Summary:
Gold mineralization at Ana Paula is largely hosted in a north-south trending corridor of intrusive rocks steeply dipping to the west, at the contacts with sedimentary rocks and hornfels, and within important breccia bodies.
Alteration
The most comprehensive studies of alteration have been completed in a series of petrographic studies by Petrascience (2005), Colombo (2012) and McComb (2023).
Petrascience (2005) describes pervasive alteration of the plagioclase phenocrysts and groundmass in intrusive rocks to K-feldspar which McComb further describes as adularia. The K-feldspar alteration is later replaced by iron-carbonate, sericite and clay. Hornblende and biotite phenocrysts are altered to carbonate chlorite pyrite titanite with minor muscovite, clay and rutile. Latest alteration of plagioclase phenocrysts comprises clay alteration which locally consists of swelling clays. The pervasive K-feldspar alteration of the intrusive rocks is a feature recognized elsewhere in the GGB by Jones (2017) who interpreted K-feldspar flooding as a retrograde skarn event that introduced the first episode(s) of gold mineralization.
Hornfels and skarn alteration are common along sediment/intrusive contacts and have been differentiated based on grain size; aphanitic calc-silicate alteration has been termed hornfels while mesoscopic calc-silicate alteration has been termed skarn. At deeper levels, the alteration of the sediments generally becomes more widespread and continuous showing a bleached pale color with bands of medium-brown-colored garnets; though they are still very fine-grained. This style of widespread skarn alteration is more indicative of being near a larger heat source, than being directly associated with mineralization like the narrower zones at shallower levels. Petrascience (2005) described retrograde skarn alteration comprising fine-grained, fractured and broken garnet, patchy aggregates of calcite-hematite replacing K-feldspar, and muscovite or chlorite or clay replacing biotite.
Rocks of the Polymictic Breccia comprise of K-feldspar (adularia) altered clasts of feldspar porphyry and sediment domain clasts in a matrix of quartz, iron carbonate (ankerite or siderite) and sulfides, which include arsenopyrite, pyrrhotite and pyrite. Work by Petrascience (2005), Colombo (2012) and more recent work by McComb (2023) have identified gold associated with arsenopyrite as free grains on, or around the grains of arsenopyrite. In the Monomictic Breccia short wave infrared (SWIR) spectroscopy confirmed an overprinting assemblage dominated by illite and other white micas and clays including the advanced argillic phase dickite.
The alteration paragenesis suggested low-sulfidation epithermal conditions in some of the samples (McComb, 2023). In one example, gold mineralization was associated with a gold-bearing adularia-quartz-calcite arsenopyrite hydraulic breccia. In another sample, AP-11-37, 317.30 m, a contact metamorphic assemblage was characterized as calcite-epidote-andalusite-garnet (Colombo, 2012). In some cases, the alteration was overprinted by adularia-bearing assemblages (adularia-calcite-quartz±pyrite±arsenopyrite). In one of the samples affected by this alteration, gold was spatially associated with arsenopyrite which in most of its occurrences tends to replace pre-existing pyrite.
Mineralization
Low grade gold mineralization at Ana Paula extends 1,150 m roughly north south along strike. The width of mineralization is highly variable, between 100 m and 300 m wide with an average width of approximately 200 m. Mineralization extends down dip approximately 950 m to the west. The high-grade mineralization amenable to underground mining methods strikes 300 m east west, is approximately 150 m wide and extends down dip to the south 600 m. Gold mineralization is still open down dip.
Re-logging of historic and current drill holes has resulted in the differentiation of a suite of mineralized and barren veins and their cross-cutting relationships have enabled paragenetic sequence to be established. Lloyd (2023c) describes up to eight veining events, of which two or three are gold mineralizing events. While two of the veining events are related to gold deposition, the same mineralized fluids responsible for the mineralized veins also deposited gold as matrix fillings and clast replacements in the Polymictic Breccia and mineralized skarn style replacement bodies along feldspar porphyry and sediment contacts. Six veining events are described below.
V1: Quartz-pyrite veinlets are hosted in the sediments, typically 1 to 2 mm in width, and composed of variable amounts of silica and fine-grained pyrite.
V2: Sulfide micro-veinlets, the main gold mineralizing event at Ana Paula, are hosted in all rock types, but more abundant outside of the Polymictic Breccia in the feldspar porphyries. This generation of micro-veinlets occur as thin breccia veins 3 to 12 mm wide, as mossy patchy halos, as sheeted veins 2 to 5 mm wide, or more commonly as wispy discontinuous micro-veinlets only 1 mm in width or less.
V3: Quartz-pyrite±ankerite micro-veinlets are contemporaneous and very similar to V2 veinlets. This generation also occurs as medium-grained pyrite in veinlets or as patches, and appears to be responsible for a few distinct intervals that are gold rich but lack arsenic. It is this medium-grained pyrite that also often creates the massive sulfide mineralization in the sediments.
V4: White quartz and massive sulfide veining usually occurs as 1 to 3 mm wide white massive quartz veinlets with sections of massive sulfides, which is almost always comprised of pyrite. This generation of veinlets appear to be post-mineral.
V5: Grey quartz veinlets are usually 1 to 10 mm in width and may or not contain fine-grained pyrite or more rarely arsenopyrite. Overall, not very common and possibly a later re-mobilization of the early gold mineralization.
V6: Epithermal quartz veins which may be quartz, quartz-adularia, quartz-carbonates, or calcite often with calcite crystals growing into open space. Widths are variable between 1 mm and 10 cm and clearly cross-cut all other veinlets and host rocks.
In summary the four sites of gold deposition are:
1. Polymictic Breccia hosted mineralization with mainly sulfide (arsenopyrite and/or pyrrhotite or later replaced by pyrite) filling the matrix.
2. Exoskarn style pyrite replacement sediments along intrusive contacts.
3. Arsenopyrite micro-veinlets that fracture all rock types, but best developed in the feldspar porphyries.
4. Disseminated sulfides in the feldspar porphyries, likely a different manifestation of the V2 arsenopyrite micro-veinlets.
All four styles of mineralization have been developed by mineralizing fluids that exploited the contact zones between the feldspar porphyries and host thin-bedded sediments and deposited arsenopyrite, pyrrhotite and gold. Petrographic work suggests that pyrite has overprinted an earlier pyrrhotite phase of mineralization and this suggests that the mineralizing fluids have evolved with time. It is interpreted that this event represents a retrograde skarn event as seen at other deposits in the GGB (Jones, 2017).