SUMMARY
The Ishkoday is an atypical Archean hydrothermal system comprising: 1) early synvolcanic polymetallic mineralization; overprinted by 2) later syn-kinematic orogenic gold mineralization. The polymetallic occurrences are focused along syn-volcanic structures within a rift zone that was reactivated roughly 30 million years later during orogenesis related to the collision of the Beardmore-Geraldton and host Onaman-Tashota greenstone belts.
This later orogenic event introduced new gold into the system within laminated quartz veins and may also have remobilized and relocated earlier gold mineralization, as well. Areas retaining the best potential for economically significant gold-mineralized rock occur where both stages of mineralization are found superimposed or in close proximity, or where there is a sufficient concentration of gold-bearing veins alone. Many such potential areas on the property remain to be tested.
Synopsis - Polymetallic Mineralization (Epithermal/Porphyry)
Polymetallic mineralization is hosted within a syn-volcanic dike swarm intruding the felsic-intermediate volcaniclastic and coherent units of the Elmhirst-Rickaby assemblage. This dike swarm marks a zone of rifting that focuses Au-Ag-Cu-Zn-Pb mineralization and alteration in discreet planar zones striking northeast-southwest. The main corridor of mineralization extends for several kilometres between a synvolcanic intrusion (the Sturgeon River stock) and a syn-kinematic intrusion (the Coyle Lake pluton).
1.ELMHIRST-RICKABY ASSEMBLAGE: Arc-like sequence of calc-alkaline, subaqueous and subaerial, intermediate to felsic effusive volcanic and volcaniclastic rocks (ca. 2740-2734 Ma) intruded by four synvolcanic plutons including gabbro, diorite-quartz diorite-tonalite, and granodiorite compositions (2736-2732 Ma), one of which hosts weak porphyry-style Cu-Au mineralized stockworks.
2. DIKE SWARM: Fractionated dike swarm containing variably feldspar-, quartz-, and amphiboleporphyritic fine-grained to aphanitic dikes. The earliest dikes host the polymetallic mineralization, while the latest dikes overprint it. They define a zone of rifting sandwiched between a synvolcanic and post-volcanic stock (Sturgeon River stock and Coyle Lake stock, respectively), along a portion of which both earlier, Ishkoday style polymetallic epithermal, and later Sturgeon River style, orogenic gold mineralization is found.
3. ISHKODAY HYDROTHERMAL SYSTEM: Evolved, three-stage hydrothermal system, with accompanying alteration spanning pre- to post-mineralization stages and a coincident, locally gold mineralized tourmaline zone, mainly within the faulted and sheared dyke swarm. On a regional scale, mineralization plunges moderately southwest.
i. Stage One – Alteration Only
Alteration – An early phase of propylitic alteration predates mineralization, characterized by epidote and chlorite. Other alteration minerals include chlorite, pyrite, and lesser actinolite, albite, quartz and sericite.
ii. Stage Two – Two Types of Mineralization and Two Types of Alteration
Mineralization - Comb-textured and related quartz veins, stockworks, and cockade breccias, with and without sulfides (sphalerite, chalcopyrite, and pyrrhotite); veins and breccias vary from isolated to concentrated with variable widths throughout several mineralized zones along strike, with similar variation in gold grades.
Mineralization - Disseminations, veins, stringers and semi-massive bodies of gold-bearing sulfides dominated by sphalerite, chalcopyrite and pyrrhotite with lesser pyrite and galena, up to several metres in width, concentrated in the northeastern portion of the dikes swarm. Gold grades vary from sub- to multi-gram with up to ten or more times silver in the sulfides.
Alteration - Discordant chlorite-quartz-sulfide alteration crosscuts the early propylitic alteration and occurs in the immediate proximity to mineralized zones. It typically forms stringer-like networks displaying pseudobreccia textures; it includes lesser sericite, actinolite, epidote and apatite.
Alteration - Calc-sodic alteration formed as vein- and breccia-fills as well as lesser replacement style alteration, and is dominated by actinolite with lesser albite, quartz, magnetite and sulfides.
iii. Stage Three – Alteration Only
Alteration - Late propylitic veinlets dominated by epidote, quartz, calcite, chlorite and pyrite.
iv. Tourmaline Zone – Alteration and Mineralization
Roughly 350 by 90 metres in surface area, it is hosted mainly in the volcanics to the southwest of the main mineralized zone. Synvolcanic and coeval with the dike swarm, it is observed as breccia fill and pervasive replacement zones. Gold occurs in folded tourmaline-quartz veins and veinlets within a zone where pervasive tourmaline replacement grades into a zone of orbicular tourmaline alteration.
Synopsis - Gold Mineralization (Orogenic)
Orogenic gold mineralization consists of two generations of gold-bearing quartz veins displaying laminated and locally vuggy textures. These were emplaced synchronous with D1 and D2 deformation, (ca. 2706-2695 Ma). The first generation of veins are dominant volumetrically and occur as fault-fill style veins emplaced in reactivated syn-volcanic structures displaying north-side up reverse movement during D1 compression.
The second generation occurs as extensional quartz-chlorite veins emplaced during D2 sinistral transpression.
4. STURGEON RIVER HYDROTHERMAL SYSTEM: Exploiting the same rift-related faults and those subsequently developed as a result of later transpressional deformation (D2), it overprints earlier Ishkoday style mineralization.
Mineralization - two main styles:
i. Centimetre- to metre-scale saccharoidal to crystalline, variably laminated gold-bearing quartz veins with strike lengths up to 1.5 kilometers (northeast trend); vein mineralogy is dominated by quartz with trace ankerite, sericite, chlorite and pyrite, with native gold in fractures and associated with sericite laminae and pyrite.
ii. Volumetrically small centimeter- to decimeter-scale saccharoidal to crystalline gold-bearing extensional quartz-chlorite veins with decimeter- to meter-scale strike lengths, primarily within boudin necks of the earlier (main) veins, typically containing quartz and fine-grained chlorite, often displaying vuggy textures.
Alteration - chlorite and sericite in envelopes from 0.5-4 meters wide, overprinting earlier Ishkoday style alteration.
Structure – vein shapes and trends – strike, dip, plunge, folded and otherwise, are related primarily to D2 deformation and sinistral shearing. The first generation of Sturgeon River veins are boudinaged and folded into F2 S-shaped closed folds. The second generation of veins are oriented at 90 degrees to the S2 fabrics and are consistent with extensional veins emplaced during sinistral D2 deformation.
CONCLUSIONS
Epithermal, Ishkoday style mineralization is associated with the rifting of an arc-like volcanic sequence, and intrusion of the dike swarm (ca. 2740-2736 Ma);
II. Ishkoday polymetallic mineralization consists of base metal sulfide stringers and Au-Ag bearing comb-textured quartz vein networks within the same faults and fractures as those exploited by the dike swarm.
III. Later, Sturgeon River style Au mineralization represents a mesothermal orogenic system that overprints earlier Ishkoday style mineralization, consisting primarily of higher grade laminated extensional Au-bearing quartz veins.
IV. The two hydrothermal systems are associated with each other and with the synvolcanic dike swarm, and is an example of ancestral structural control on orogenic systems.
Exploration potential for concentrations of Au mineralization of possible economic significance exists in areas where both earlier and later mineralization is found overlapping and in close proximity, or where there is a sufficient concentration of orogenic gold-bearing veins alone. Many such potential areas on the property remain to be tested.