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The
copper-molybdenum deposits in Tumbagahan,
Boac, Marinduque are disposed along the
southem periphery of a
northwest-trending quartz diorite stock
that has been intruded into a thick
sequence of thermally metamorphosed
basic flows and graywackes. Important
deposits of iron are also known to occur
elsewhere along the contact between the
diorite and the invaded rocks. Augite
andesite dikes that have been intruded
at some later stage following
rejuvenation of movement parallel to the
contact zone, are not known to be
mineralized. On the basis of their
mineralogy, texture, structure and
geometrical position with respect to the
diorite stock, the deposits clearly
belong in the contact pyrometasomatic
class of ore deposits.
Mineralization in the Tumbagahan area
occurred in two distinct stages. The
early stage of mineralization,
consisting in the deposition of aplitic
material and/or quartz, and sulfides,
chiefly chalcopyrite, bornite and
molybdenite, took advantage of marginal
(tension) fissures developed in the
solidified outer shell of thé diorite
intrusive. Second stage-deposition of
molybdenite, chalcopyrite and/or bornite
in increased concentration, attended by
appreciable silicification and feeble
sericitization of the dioritic wall
rock, took place with rejuvenation of
movement along the contact zone before
the consolidation of the diorite.
Post-mineral shearing movement along the
same structure produced notable smears
of molybdenite along slickensided fault
planes. |
