered from its gangue either by cyanide leaching or by
gravity concentration, which is then amalgamated and refined to its
In the
southwest Pacific region, studies conducted by Garwin et. al., 2005;
and Carlile, 1996 on metals endowment for copper and gold show that
59 % of the gold comes from porphyry copper and 17% from skarn. The
remaining comes from epithermal vein and replacement deposits; i.e.,
low sulfidation, 12%; intermediate sulfidation, 18%; high
sulfidation, 3%; and 1% for each of the following: disseminated
sedimentary-rock hosted, polymetallic epithermal vein, and
volcanogenic massive sulfides.
In the
Philippines, from 1975 to 2005, a little more than 55% of the gold
that was produced came from the porphyry copper and the rest from
epithermal veins and replacement deposits which include: high and
intermediate sulfidation. A small amount also comes from skarn,
limestone replacement, volcanogenic massive sulfides, and the
so-called “complex types”.
This
review is an attempt to classify the different types of gold
deposits under the various geological processes outlined herein as
headings, and gives an overview of the geologic settings and models
of gold orebodies that were only recovered or produced in the last
30 years of mining operations. The geologic processes include:
hydrothermal, sedimentary and complex types. Under each geological
process are sub-headings referring to geological classification with
emphasis to their host lithology. Subsequently, under each
lithologic sub-headings come the actual deposit classification with
respect to their depositional environment, including their
corresponding examples from the various goldfields in the
Philippines. |