Documentation of variable trace- and rare-earth-element abundances in carbonates from auriferous quartz veins in Meguma lode-gold deposits, Nova Scotia

Samples of vein carbonate from 16 Meguma lode-gold deposits in Nova Scotia have been analyzed by solution chemistry (ICP-MS) and laser ablation micro- analysis (LAM ICP-MS) for their trace- and rare-earth-element (REE) abundan ces in order to assess the potential use of such data to decipher the origi n of the mineralization. The gold deposits, hosted by metasedimentary rocks of the Meguma Group, are well suited to such a study because carbonate is the second most abundant phase (after quartz) in these mesothermal lode-gol d deposits. Trace-element abundances in the carbonates are low for all elem ents except Sr (100 to 15 000 ppm), which has larger intra- and inter-depos it variation and is zoned within individual grains (LAM analysis). The REE data on vein carbonates are variable, both in terms of abundance (ca. 5 to 100 times chondrite) and degree of fractionation, with patterns generally f lat to slightly fractionated [(La/Sm)(N) and (Gd/Yb)(N) both generally less than or equal to 1-2] and Eu/Eu* of ca. 1 to 2; however, excursions to ext reme enrichment in either light or heavy REE may occur. The fact that the l arge chemical variations obtained via solution chemistry (i.e., bulk separa tes) are replicated by LAM ICP-MS analysis precludes putative contamination by microinclusions as a critical factor controlling the REE content of the carbonates. Instead, a crystal-chemical control is suggested. The data are interpreted to reflect modification of a primary REE signature inherited f rom a reservoir by secondary processes operating at the vein scale. These s econdary processes may include one or all of the following: (1) precipitati on and dissolution of REE-bearing phases, including carbonate, (2) developm ent of narrow wallrock-alteration zones marginal to veins, (3) changes in f luid composition [i.e., pH, f(O-2)], (4) recharge of the vein fluid, and (5 ) rate of fluid flow. Given that the carbonate REE patterns deviate markedl y from the REE signature of the host metasedimentary recks, the data are in permissive agreement with vein fluids originating from a non-Meguma Group reservoir.