The Paradise Peak gold-silver-mercury deposit is located in the Fairplay District of north-western Nye County, south-western Nevada, USA. It is approximately 45 km to the south-east of the Rawhide mine and 85 km due west of Round Mountain.
The Paradise Peak mineralisation is hosted by Oligocene to Miocene andesitic and rhyolitic volcanic rocks within which it is broadly stratabound.
Published production and reserve figures include:
Total Production, 6 pits, to 1993 - 20.6 Mt @ 2.3 g/t Au, 61 g/t Ag = 48 t Au ( Sillitoe, 1994).
Mill Ore Reserve, 1986 - 6.3 Mt @ 4.2 g/t Au, 124 g/t Ag (Min. J., Gold Serv, v. 32, no. 1, 1989).
Leach Ore Reserve, 1986 - 5.1 Mt @ 0.92 g/t Au, 17.5 g/t Ag (Min. J., Gold Serv, v. 32, no. 1, 1989).
The deposit is composed of six separate orebodies, spread over a distance of 6 km, as follows (Sillitoe & Lorson, 1994):
Paradise Peak - 9.06 Mt @ 3.94 g/t Au, 125 g/t Ag Mined ore.
Ketchup Flat - 8.24 Mt @ 1.03 g/t Au, 10.36 g/t Ag Mineable Reserve.
Ketchup Knob - 0.16 Mt @ 1.03 g/t Au, 25.10 g/t Ag Mined ore.
Ketchup Hill - 0.37 Mt @ 1.54 g/t Au, 30.38 g/t Ag Refractory, not mined.
County Line - 2.38 Mt @ 1.06 g/t Au, 10.30 g/t Ag Mineable Reserve.
East Zone - 0.39 Mt @ .058 g/t Au, 0.65 g/t Ag Mineable Reserve, half mined.
Mining ceased at the Paradise Peak set of mines in mid 1993, after the production of 47.7 t of Au, 1225 t of Ag and at least 457 t of Hg (Sillitoe & Lorson, 1994).
Geology
The Paradise Peak deposits occur within upper Oligocene and lower Miocene volcanics, near the north-eastern corner of an early Miocene volcanic field that originally covered approximately 100 sq. km. These rocks, which are primarily intermediate to felsic lavas, unconformably overlie Oligocene and lower Miocene volcanics and pre-Tertiary rocks and were erupted during and shortly after a brief, but widespread episode of normal faulting and extension. The nearest pre-Tertiary outcrops are some 4 km from mine. The Paradise Peak orebodies are closely associated, both spatially and temporally, with early Miocene intermediate volcanism and normal faulting (John, et al., 1990).
The geologic history of the Paradise Peak district is as follows, from the base (from John, etal., 1990):
Permian to Jurassic, sediments - limestone, dolomite, conglomerate, sandstone, shale, quartzite, rhyolitic to andesitic lavas and breccias, diorite and gabbro. These rocks are metamorphosed to lower greenschist to amphibolite facies.
Late Cretaceous, plutons, ranging in composition from leucocratic granite to diorite.
Unconformity. The contact with the Tertiary sequences however, are usually faulted.
Oligocene to Miocene Volcanics, which have been subdivided into:
- Older Andesites, 26 to 24 Ma, thickness unknown, but probably 'many hundreds of metres' - a thick series of andesite to rhyodacite flows and flow breccias that are locally intercalated with volcaniclastic sediments and dacite ash flow tuffs. The overlying tuffs are generally above a low angle fault.
- Middle Tuff, 23 Ma, up to 2000 m thick - a series of silicic ash flow tuffs, locally intercalated with intermediate lavas and epiclastic sediments. There are as many as eight ash flow tuff units present, probably representing outflow facies rocks from three or more separate sources. The tuffs are dominantly rhyolitic.
- Younger Andestite, 20 to 15.5 Ma, 200 to >500 m thick - a series of intermediate to felsic lavas and sub-volcanic intrusive rocks. These rocks are dominantly coarsely porphyritic lavas, but include minor epiclastic and volcaniclastic sedimentary rocks, block-and-ash flows and small dykes and plugs. Most of the rocks are andesites and dacites, although they range from trachyandesites to rhyolites. The sequence is no more than 200 m thick at Paradise Peak, but may be up to more than 500 m thick some 8 km south-west of the mine and 750 m thick 8 km to the north-east of Paradise Peak.
Miocene Sediments, 15 to 12 Ma - South of Paradise Peak, the younger andesites are locally overlain by a thick sequence of lacustrine and fluvial sediments that were deposited in an inland basin.
Miocene Volcanics, 12 Ma - andesite and basaltic-andesite lavas that are the youngest volcanics in the area.
The volcanic stratigraphy within the mine is as follows, from the base:
Older Andesites - which are not exposed in the mine. These are mainly andesite to rhyodacite lava, flow breccias with minor volcaniclastic and epiclastic sediments and welded tuff that have been propylitised.
Tuff of Goldyke, 23.6 to 23 Ma, at least 100 m thick - a fine to medium grained, crystal rich, densely welded, rhyolitic ash-flow tuff. It is commonly a bright purple at the surface, but in fresh exposure is a propylitically altered green colour. It contains 20 to 30% phenocrysts of feldspar, biotite and quartz, is pumice rich, and is only known from deep drilling in the mine.
Dacite Porphyry Flows, up to 100 m thick - medium grained, biotite-hornblende-plagioclase dacite to rhyodacite porphyry flows, flow breccias and minor welded tuffs. They are generally crystal rich, with 20 to 40% fine to coarse grained phenocrysts of plagioclase, hornblende, book biotite and locally quartz. The lavas are commonly flow foliated and are generally coarser grained with less quartz than the overlying porphyry. It is only known from drill holes and a few small exposures near the bottom of the pit.
Quartz-Feldspar Porphyry Flows, up to 30 m thick - This unit is composed of lava flows and flow breccias of fine to medium grained, feldspar (plagioclase?)-hornblende-biotite-quartz rhyodacite porphyry. It is generally crowded with phenocrysts (15 to 40%) and has a well developed trachytic texture defined by fine grained feldspar and hornblende crystals. Rounded quartz phenocrysts and biotite books are abundant. Its stratigraphic position is uncertain and it is not known outside of the mine.
Tuff, up to 75 m thick - This is the main host to precious metal mineralisation and comprises air fall(?) and ash flow tuffs, fine grained sedimentary rocks and probably minor intermediate lava flows. The tuffs range from aphyric to strongly porphyritic, pumice poor to pumice-rich and poorly welded to densely welded. Phenocrysts include quartz, feldspar and zircon. The original composition is masked by strong alteration. Tuffs are generally well laminated, with concentrations of fine grained anatase or leucoxene forming distinct bands. Sedimentary strata range from very finely laminated siltstone or mudstone to finely bedded sandstone. Lava flows are well laminated aphyric to sparsely porphyritic andesite or dacite.
Younger Andesite Flows, 19 to 18 Ma in the mine, and up to 100 m thick - occur as coarsely porphyritic to aphyric andesite and dacite lava flows, flow breccias and minor block and ash flows, air fall tuff and volcani-clastic sandstone and siltstone. Phenocrysts in the lava generally consist of coarse grained plagioclase and hornblende with local biotite and minor quartz, and range from 0 to 40% of the lava. Flow banding is common in less altered exposures. These andesites are generally altered, although barren and commonly overlie mineralised tuff.
Alteration and Mineralisation
Precious metal mineralisation at Paradise Peak is primarily hosted in hydrothermal brecciated, strongly silicified, tuffaceous rocks that probably are a local unit in the Middle Tuff Member. Several thin intercalations of finely laminated siltstone and sandstone occur within the tuffs. The host silicified rocks are stratabound, and in the case of the largest deposits, Paradise Peak and Ketchup Flat, are up to 450 m long and 110 m thick. These silicified rocks contain 90 to 95% SiO2. Mineralisation is interpreted to have been formed at about 19 to 18 Ma and is closely associated, both spatially and temporally, with intermediate volcanism and extensional faulting (John, et al., 1990).
The main alteration types at Paradise Peak are:
i). Silicification, is the most common type of alteration in the mineralised parts of the deposit, although there are large variations in the textures and mineralogy of silicified rocks. Two main types are distinguished, namely,
a). Dense silicified rocks which are very fine grained quartz (10 to 50 µm) and/or opal with fine grained disseminated marcasite and/or pyrite (typically <25 µm), and a TiO2 phase that totally replaces the rock, with the exception of the quartz and zircon phenocrysts. Un-oxidised varieties are a dark blue-grey due to the sulphides, although most of this type has been oxidised leaving vugs where the sulphides are oxidised, now filled with barite, quartz and locally, native sulphur and cinnabar. Generally the texture is destroyed although the outlines of feldspar and mafic phenocrysts may be preserved; and
b). Powdery silicification which grades into leached, oxidised, dense silicification and results from extreme leaching and recrystallisation of early dense silicification, or by local replacement of argillic alteration. The rocks affected by this alteration style range from highly porous indurated to intense powdery forms. They are composed of quartz, opal, anatase and local jarosite. Vugs, if present, are partially filled with barite, and locally, native sulphur and cinnabar. Remnant pods of dense silicification occur within the powdery zones (John, et al., 1990).
The higher grade ore commonly occurs within the friable, powdery silicification, whereas lower grades are present in the dense silicified variety (Sillitoe & Lorson, 1994).
Silicified tuff is commonly bordered above and below by a discontinuous selvage of andesite flows, altered to quartz-alunite. This quartz-alunite rind passes outwards, gradationally, to an argillic zone, as described below, which has an inner kaolinite rich interval, giving way in turn to smectite-chlorite alteration. Locally quartz-alunite may be absent and siliceous tuff passes directly into kaolinised andesite. Compared to the other orebodies, the quartz-alunite alteration above silicified tuffs is much thicker at the main Paradise Peak pit, where it locally contains ore grade mineralisation (Sillitoe & Lorson, 1994).
ii). Alunite-jarosite, which varies from almost pure alunite or jarosite to rocks containing roughly equal amounts of the two minerals, combined with fine grained silica, TiO2 and locally, kaolinite. Alunite may occur as aggregates replacing feldspar phenocrysts; as fine, irregular, powdery alunite associated with powdery silicification; or rarely as veinlets. Both minerals may be either hypogene or supergene. Probable hypogene alunite-jarosite alteration primarily occurs as a crudely stratabound layer 20 to 40 m thick below hydrothermal breccias that cap the deposit, and above the main silicified ore zone. K-Ar dating of hypogene alunite yielded ages of 18.0±0.5 Ma and 18.8±0.8 Ma, which is though to closely approximate the age of the hypogene mineralisation (John, etal., 1990).
Numerous structurally controlled chalcedonic-quartz and quartz-alunite zones which strike east-west, north-west and locally north-east are present in the Paradise Peak district. In addition, extensive stratabound zones, the largest being >1 km long, of lithologically localised quartz-alunite alteration in the Middle Tuffs have been mapped between Paradise Peak and the County Line deposit. The alunite is coarsely crystalline and considered hypogene. None of these quartz-alunite bodies however contain more than weak concentrations of precious metals (Sillitoe & Lorson, 1994).
iii). Argillic Alteration, which is gradational with, and locally intermixed with, alunite-jarosite alteration. It is well developed in the Younger Andesite that overlies the main silicified zone and in the dacite porphyry below the main mineralised zone. Oxidation is widespread in the upper layer, while in the lower zone there are 5 to 15% disseminated fine grained pyrite and/or marcasite. Argillic alteration consists of smectite±opal±quartz+leucoxene with minor mixed layer illite-smectite and/or kaolinite locally. The intensity of argillisation is variable. Thin late seams of jarosite and still later gypsum veins up to 0.5 m thick are abundant in oxidised sections. Gypsum veinlets are also common in un-oxidised argillic rocks (John, et al., 1990).
iv). Propylitisation, is gradational with argillic alteration and only locally exposed within the mine and in deep drilling. It is widespread outside of the mine in the Older Andesite and much was formed prior to mineralisation and the Paradise Peak hydrothermal system. Plagioclase is altered to calcite and smectite/illite; biotite is replaced by chlorite; and hornblende has become calcite or chlorite, iron oxide and leucoxene, all in a smectite matrix. Disseminated pyrite and marcasite is common and epidote is present locally (John, et al., 1990).
Field relationships indicate a number of major episodes of hydrothermal activity within the ore zone. These are, from oldest to youngest:
Early Silica-Sulphide Alteration - present as dense fine grained silicification characterised by opal and/or quartz, and accompanied by fine grained iron sulphides. It is pervasive in the Tuff and Quartz-Feldspar Porphyry units and largely absent in the andesite and underlying Dacite Porphyry. It forms a tabular, crudely stratabound layer overlain by alunite-jarosite and underlain by argillic alteration. Only low Au and Ag values generally accompany this style of alteration where preserved (John, et al., 1990). The majority of this phase has been oxidised, with only kernels of the un-oxidised alteration remaining. Below the base of oxidation larger, un-oxidised masses of this style of alteration are preserved in the eastern lobe of Paradise Creek and at Ketchup Hill, as described below.
At Paradise Peak, argillic alteration and alunite-jarosite is apparently in part a time equivalent to this phase and is present in relatively mafic rocks (andesites and dacite porphyries) above, below and peripheral to the early silica-sulphide alteration zone. Alunite-jarosite forms a tabular zone overlying dense early silica-sulphide alteration, whereas the argillic alteration lies peripheral to and below it (John, etal., 1990).
Early White-Silica Matrix Hydrothermal Breccia - consists of fragments of densely silicified Quartz-Feldspar Porphyry and the Tuff in a matrix of fine grained white silica (mostly opal) and silicified rock flour. Most fragments are not strongly leached and commonly have a siliceous sulphidic core. Clasts of argillised or alunite-jarosite are also found. These breccias are locally in fault contact with un-brecciated, argillised, Younger Andesite where both have had subsequent powdery silica alteration superimposed on the earlier alteration (John, et al., 1990).
Individual breccia masses may be of the order of 40 m thick, 60 m wide and 200 m long, dipping at 40° S as in the northern part of the main Paradise Peak pit. The breccia may be either clast or matrix supported and has angular to sub-angular clasts. They are usually monolithic. The clasts are shattered with matrix injected between the fragments. Most fragments are <10 cm across and may have a thin <1 cm oxidised rind surrounding a sulphidic, densely silicified core (John, etal., 1990). This breccia forms a lensoid mass on the northern edge of the Paradise Peak orebody (Sillitoe & Lorson, 1994).
Black Matrix Hydrothermal Breccia - crosscuts the earlier white silica matrix hydrothermal breccias, early sulphide-silica, alunite-jarosite and powdery white silica alteration types described above. These breccias are generally heterolithic and commonly contain clasts of rocks that show evidence of all earlier hydrothermal events, although most clasts are oxidised and strongly leached. The major stages of mineralisation are interpreted to have occurred during the cementation of these breccias (John, et al., 1990).
Clasts are sub-angular to well rounded and from 1 cm to >1 m in diameter. The breccia is highly irregular in fabric, ranging from large areas tens of metres across with nearly 100% matrix, to areas of clast rich but matrix supported breccias, to areas of crackle breccia consisting of small amounts of matrix material filling irregular fractures in the wall rock. The matrix of the breccias is dark grey or black, fine grained sugary quartz (<50 µm), anatase and locally fine grained sulphides (mainly pyrite or marcasite). Vugs are common and are partially filled with coarser grained quartz, barite and locally, sulphide minerals. Visible gold is occasionally found lining vugs (John, et al., 1990).
The main black-matrix breccia forms a steeply south-east plunging, irregular pipe defining the orebody's core. It penetrates upward from the silicified tuff, through the quartz-alunite cap to crop out on the pre-mining summit of Paradise Peak (Sillitoe & Lorson, 1994). The discovery outcrop was a black matrix breccia, occurring as an oval shaped, north-west elongated body about 65 x 130 m in plan and is 25 m thick (John, et al., 1990).
Opal-Jarosite Matrix Hydrothermal Breccia - have only been identified near the northern end of the pit where they are commonly associated with the margins of the earlier formed hydrothermal breccia bodies. They contain clasts of both other breccias as well as un-brecciated, angular, generally un-oxidised clasts of densely silicified Tuff and Quartz-Feldspar Porphyry. These breccias are barren and frequently contain clasts of previously mineralised rocks. These clast supported breccias have a vuggy matrix composed of fine grained jarosite and opaline silica and occur as podiform to tabular bodies. Opal-jarosite matrix breccias are locally replaced by powdery silica alteration, principally along high angle faults (John, et al., 1990). These form steep, irregular, but broadly tabular bodies of powdery cristobalite aggregates accompanied locally by alunite and/or kaolinite around parts of Paradise Peak, and to a lesser extent at the three 'Ketchup' deposits (Sillitoe & Lorson, 1994).
Oxidation and Leaching - is common in the tuff unit throughout the ore zone. Nearly all sulphides formed in the early silica-sulphide period are oxidised and leached to a grey to black, porous quartz or opaline-silica, to light pink, tan or white, dense fine grained quartzose rock that only contains a few small vugs. Anatase and leucoxene are abundant pseudomorphing earlier textures. Barite, and locally, native sulphur and cinnabar partially fill vugs. In places more intense alteration has totally destroyed primary textures and formed white powdery opaline silica. Many of the leached tuffs contain low to high grade ore (John, et al., 1990). This oxidation and leaching was regarded as hypogene by John, et al., (1990), occurring as an early stage following the white-silica matrix breccia stage, but prior to the black matrix breccia. Sillitoe & Lorson (1994) however, suggests that the oxidation and leaching is not hypogene.
Oxidation and leaching has also resulted in the local destruction of the black matrix breccias leaving a grey to black to orange "sand" that contains fragments of strongly leached and partially disaggregated black matrix breccia. "Sandy" zones primarily consist of fine grained quartz with locally abundant Fe oxides and/or jarosite. They are commonly highly enriched in Ag, Au, Fe and many other metals. Much of the Fe appears exotic. It is not certain whether this is hypogene or supergene, although it grades into late stage powdery silicification which appears to be hypogene. The late powdery silicification is also superimposed on argillic and early silica-sulphide alteration, early oxidation and leaching and all three types of breccia. The late stage powdery opaline silicification is generally barren, in contrast to the late oxidation and leaching (John, et al., 1990). Again, although John, et al., (1990) indicate that this oxidation and leaching is late stage hypogene process, following the opal-jarosite breccia, Sillitoe & Lorson (1994) suggest that it is not hypogene.
Supergene oxidation has resulted in most of the formerly sulphidic, argillised, intermediate lavas peripheral to the silicified tuffs being oxidised to a depth of 50 to 150 m below the pre-mine surface. Variable amounts of jarosite and/or iron oxides are present as major groundmass constituents, as replacements of mafic phenocrysts, and as fracture fillings. The silicified ore occurs almost entirely above the base of oxidation and has been largely oxidised and leached as described above (John, et al., 1990).
Where geologically similar silicified ore occurs beneath the main base of oxidation, as in the eastern lobe of the Paradise Peak orebody and at Ketchup Hill, it is present as high sulphide, refractory mineralisation, with no associated oxidation or leaching. In this ore, sulphides comprise 10 to 90% of the un-oxidised material, and upon oxidation produced the friable, powdery ore common in the orebodies. Weathering apparently only resulted in very local redistribution of Au and Ag (Sillitoe & Lorson, 1994).
A low permeability gouge zone at the base of the Paradise Peak deposit, which is related to the flat, post ore detachment that passes below the orebody, has apparently had a strong local influence on the base of oxidation which corresponds closely with the current water table (Sillitoe & Lorson, 1994).
At least three stages of hypogene gold and silver mineralisation are inferred at Paradise Peak, forming an elliptical ore zone which is about 200 x 450 m in plan area and elongated in a north-west direction. It is as much as 125 m thick and plunges at 15° to the south-east. Ore is crudely stratabound within the Tuff Unit which contains 85% of the ore. Much of the high grade ore occurs within black matrix hydrothermal breccia developed within this unit (John, etal., 1990).
Most of the deposit is oxidised, with only localised pods of fine grained sulphide mineral remaining within silicified ore zones above the base of oxidation. Mercury deposition apparently followed the main gold and silver emplacement (John, et al., 1990). Below the base of oxidation however, within the un-oxidised, siliceous and sulphidic sections of the eastern lobe of the Paradise Peak orebody and at Ketchup Hill, the metal grades are similar to those in the oxidised ore. Due to its refractory nature however it is not considered to be ore. The sulphidic rock comprises 10 to 90% sulphide, predominantly marcasite, which, where nearly massive, commonly displays a brecciated texture defined by sulphide clasts set in a matrix of quartz and sulphide. Sulphides, including those constituting clasts, are locally well laminated. Marcasite and pyrite are very fine grained, averaging 50 to 150 µm, and are commonly porous and colloform in texture. Native sulphur is intergrown intimately with marcasite and pyrite, and does not appear to be a late stage addition. Small amounts of stibnite, bismuthinite, argentite and native gold along with barite and anatase occur in the sulphide zone, as well as in sulphidic patches above the base of oxidation (Sillitoe & Lorson, 1994).
Gold and silver minerals are locally associated with early silica-sulphide alteration; are common in rocks that experienced leaching and oxidation; but at Paradise Peak generally occur in black matrix breccias. High concentrations of gold and silver are also common in iron rich zones of leached black matrix breccias, possibly reflecting supergene enrichment. Au and Ag values may commonly be low in remnants of early silica-sulphide altered rocks however, with values in un-oxidised, un-brecciated rocks of this type being less than 0.05 ppm Au and <4 ppm Ag (John, et al., 1990). The early silica-sulphide altered rocks generally comprise dense silicification which commonly only has low grade mineralisation in contrast to the powdery variety (Sillitoe & Lorson, 1994).
The silicified, oxidised and variably leached tuffs usually contain low to high gold and silver values and constitute much of the ore of the deposit. These range from black to dark grey, extremely porous 'sponge rock', to dense, variably coloured, quartzite like rocks. In these rocks Au occurs as native gold with little Ag, while Ag is present as cerargyrite and acanthite (John, et al., 1990). This style of alteration and leaching is characterised by powdery and porous silicification which are more commonly associated with higher grades of precious metals (Sillitoe & Lorson, 1994).
At Paradise Peak the high grade ore is commonly in black matrix breccia, with a major stage of mineralisation during cementation of the breccias, where values of >30 g/t are found locally. Gold occurs as native Au that commonly lines vugs and coats quartz. It is frequently overgrown by coarse grained barite. Most gold is <20 µ:m, although visible gold is occasionally observable. No detectable Ag alloys with the Au in this ore type. Silver occurs as cerargyrite and embolite1, although acanthite, native silver and iodyrite have also been identified. Barite, stibnite and horobetsuite and locally marcasite are associated with gold and silver bearing breccias (John, et al., 1990).
Visible gold is present in silicified, partially un-oxidised rocks as a supergene fracture coating associated with crystalline jarosite, cinnabar and native sulphur. Very high Ag values of up to several thousand ppm and high Au contents of up to 100 ppm are found in zones of strongly leached, black to orange, sandy quartz, as described above. The Ag minerals have not been seen, but the gold is present as <4 µ:m crystals coating quartz, jarosite and iron oxides. The presence of exotic Fe suggests that this may be supergene (John, et al., 1990).
Mercury mineralisation appears to have formed late and is commonly associated with structurally controlled powdery silica alteration that post-dates black matrix breccia formation. Hg minerals and native sulphur also commonly fill vugs in strongly leached silicified rocks and in black matrix and opal-jarosite breccias, as well as coating fractures in partially oxidised, early silica-sulphide altered rocks. Cinnabar is the most common Hg mineral, although meta-cinnabar, corderoite, montroydite and calomel and possibly a Bi-Sb-Hg sulphosalt has also been reported (John, etal., 1990).
Significant amounts of Ti, Zr, Ag, Au, Ba, Bi, Pb, Sb and Sn were added to the orebody during the black matrix breccia stage. These breccias are highly anomalous in Ag, Au, Bi, Pb, Sb and locally Mo, Sn and Tl. Mineralogy, alteration, textural and structural features and limited fluid inclusion studies suggest the orebody at Paradise Peak formed at depths of <200 m and at temperatures of 150 to 200° C, from acidic, low salinity, periodically boiling fluids (John, et al., 1990).
The main Paradise Peak deposit, which has been the main subject of the description above, is very similar to the other satellite orebodies at Ketchup Hill, Ketchup Knob and Ketchup Flat which are located in the same stratigraphic horizon, near the base of the Younger Andesite. These three latter deposits comprise a linear, north-trending zone, 600 m long, which is located 800 m south-east of the Paradise Peak orebody. In contrast the County Line deposit is near the base of the Middle Tuffs, some 4 km WNW of Paradise Peak. At all of the deposits the ore is hosted by stratabound bodies of pervasively silicified, welded ash-flow tuffs. The ore bearing tuff is bounded on all sides, both stratigraphically and structurally, by aquitards, mainly andesite flows, although an altered vitrophyre is also present at the County Line orebody, forming the upper limit to ore (Sillitoe & Lorson, 1994).
At the East Zone occurrence the Older Andesite sequence, beneath the mineralised tuffs horizon, is the host to a large zone of low grade gold mineralisation of porphyry type, with gold present as a quartz veinlet stockwork cutting sericitised andesite flows. It has been suggested that a porphyry stock may occur at depth below this zone (Sillitoe & Lorson, 1994). They further suggest that the main orebodies are a higher, more distal zone to a porphyry system above a concealed plug.
Sillitoe & Lorson (1994) suggest that acid leaching within the Paradise Peak deposits occurred as a late stage process, induced by steam heated ground waters above the palaeo-water table, and the earlier emplaced hypogene mineralisation. This acid leaching, they suggest, was responsible for the quartz-alunite and argillic altered zones and took place during the late stage descent of the palaeo-water table, within still reactive rocks, mainly andesites, which occurred around the altered tuffs containing the hypogene mineralisation. It was also responsible for the deposition of mercury and native sulphur. The acid leaching, they say did not modify the hypogene mineralised and altered zone. The rocks were all subsequently oxidised by supergene processes as the water table was further lowered, producing the oxidation and powdery silicification found within most of the orebodies.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 1994)
