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Preble |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Preble gold deposit is located 25 km to the east of the town of Winnemucca, Humboldt County, in north-central Nevada, USA. Like the other deposits of the Getchell Trend, it occurs on the eastern margin of the Osgood Mountains, within the Potosi Mining District. It is about 19 km south-west of the Pinson orebody.
Preble was discovered in 1972 by the Cordex I Syndicate during the same exploration program that found Pinson (see the Pinson description above). The orebody was apparently a virgin discovery. The mine is owned and operated by the Pinson Mining Company which was formed by the Cordex Syndicate in the early 1980's to mine the Preble and Pinson deposits (Kretschmer, 1986).
Published resource and reserve figures include:
1.4 Mt @ 2.4 g/t Au = 3.4 t Au (Initial Resource, 1972, Kretschmer, 1986).
1.2 Mt @ 3.1 g/t Au (Reserve, 1984, Bagby & Berger, 1985).
Geology
The Preble gold deposit is localised wholly within carbonaceous shales and silty limestones belonging to the middle member of the Cambrian Preble Formation. Ore occurs within and along a broad shear zone which may be up to 50 m thick and is parallel to bedding (Kretschmer, 1987).
The host Preble Formation conformably, but often structurally, overlies the Cambrian Osgood Mountain Quartzite, and is in turn succeeded by the late Cambrian to middle Ordovician Comus Formation. All three belong to the Transition Assemblage. To the north these units are overlain by the Ordovician Valmy Formation of the Western Assemblage, which may have been structurally emplaced from the west during the Devono-Carboniferous Antler Orogeny. All of these rocks are unconformably overlain by the Overlap Assemblage Antler Sequence, and by the structurally emplaced Havallah Sequence, which was thrust eastward over the Golconda Thrust, during the Permo-Triassic Sonoma Orogeny. All of these rocks are intruded by the Cretaceous (90 to 85 Ma) granodiorite of the 10 km long, north-south elongated, dumb-bell shaped, Osgood Mountain Stock (Kretschmer, 1987).
The southern tip of the main Osgood Mountain Stock is about 16 km to the north-east of Preble. Aero-magnetic data however, indicates the presence of concealed granitoid stocks which are 3.5 to 9 km to the north-east of Preble, and around 7 km to the south, generally lying along the line of the Getchell Trend (Grauch & Bankey, 1990).
The regional geology and setting of the Getchell Trend and of the host Preble Formation are described in the 'Getchell Trend - Geology' record.
The Cambrian Preble Formation can be broken into three distinct units, (Kretschmer, 1987):
- Lower Member - sandy shale, quartzitic sandstone and phyllitic shale.
- Middle Member - which is the host to the Preble orebody, is composed of limestone, carbonaceous shale and calcareous shale, with subordinate phyllitic shale and quartzitic sandstone.
- Upper Member - phyllitic shale, sandy shale and a few carbonaceous beds.
Structure
The Preble deposit, like the Pinson and Getchell orebodies, lies within the Getchell Fault System, a curvilinear structure that parallels the eastern margin of the Osgood Mountain Stock, and flanks the Osgood Mountain range to the south-east. It is about 40 km long and swings from north-west trending in the north at Getchell, where it cuts across the range, to south-west at Pinson, continuing on in the same direction to Preble. This fault is believed to be an older structure that is at least Cretaceous in age, but has been reactivated during the Basin and Range block faulting episode of the Miocene. It is made up of a series of sub-parallel en-echelon faults, and often broad shear zones, that dip at moderate angles to the east. Bedding is roughly parallel to the faulting within the structural zone over much of its length, making displacement hard to determine (Kretschmer, 1987).
At Preble, the broad shear zone that hosts the ore, is part of this structure. It strikes north-easterly, and dips at 30° to the south-east, parallel to bedding (Kretschmer, 1987). The same structure encloses several strongly altered porphyry dykes as described below (Berger & Bagby, 1985).
The sequence is both folded and faulted, with crenulation cleavage being well developed in the host shale. At least two period of deformation have affected the rocks. Early, either pre- or syn-metamorphic veins are folded. These veins consist of calcite where they cut limestone and quartz+calcite in phyllitic shales (Berger & Bagby, 1985).
Mineralisation and Alteration
The orebody is characterised by very uniform dissemination and grade, terminating in sharp cut-offs at the footwall and hangingwall. It is suspected that this cut-off may also be related to chemical boundaries (Kretschmer, 1987).
A small, highly argillised sill is found along the footwall shear in the centre of the deposit. This sill occupies a fault zone that has had recurrent movement subsequent to emplacement. Some higher grade ore occurs along the footwall of the shear zone in the vicinity of the sill (Kretschmer, 1987; Bagby & Berger, 1985).
Thin bedded, carbonaceous, calcareous and silty shales of the middle member of the Preble Formation are the host to mineralisation. These beds have been silicified to varying degrees within the ore horizon, and laterally outwards from it. The host shale has commonly only been subtly silicified, with the rock retaining its fissile texture. Where fresh and un-altered these rocks are composed of variable amounts of muscovite, quartz, chlorite, carbonate and smectite/montmorillonite. Metamorphic folded quartz veins occur throughout the phyllitic shale sequence. Quartz overgrowths on these veins contain gold and are evidence of later silica addition associated with gold deposition. Gold is strongly associated with replacement quartz, either as patchy replacement of limestone or phyllitic shale to form jasperoids, or as quartz veining (Kretschmer, 1987; Bagby & Berger, 1985).
There are several generations of veining at Preble, some of which are definitely associated with gold deposition. These vein types include metamorphic quartz, quartz-carbonate and calcite veins, and later quartz, quartz-calcite, jasperoid, dolomite and calcite veins. All are recognisable in the field with the possible exception of differentiating between metamorphic and gold bearing quartz veins (Bagby & Berger, 1985).
In addition to silicification, the host shale/phyllite also changes colour in the alteration zone, characterised by bleaching of the original greenish sheen to a buff colour. Dolomitisation of limestones in the sequence is also common near high angle faults. Jasperoid veins cut the limestones, and silica replacement of limestone forms massive jasperoid (Bagby & Berger, 1985).
The orebody is oxidised to a depth of approximately 60 m, below which it becomes very refractory. Gold is fairly uniformly distributed within the host, except for very near the surface where some is apparently leached. The remainder of the oxidised portion of the ore zone exhibits more silicification and 50% higher grade than the near surface material. In the oxidised zone, gold, limonite and minor pyrite are seen in the silicified shale. The gold that is visible in polished section from the oxidised zone, has an average size of 2 µm and is associated with silica. Within the oxidised zone carbon and sulphide have been largely removed, leaving a limonitic shale with a very phyllitic texture, which is characteristic, even when highly silicified. The limonite is composed of intergrowths of goethite and lepidocrocite (Kretschmer, 1987).
The removal of carbon by oxidation may have been enhanced by acid from the oxidising sulphides of the ore zone. In the un-oxidised portion of the orebody the rock is sooty black with 2 to 3% pyrite and minor chalcopyrite. Carbonaceous matter has been heated to high temperatures and is almost graphite. Drilling outside of the orebody indicates that most of the sulphides are pre-ore in age, possibly diagenetic components of the host sediments. Two types of pyrite have been noted, namely crystalline and botryoidal. The gold that has been seen in the un-oxidised zone has been restricted to quartz grains, while some is tentatively identified as a thin rim on pyrite grains. Crystalline pyrite is seen in three rock types of the un-oxidised zone, these are siliceous, carbonaceous and calcareous lithologies. The framboidal pyrite is seen in carbonaceous rocks. Minor amounts of arsenopyrite, marcasite, chalcopyrite and sphalerite have also been identified (Kretschmer, 1987; Bagby & Berger, 1985).
Arsenic minerals, and to a lesser extent Hg, Ba and Sb are seen to accompany silicification and mineralisation. Assays of drill cuttings indicate that As, and to a lesser extent Hg, are associated with silicification and gold mineralisation. Tl and F similarly show an association with Au. Sulphur assays indicate 2 to 5% pyrite throughout the area (Kretschmer, 1987).
The most recent source geological information used to prepare this decription 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.
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Kretschmer E L, 1987 - Geology of the Preble gold deposit, Humboldt County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 348-351
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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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