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) abundances in order to assess the potential use of such data to decipher the origin 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-gold deposits. Trace-element abundances in the carbonates are low for all elements except Sr (100 to 15 000 ppm), which has larger intra- and inter-deposit 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 flat to slightly fractionated [(La/Sm)(N) and (Gd/Yb)(N) both generally ≤1-2] and Eu/Eu(*) of ca. 1 to 2; however, excursions to extreme enrichment in either light or heavy REE may occur. The fact that the large chemical variations obtained via solution chemistry (i.e., bulk separates) 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 from a reservoir by secondary processes operating at the vein scale. These secondary processes may include one or all of the following: (1) precipitation and dissolution of REE-bearing phases, including carbonate, (2) development of narrow wallrock-alteration zones marginal to veins, (3) changes in fluid composition [i.e., pH, f(O₂)], (4) recharge of the vein fluid, and (5) rate of fluid flow. Given that the carbonate REE patterns deviate markedly from the REE signature of the host metasedimentary rocks, the data are in permissive agreement with vein fluids originating from a non-Meguma Group reservoir.