Geological Description
At Renfrew, numerous Au-bearing quartz veins of the interbedded, or saddle reef type are found intruding the Goldenville Formation around a northeast-trending unsymmetrical anticlinal dome. This dome is an extension of the same anticlinal structure that hosts the Mt. Uniacke Gold District thus the structure is known locally as the Mt. Uniacke-Renfrew Anticline. Strata on the north flank of the dome have average dips of 20° to 30° northwest, while the south limb has generally steeper dips of 30° to 60° southeast. Several parasitic folds or undulations radiate from the centre of the dome and faulting is commonly associated with them. There is a gradual increase in width of individual quartz veins as they approach the parasitic folds. Also, angular and fissure veining becomes more common near parasitic folds.
Most of the area of productive veining is more or less restricted to an area of 2 x 2 km found along the southeast limb, and at the southwestern terminus, of the fold. No veins of any economic importance have been found on the north limb of the anticline. The strike length of veins mined was up to 4.1 km and to a maximum depth of about 150 m. There is no doubt that gold mineralization in the veins extends to depths below this.
A strong schistosity is discernable within the metasedimentary wallrocks particularly within metasiltstone and slate units. This foliation strikes 60° to 85° and dips approximately 60° to 70° south. Several Z-drag folds are observed and are mostly within more sheared sections. The folds are confined within the schistosity plane: the fold axes trend southwest to west-southwest and dip from 21° to 48° to the southwest while the axial planes dip steeply to the south. The drag folds suggest a reverse sense of vertical movement along with a significant dextral horizontal displacement component. Asymmetric crenulations and offsets in the quartz veins would also be supportive of dextral horizontal displacement.
A brittle fracture cleavage is found in both the massive and sheared sediments suggesting their formation occurred during, or after, the main shearing episode. The fractures occur in a wide variety of strikes but most dip subvertically. Two sets of joint surfaces are distinguished in the massive greywacke: one set strikes roughly northwest to north-northwest and dips 72° to 78° west, while the other set strikes northeast and dips 42° to 60° southwest. Quartz veins appear to be more numerous and thicker in the western extremity of the district, and many of the veins diminish in size and sometimes pinch out to the east.
Mineralization
Most gold-bearing veins in the district are Interbedded Quartz Veins. These veins vary in thickness from a few mm up to a meter and are bedding parallel, laterally and vertically persistent, and occur mostly within slate or metasiltstone units. Carbonate is often associated with the vein material as well as generally minor amounts of arsenopyrite and pyrite and even lesser amounts of galena, chalcopyrite, pyrrhotite and sphalerite. Tourmaline, scheelite, stibnite and cassiterite have also been reported associated with auriferous veins. These veins often have a laminated or crack-seal appearance due to the presence of wispy to distant inclusions of slate or schist. Gold occurs as fine to coarse free particles in the quartz, carbonate, sulphides and immediately surrounding slate wall rock.
An important gold concentration mechanism occurred during the later stages of deformation due to buckling or kinking of the slate or schist units, and coincidental fracturing of more competent rocks. These crumples occur as bands oriented at an angle to the axial plane of the regional fold. The phenomenon causes foreshortening of the kinked of buckled area. Sometimes silica was remobilized into the adjacent rocks during this process and result in a net gold enrichment within the vein and surrounding slate or wall rock.
Mining
Gold was first discovered at Renfrew in 1861 by a William Thompson who found an auriferous quartz boulder on the banks of the brook near his sawmill. Later that year, a John McPhee discovered additional quartz veins near Thompson’s mill. These discoveries lead to a swirl of activity by many small companies and prospectors staking independent small claims. Production began in 1862 and continued to 1927 followed by intermittent production to 1958.
The Renfrew Gold District has the distinction of having what was probably one of the richest leads known to the province during the years 1862-1913. From this lead, the Preeper Lead, an open cut 5.4 meter long and 2.4 m deep produced 180 oz of gold from 83 tons of quartz.
Mining operations in the Renfrew Gold District resulted in a total reported production of 51,985.9 oz of gold, the seventh largest in the province, from milling of 66,567 tons of ore. The resultant grade was 0.78 oz per ton. Even during the low production periods, the grade never fell below 0.40 oz per ton.
The last recorded exploration diamond drilling was in 1985.