Alteration Zoning In Philippine Porphyry Copper-Gold Deposits In Relation With Time And Space



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The spatial lateral and vertical distribution of alteration mineral assemblages referred to as zoning in porphyry Cu-Au-Mo systems in continental and oceanic island-arcs are concentrically arranged from a central potassic core outward into sericitic and argillic zones. These are in turn enveloped by propylitic alteration. As we view this in the context of zoning, it gives us the impression that from a cooling and consolidating magmatic source, potassic to phyllic and argillic, and finally to propylitic types of alterations (Lowell and Guilbert, 1970) developed outwards and away from its heat source as a single cooling event.


However, the spatial sequence of zoning does not explain the observed temporal and paragenetic relationships of the various alteration mineral assemblages, specially in Philippine deposits. From a central magmatic heat source, a central potassic core followed by a retrogressing propylitic alteration halo develops in the early phase of porphyry alteration. Copper, gold and silver formed concomitant with potassic alteration. As the magmatic-hydrothermal ore fluids mix with cooler meteoric ground water along heavily fractured zones along the interface of potassic and propylitic zones, feldspar-destructive alteration consisting of intermediate argillic and phyllic alteration overprints both earlier potassic and propylitic assemblages. In some deposits, it is in this zone where much of the copper and pyrite are found together with some gold and silver and trace molybdenum.


Much later in time, as magmatic and meteoric waters contact with acidic near-surface groundwater, epithermal advanced argillic alteration tends to be superimposed on the earlier potassic, propylitic, intermediate argillic and phyllic alterations. This marks the porphyry-epithermal transition in which high grade copper, gold and silver are developed in association with arsenic.


Finally, in a complex subduction-related, young and active volcanic island-arc setting like the Philippines, almost all the porphyry Cu-Au orebodies are overprinted and telescoped by later hydrothermal events. Typical examples are: Far Southeast, Dizon, Boyongan and Tampakan.


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