Jerritt Canyon Mineralisation - Enfield Bell, Marlboro Canyon, Alchem, Generator Hill

Nevada, USA

Main commodities: Au Ag
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The Jerritt Canyon group of deposits continued from Jerritt Canyon - Geology & Structure.

Alteration and Mineralisation

Within the Jerritt Canyon district ore occurs in a variety of forms in differing structural and stratigraphic positions. Daly, et al. (1990), list four cases, namely: 1). deposits occurring within imbricate thrust sequences that have translated Member 3 of the Hanson Creek Formation over the Roberts Mountains Formation. These typically form the highest grade and most continuous ore zones, as at Marlboro Canyon, and are associated with argillic alteration and potassium metasomatism. They form elongate, steeply to moderately dipping, tabular zones; 2).nbsp;Orebodies such as that at North Generator Hill, are localised at the contact between Members 2 and 3 of the Hanson Creek Formation and are commonly bounded above and below by tabular jasperoid bodies; 3). At West Generator Hill, ore occurs below the Roberts Mountains Thrust which separates the allochthonous silici-clastics from the autochthonous lower plate carbonates. Mineralisation is found in both un-silicified and silicified parts of the Roberts Mountains Formation, and in silicified sections of the underlying Hanson Creek Formation, across the Saval discontinuity. The best grades are in the Roberts Mountains Formation. The ore zone may be highly undulatory. Ore in this example is partially oxidised, but is more typically sulphide bearing and carbonaceous. The western Marlboro Canyon and Alchem deposits also have the same configuration; 4). In the Saval/Burns deposits, gold occurs within the lower part of the Hanson Creek Formation Member 3, or along the contact between Members 3 and 4. The ore at Saval/Burns is generally sulphide rich and refractory (Daly, et al., 1990).

Gold mineralisation in all of the ore variations is controlled by either structural, stratigraphic or alteration discontinuities, or a combination of two or more these (Daly, et al., 1990). The majority of the ore is hosted by Member 3 of the Hanson Creek Formation and by the Roberts Mountains Formation. Less important hosts are the Upper Member and Member 2 of the Hanson Creek Formation. Ore is found within these favourable beds along east-west, north-west and north-east trending faults, and especially at the intersection of two or more of these fault directions. All of the gold is found below the Roberts Mountains Thrust within the lower plate carbonates of the Eastern Assemblage (Birak & Hawkins, 1986; Hofstra, et al., 1990).

Minor mineralisation may also be found within altered intrusives which occur as dykes and sills of olivine basalt and monzodiorite. These dykes and sills have generally been highly argillised near mineralisation, grading outwards to sericitic and distal propylitic alteration. It is uncommon to find un-altered dykes or sills. A limited number of age datings suggest that these are Tertiary. Most are barren, although some carry up to 6 g/t Au locally (Weideman, et al., 1990; Daly, et al., 1990).

In all of these examples gold is disseminated and fine grained. Gold grains observed in thin section may exceed 5 µm, but are generally <2 µm in diameter. Where large enough to be observed visually, gold is commonly spatially associated with goethite which may be pseudomorphed after pyrite (Birak & Hawkins, 1986).

Alteration affects within and peripheral to the Jerritt Canyon orebodies include 1). silicification; 2). dolomitisation; 3). remobilisation; 4). reconstitution of organic carbon; 5). decalcification; 6). illite development; 7). argillisation; 8). hypogene oxidation; and 9). supergene oxidation and bleaching (Daly, et al., 1990). The most important of these are:

Silicification and Decalcification - Two main stages of silicification have been recognised. The older of these is a widespread, pre-ore stage, possibly related to the Antler Orogeny, resulting from carbonate replacement and chert recrystallisation during diagenesis and low grade metamorphism. The second stage appears to be separated temporally, but not spatially from the first stage of jasperoid development. Stage 2 silicification is characterised by coarse grained jasperoid that is typically fracture controlled. The event is partly related to gold mineralisation. Jasperoids are stratabound and make up around a third of the rocks in the mine area, but only host 10 to 20% of the ore. Each formation in the mine area contains some jasperoid, modifying both the Roberts Mountains and Hanson Creek Formations, although the Hanson Creek Formation Upper Member and Member 2 are preferentially silicified. The jasperoids in the mine area stand out in strong relief (Daly, et al., 1990; Weideman, et al., 1990, Birak & Hawkins, 1986).

Gold ore does not normally occur in jasperoids, but where it does, it is generally homogeneously distributed and of lower grade relative to the range of values found in the main ore zones. Locally grades of 30 g/t Au may be found in jasperoids, although more commonly they contain 0.05 to 1.5 ppm Au (Birak & Hawkins, 1986).

Oxidation and Argillic Alteration - These alteration forms appear to have been the most important economically. The oxidised and argillised rocks contain the highest gold values and are the easiest to treat with conventional cyanide techniques. Oxidation of the Roberts Mountains Formation produced a tan to light orange-brown semi-friable siltstone, in which pyrite grains are well oxidised to limonite and goethite, and carbon is usually absent. The rock may also be non-calcareous, owing to associated, but limited decalcification. Oxidation in the Hanson Creek Formation is similar to that in the Roberts Mountains Formation, with two exceptions, 1) local oxidation of black, carbonaceous chert lenses and beds to light brown chert or black chert with brown rims; and 2) preferential oxidation of the more permeable laminated Member 3. Again as in the Roberts Mountains Formation, oxidation may have been accompanied by decalcification (Birak & Hawkins, 1986).

Apart from blanket like surface affects, oxidation typically forms 'blooms' around faults and some contacts. In its most advanced stages oxidation resembles intense argillisation, although the clay like material is dominantly detrital sericite (Weideman, et al., 1990).

Argillisation at Enfield Bell took place along structurally controlled zones and is considered an advanced, more localised stage of the oxidation process. This alteration produced clay rich zones within both the Hanson Creek and Roberts Mountains Formations that are strongly tabular, with vertical and horizontal continuity. Their shape is believed to be due to formation along high angle structures such as the Marlboro Canyon and Bell Faults where argillised zones of both formations are juxtaposed. Argillisation is also recognised in rocks directly above and below jasperoid bodies. Primary rock textures are commonly preserved in argillic altered zones. The component minerals are kaolinite, sericite, illite, smectite, alunite, jarosite, quartz and iron oxide. Carbonate, pyrite and carbon are rare. Gold values in the argillised and oxidised rocks range from <0.05 to >150 g/t Au. The highest gold value obtained to 1985 was 685 ppm Au in a Roberts Mountains Formation siltstone (Birak & Hawkins, 1986).

Most of the observed effects of oxidation and argillisation are believed to have formed nearly contemporaneously, but after silicification. Argillised rocks grade into oxidised beds, whereas the contacts between argillised and silicified or carbonaceous lithologies are razor-sharp. Some oxidation may have occurred before argillisation and silicification, as evidenced by oxidised, decalcified, un-silicified beds of limestone, interbedded with silicified limestone. Replacement of carbonate by silica normally results in a highly impermeable rock that resists subsequent argillisation and oxidation (Birak & Hawkins, 1986).

Carbonisation - studies of the distribution of carbon in the orebodies infers a two stage evolution of carbon species. The first involves graphitisation of organic material during regional greenschist metamorphism, possibly related to the Antler Orogeny. The second event entails the mobilisation of the microcrystalline graphite grains that were formed during stage one graphite development (Daly, et al., 1990; Weideman, et al., 1990, Birak & Hawkins, 1986).

Carbonisation is strongly structurally controlled and is probably the result of enrichment of primary sedimentary hydrocarbons, transported by the hydrothermal mineralising fluids and concentrated in permeable fault zones. Black, sheared, sooty, carbonised zones, often with accompanying arsenic sulphides (orpiment and realgar) and high gold grades, occur within the mine area, but rarely outside of the mineralised zones. Gold values within the mobilised carbon zones may be high along the main ore trends, generally ranging from <0.05 to 35 ppm Au. Decalcification is typically associated with carbonisation and intense oxidation, and is in both cases, commonly spatially associated with ore (Daly, et al., 1990; Weideman, et al., 1990, Birak & Hawkins, 1986).

Several gangue minerals occur within the Enfield Bell Mine area. Some are excellent indicators of gold mineralisation, while others indicate hypogene activity on a district wide scale. The most reliable indicators of gold mineralisation are orpiment and realgar. Realgar is the most widespread, while orpiment is only formed as an oxidation product of realgar. Minor arsenopyrite has been detected via X-ray diffraction. Realgar and orpiment are found as veins in carbonaceous rocks, intermixed with sparry calcite, and as small scattered grains associated with remobilised carbon in fractures and shears. These sulphides however are not found in argillised or oxidised rocks. District wide geochemical sampling values range from trace amounts to 1000 ppm As, with higher values generally associated with ores.

Cinnabar is also a good indicator of gold mineralisation, although less abundant. It has been found in high grade disseminated ores. District wide geochemistry yielded trace to 50 000 ppb Hg values. The highest Hg values accompany high As levels in remobilised carbon zones.

Barite and stibnite constitute the rest of the important accessory minerals. Although they are not reliable indicators of gold, they are abundant in the mine area, where they are almost entirely restricted to jasperoids. Barite occurs as euhedral encrustations in vugs or on fractures, as thin white veinlets or as massive pods and veins in jasperoids, or less commonly siltstones. Stibnite forms euhedral, acicular crystals to subhedral grains. Veins of stibnite range from <1 to >15 cm in width. Barite and stibnite are commonly intermixed.

Base metal sulphides have not been reported in the Enfield Bell mine, although anomalous Cu, Pb and Zn have been obtained from surface samples. Silver values typically range from <0.05 to 1.5 ppm Ag. The Au:Ag ratio normally exceeds 20:1.

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:
Hofstra A H, Snee L W, Rye R O, Folger H W  1999 - Age constraints on Jerritt Canyon and other Carlin-type Gold deposits in the western United States - relationship to mid-Tertiary extension and magmatism: in    Econ. Geol.   v94 pp 769-802
Peters S G, Armstrong A K, Harris A G, Oscarson R L, Nobel P J  2003 - Biostratigraphy and structure of Paleozoic host rocks and their relation to Carlin-type Gold deposits in the Jerritt Canyon Mining District, Nevada: in    Econ. Geol.   v98 pp 317-337
Robbins E I, DAgostino J P, Haas Jr. J L, Larson R R and Dulong F T,  1990 - Palynological assessment of organic tissues and metallic minerals in the Jerritt Canyon gold deposit, Nevada (U.S.A.): in    Ore Geology Reviews   v5 pp 399-422

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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