Bulletin 29: Geology and Ore Deposits of the Sandon Area, Slocan Camp, British Columbia
Table of Contents
Map and Figures
(PDF document, 3.1MB) Note: This PDF file also contains the document title page and Table of Contents.
Chapter 1 and 2
Introduction and General Geology.
(PDF document, 12.5 Mb)
Chapter 3, 4, and 5
Structural Geology, Economic Geology, and Descriptions of Properties.
(PDF document, 16.5 Mb)
Bulletin 29 covers the geology and ore deposits of the Sandon area, Slocan Mining Camp, British Columbia. Sandon has been a centre of production of silver-lead-zinc ores for sixty years. Mines in the area have contributed approximately half the entire output of silver and lead of the Slocan Mining Division. Mining activity has fluctuated widely with the condition of the metal market, and interest in the area has been renewed with the most recent rise in prices.
The area is 2 miles north of the northern boundary of the Nelson batholith and is underlain by sediments of the Slocan series, intruded by dykes and small stocks.
The Slocan series is of great thickness, only part of which is exposed within the Sandon area. In the area, tuff occurs in the upper part of the geologic column. The sediments are mainly argillites, quartzites, and limestones, or intimate admixtures of these rock types. For the most part the sediments are silty, estuarine deposits with rapid alternations in character; some are rhythmically banded. Cross bedding and evidence of turbulent deposition are common in many parts of the geologic column. The rocks are locally slaty.
Intrusive bodies are for the most part sill-like and some stocks are elongated with the structure. The general rock type is a quartz diorite, with a highly variable texture. Lamprophyres of the same general composition, but relatively rich in biotite, are common and grade into the quartz diorites.
Metamorphism of contact type is not marked. The principal product, which occurs locally, is a brownish rock produced by the development of biotite in some of the argillite. Silicification is widespread, affecting the rocks patchily and chiefly in the southern and western parts of the area. The rock most affected is limestone, which may be converted to a product very similar in appearance to quartzite. Silicification is not related to the lodes but to the general process of intrusion, and is only locally related to specific dykes or sills.
The structure is of a type not heretofore described. It is more Alpine than has previously been recognized in British Columbia and has far-reaching regional implications, although the implications are not considered in this bulletin. Certain details of the structure are believed to be closely related to the emplacement of the Nelson batholith but, as time has prevented study of the batholith itself, the relationship has not been proved.
The folding of the Slocan sediments is of recumbent type, the individual folds having axial planes with little or no dip and axial lines with little or no plunge. The entire assemblage is involved in one huge recumbent fold, striking northwesterly, which is open or concave to the southwest. This is termed the Slocan fold, and includes many smaller folds of various sizes and degrees of complexity.
The large complex Slocan fold is wrapped round the northeastern end of the Nelson batholith, east of the Sandon area. South of the area, and also east of it, the rocks deviate markedly from the northwesterly strike along the general east-west course of the batholith. The folded rocks swing through a right angle in strike in a huge crumple that is termed down buckling, and ultimately meet the batholith tangentially. This folding of folded rocks is a modification of the Slocan fold structure, and there is no evidence that it was produced by a second period or separate generation of orogeny; rather, it is believed to have resulted from a progressive change in stress application during late stages of the Slocan folding. A further modification of the Slocan fold is a broad cross-warp that extends across the entire structure from the vicinity of Idaho Peak to the vicinity of Retallack. The cross-warp is anticlinal, with low regional plunges to northwest and southeast on either side of it. It extends through the main productive belt of the Slocan mining camp.
Drag folds of all sizes are developed in characteristic relation to the large folds. The geometry of recumbent folding is such that all upper beds have moved relatively downward, the reverse of the movement observed in upright folds. Cleavage is developed locally, and in the main Whitewater and Payne slate belts it is produced at least in part by excessive interbed slippage which amounts to shearing. Axial-plane cleavage and shear cleavage cannot be differentiated except by structural reference, and some cleavage is of dual origin.
There are many faults, all of which are related to the structural complex and the forces that produced it. Two general classes of faults are recognized: tangential and crosscutting. The tangential faults are parallel in strike to the formation and commonly are bedded. The crosscutting faults cross the regional strike at large angles. The two classes are closely related in time of origin. All faults appear to be normal, and most have a lateral as well as a normal component of movement. The displacement on the tangential faults is a consequence of the folding and bears the same relative movement as the interbed slippage of which the faults are, in a sense, an extreme expression. The crosscutting faults are tear faults with a component of underthrusting.
The lodes are almost without exception crosscutting faults, of small and large size. In the Sandon area they cut across the structure but, immediately to the south in the valley of Silverton Creek, they are tangential to the down-buckled strata. It is believed that the larger faults of this class originated parallel with the down-buckled rocks as an extreme of this late phase of folding and in partial relief of the stresses that produced it. The larger faults or lodes stemmed from a focal area in lower Silverton Creek with steep dips to the northwest, rolled with the down buckle to a southeasterly dip and, in the zone of maximum curvature, continued across the northwesterly striking rocks.
Most of the lodes are complex, inasmuch as they represent zones of rupture with more than one locus of movement. They are to a considerable extent influenced by the structure they cross, in both strike and dip. They are zones of both fracture and shear.
Orebodies are related to zones of fracture rather than of shearing and as a rule occur in places of decreased confining pressure. The factors that may be said to have controlled ore deposition are many, and most orebodies were formed as a result of the conjunction of a number of favourable circumstances.
Metal distribution within the area and within individual lodes is of great importance. Factors which might limit the occurrence of ore at depth and which might produce lateral or vertical segregation are vital to considerations of development. No marked pattern of zoning or segregation is recognized, except for the fact that the margins of orebodies are relatively richer in zinc than in lead. The control of ore deposition is structural, and temperature and load pressure due to depth are of minor importance. The most important single factor was probably that of local confining pressure in a structurally complex environment.
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