Ml. Williams et Pw. Bauer, THE COPPER-HILL CU-AG-SB DEPOSIT, PICURIS RANGE, NEW-MEXICO - RETROGRADE MINERALIZATION IN A BRITTLE-DUCTILE TRAP, Economic geology and the bulletin of the Society of Economic Geologists, 90(7), 1995, pp. 1994-2005
The strata-bound copper-silver-antimony deposit at Copper Hill, New Me
xico, developed in a distinctive structural trap, here termed a ''brit
tle-ductile trap,'' during regional retrograde metamorphism of Early P
roterozoic metasedimentary rocks. The deposit is localized near the co
ntact between massive orthoquartzite (Ortega Formation) and the overly
ing schists (Rinconada Formation). Peak regional metamorphism at 4 kba
rs and 500 degrees C occurred near the end of a regional shortening ev
ent that formed the upright, west-plunging Copper Hill anticline. At C
opper Hill, late metamorphic, vertical, north-south-striking faults an
d fractures cut the Ortega Formation quartzite and terminate against f
olded Rinconada Formation schists. Synmetamorphic quartz veins, severa
l centimeters to 1 m thick, fill many of the fractures and terminate a
gainst the overlying schists. At the quartzite-schist contact, many of
the veins merge into mushroom-shaped bodies that project along the co
ntact. Fracture-controlled and disseminated Cu-Ag-Sb minerals occur wi
thin the quartz veins and in quartzite near the veins. The mineral ass
emblage includes malachite, chrysocolla, stibiconite, cuprite, and min
or chalcocite and covellite, but some of these are late-stage alterati
on-oxidation minerals derived from an originally sulfide- or oxide-dom
inated suite. A syn- to late metamorphic age for the primary deposit i
s indicated by: (1) the crosscutting nature of the mineralized veins,
(2) local replacements of kyanite and staurolite by economic minerals,
and (3) evidence for postmineralization annealing of the veins and qu
artzite. During retrograde metamorphism, the quartz veins and massive
quartzite behaved brittlely whereas the overlying schists deformed duc
tilely. SiO2-bearing and then Cu-Ag-Sb-bearing metamorphic fluids migr
ated through the fractured quartzite and ponded below the folded imper
meable schists. Although the Copper Hill deposit is not currently econ
omically viable, it presents a model for retrograde metamorphic minera
lization in rocks with varying mechanical properties that may be impor
tant in other regions.