| Abstract |
- The Washougal Mining District is approximately 50 km (30
miles) northeast of Vancouver, Washington, along the western
slopes of the Cascade Range. Although the district has produced
the unostentatious amount of $572 in metals since 1903, the presence
of porphyritic intrusions, hydrothermal alteration, breccia pipes,
and base-metal mineralization typical of many porphyry copper-molybdenum
deposits give this area increased exploration significance.
Bedrock in the district consists primarily of mafic to intermediate
flows and volcaniclastics of probable Eocene to Oligocene
age, which have been intruded by the Silver Star plutonic complex.
This complex is composed of a chilled border phase, diorite, quartz
diorite, granodiorite, quartz diorite porphyry, gran.odiorite porphyry,
and granite aplite, in probable order of emplacement. In addition,
at least 21 tourmaline-bearing breccia pipes, interpreted to have
formed by late-stage magmatic fluids, are associated primarily with
the granodiorite phase. The phases of the complex display the
systematic and continuous variations in mineralogy and chemical
composition that are characteristic of a rock suite differentiated
from a single magma which underwent late-stage silica and potassium
enrichment.
Northwest-trending folds and faults are found throughout the
Washougal District. There is a close correlation between joint plane
orientations, and lineaments determined from aerial photographs.
This association suggests that pre-existing north-northeast- and
no structures controlled the emplacement of the
intrusion and localized the related hydrothermal mineralization.
Prophylitic, phyllic, potas sic, and tourmaline alteration as
well as the associated copper, molybdenum, and zinc mineralization
accompanied and (Or) closely followed the emplacement of the granodiorite
porphyry phase. The ore minerals, which are generally restricted
to the hydrothermal breccias and the surrounding host rock,
are sporadically zoned throughout the district. Anomalously high
concentrations of copper and molybdenum are broadly antipathetic
with those of silver, lead, and zinc. The association of calc-alkalic
magma including porphyritic phases, tourmaline-bearing breccias,
and erratic hydrothermal alteration and metallization suggests that the Silver Star plutonic complex as presently exposed is a high
level porphyry copper-molybdenum deposit.
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