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Questa

New Mexico, USA

Main commodities: Mo
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The Questa porphyry molybdenum deposit is located in northern New Mexico, USA.

Geology

Molybdenite mineralisation occurs in the hood zone of the early Miocene Questa Mine aplite-porphyry intrusive, one of three silica-rich intrusives found along the east-west trending Red River Trench, which forms the northern margin of the circular Gold Hill Block in New Mexico (Carpenter, 1968). The three intrusives are the Bear Canyon, Questa Mine and Red River Intrusions.

The aplite porphyry bodies intrude a sequence comprising:

Middle Proterozoic basement of biotite gneiss and schists, amphibolite gneisses and schists, feldspar gneisses, acid granite gneiss, quartzite and granite;
Upper Carboniferous (Pennsylvanian) arkose, shale and sandstone from 0 to 100 m thick;
Cretaceous sandstone, siltstone and grits from 0 to 30 m thick;
Early Miocene volcanics comprising,
 - Lower Andesites - 150 to 900 m of dark green to grey, fine grained featureless hornblende andesite with infrequent, finely crystalline tuffaceous lenses, and an uppermost 0 to 60 m thick band of medium to coarse grained tuffaceous andesite porphyry;
 - Latite - generally around 200 m thick, composed of porphyritic, quartz bearing, hornblende-biotite latite tuffs and flows with agglomerates towards the top;
 - Lower Rhyolite Tuff - 500 m of coarse grained, welded tuff with alternating dark grey glassy and light grey pumiceous bands up to 2.5 cm thick;
 - Upper Rhyolite Tuff - 400 m of fine grained, light to dark grey, banded, glassy and pumiceous, welded rhyolitic tuff, becoming fragmental towards the top;
Pliocene light grey, banded, porphyro-aphanitic rhyolite and banded, spherulitic, highly contorted, glassy rhyolite around 100 m thick. The intrusive aplite porphyries occur largely within the andesites, and are interpreted as having been emplaced by metasomatism rather than by stoping (Carpenter, 1968).

The intrusive phases in the area include, from oldest to youngest,

* Hornblende-biotite monzonite porphyry - which occurs largely as dykes, and contains phenocrysts of sanidine over 30 mm long, plagioclase 5 to 6 mm in length, ferro-magnesian minerals up to 4 mm and quartz 8 mm in diameter. All are set in a fine grained aphanitic matrix of quartz and alkali feldspar which makes up about 50% of the rock;
* Hornblende bearing biotite granite porphyry - occurs as plugs, dykes and sills in the volcanics. It is composed of potash feldspar crystals more than 30 mm long, plagioclase to 12 mm in length, quartz 6 mm in diameter with biotite 4 mm and hornblende up to 12 mm in length. These phenocrysts are separated by small ferro-magnesian crystals, plagioclase laths, and anhedral crystals of potash feldspar, quartz and plagioclase;
* Questa Mine Aplite body, which covers an outcrop area of approximately 1500 x 1200 m, and plunges to the south. This and two other nearby similar aplites may represent the peaks of a concealed aplitic batholith. Emplacement of these aplite bodies is interpreted as having been accomplished in part by 'dyking' and stoping, and in part by flooding and metasomatism by potash-silica rich fluids representing an advancing segregation from depth. Large volumes of volcanics and granite porphyry have been 'digested' by the advancing 'fluid front'. Dense siliceous aplite is found in the hood zone of the Questa Mine Aplite body as cross-cutting dykes and in areas of flooding;
* Biotite granite - which makes up the deeper phases of the aplitic bodies, grading upwards into the porphyritic aplite. It is somewhat porphyritic with 4 to 10 mm phenocrysts of plagioclase, potash feldspar, quartz and biotite in a matrix of the same components;
* Quartz porphyry or rhyolite porphyry - which is found as a plug and a set of dykes. It is light grey and porphyritic with numerous phenocrysts of potash feldspar making up two thirds of the phenocrysts, with the remaining one third being quartz. The matrix is aphanitic, consisting entirely of potash feldspar and quartz, with accessory zircon, monazite, apatite and sphene; and
* Aplite porphyry - which is found as dykes cutting through the hood zone of the Questa mine intrusive (Carpenter, 1968).

Mineralisation & Alteration

Hydrothermal alteration occurs as:

1). blanket propylitisation in the volcanics,
2). sericitisation and kaolinisation in the porphyries, and as
3). a halo-type alteration outwards from veins and shear zones resulting in strong biotitisation in the volcanics and sericitisation and kaolinisation in the aplitic rocks.

Propylitisation of the volcanics has produced a suite including sericite, carbonate, kaolinite, epidote, biotite, chlorite and iron oxides. Molybdenite is the only ore mineral at Questa, commonly accompanied within the aplite by pyrite, quartz, sericite, and less frequently fluorite, topaz, adularia, rhodochrosite, calcite, dolomite, potash feldspar, biotite, kaolinite and illite. Sphalerite, galena and chalcopyrite infrequently occur in small veinlets or as fillings in re-opened molybdenite veinlets. Drilling had not penetrated more than 300 m below the volcanic-aplite contact as of 1968. No ore or gangue zoning had been noted over this vertical interval. Regionally a lateral zonation from molybdenite to zinc to lead and finally to silver is indicated (Carpenter, 1968).

The low grade orebodies are irregular in size and shape, with margins being established by assay boundaries. Small orebodies mined prior to the extensive lower grade ore, consisted of high grade molybdenite (around 5% MoS2) occurring as quartz veins along the contact between the intrusive aplites and the andesites. The main mineralisation occurs as 1). disseminated molybdenite, found as small accessory sulphide flakes in the aplitic intrusive rocks; 2). molybdenite-quartz veinlets and molybdenite in quartz-biotite-potash feldspar veinlets; 3) zoned molybdenite-quartz veins, frequently with biotite and potash feldspar along the walls. These occasionally carry minor Pb, Zn and Cu sulphides, followed by fluorite and carbonates when subsequently re-opened; 4). high grade molybdenite locally occurring along shear zones, with minor associated quartz and pyrite; and 5). molybdenite paint along joint and fracture surfaces. The orebodies are situated largely in the silica rich intrusives and locally silicated volcanics that have been sufficiently well cracked, shattered and mineralised to contain 0.25 to 0.3% MoS2. Of the five types described above, types 4) and 5) account for the majority of the contained molybdenite. This mineralisation is apparently associated with faulting related to numerous adjustments along the trench, and by major shear systems trending north-east, east-west and north-south to north-west (Carpenter, 1968).

Molybdenite has been well oxidised to ferri-molybdite in near surface exposures, and the molybdenum bearing ferric hydroxide, akaganeite. The abundant pyrite of the ore has also been well oxidised (Carpenter, 1968).

Published reserve figures for the Questa orebody are: 110 Mt @ 0.183% Mo (Res. 1979, start-up 1966, USBM).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1996.    
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.


  References & Additional Information
   Selected References:
Cline J S, Bodnar R J  1994 - Direct evolution of brine from a crystallizing silicic melt at the Questa, New Mexico, Molybdenum deposit: in    Econ. Geol.   v89 pp 1780-1802
Klemm L M, Pettke T and Heinrich C A,  2008 - Fluid and source magma evolution of the Questa porphyry Mo deposit, New Mexico, USA: in    Mineralium Deposita   v.43 pp. 533-552
Ross P-S, Jebrak M, Walker B M  2002 - Discharge of hydrothermal fluids from a magma chamber and concomitant formation of a stratified breccia zone at the Questa Porphyry Molydenum deposit, New Mexico: in    Econ. Geol.   v97 pp 1679-1699


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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