GEOLOGICAL SOCIETY OF THE PHILIPPINES

HYDROGEOLOGICAL SURVEY OF USON, MUNICIPALITY OF MASBATE

 

Diosdado Dizon

Mines and Geosciences Bureau-Region V

 

Assessment of ground water resource of Uson, Masbate was undertaken to determine the extent and locations of water bearing zones in the municipality. A combination of hydrogeological & geophysical surveys were employed to define the character and extent of water bearing zones in the study area.

The survey campaign focused on 5 areas where springs and water locations have high to moderate capacities which range from 10,000 to 15,000 liters per day and could ultimately be recommended for development.

These areas are Puro, Pepita, Badling, Del Carmen and Mabini, Georesistivity survey in the area initially disclosed an unconfined aquifer at pepita area and a confined aquifier at puro area. Four (4) of this areas are sourced from limestone aquifer aged as Pleistocene with unknown thickness, while the source at the Del Carmen area is derived from an unexplored sedimentary formation.

Pumping tests and further exploration drilling on this sites are being eyed as recommendatory activities prior to development.

In POC massive podiform chromitites (Cr# - 0.78 to 0.84) mostly having orbicular textures, yield Re-Os ages of around 500Ma in contrast to the schlieren, more aluminian chromitites (Cr#- 0.20 to 0.69) which are around 60 Ma. The schlieren variety exhibits generally euhedral grains with relatively higher frequency of solid inclusions (amphiboles, phlogopites, pyroxenes, and various base metal sulfides and PGEs). The podiform chromitites on the other hand generally contain highly fractured grains probably due to post-depositional brittle fracturing. Within the complex these two chromitite groups are of contrasting distribution; the schlieren in the upper levels close to the dunite-gabbro interface whereas the podiform variety is deep (approximately 1.5 km from the interface) within the tectonite harzburgite section. 

In DOC, the chromitites are generally of schlieren (Cr#-0.68 to 0.78) variety occurring within a zone of about 20-30 meters thick and are mainly confined in the upper level dunite. Solid inclusions are similar to that of POC chromites but with the absence of phlogopites.  

Consideration of the 187Os/188Os ratios of the chromitites and peridotites as against their Al2O3wt%, the harzburgites are mainly confined in the depleted end of the Ronda ophiolite trend – a feature generally exhibited by depleted mantle materials. Using the same parameters, the contributing factors for chromitite precipitation could also be delineated; in DOC chromitite formation could be attributed to the fluid addition process whereas the melt addition process is mainly responsible for the chromitite schlieren of POC. The podiform chromitite plot of POC does not define a very distinct trend but apparently it covers the “enriched end” in the partial melting trend of the Ronda ophiolite. 

The schlieren chromitites in general contains elevated 187Os/188Os with respect to present day chondritic ratio (0.1270) while the podiform chromitites are at sub-chondritic levels. 

Taking into consideration the initial 187Os /188Os ratios of the chromitites as plotted against the reciprocal of their total Os contents, it could be inferred that podiform chromitites could have been formed mainly due to melt accumulation probably consequent to harzburgite (on mid-ocean ridge setting)formation whereas schlieren chromitite formation my have been due to melt –rock reaction of mantle wedge material with melts from a subducted slab (supra-subduction zone setting).

 
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