Summary:
The gold-silver-zinc mineralization at Minera Florida presents characteristics common to several deposit models but they do not fit any unique model, likely due to evolution of a long-lived mineralizing system over time. Although it displays characteristics of epithermal deposits (Moncada et al., 2015) such as structurally controlled banded and cockade textured polymetallic veins with haloes of illite alteration in a near-surface volcanic setting, it also shares characteristics with structurally controlled iron oxide-copper-gold (IOCG) and skarn deposits that indicate an intrusion- related environment, suggesting an evolution from a hot, intrusion-related setting to a cooler epithermal environment over the span of the magmatic and hydrothermal system.
Intrusion-related deposits are a broad category that encompasses the various deposit types that can be found in such an environment. Some epithermal (both high-and low-sulphidation) as well as skarn, and IOCG deposits are intrusion-related; features common to these deposit types are found at Minera Florida (Moncada et al., 2015).
In light of this, the following sequence outlines the important stages of the formation of Minera Florida deposit and is proposed as a guide for exploration for additional mineralization of this type:
- The district was affected by an early very-high-temperature regional metasomatic event caused by the transport of magmatic fluids along the steep El Roble fault and through subsidiary faults throughout the district, both laterally and vertically.
- With time, fluid temperatures declined from >800°C down to about 500°C, producing retrograde alteration assemblages.
- The mineralization was emplaced along both high- and low-angle faults by fluids whose temperatures ranged from 300 to 500°C. Later fluid pulses at lower temperatures (200-300°C) were mostly barren.
- The eastern domain, between the Maqui and El Roble faults, is characterized by structurally controlled IOCG-like mineralization. The alteration haloes observed throughout the entire district (potassic, sodic-calcic, and propylitic) are also typical of IOCG-type deposits.
- The western part of the deposit, between the Agua Fría and Maqui faults, is characterized by structurally controlled polymetallic, skarn-like mineralization containing calcic garnets, hornblende, rhodonite, epidote, and calcic zeolites.
Mineralized zones developed along primary and secondary structures as hydrothermal breccias and zones of stockwork and veinlets. The form of the mineralized bodies is roughly vetiform or developed as pipe- like bodies in sigmoidal jogs and structural intersections. Classic vein geometry with well-developed vein walls is rare; in many cases gold, silver, and base metal grades drop off gradually at the boundaries of the orebodies, as breccias and stockwork zones pass outwards into gradually weakening veinlet zones.
The mineralization is always multi-episodic with re-brecciation of previous pulses and multiple cross- cutting relationships of veinlets; it can also be reworked in cataclastic zones. Disseminated pyrite occurs in alteration haloes but does not contain gold or silver.
In the core mine area, the mineralization is multi-episodic and polymetallic. It consists of pyritesphalerite-galena-chalcopyrite; it contains precious metals in the form of native gold and silver, electrum, silver sulphosalts, and acanthite and is associated with quartz-epidote-chlorite- amphibole-calcic garnet-magnetite-(rhodonite). Magnetite gives way upwards to magnetite pseudomorphs after hematite and specular hematite.
Mineralized zones have highly variable widths of <1 m to 20 m and have horizontal and vertical continuities of up to 300 m. Mineralization hosted in the Lower Tuffs and Lower Andesites usually (but not always) presents the highest precious metal grades; this is thought to reflect the availability of iron in these rocks to form pyrite and remove H2S from the mineralizing fluids.
Gangue minerals such as amphibole, rhodonite, scapolite, albite, and tourmaline are important vectors to mineralization.
Fluid flow from the hydrothermal system produced strong hydrothermal alteration in the rocks located at high stratigraphic levels above the mineralization. The Upper Tuffs are strongly silicified and contain hypogene white mica, chlorite, and pyrite and supergene kaolinite and smectite. The Upper Andesites are affected by structurally controlled narrow zones of epidotechlorite-zeolite-calcite-hematite alteration with associated broad haloes of phengitic white mica replacing early metasomatic biotite. These alteration zones can be correlated with mineralized zones at depth (e.g., in Tribuna East). These features are being used at surface to target potential underground mineralized zones at deep levels in the eastern block.
In Cerrito Metal, immediately to the east of the Maqui–Los Patos intersection, hydrothermal breccias are intensely silicified and present advanced argillic alteration, with dickite, pyrophyllite, andalusite and topaz surrounded by a halo of illite and kaolinite, associated with abundant pyrite and arsenic and molybdenum anomalies. The temporal and genetic relationships between this event and the mineralization at depth are currently unknown.
Post-mineral veinlets contain calcite, chlorite, and zeolites with hematite and scarce actinolite; they are associated with the formation of white mica and smectites, mostly replacing potassium feldspar. This late phase of veining and alteration is associated with post-mineral movements along the principal faults, such as the 200 m downthrow on the eastern side of the Maqui Fault. The actinolite Ar/Ar ages of 71–78 Ma probably represent the timing of this phase.