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Groundhog |
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New Mexico, USA |
| Main commodities:
Cu Zn Pb Ag
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Super Porphyry Cu and Au
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The Groundhog Zn-Pb-Cu-Ag mine, which is 2 km west of the Santa Rita Stock (and mine), 15 km east of Silver City in the Central Mining District of south-western New Mexico, USA.
(#Location: 32° 46' 10"N, 108° 6' 20"W).
Veins were exploited from a near surface 200 m vertical interval in a flat pitching zone in the steep Groundhog Fault where it cuts Cretaceous sediments of the Beartooth and Colorado Formation, principally sandstones and shales with minor carbonates, and interleaved diorite sills. The vein system occupies fractures and sills associated with the fault.
The Groundhog Fault is a regional NNE trending structure. Anastomosing granodiorite porphyry dykes from 1 to 60 m wide, similar in composition to the Santa Rita Stock which hosts porphyry copper mineralisation, are found in association with the veining in the fault, occupying a structural zone with a width of 100 to 400 m, over a distance of 3000 m. Three pulses of alteration are recognised: i). an early sericite phase accompanying barren quartz-pyrite prior to porphyry intrusion and mineralisation; ii). sericite and chlorite related to main stage mineralisation associated with and following granodiorite intrusion; and iii). weak quartz-sericite-pyrite alteration following injection of late quartz monzonite porphyry dykes. Propylitic alteration characterised by albite, chlorite, calcite, epidote and sericite accompanied the granodiorite porphyry dykes.
The mineralogy of the veins is basically early quartz and pyrite, followed paragenetically by sphalerite, galena, chalcopyrite and pyrite. The quartz-sulphide veins are enveloped by quartz-sericite-pyrite assemblages. Locally the main shoot grades laterally into stockwork quartz±pyrite veins with associated quartz-sericite-pyrite alteration. The quartz-sericite-pyrite alteration occurs along the Groundhog Fault for more than 4 km. Overlapping alteration on numerous fractures forms wide zones. This alteration is best reflected in the diorite porphyries, locally extending from 0.1 up to 20 m from veining, but is less evident in the granodiorite porphyry where chlorite-quartz-hematite-pyrite-sericite overprints earlier propylitisation adjacent to veins. In sediments, both sandstones and shales are variably sericitised and may contain up to 3% disseminated pyrite and local matrix silicification, although strong sericitisation and disseminated pyrite rarely extends for more than 2 m into the sedimentary wall (Hawksworth & Meinert, 1990).
Deeper in the mine extensive replacement skarn orebodies were mined from 400 to 700 m below the surface, inferred to be within the underlying Carboniferous carbonates. These skarns are characterised by an assemblage of mangano-hedenbergite, bustamite, rhodonite and local ilvaite located on the marble side of the andradite garnet zone and have accompanying Zn-Pb-Cu mineralisation enveloping the zone of sphalerite rich vein systems (Einaudi, 1982).
Within the skarn, metal ratios, mineralogy and composition of the skarns are systematically zoned from NE to SW along the >3 km length of the vein system and relative to the original dyke-limestone contact. Ratios of Zn:Cu, Zn:Ag, Pb:Cu, Pb:Zn and Pb:Ag all increase to the SW where Zn, Pb and total metal grades are highest. To the NE the skarn zonation outwards from the dyke is garnet to pyroxene to pyroxenoid to marble. In the centre of the system garnet is absent and pyroxene begins the zonation, while to the SW pyroxene is less common and pyroxenoid is found at the contact. The pyroxenes become more Mn rich: i). towards the marble front, ii). to the SW, iii). at higher levels and iv). in coarser grained purer limestones. Distal characteristics such as the presence of pyroxenoids and the development of mantos are also more common to the SW.
The Groundhog Mine produced 3 Mt @ 12.3% Zn, 3.6% Pb, 1.3% Cu, 75 g/t Ag, the largest shoot of which was 330 x 120 x 7 m.
For more information consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1995.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Groundhog
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Selected References:
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Hawksworth M A, Meinert L D 1990 - Alteration and fluid inclusion study of the Groundhog vein system, Central Mining District, New Mexico: in Econ. Geol. v85 pp 1825-1839
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Meinert L D 1987 - Skarn zonation and fluid evolution in the Groundhog mine, Central Mining district, New Mexico: in Econ. Geol. v82 pp 523-545
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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