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A detailed study of the geochemical and isotopic characteristics of the volcanic rocks of the Hidden and Louis formations, which make up the hanging wall to the volcanogenic massive sulphide deposits at Flin Flon, Manitoba, was carried out on a stratigraphically controlled set of samples. The stratigraphy consists of the lowermost, dominantly basaltic, Hidden formation, and the overlying, dominantly basaltic, Louis formation. Of importance petrogenetically, is the 1920 unit a basaltic andesite with Nb/Thmn = 0.54-0.62, 3Nd(1.9Ga) = +3.6-+5.9, 3Hf(1.9Ga) = +8.5-+9.6, and 204 Pb/ 206 Pb = 23.9. The basaltic flows that dominate the Hidden formation have Nb/Thmn = 0.16-0.29, 3Nd(1.9Ga) = +1.7-+4.4, 3Hf(1.9Ga) = +7.0-+11.8 and 204 Pb/ 206 Pb = 16.9-18.6). The Carlisle Lake basaltic-andesite (top of Hidden formation) is characterized by Nb/Thmn = 0.16-0.14, and 204 Pb/ 206 Pb = 21.4. The rhyodacitic Tower member (bottom of Louis formation) has Nb/Thmn = 0.23, 3Nd1.9Ga = +4.6, 3Hf1.9Ga = +9.3, and 204 Pb/ 206 Pb = 22.2. The basaltic flows that dominate the Louis formation have Nb/Thmn = 0.18-0.25, 3Nd(1.9Ga) = +3.6-+4.2, 3Hf(1.9Ga) = +8.4-+11.3 and 204 Pb/ 206 Pb = 17.9. The Hidden and Louis formations show dominantly transitional arc tholeiite signatures, with the 1920 unit having arc tholeiite characteristics. It is interpreted to have formed through extensive fractional crystallization of differentiated magmas at shallow levels in oceanic crust. Given the geological, geochemical, and isotopic characteristics of the Hidden and Louis formations, they are interpreted to represent subducted slab metasomatism with minor contamination from subducted sediments.Résumé : Une étude détaillée des caractéristiques géochimiques et isotopiques des roches volcaniques des formations de Hidden et de Louis, lesquelles forment l'éponte supérieure des gisements de sulfures volcanogènes massifs à Flin Flon, Manitoba, a été effectuée sur un ensemble d'échantillons contrôlés par la stratigraphie. La stratigraphie comprend, tout à la base, la formation de Hidden, principalement basaltique et la formation de Louis sus-jacente, principalement basaltique. L'unité 1920, une andésite basaltique, est importante d'un point de vue pétrogénétique; elle possède les valeurs suivantes : Nb/Th mn = 0.,54-0,62, 3Nd (1.9Ga) = +3,6 à +5,9, 3Hf (1.9Ga) = +8,5 à +9,6, et 204 Pb/ 206 Pb = 23.9. Les coulées basaltiques qui dominent la formation de Hidden ont les valeurs suivantes : Nb/Th mn = 0,16-0.29, 3Nd (1.9Ga) = +1,7 à +4,4, 3Hf (1.9Ga) = +7,0 à +11,8 et 204 Pb/ 206 Pb = 16,[9][10][11][12][13][14][15][16][17][18]6). L'andésite basaltique Carlisle Lake (sommet de la formation de Hidden) est caractérisée par Nb/Th mn = 0,16-0,14 et 204 Pb/ 206 Pb = 21,4. Le membre Tower (bas de la formation de Louis) a les valeurs suivantes : Nb/Th mn = 0,23, 3Nd 1.9Ga = +4,6, 3Hf 1.9Ga = +9,3 et 204 Pb/ 206 Pb = 22,2. Les coulées basaltiques qui dominent la formation de Louis ont les valeurs suivantes : Nb/Thmn = 0,18-0,25, 3Nd(1.9Ga) = +3,6 à +4,2, 3Hf(1.9Ga) = +8,4 à +11...

The hanging wall to the Flin Flon, Callinan, and Triple 7 volcanogenic massive sulphide deposits of the Flin Flon district is composed of the Hidden and Louis formations. The contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism. The formations form a composite volcanic edifice that was erupted and grew within a large, volcanic–tectonic subsidence structure (hosting the deposits) that developed within a rifted-arc environment. The formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows, sills, and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks. The Hidden formation is interpreted to represent a small shield volcano and the Louis formation a separate shield volcano that developed on its flank. Both the Hidden and Louis volcanic edifices were constructed by continuous, low-volume eruptions of pillow lava. A gradual change from a dominantly extensional environment during the formation of the footwall Flin Flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the Hidden and Louis formations are unlikely to host significant volcanogenic massive sulphide-type mineralization. However, synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization, indicating their control on ore formation, longevity,and reactivation as magma and fluid pathways during the growth of the Hidden and Louis volcanoes.