PHANEROZOIC FLUIDS PERMEATING A PRECAMBRIAN GOLD DEPOSIT EXAMINED BY SIMS: IMPLICATIONS FOR REMOBILIZATION OF GOLD

The Reef Deposit is an anomalous Au-Cu occurrence in the Paleoproterozoic terranes of northern Wisconsin. Previous work using lead isotopes has identified a protracted development of the Au mineralization from initial formation as the root zone veins of a volcanogenic massive sulfide (VMS) deposit during the Penokean orogeny (~ 1.9 – 1.8 Ga), with the most recent Au mineralization/remobilization activity associated with late Paleozoic fluid flow (Haroldson et al., 2016). Here we use the oxygen isotopic history of the Reef Deposit to verify and further examine the deposit’s protracted development.

Laser fluorination measurements of quartz veins at mm-scale range in δ¹⁸O from 6.8 to 10.8 ‰ VSMOW, and a trend is observed of increasing δ¹⁸O values of (northeast trending) vein zones located west to east, likely signifying a temperature gradient during initial formation.

In-situ oxygen isotope measurement by Secondary Ion Mass Spectrometer (SIMS) of quartz and carbonate veinlets cross-cutting the primary quartz veins range in δ¹⁸O from 8.5 to 28.4 ‰ for quartz, 25.3 to 28.1 ‰ for dolomite, and 9.6 to 29.3 ‰ for calcite. High δ¹⁸O values (>19 ‰) are measured in a crustiform textured quartz stockwork veinlet, in dolomite observed in late cross-cutting carbonate veinlets, and calcite in settings associated with the late quartz and dolomite and a separate calcite-only setting directly linked with gold mineralization/remobilization.

The lower δ¹⁸O measured by SIMS verifies the laser fluorination values for the primary quartz veins as related to a high temperature portion (root zone) of a VMS system, whereas the higher δ¹⁸O is verified with fluid inclusions in late cross-cutting calcite to have formed from a low temperature (< 103 °C) high salinity (22-24 wt. % NaCl equiv.) fluid. The system related to high δ¹⁸O values measured in open space growth quartz and dolomite is less clear, however, support for the same system which formed the late calcite is found in chalcopyrite mineralization observed in all three settings. We therefore interpret the late quartz and carbonate gangue mineralization to be related to an overprinting MVT fluid.

References

Haroldson, E.L., et al., 2016, Phanerozoic Au in a Paleoproterozoic Cu Vein deposit of Central Wisconsin; Evidence from Pb Isotopes Using LA-ICP-MS, GSA. Vol. 48, No. 7, paper no. 212-13