Origin and evolution of gold-bearing fluids in a carbon-rich sedimentary basin: A case study of the Algamarca epithermal gold-silver-copper deposit, northern Peru

Sediment-hosted gold deposits account for the major part of economic gold in the Earth's crust. However, the origin of the gold-bearing fluid and its evolution in sedimentary basins in the presence of organic carbon and its metamorphosed products such as graphite are poorly known. In an attempt to clarify these issues, we performed an integrated mineralogical, geochemical, and fluid-inclusion study of the Algamarca epithermal Au-Ag-Cu deposit, hosted by Mesozoic sediments corresponding to an over-mature petroleum system within the Marañón fold and thrust belt (northern Peru). Results show that mineralization started with a pre-gold stage characterized by quartz veins containing gold-poor pyrite and chalcopyrite. Most gold was deposited afterwards, during the main gold stage in an “invisible” form within arsenian pyrite, followed by minor visible native gold with sulfosalts and chalcopyrite at a later stage. Fluid inclusions in quartz from the pre-gold and gold stages show features analogous to those observed in porphyry Cu-Au systems such as vapor–liquid immiscibility, enrichment in K, Rb, Cu, As, and Sb, a wide range of salinity (5–35 wt% NaCl eq.), and similar elemental (atomic) ratios (Zn/Pb ∼ 4, 0.1 < K/Na < 5, Br/Cl ∼ 0.06), all consistent with a fluid of magmatic origin. In addition, the fluid inclusions from the pre-gold stage are highly enriched in CO₂ (∼60 mol% in gas phase), CH₄ (∼10 mol%) and H₂S (∼30 mol%). Such high volatile contents are rather unusual for typical porphyry-epithermal systems and likely reflect reactions between the magmatic fluid and carbon-bearing sediments. This conclusion is independently supported by the temperature values of graphite metamorphic peak determined by Raman spectroscopy, which are similar to those derived by fluid-inclusion microthermometry in quartz veins. Our findings imply that strong interactions of magmatic fluid with carbonaceous matter favored gold transport through the sedimentary basin and its subsequent concentration in arsenian pyrite. Furthermore, our results point to a possible presence of porphyry-style mineralization beneath the sedimentary sequence hosting the epithermal Algamarca deposit, thereby providing new potential for exploration.