GEOLOGY OF THE COPPER-MOLYBDENUM DEPOSITS
AT TUMBAGAHAN, BOAC, MARINDUQUE


LUIS M. SANTOS-YNIGO

 

 

 

 

ABSTRACT

 

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.

 

 

 

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