Abstracts concerning the geology of Hillsboro district, New Mexico
(Also see my summary of Hillsboro district)
back to Ginger McLemore's homepage
McLemore, V. T., Munroe, E. A., Heizler, M. T. and McKee, C., 1999,
Geochemistry of the Copper Flat porphyry and associated deposits in the
Hillsboro mining district, Sierra County, New Mexico, USA: Journal of Geochemical
Exploration, v. 66, p. 167-189.
ABSTRACT
New geochemical, geochronological, and geological data, combined with earlier
studies, have provided a refinement of the evolution of mineralization
in the Hillsboro district in central New Mexico. Laramide (polymetallic)
vein, placer gold, carbonate-hosted Ag-Mn and Pb-Zn, and porphyry-copper
deposits are found in this district. The Hillsboro district is dominated
by Cretaceous andesite flows (75.4±3.5 Ma), breccias, and volcaniclastic
rocks that were erupted from a volcano. The mineralized Copper Flat quartz-monzonite
porphyry (CFQM, 74.93±0.66 Ma) intruded the vent of the volcano.
The unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma) and
a third altered, 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; the
polymetallic vein deposits are associated with these dikes. The igneous
rocks are part of a differentiated comagmatic suite. Alteration of the
igneous rocks consists of locally intense silicification, biotite, potassic,
phyllic, and argillic alteration. Large jasperoid bodies have replaced
the El Paso Formation, Fusselman Dolomite, Lake Valley Limestone, and Percha
Shale in the southern part of the district. Many workers in the district
have recognized district zoning. The low sulfur (<7%) porphyry-copper
deposit forms the center. Trending radially from the CFQM are Laramide
Au-Ag-Cu veins. Carbonate-hosted replacement deposits (Ag, Pb, Mn, V, Mo,
Zn) are located in the southern and northern parts of the district, distal
from the center. Collectively, the evidence suggests that the deposits
found in the Hillsboro district were formed by multiple convective hydrothermal
systems related to the Copper Flat volcanic/intrusive complex.
KEYWORDS: Porphyry-copper deposit, New Mexico, polymetallic veins
Munroe, E. A., McLemore, V. T., and Dunbar,
N. W., 2000, Mine waste rock pile geochemistry and mineralogy in southwestern
New Mexico, USA; in ICARD 2000-Program from the 5th international conference
on acid rock drainage: Society for Mining, Metallurgy, and Exploration,
Inc., Littleton, Colo., pp. 58.
Munroe, E. A., McLemore, V. T., and Dunbar, N. W., 2000, Mine waste
rock pile geochemistry and mineralogy in southwestern New Mexico, USA;
in ICARD 2000-Proceedings from the 5th international conference on acid
rock drainage: Society for Mining, Metallurgy, and Exploration, Inc., Littleton,
Colo., pp. 1327-1336.
ABSTRACT
In New Mexico, there are at least 100,000 abandoned mine waste rock piles
with widely ranging geochemical and mineralogical compositions. A geochemical
and mineralogical study was implemented for five waste rock piles, some
which contained sulfides, in the Hillsboro Mining District. Chemical and
physical characteristics of mineral grains and their secondary mineral
oxidation rinds were examined with the electron microprobe. Pyrite and
chalcopyrite appear to be the most reactive to oxidizing conditions. Clays,
iron oxide, iron hydroxide, iron sulfate, and primary texture rinds exist
in the waste rock piles analyzed.
McLemore, V. T., Munroe, E. A., Heizler, M. T. and McKee, C., 2000,
Geology and evolution of the Copper Flat Porphyry Copper system, Hillsboro
mining district, Sierra County, New Mexico; in Geology and Ore Deposits
2000, The Great Basin and Beyond: Geological Society of Nevada, Program
with abstracts, p. 63.
McLemore, V. T., Munroe, E. A., Heizler, M. T. and McKee, C., 2000,
Geology and evolution of the Copper Flat Porphyry-Copper and associated
mineral deposits in the Hillsboro mining district, Sierra County, New Mexico;
in Cluer, J. K., Price, J. G., Struhsacker, E. M., Hardyman, R. F. and
Morris, C. L. eds., Geology and Ore Deposits 2000, The Great Basin and
Beyond: Geological Society of Nevada, Symposium Proceedings, in press.
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 CQFM stock. Polymetallic
veins hosted by many of the latite/quartz latite dikes trend outward radially
from the Copper Flat porphyry copper deposit. Carbonate-hosted 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.
KEYWORDS: porphyry copper deposit, Laramide, Hillsboro, Copper Flat,
vein deposits, New Mexico