CHANGES IN STRESS FIELD DURING TEXAS CREEK MINERALIZATION IN NORTHWESTERN BRITISH COLUMBIA: AN EXPLORATION TOOL AND POSSIBLE RECORD OF MICROCONTINENT COLLISION

The Early Jurassic (197-185 Ma) Texas Creek Plutonic Suite in the Stikine terrane of northwestern British Columbia is one of the most fertile gold-copper metallogenes in the Canadian Cordillera. At least 10% of nearly 1000 mineral occurrences in the area, including the past-producing Premier and Snip deposits and the Kerr, Sulphurets, Mitchell and Brucejack (VOK) deposits in the “Golden Triangle” formed during this metallogenic event.

The past-producing Georgia River deposit, 18 km south of Stewart near the southern apex of the Golden Triangle, is hosted by volcanosedimentary rocks correlative with Triassic Stuhini Group and folded with northeast vergence. These folded strata are intruded by irregular stocks of Texas Creek Plutonic Suite. The intrusions are not folded, nor is there evidence of strain release peripheral to contacts, but they carry a southwest-dipping foliation parallel to the axial planar cleavage in the supracrustal rocks. Similarly-oriented dextral shear zones increase in thickness and abundance near the deposit, culminating in a 500 m-thick shear zone 150 m southwest of the mine workings. The shear zones host low-grade mineralization in the mine. North-striking dilational veins hosting higher grades of Au are interpreted as extension structures formed during the dextral shearing.

Johnny Mountain, 120 km northwest of Georgia River, is underlain by volcanosedimentary rocks of the Stuhini Group, intruded by the Red Bluff Porphyry (Texas Creek Suite). The SNIP gold deposit is hosted by a southwest-dipping shear zone parallel to axial planar cleavage of northeast-vergent folds in the Stuhini rocks. The SNIP structure is widest at its shallowest-dipping part, strongly suggesting compressional structural preparation. Dextral offset subsequent to intrusion of the Red Bluff Porphyry (Texas Creek Suite) is shown by airborne geophysics. The latest stages of mineralization at SNIP are in extensional settings.

The commonality of structures in widely separated metallogenic systems suggests that dextral shearing during or immediately following Texas Creek intrusion is a regional phenomenon and therefore a useful exploration tool. Moreover, the apparent change in stress field before and during Texas Creek time documents a possible pre-accretion microcontinent collision.