Zortman, Landusky

Montana, USA

Main commodities: Au
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Low grade, heap leachable, gold-silver deposits at Zortman and Landusky are hosted by the Tertiary calc-alkalic to alkalic rocks of the Little Rocky Mountains. They are located in north-central Montana, some 300 km to the north-west of Butte.

Gold was first discovered in the Little Rocky Mountains in the late 1880's by prospectors from the Black Hills. Only minor placer gold were exploited, although the source lodes were soon found and mining commenced shortly thereafter, first in the Landusky area and then at Zortman. Development of the vein deposits accelerated in the early 1900's with the advent of cyanide processing, and continued into the 1920's when the operation became marginal, with sporadic working until 1959. More recently a number of the major companies have explored in the area. Pegasus Explorations Ltd commenced development of the shallow, low grade stockwork mineralisation in 1979. Major heap leaching began in 1980 with 1.18 Mt of ore being mined to produce 1.06 t Au and 1.65 t Ag. By 1985 the annual mining rate was 4.85 Mt of ore, yielding 1.88 t Au and 4.90 t Ag (Hastings, 1988). In 1994 13.46 Mt of ore were treated to produce 3.35 t Au at a cost of $US 9.75/g Au [$US 303/oz] (AME, 1995).

The Little Rocky Mountains are a dome shaped, 230 sq. km inlier of Proterozoic, Palaeozoic and Mesozoic rocks, encircled by the Cretaceous sediments of the Interior Platform which form the floor of the surrounding plains. The Little Rocky Mountains protrude up to 900 m above the plains and were formed by up-doming accompanying the intrusion of Palaeocene calc-alkalic to alkalic rocks through the underlying basement into the Palaeozoic and Mesozoic rocks above. This intrusive centre is one of twelve separate centres that are aligned in a north-east trending belt measuring 200 x 400 km which makes up the Central Montana Alkalic Province (Hastings, 1988).

The oldest rocks in the Little Rocky Mountains are Proterozoic gneisses and schists of amphibolite grade with radiometric ages of 1750 to 1710 Ma. Overlapping Palaeozoic sediments are more than 900 m in thickness and comprise Cambrian sandstone and shale, Ordovician dolomites and Devonian to Carboniferous shales and limestones. Mesozoic rocks mostly ring and generally dip outwards from the intrusive centre. They include 1220 m of sandstones, shales and conglomerates (Hastings, 1988).

The alkalic and calc-alkalic intrusives range in composition from diorite to granodiorite. The majority are syenite and quartz-latite/rhyodacite. Age dating determinations average 62.6±1.5 Ma. The intrusives are commonly porphyritic with phenocrysts of feldspar and quartz. Quartz-monzonite porphyry or quartz-latite porphyry is believed to be the oldest, followed by trachyte dykes and sills that cut all other intrusives and generally occupy mineralised shears. Breccias are locally important host rocks and vary from simple tectonic breccias of a single rock unit, to possible intrusive breccias composed of different rock types, including Proterozoic metamorphics and Tertiary volcanics and intrusives (Hastings, 1988).

The ore deposits are structurally controlled and vary from narrow veins and restricted stockworks at depth to oxidised low grade stockworks, breccias and intensely fractured sheeted zones near the surface. Primary mineralisation consists principally of native gold and silver associated with pyrite, sylvanite, calaverite and hessite. Coarse grained gold is found in the deeper veins, but is scarce to non-existent in the shallower, lower grade zones where it occurs as micron sized particles. Gangue minerals are kaolinite, quartz, marcasite, pyrite, calcite, arsenopyrite and fluorite. Other sparse sulphides include chalcopyrite, molybdenite, sphalerite, galena and covellite which are found both in the veins and in the upper disseminated zones. Native Bi has been identified in a polished section of vein material. Metal zoning is erratic and the Au:Ag ratio varies from 4:1 to 20:1. Generally Landusky has a higher ratio than Zortman (Hastings, 1988).

Gold is closely associated with early pyrite and marcasite mineralisation in veins and veinlets. A paragenetic sequence at both deposits comprises an initial stage of quartz-pyrite-sericite with accompanying gold, followed by renewed movement along the veins, which is in turn succeeded by coarser grained quartz, native gold, sylvanite and fluorite. The major veins from both Zortman and Landusky are continuous along strike for up to 950 m and were stoped to vertical depths of more than 200 m. Oxidation depth were variable, but extended in places to 150 to 200 m below the surface. The veins vary from sheeted zones up to 12 m wide to well defined faults 0.5 to 2 m in thickness. The average past production from the veins was 3.4 g/t Au, although values of up to 1000 g/t were recorded. Drusy vein quartz, comb structures, chalcedony and a fine grained nature characterised the vein mineralisation (Hastings, 1988).

The low grade stockwork presently being mined overlies the veins and occurs as large shear zones up to 600 m wide and extending up to 1800 m along strike. The Zortman and Landusky trends, although similar in overall aspect, exhibit some differing characteristics. At Zortman the host rocks are principally syenite and quartz-latite/rhyodacitic porphyries, and locally gneiss. Trachyte dykes are rare. The overall shear trend is NNW, with associated shears and veins generally striking NNE. Where the latter veins cut the major shears and are sufficiently numerous, the intensity of mineralisation increases as well as the depth of oxidation. This is a major factor in the localisation of ore (Hastings, 1988).

At Landusky the host rocks are undifferentiated Tertiary intrusives, including porphyritic rhyodacite/quartz-monzonite/quartz-latite and syenite, as well as Proterozoic quartz-feldspar gneiss and 'shale'. The major structural trend is north-east, with intersecting northerly striking shears and veins. As at Zortman, this structural pattern localises ore and increases the depth of oxidation. Trachyte dykes are common, especially along major shear zones, and are considered an indication of strong structural preparation (Hastings, 1988).

The degree of oxidation is important in the leachability of the ore. Mainly sulphide ore was not being leached in the late 1980's, although mixed oxide/sulphide ore was. In the oxidised sections of the orebodies, Fe and Mn oxides are the most common gangue, accompanied by variable amounts of clays, quartz and fluorite. The oxidation tends to give many deposits a funnel shape, locally extending to 250 m, depending on the degree of fracturing and permeability. In the oxidation zone the finely divided gold and silver particles are distributed in the Fe and Mn oxides as well as in clay minerals (Hastings, 1988).

Hydrothermal alteration of the central intrusive zone varies from practically non-existent in areas distant from ore to weak argillic within and adjacent to the deposits. Local strongly altered zones are generally associated with limited breccia and crushed zones. Structure largely controls the degree of alteration with the major Zortman and Landusky shear zones which contain the most persistent alteration. Pyrite and argillisation are the most frequent alteration forms, generally being most intense accompanying the mineralised intervals, although commonly extending well beyond ore. Orthoclase alteration as veins is frequently observed and is considered part of the hydrothermal system. Alteration of the intrusive wall rocks generally commences with pyrite followed by marcasite-sericite-orthoclase-kaolinite (argillisation). Chloritisation is occasionally present, but is not common, while silica dissolution and subsequent re-deposition accompanies, and appears to overlap, the sericite-orthoclase stage. Sericite replaces mafic minerals and plagioclase to varying degrees. Coarse to very fine pyrite with quartz and/or orthoclase cut earlier phases of alteration, with orthoclase invading the groundmass. Fluorite, occasionally with adularia, often accompanies this veining. Late stage fluorite also replaces matrix plagioclase in more heavily broken and brecciated zones (Hastings, 1988).

The trachyte dykes are the least altered intrusives. The sediments commonly occur as roof pendants in the intrusives, with weak to moderate alteration and stronger zones coinciding with intervals of more intense fracturing. Alteration distal from mineralisation is usually restricted to veins (Hastings, 1988).

The dominant alteration is weak to moderate argillisation, accompanied by iron oxide replacement of fine pyrite. Fluorite is frequently associated with breccia zones. Host rocks are commonly silicified, and on the margin of one pit jasperoid was present prior to mining (Hastings, 1988).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Hastings J S,  1988 - Gold deposits of Zortman-Landusky, Little Rocky Mountains, Montana: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno,    pp 187-205
Wilson M R, Kyser T K  1988 - Geochemistry of porphyry-hosted Au-Ag deposits in the Little Rocky Mountains, Montana: in    Econ. Geol.   v83 pp 1329-1346

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