Abstract:
The Sand Creek Prospect is located within the eastern
exposed margin of the Coast Plutonic Complex. The occurrence
is a plug and dyke porphyry molybdenum deposit. The rock
types, listed in decreasing age: 1) metamorphlc schists
and gneisses; 2) diorite suite rocks - diorite, quartz
diorite, tonalite; 3) rocks of andesitic composition;
4) granodiorites, coarse porphyritic granodiorite, quartzfeldspar
porphyry, feldspar porphyry; and 5) lamprophyre.
Hydrothermal alteration is known to have resulted from
emplacement of the hornblende-feldspar porphyry through
to the quartz-feldspar porphyry. Molybdenum mineralization
is chiefly associated with the quartz-feldspar porphyry.
Ore mineralogy is dominated by pyrite with subordinate
molybdenite, chalcopyrite, covelline, sphalerite, galena,
scheelite, cassiterite and wolframite. Molybdenite exhibits
a textural gradation outward from the quartz-feldspar porphyry.
That is, disseminated rosettes and rosettes in quartz veins
to fine-grained molybdenite in quartz veins and potassic
altered fractures to fine-grained molybdenite paint or
6mears in the peripheral zones.
The quartz-feldspar porphyry dykes were emplaced in
an inhomogeneous stress field. The trend of dykes, faults
and shear zones is 0^1° to 063° and dips between 58° NW
and 86* SE. Joint Pole distribution reflects this fault
orientation. These late deformatior maxima are probably superimposed upon annuli representing diapiric emplacement of the plutons.
A model of emplacement involving two magmatic pulses
is given in the following sequence: Diorite pulse (i) dioritequartz
diorite, (ii) tonalites; granodiorite pulse (iii)
hornblende-fildspar microporphyry, hornblende/biotite porphyry,
(iv) coarse grained granodiorite, (v) quartz-feldspar porphyry,
(vi) feldspar porphyry, and (vii) lamprophyre.
The combination of plutonic and coarse porphyritic
textures, extensive propylitic overprinting of potassic
alteration assemblages suggests that the. prospect represents
the lower reaches of a porphyry system.
