Geology and Evolution of the Mineral Deposits in the Hillsboro District, Sierra County, New Mexico

ABSTRACT
Unlike many porphyry copper deposits in southwestern United
States, the Hillsboro district, in central New Mexico, is well
exposed, with minimal erosion, and offers an excellent opportunity
to examine a relatively intact magmatic-hydrothermal system with
multiple zones of different mineral deposit types. New geochemical,
geochronological, and geological data, combined with earlier published
studies, have provided a refinement of the evolution of mineralization
in the district. Past production has been predominantly
from the polymetallic veins and placer gold deposits, although
minor production has occurred from the porphyry copper and carbonate-
hosted Pb-Zn and Ag-Mn replacement deposits. The geology
of the Hillsboro district is dominated by Cretaceous andesite flows
(75.4±3.5 Ma, 40Ar/39Ar), breccias, and volcaniclastic rocks that
were erupted from a volcano. The Copper Flat quartz monzonite
porphyry stock (CFQM, 74.93±0.66 Ma, 40Ar/39Ar) intruded the
vent of the volcano and hosts porphyry copper mineralization. The
unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma,
40Ar/39Ar) and a third altered, but unmineralized quartz monzonite
intruded along fracture zones on the flanks of the volcano. Younger
latite and quartz latite dikes intruded the andesite and CFQM and
radiate outwards from the CFQM. Hydrothermal alteration of the
igneous rocks consists of locally intense biotite-potassic, potassic,
sericitic, propylitic, and argillic alteration. Large jasperoid bodies, as
well as smaller masses of skarn and marble, replaced limestones in
the southern part of the district. The porphyry copper deposit is
characterized by low-grade hypogene mineralization that is concentrated
within a breccia pipe in the CFQM stock. Polymetallic veins
hosted by many of the latite/quartz latite dikes trend outward radially
from the Copper Flat porphyry copper deposit. Carbonatehosted
replacement deposits (Ag, Pb, Mn, V, Mo, Zn) are found in
the southern and northern parts of the district, distal to the Copper
Flat porphyry copper deposit. Collectively, the evidence suggests
that the mineral deposits found in the Hillsboro district were formed
by large, convective magmatic-hydrothermal systems related to the
Copper Flat volcanic/intrusive complex. The Copper Flat porphyry
copper deposit exhibits very little supergene alteration and enrichment,
in contrast to the extensive supergene alteration and enrichment
at the Santa Rita and Tyrone, New Mexico and Morenci,
Arizona porphyry copper deposits. This may be a result of a lower
pyrite content (<2%) at Copper Flat, thereby preventing the supergene enrichment process from occurring. Another explanation for this difference is that the Copper Flat porphyry copper deposit remained buried in the subsurface until uplift at about 21-22 Ma (Kelley and Chapin, 1997), whereas the Santa Rita, Tyrone, and Morenci deposits were exposed to multiple periods of surface erosion, alteration, and supergene enrichments.