Metalore Resources’ Gold Discovery, formerly known as the Brookbank occurrence, is in northern Ontario on an all-weather road about eight miles northeast of the town of Beardmore. Mineralization occurs in a shear zone lying along the faulted contact between east-west-trending polymictic conglomerates to the north and mafic volcanics to the south. A diorite intrudes the mafic volcanics in close proximity to the fault contact. All rocks are of Archaean age. Who discovered it? The first recorded exploration work on the Brookbank zone dates back t o 1934 and consisted of trenching and drilling, conducted by F. M. Connell Mining and Exploration. Subsequently, extensive trenching and drilling was conducted by Noranda in 1944. Forty shallow holes were completed, totalling 6,092 ft, and obtained intersections as significant as 17.7 ft of 0.288 oz gold per ton. No further work was done until 1974, when Lynx- Canada Explorations conducted a limited program of mapping, stripping and drilling. Metalore optioned the property in 1981 and has maintained an ongoing program since that time. What is the geological setting?
The Beardmore-Geraldton greenstone belt is within the Superior Province of the Precambrian Shield. It comprises three sequences of volcanic and sedimentary rocks with local mafic and felsic intrusives that are all cut by a number of sub-parallel, east- west-trending regional faults. The first sequence, south of the Metalore discovery, consists of sub-aqueous mafic volcanics and clastic sediments. What other deposits are found in this setting?
Several gold deposits are located in this southern series, including:
* The Leitch and Northern Empire deposits, both hosted in steeply dipping quartz vein systems; the former in sediments and the latter in mafic volcanics. Both are near Beardmore.
* The MacLeod Cockshutt and Hard Rock mines, consisting of quartz stringers in iron formation and quartz veins associated with felsic intrusives, both near Geraldton. * The Solomons Pillars occurrence, consisting of quartz strin gers associated with iron formation, located near Jellicoe.
Regionally, all of the above deposits lie in close proximity to east-west- trending fault zones and intrusives that may be associated with these structures.
A second volcanic-sedimentary sequence hosts the Metalore discovery, which is localized along a sheared contact zone between clastic sediments and mafic volcanics in close proximity to the regional Paint Lake Fault. A diorite intrudes the volcanics and has caused incipient deformation, alteration and mineralization in the sediments and volcanics.
The third sequence, north of the Metalore discovery, comprises a series of felsic to intermediate volcanics with felsic and mafic intrusives. An example of a gold deposit in this unit is the Sturgeon River mine, which consists of north-south-trending quartz veins in felsic volcanics. The above sequences represent three separate volcanic-sedimentary cycles which were subsequently folded, locally overturned (as at the Brookbank Zone) and sheared. What is the geology of the Metalore discovery itself?
The Metalore zone occurs within a sequence of massive mafic volcanics overlain by a polymictic metaconglomerate. These rocks are isoclinally folded and overturned. They dip steeply to the south, with pillow tops facing north.
The conglomerate forms the basal section of a debris flow, clasts are disorganized, and there are no graded sections and no evidence of stratification or clast imbrication. Fragments are of variable composition (granitic, feldspathic, quartz, mafic volcanics and jasper) and range in size from pebbles to cobbles set in a homogeneous, green greywacke matrix, probably derived from mafic volcanics. The basal conglomerate unit has a minimum thickness of 4-6 ft and is overlain by a pebbly sandstone conglomerate where the matrix is intensely altered to quartz and sericite. The pebbly sandstone is in turn overlain by a greywacke, interbedded with siltstone and shaly units.
A diorite intrudes the mafic volcanics and occasionally protrudes through to the conglomerates. The diorite is coarse-grained; and while no strongly defined chill margin has been seen on surface, chilled contacts have been observed in two thin mafic flows which are proximal to and concordant with the northern volcanic- sedimentary contact. This elongate intrusion extends for several thousand feet along the south flank of the contact zone hosting the Metalore deposit. Is the deposit controlled structurally?
Subsequent to isoclinal folding, the whole assemblage of volcanic, sedimentary and intrusive rocks in the Beardmore-Geraldton area underwent widespread ductile deformation including the development of regional shear zones. These are closely associated with major faults, such as the Watson Lake and Bankfield-Tombill Faults to the south of the Metalore discovery, the Paint Lake Fault on the Metalore property and the Musca Lake Fault to the north. Anastomosing faults are common throughout the greenstone belt.
The Metalore shear zone occurs along the mafic volcanic-sedimentary contact and probably lies on a splay or subsidiary of the main Paint Lake Fault. It is an east-west-trending zone of intense lateral strain. Ductile deformation is best demonstrated by the following features:
* Schistosity, which is penetrative, strikes 90 degrees and dips –78 degrees south;
* Flattened pillow selvages, vesicues and amygdules; and * Late, small-scal e structures such as sigmoidal tension gashes, strike-slip movements along planar features, S- and Z-shaped folds and, particularly, kink folds in the sheared and altered conglomerates.
Brittle deformation in the Metalore zone is best demonstrated by brecciation and small-scale cross-cutting quartz-carbonate-filled fractures trending in northwest and northeasterly directions.
The late brittle fracturing that occurs along the footwall of the Metalore zone has also been observed in drilling thus far over a 6-mile strike length on Metalore’s property. The fault is probably localized along a time-stratigraphic horizon and is represented by a narrow zone of breccia and gouge, which has been silicified into a very hard and distinct black- and-white-banded marker unit. The black is caused by amorphous graphite which contrasts with the white from silica flooding. The fault is 1/8-in to one foot in width near the Metalore zone but has been observed in drilling to be as much as several feet wide, two miles west of the main mineralized area. It generally follows the contact between the volcanics and conglomerates but can meander away from the contact several feet into the conglomerates. The fault always represents the structural footwall of the mineralized zone and the limit of gold values. What chemical alteration occurs in and around the deposit?
An alteration halo involving several mineralogical facies traverses the mineralized zone from hangingwall to footwall as follows:
* Intrusive — The diorite contains visible magnetite crystals and is generally altered with epidote, pink potassium feldspars, quartz, calcium and iron carbonate veinlets. Localized zones of iron-carbonate occur sporadically within the diorite and are often associated with specularite veinlets, reddish hematite and silicification. Coarse-grained pyrite (2%-7%) accompanies this alteration but carries negligible gold values. Some of this pyrite probably pre-dates the gold mineralizaton and may not be related to it. Other sulphides and oxides include minor chalcopyrite, galena and ilmenite.
* Mafic Volcanic and Conglomerates — Calcite and chlorite exist as prominent alteration minerals in close proximity to the main mineralized zone in the volcanics. They are principally confined to the volcanics but may extend minimally into the conglomerates. Calcite occurs as early to late stringers and veinlets. Chlorite occurs as pronounced veinlets in the volcanics and as minor veinlets with associated pyrite in the conglomerates. This assemblage is also characteristic of a regional alteration facies common in most rock
types in the area.
* The second major alteration facies is characterized by the development of up to 20% ankerite in the volcanics, commencing at the hangingwall of the main mineralized zone and gradually decreasing across the contact to less than 1% at the footwall in the conglomerates. The ankerite alteration is an essential constituent and indicator of gold values in the main mineralized zone.
* Approaching the footwall, hematite is a distinct but minor constituent (2%-5%) of the main mineralized zone within the mafic volcanics and appears to be associated with ankerite. The hematite occurs both as a reddish stain and as distinct metallic specularite crystals in veinlets and stringers.
* Generally the mafic volcanics are potassically altered and contain up to 20% pink potash feldspars which are associated with the ankerite and hematite alteration. The characteristic light pink color of the potassic alteration is frequently masked by the darker red hematite stain, but the potassium can read ily be detected with a spectrometer on outcrop or in drill core. Whole rock chemical analysis of the altered volcanics have shown up to 6% potash content versus 2% potash in the unaltered material.
Sericite and, to a much lesser degree, muscovite, have been metasomatized from the potash feldspars. The sericite occurs as a very minor alteration in the mafic volcanics; however, approaching the contact of the conglomerates it increases to 5%, and within the conglomerate it increases up to 20% locally. The sericite occurs primarily along slips and in veinlets in the conglomerates often in association with sulphides.
* Silicification is the principal alteration facies in the main zone. It is associated with pink potash feldspar, ankerite and, to a lesser degree, hematite. Silicification, in combination with the latter three minerals, also occurs in a patchy pattern in the mafic volcanics. In these rocks, near the contact with the conglomerate, the silica content increases up to 85% versus 50%-60% in the unaltered volcanics. Within the conglomerates, the silicification progressively increases toward the footwall of the zone and silica may reach 90% of the total rock composition. The precise contact between the mafic volcanics and conglomerates is not always readily discernible because of the intensity of the alteration, and primary textures are rarely retained. The silicification has a vertical zoning with widths of only a few feet at surface progressively increasing to 30 ft or more below about 1,200 ft from surface.
The development of very fine- grained disseminated pyrite is generally associated with the silicification. Generally 1%-2% of very fine-grained disseminated pyrite occurs in the mafic volcanics and increases to 2%-5% in the conglomerates at the footwall. Gold values increase with the greater percentage of silicification and very fine-grained pyrite. How does gold occur in the deposit?
Generally where the mineralized zone is hosted by mafic volcanics, gold values grade between 0.15-0.35 oz gold per ton. However, toward the conglomerate contact, in a transitional zone, the grade diminishes to 0.10 oz gold per ton, and within the conglomerates it increases again to 0.30-0.50 oz gold per ton and may increase to more than 2 oz gold per ton. The gold is homogeneously disseminated in a free state within the gangue material and/or is associated with the pyrite, interstitial to the pyrite, or encapsulated within it. The gold is very fine and is invisible to the naked eye even where very high assays predominate (i.e. more than 2 oz gold per ton). Other sulphides and oxides which occur in trace amounts in the zone include argentite, chalcopyrite, chalcocite, galena and ilmenite. These are not expected to cause metallurgical problems. How does the company’s chief geologist interpret the deposit?
My interpretation of the gold deposition in the main mineralized zone is that the intrusion of the diorite was accompanied by structural deformation of the wall rocks resulting in folding, faulting and/or shearing of the mafic volcanics along the sedimentary contact. This opened channelways for hydrothermal solutions and the ensuing migration of gold. It bears noting that the main gold-bearing zone dips steeply to the south toward the diorite. The volcanic-sedimentary contact has been drilled along strike in both directions for several miles and a peculiarity is that wherever the contact dips to the north, away from the diorite, good silicification, alteration and faulting have been encountered without any significant gold enrichment. Are there any geophysical responses which could lead to other discoveries?
There is a moderate magniphase and vlf signature along the main contact between the mafic volcanics and sediments, but this may be due in part to a zone of sericite schist that occurs near the contact within the sediments.
The diorite is represented by a 1,000-3,000-gamma increase in magnetic intensity. This usually decreases in the mafic volcanics and becomes a slight magnetic depression along the contact and into the conglomerate.
Total count radioactivity of up to three times background can be detected where there is potassium alteration in the mafic volcanics. Nominal radioactivity is ubiquitous in the sediments. Are there any other important guides to gold exploration in the area?
Metalore’s experience in the Beardmore area suggests that the following factors are important gold exploration guides:
* Contact areas between two distinct lithologies.
* Proximity of mafic or felsic intrusives.
* Faulting, folding and/or shearing along or parallel to contacts.
* Presence of iron carbonate and silicification with 0.5%-2% fine sulphides and anomalous gold (0.03 oz gold per ton or higher). Anomalous radioactivity in the mafic volcanics indicating potassic alteration. Barbara Kowalski is chief geologist of Metalore Resources.
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