Virginia T. McLemore

Intermittent 1630–1220 Ma magmatism in central Mazatzal province: New geochronologic piercing points and some tectonic implications

The northern Burro Mountains in southwestern New Mexico reveal three distinct, intimately juxtaposed Mesoproterozoic magmatic suites in southern Laurentia. At 1633 Ma, the newly formed Mazatzal crust was intruded by tholeiitic diabase with a depleted-mantle–type Nd isotope composition but with enriched incompatible trace element abundances. A potassic granite-minette suite was emplaced ca. 1460 Ma, and a tholeiitic A-type granite-anorthosite suite intruded ca. 1225–1220 Ma. The diabase-minette-anorthosite sequence and the associated silicic rocks record dominantly juvenile additions to the cratonic margin and imply subcontinental enrichment events ca. 1650 Ma (accretion), prior to 1460 Ma (potassic metasomatism), and ca. 1220 Ma (magmatic underplating). The latter two may have been controlled by a major transcurrent structure along the south margin of Laurentia.

Syntectonic 1.46 Ga magmatism and rapid cooling of a gneiss dome in the southern Mazatzal Province: Burro Mountains, New Mexico

Large-volume granitic plutons in the Burro Mountains, southwestern New Mexico, cover an area of 1300 km 2 and include biotite leucogranite and biotite-hornblende granodiorite. These intrusions are part of the ca. 1.4 Ga granite and rhyolite province stretching across Laurentia. U-Pb zircon dating of five samples of the biotite leucogranite yielded ages ranging from 1469 ± 12 to 1455 ± 11 Ma (2σ uncertainty). Three samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the zircon ages are within error and together have a weighted mean age of 1462 ± 3 Ma, but the granodiorite is older than the granite based on crosscutting relationships. Pressure and temperature estimates from metapelitic country rock, the first from southern New Mexico, are 4–6.5 kbar and 575–650 °C, consistent with their mineralogy. Decompression textures on garnets indicate exhumation following peak conditions, interpreted to have been reached during magmatic heating at 1460 Ma. Decompression had a magnitude of 1–3 kbar. An amphibolite from the country rock has metamorphic zircons dated at 1458 ± 9 Ma, with low Th/U and soccer-ball morphology enclosed within hornblende; this is the age of amphibolite-facies metamorphism. Electron microprobe monazite U-Pb dates from metapelite country rock mostly range from 1500 to 1400 Ma, with a peak at 1474 Ma. This is interpreted as indicating that the area experienced high-temperature metamorphism associated with pluton emplacement at this time. The 40 Ar/ 39 Ar dates from granite and amphibolite form two main groups. The older group includes four hornblende dates of 1477–1467 Ma and biotite and white mica ages from 1472 to 1459 Ma. These are interpreted as reflecting cooling of the plutons and their country rock soon after intrusion. The younger group of hornblende and biotite ages is at 1228 ± 3 Ma and is close in age to the adjacent Redrock granite, which intruded at 1225 Ma. There is a dominant strong foliation in the country rock gneisses and metasedimentary rocks. Foliated granite and gabbro both yield ca. 1630 Ma U-Pb zircon ages. The Burro Mountain granite is not pervasively deformed, but the granodiorite has strong augen gneissic foliations and mylonitic shear zones with S-C fabrics. This fabric formed prior to intrusion of undeformed dikes of Burro Mountain granite and thus is synmagmatic within geochronologic uncertainty. The fabric is identical to the fabric in the country rock, and xenoliths of country rock are foliated with orientations parallel to the foliation in the granodiorite. The similar orientations and the timing of metamorphism suggest that most of the deformation in the Burro Mountains occurred at 1460 Ma and was synchronous with intrusion of the granodiorite. We interpret the 1460 Ma plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development within an extensional tectonic setting, although a transpressional setting is also permissible.

Geochemistry of the Copper Flat porphyry and associated deposits in the Hillsboro mining district, Sierra County, New Mexico, USA

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 (

Waste rock pile characterization, heterogeneity, and geochemical anomalies in the Hillsboro Mining District, Sierra County, New Mexico

Abstract Mine waste rock piles pose an environmental hazard which is difficult to assess without an appropriate sampling procedure. A sampling strategy was devised for four waste rock piles representing different types of mineral deposits in the Hillsboro mining district in Sierra County, New Mexico to characterize their geochemistry. The sites consisted of a placer gold waste rock pile (Site A), a Laramide polymetallic vein waste rock pile (Site B), a carbonate-hosted Pb–Zn waste rock pile (Site C), and a carbonate-hosted Ag–Mn waste rock pile (Site D). In addition, another Laramide polymetallic vein waste rock pile (Site E) was studied to compare physical and chemical characteristics with Site B. The waste rock piles have highly variable grain sizes. Chemical analyses of six size fractions indicated that the less than 0.25 mm size fraction typically contained the highest Cu, Pb, Zn and As concentrations. The four waste rock piles were sampled using grids with dimensions specific to the size and shape of each site. Each of the waste rock piles was sampled, using a 0.25 mm stainless-steel sieve, with sample densities of 15, 30 and 45 for each of the respective grids. The data revealed that sampling 45 samples per grid area produced metal concentrations similar to those for 15 and 30 samples. Therefore, the most economic sampling strategy to adequately characterize the waste rock piles, is a homogeneous composite of 15 to 30 samples.

A study of the analytical variation of sampling and analysis of stream-sediments from areas contaminated by mining and milling

During an environmental study of stream-sediments, large variations in metal values were noticed in contaminated sediments taken from the same site at different times. The question arose as to whether these differences were due to variations in contaminate input or merely represented the heterogeneity of the contaminated sediments at the site. Large numbers of replicate samples and measurements could be utilized to help answer the questions, but this is time-consuming and costly. Random errors can be estimated by the use of a duplicate analyses scheme and much use has been made of duplicate analyses in geochemical surveys and prospecting. In the work reported here, use was made of duplicate analyses to study Cu, Pb, and Zn variation in environmental stream-sediment samples. Samples were collected from three areas: (1) adjacent to the La Bajada mine along the Santa Fe River, (2) adjacent to the Pecos mine along the Pecos River, and (3) downstream from the Alamitos mill located on a tributary of the Pecos River. Both rivers are located in northern New Mexico. The mines and the mill have been inactive for many years. ANOVA statistics were used to evaluate differences in Cu, Pb, and Zn values between contaminated sites that were immediately adjacent. The large variations in metal values noted on earlier trips at the sites on the Pecos River were not noted in this study. Analytical error was the greatest contributor to total variance. Differences in the adjacent site means were most apparent in the smallest size fraction (<63 μm). But even these differences were shown to be mostly due to random error. The smallest size fraction contained the highest metal values at all three sites. Stream-sediments at the La Bajada mine site did not appear to be contaminated.

Distribution and partitioning of copper, lead and zinc in stream sediments above and below an abandoned mining and milling area near Pecos, New Mexico, USA

A reconnaissance study of base metals and mercury in stream sediments from along the Pecos River from North-eastern to South-eastern New Mexico showed elevated levels of copper, lead and zinc at two areas: below an abandoned mine waste dump and below an inactive mill. Zinc was also present at elevated levels in the area of a fish hatchery. During subsequent sampling trips, stream-sediment samples were collected from tributaries as well as the main stem of the river from the Pecos Wilderness boundary south to Villanueva State Park, a distance of approximately 88 km. Sediments were sieved to obtain the 2 mm to 63 µm and <63 µm size fractions and analysed for copper, lead and zinc using aqua regia dissolution and flame atomic absorption spectrometry. The smallest size fraction, which typically constituted 10% or less of the mass of the sample, contained the highest concentrations of copper, lead and zinc with the exception of Willow Creek at the mine waste dumps and San Miguel, a site downstream. Partial dissolution techniques were employed on the size fractions from selected sites to estimate exchangeable, organic and oxide-bound metal forms. Copper was the only metal with a significant association with the organic fraction (about 26% with more than half the sites having 20% or greater). The largest amounts of copper and lead (about 56%) were in a form solubilized by aqua regia but not by the three extraction techniques. In contrast, the largest amount of zinc (about 52%) was associated with the iron and manganese oxide fraction. A significant amount of lead (about 38%) was associated with the oxide fraction with very little associated with either exchangeable or organic fractions. Zinc fractionation was low for the exchangeable and organic forms with the exception of significant amounts of exchangeable zinc at sites below the mine waste dump.

Geology and Regional Implications of Carbonatites in the Lemitar Mountains, Central New Mexico

More than 100 carbonatite dikes and veins intrude a complex Precambrian terrain in the Lemitar Mountains, New Mexico. These Ordovician carbonatites are not associated with any alkalic rocks or kimberlites, but they exhibit textures, mineralogy, chemistry, and alteration characteristic of carbonatites. Despite variations in texture, the Lemitar carbonatites can be grouped on the basis of mineralogy, chemistry, and texture as primary-magmatic and replacement silicocarbonatites and rodbergs. The Lemitar silicocarbonatites consist of more than 50% carbonate minerals and varying amounts of accessory minerals, and they are enriched in LREE. The rodbergs consist of more than 50% carbonate minerals, are stained red by hematite, and exhibit flat REE patterns. Carbonate and barite-fluorite-galena veins of uncertain origin are found in the carbonatites and country rocks. Periodic episodes of alkalic magmatism occurred in New Mexico and southern Colorado during the Proterozoic, late Paleozoic, and Tertiary. The Lemitar carbonatites are part of a regional episode of alkalic and carbonatite magmatism which affected New Mexico and southern Colorado during the late Paleozoic. Continental rifting may have occurred during the Precambrian and late Paleozoic in central New Mexico, although exact geographical limits of paleorifts cannot be determined because overlapping periods of tectonism may have affected the area since Paleozoic times.

CHARACTERIZATION OF GOATHILL NORTH MINE ROCK PILE, QUESTA MOLYBDENUM MINE, QUESTA, NEW MEXICO 1

Rarely do rock pile characterization methods allow for examination and sampling of the interior of large rock piles in-situ. The regrading of the Goathill North (GHN) rock pile at the Questa mine provided a unique opportunity to examine and sample the interior of a large rock pile through the construction of trenches cut into the rock pile as earth-moving progressed. Maps of each bench were created to document the different stratigraphic units, including the thickness, dip, and extent of the units. Units were defined based on grain size, color, and other physical properties. Units were correlated between benches and downward through the series of successively excavated trenches. Typically, paste pH increased with distance from the outer, oxidized zone (west) towards the interior units (east) of the GHN rock pile. The outer zone was oxidized (weathered) based upon the white and yellow coloring, low paste pH, presence of jarosite and gypsum, and absence of calcite. However, the oxidation/reduction (weathering) state in the interior zone is not yet determined. The base of the rock pile closest to the bedrock/colluvium surface represents the oldest part of the rock pile since it was laid down first. Portions of the base appeared to be nearly or as oxidized (weathered) as the outer, oxidized zone, suggesting that air and water flow along the basal interface occurred and possibly was an active weathering zone. Analyses of samples from unweathered, unoxidized drill core samples and from the GHN rock pile are similar in clay mineralogy as determined by XRD and electron microprobe analyses, which suggests that the majority of clay minerals in the GHN samples were derived from the original, pre-mined hydrothermal alteration and not post-mining weathering.

Development of a Modified in situ Direct Shear Test Technique to Determine Shear Strength Parameters of Mine Rock Piles

A modified direct shear test apparatus was designed and used to measure cohesion and friction angle of rock pile materials. Two test apparatuses were constructed, a 30-cm square metal shear box and a 60-cm square metal shear box. In addition to the shear box, the testing apparatus has a metal top plate, a fabricated roller plate, normal and shear dial gages with wooden supports, and two hydraulic jacks and cylinders with a maximum oil pressure of 70 MPa (10,000 psi) and a load capacity of ten tons. The main difference between the in situ shear box and its conventional laboratory equivalent is that the in situ shear box consists of a single box that confines an excavated block of rock pile material. The lower half of the block consists of the rock pile material underneath the shear plane that is a semi-infinite domain. This modification in the shear test apparatus reduces the time needed for block preparation, helps perform several tests at different levels of the same sample block, and allows for accommodating large shear displacement with no reduction in the area of the shear plane.

PETROGRAPHIC, MINERALOGICAL AND CHEMICAL CHARACTERIZATION OF GOATHILL NORTH MINE ROCK PILE, QUESTA MOLYBDENUM MINE, QUESTA, NEW MEXICO 1

Tachie-Menson, Vanesssa Viterbo, Virgil W. Lueth, and Andrew R. Campbell Abstract. Rarely do rock pile characterization studies allow for petrographic, mineralogical, and geochemical characterization of the undisturbed interior of large rock piles in-situ. The regrading of the stable portion of Goathill North (GHN) rock pile at the Molycorp Questa molybdenum mine, New Mexico, provided an opportunity to examine and sample rock pile material in-situ through the construction of trenches cut into the rock pile as earth-moving progressed. Maps of each bench were created to describe the different stratigraphic units, including the thickness, dip, and extent of the units. Units were defined based on grain size, color, texture, stratigraphic position, and other physical properties that could be determined in the field. Units were correlated between benches and to both sides of each trench, and several units were correlated downward through the excavated trenches in the rock pile. Characterization of original rock pile material was accomplished by petrographic, mineralogic, and chemical analysis and includes descriptions of rock fragment lithology, mineralogy, texture, and alteration type and intensity. Rock fragment lithology is generally consistent within mapped units and correlates well with mineralogy and chemistry. Typically, paste pH and carbonate abundance increases with distance from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile. Conversely, authigenic gypsum, which is likely a product of in-situ weathering, usually exhibits a decrease in abundance from the outer to the inner portion of the rock pile. CaO and Sr also typically increase in concentration from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile, suggesting dissolution of feldspars and calcite to form gypsum in the outer, oxidized portions of the rock pile. Clay mineral and electron microprobe analyses of soil samples and unweathered drill core imply that the majority of clay minerals in the GHN samples are derived from the original pre- mining hydrothermal alteration and not by oxidation of minerals within the rock pile.