The property is located at the International Boundary on the east side of Goosmus Creek, 10 kilometres south-southeast of Greenwood. Access to the property is mainly by a network of farm, logging and old mining roads connected to the Phoenix to Lone Star haulage road that passes through the property.
The deposits are at the head of the valley of Goosmus Creek, a southeasterly flowing tributary of the Kettle River in Ferry County, Washington State. The important mineral exposures are on the southerly and southwesterly facing slopes overlooking Goosmus Creek. Elevations in the area range from 1,000 metres on the creek to 1,500 metres on surrounding ridges. Outcrops are relatively scarce and glacial till is as much as 75 metres thick in places. Geological knowledge from surface work is augmented by abundant information from diamond drilling.
The copper-gold bearing Lexington quartz porphyry and associated veins have been explored since 1890 by numerous adits, shafts and drill holes on both sides of the Canada - U.S.A. border. In 1900, development at the City of Paris mine, above what is now known as the Lexington Main zone, yielded 1,900 tonnes of ore grading 13.7 grams per tonne gold, 71 grams per tonne silver and 3.12 per cent copper. In a similar geological setting, the Lone Star mine in Washington State produced 5,900 tonnes of ore from 1890 to 1920 that yielded 1.1 grams per tonne gold, 6.5 grams per tonne silver and 2.6 per cent copper. An additional 360,000 tonnes was mined from the Lone Star open-pit by Granby Mining and Smelting Company in 1977-78. United States Borax and Chemical Corporation and Ryan Exploration Company Limited continued exploration, including diamond drilling, from 1989 to 1992. In 1993, Britannia Gold Corporation acquired ownership of the mineralized zones on both sides of the boundary. To the end of 1993, 474 drill holes representing a total of 31,720 metres of drilling, were completed. Britannia Gold Corporation's 1993 Work Program Review reports "an extensive and detailed geological and geophysical program" and "13 B.Q. Drill Holes - 1862.2 metres" indicating good results including "a significant copper-gold intercept in B-93-6 with 11.8 grams per tonne gold and 1.094 per cent copper over 14.3 metres."
The upper valley of Goosmus Creek, at the northern extremity of the Republic graben, is underlain by a southeasterly striking belt of pre-Permian to early Mesozoic rocks, comprising gneiss, schist, chert, argillite, limestone and metavolcanic rocks. These units are cut by a wide variety of intrusions including Jurassic to Tertiary felsic stocks and dikes, and several large fault bounded serpentinite bodies.
The most important structure is a southeast trending fault zone that hosts the principal mineral deposits and separates Permian rocks (Attwood Group) on the southwest from the predominantly pre-Permian rocks (Knob Hill Group) on the northeast. This fault zone is part of a regional northeasterly dipping thrust system into which ultramafic rocks have been emplaced before intrusion of the Lexington porphyry.
The inferred location of the Bacon Creek fault is the western boundary of the Republic graben. Although this northerly trending fault is of regional importance, movement is Tertiary age and related to the pulaskite and other relatively young crosscutting dikes that clearly post-date the principal copper-gold mineralizing event at Lexington and Lone Star. Copper is not an important constituent of the epithermal gold-quartz fissure veins associated with the Bacon Creek Fault at Republic or other mineralized localities in the Republic graben.
The oldest rocks are assigned to the Knob Hill Group. This group comprises a great thickness of highly deformed Devonian-Carboniferous ribbon cherts, phyllites, thin limestone lenses and some greenstone. These rocks are east of, and form the hangingwall of the serpentinite and Lexington quartz porphyry. The Attwood Group, composed mainly of black argillite, sandstone and andesites, occurs west of the serpentinite and Lexington quartz porphyry. The 'Anarchist Series' comprises a poorly defined mainly Paleozoic assemblage that includes Knob Hill and Attwood rocks.
Serpentinite underlies much of the headwater area of Goosmus Creek. This unit is believed to be part of a dismembered ophiolite of probable Paleozoic age that was emplaced prior to the Lexington quartz porphyry. In the map-area the unit comprises two subparallel easterly-dipping lenses, each several kilometres in length, following Goosmus Creek valley. The westerly (footwall) lens is suspected to merge with the north end of the Bacon Creek fault. The easterly (hangingwall) lens follows a separate fault strand that lies immediately east of the Lone Star and City of Paris mines. Alteration consists mainly of talc and carbonate (listwanite) in lenses and seams associated with the principal faults and fissures. Magnetite is concentrated locally, possibly by metasomatic processes, at the contacts of the Lexington quartz porphyry.
The Lexington intrusion is an elongate quartz porphyry emplaced in a shear zone that extends at least three kilometres southeast from the source area of Goosmus Creek, through the City of Paris mine, across the International Boundary to the Lone Star mine. The intrusion follows the serpentinite and is postulated to be cogenic with the relatively undeformed larger quartz feldspar porphyry body exposed one to two kilometres to the west and similar rocks in the vicinity of the Midway mine 14 kilometres further west. These bodies intrude various Paleozoic units including chert, schist, argillite, limestone and greenstone of the Knob Hill and Mount Attwood groups (rocks that also occurs as xenoliths and screens within the intrusion). The Lexington quartz porphyry contains subhedral quartz phenocrysts and composite quartz eyes set in a matrix of small polygonal plagioclase crystals (mostly altered to clay), chloritized biotite and interstitial fine-grained quartz and feldspar. Where strongly altered such as in the Lone Star pit the unit has been transformed locally into a quartz-sericite or chlorite schist. Quartz comprises 35 per cent of the rock and the chemical analysis shows SiO2 72 per cent and Al2O3 16 per cent. Mylonitization has commonly destroyed or reduced the size of phenocrysts, and because of this some cataclastic facies resemble fine-grained felsic volcanic rock but without the embayed quartz and fresh, zoned feldspar phenocrysts typical of the Tertiary rhyolite and dacite in the region.
A series of small dikes, stocks and sills, petrologically similar to the Coryell Intrusions, Shasket Creek alkalic intrusions and lavas of the Marron Formation occur throughout the area. These are relatively fresh and undeformed and thought to be early Tertiary age. An unusually high concentration of diorite and pulaskite dikes of this suite is found on Mount McLaren and Rusty Mountain. The dikes commonly trend northerly parallel to a prominent set of cross joints that are generally associated with the tension fractures of the Republic graben. Northeast of Goosmus Creek they coalesce to form irregular-shaped bodies adjacent to the serpentinite.
The age of the Lexington quartz porphyry was previously thought to be Cretaceous or early Tertiary; however, determination of U/Pb isotope ratios on accessory zircon from diamond-drill core from the City of Paris area gives an Early Jurassic and a Precambrian age. The lower concordia intercept (200 Ma) indicates the age of intrusion; the upper concordia intercept (2445 Ma) is believed to be the result of a relict zircon fraction assimilated from early Proterozoic basement rocks.
The Lexington-Lone Star copper-gold deposits are spatially and temporally related to the Lexington quartz
The copper-gold mineralization on the Richmond property and at the Lone Star mine is confined to two bodies: the Pit zone and the Northwest zone. The Pit zone mineralization at the Lone Star mine is characterized by disseminated and stockwork sulphides that generally comprise several per cent of the rock grading > 0.3 per cent copper. Weak molybdenite mineralization (mostly on slip surfaces) is also scattered throughout the Pit zone. Walls in the pit are altered to green chloritic rocks. Pit zone mineralization occurred at the same time or soon after the emplacement of the quartz porphyry along the principal northwest trending shear zone, possibly with some later remobilization into limbs of a northerly plunging arch delineated by structure contours at the top of the footwall serpentinite.
Northwest zone mineralization on the Richmond property, occurs predominantly within the top portion of the footwall serpentinite. Although this zone is known only from drilling through overburden, the foliated textures in the core indicate the mineralization may be controlled by a gently-dipping thrust at the contact of the footwall serpentinite and the quartz porphyry. Furthermore, some of the best intercepts in the Northwest zone appear to coincide roughly with the axis of the northerly-plunging arch which marks the top of the footwall serpentinite in the pit area.
Exploration on the Lexington claims has focussed on gold and silver bearing quartz veins and stockworks; low grade sulphide disseminations and fillings on fractures and shear zones. The most widespread mineralization is pyrite (1 to 5 per cent) and chalcopyrite occurring as disseminations and fillings on lacy fractures in the Lexington quartz porphyry. The general tenor of this low grade mineralization is shown by analyses of 120 core samples which assayed from 0.1 to 0.3 per cent copper and 0.05 to 0.25 gram per tonne gold. The rock is commonly leached at surface with fractures being coated with limonite and malachite or black manganese oxide. Fractures are well developed locally within the quartz porphyry and the intensity of mineralization appears to be proportional to this development. Pyrite disseminations occur most commonly near the margins of the intrusion. The higher concentrations of copper mineralization are confined mostly to the upper and lower margins of the quartz porphyry and within about 30 metres of the enclosing serpentinite. However, detailed diamond drill cross sections indicate as many as three separate zones.
Workers at the City of Paris mine explored and developed a system of discontinuous quartz veins extending for about 300 metres along the upper contact of the Lexington quartz porphyry and in the overlying serpentinite. The accessory minerals in these veins include pyrite, chalcopyrite, galena, sphalerite and, less commonly, tetrahedrite. Up until 1969. the hangingwall of the quartz porphyry was thought to be the best locus for concentrations of chalcopyrite but later drilling by Lexington Mines Ltd. has shown that the footwall is also favourable. Of the 28 diamond drill holes and 18 percussion holes completed in the vicinity of the City of Paris workings, 13 intersected mineralization that appears to lie in a continuous zone, known as the Lexington Main zone. Abundant copper mineralization has also been discovered in the serpentinite adjacent to the Lexington quartz porphyry. For example, the footwall serpentinite exposed immediately west of the Lexington portal on Goosmus Creek contains pyrite, magnetite and chalcopyrite impregnations within talc alteration in shear zones with copper grades ranging from 0.36 to 0.76 per cent.
The Lexington Main zone is a gently plunging, sinuous deposit 365 metres long, enriched in pyrite, chalcopyrite and hematite that lies near the footwall of the Lexington quartz porphyry below the City of Paris portal. At the west end, the Lexington Main zone projects to surface. At the southeast end it is cut off by a pulaskite dike. Possible mineralized offsets have been found in isolated drill holes further south. A diamond drill hole in the middle of the zone returned an assay result of 15.3 grams per tonne gold and 2.0 per cent copper across 11.6 metres. However, the average grade of 4.5 grams per tonne gold, 4.1 grams per tonne silver and 0.93 per cent copper reported by Phendler (1974) is probably more representative of the reserve. Microprobe analyses of relatively fresh, slightly fractured ore shows that the gold is associated with pyrite occurring as discrete inclusions and along grain boundaries. No gold has been observed on fractures or associated with chalcopyrite.
The age of mineralization is mainly older than the Tertiary dikes that cut across the ore and younger than the listwanite alteration associated with thrusting and probably about the same age as the Lexington quartz porphyry. In the Midway mine area, 16 kilometres to the west, reports that sills and dikes, correlative with the Lexington quartz porphyry, intrude the serpentinite and a slightly older microdiorite body.
Very commonly, these intrusives are altered, with saussuritizated feldspars, pervasive clay and quartz-pyrite- sericite alteration, and less often, silicification. The very strong correlation between this alteration and the presence of the quartz-feldspar porphyry, not only at this location but elsewhere on the grid and in the Greenwood Camp, suggests that the emplacement of the intrusion was responsible for the alteration.
Pyritic lenses and concentrations of magnetite near the contacts of the porphyry are believed to be related to the intrusive event.
Reserves in the mineralized zones, reported by Ebisch (1991), are as follows:
-------------------------------------------------------------------Zone Tonnes Copper Gold (grams per tonne) (per cent) Lone Star Pit 17.60 million 0.52 0.3 Northwest (Border) 0.95 million 1.04 1.0 Lexington 1.10 million 0.93 4.5 -------------------------------------------------------------------- |
