The phonolites s.s. (Figure 3(c), Figure 3(d)) are well exposed in points 56, 57 and 118 (Figure 2). They present macroscopic features such as: gray or greenish gray color when unaltered and beige when altered; Porphyritic inequigranular texture, with about 5% phenocrysts (mainly of nepheline and potassic feldspar) and 95% of very fine matrix (aphanitic). The structure of these rocks is mainly massive and rarely of incipient magmatic flow, defined by oriented phenocrystals.

For this lithology, three petrographic thin sections were made. The microscopic characteristics in common are porphyritic inequigranular texture and flow structures defined by the orientation of the phenocrysts. Another observed feature is the unusual presence of thin nepheline veins discordant with rock structures.

The main mineral component of the phonolites s.s. is nepheline. It accounts for 30% to 60% of the rocks, 50% being a representative value. Nepheline is found as phenocrysts (Figure 4(a), Figure 4(b)) that can reach up to 2 cm along the major axis, but is also found as microphenocrysts (Figure 4(c), Figure 4(d)) in the matrix.

Sanidine is identified as the primary alkali feldspar in this lithology. Like nepheline, it may occur as phenocrysts, but it is predominantly the mineral constituent of the matrix. It can represent between 15 and 45% in the range and an average of 30% is a valid mean for Sanidine.

Clinopyroxenes were also described as modal minerals of phonolites s.s. Identified in the slides as aegirine-augite (Figure 4(e), Figure 4(f)), this mineral is exclusively in the matrix and corresponds for 20% to 25% of the rocks. Sometimes, they are associated with features, such as aggregates of pyroxenes, sometimes included in crystals of nepheline or sanidine, and sometimes in hexagonal aggregates, which are common in all samples. These features define a texture of rapid cooling and growth for the minerals (spherulitic texture).

Trace minerals, such as biotite, diopside and opaques are commonly observed, and may occasionally contain reaction edges with aegirine-augite (Figure 6(f)).

The most common accessories in phonolites s.s. are titanite, zircon, epidote and apatite; the last three as inclusions, mainly in nepheline and sanidine phenocrysts.

Phonolites with amygdaloidal structures are found mainly in points 56 and 57, occur intercalated with phonolites s.s. (Figure 3(a), Figure 3(b)), and are controlled by a penetrative system of fractures N55E/70SE, probably columnar jointed. This lithology presents specific macroscopic characteristics, generally observed as three levels of different textures and colors, with centimetric banding. The individualization of this lithotype, in addition to the structures mentioned, point to at least two alteration phases in these rocks, which will be discussed below.

In macroscopic samples, this lithotype mainly presents three levels of colors and different textures. In the first level are observed colors in between beige and light brown, very fine granulation (aphanitic), containing figures of oxidation. The second level has purple color, fine granulation and internal banding with levels that have a stronger or weaker shade of purple. The third level is also characterized by a purple color, very fine granulation and higher concentration of purple spheres or amygdala (Figure 5(c), Figure 5(d)).

Nepheline is also the main constituent in the amygdaloidal phonolites. They are exclusively matrix crystals, showing anhedral habits, probably because the substitution of this mineral by the zeolite in these rocks is larger. It represents between 40% and 60% of the composition and is homogeneously distributed at all rock levels.

Sanidine is found making up between 20% and 30% of the matrix (a single phenocrystal observed) with a subhedral euhedral.

In the amygdaloidal phonolites, clinopyroxenes are not observed, being completely replaced by opaque oxides or hydroxide (goethite), which together can represent 20 to 30% of the samples.

Fluorite traces are interpreted as present in the rock. This mineral would be responsible for the purple levels (Figure 5(a), Figure 5(b)). On thin section, levels containing a greater concentration of holes with square or rectangular forms are observed. It is possible that these holes were created with the removal of low hardness fluorite during preparation.

Biotite, calcite and celadonite are also traces in the amygdaloidal phonoliths. Biotite was observed as a single crystal in one sample, with acicular euhedral habit. Calcite and celadonite are unique minerals in filling the amygdala, accompanied by zeolites. Epidote is the only accessory mineral observed.

This group of rocks is very similar to the phonolites s.s. and was individualized for textural reasons, besides having different geochemical characteristics. The outcrops are located mainly in the northeastern portion of the alkali dyke, and have an exposure at point 125 (Figure 3(c), Figure 3(d)). These rocks were initially called altered phonolites and, macroscopically, show very light gray color, porphyritic inequigranular texture, fine granulation of the matrix (aphanitic) and massive structure, often showing cavities where there once were feldspar or feldspathoid crystals.

Microscopically (Figure 6(c), Figure 6(d), Figure 6(e)), a trachytic porphyritic inequigranular texture is observed, defined by the strong orientation of sanidine crystals in the rock matrix. One sample in which nepheline is practically absent was analyzed; only the habit of the crystal remained, the mineral having been entirely replaced by zeolites (Figure 6(a), Figure 6(b)).

The zeolite is the main mineral present in this rock, comprising about 50% in the samples. Sanidine is an important mineral as well, making up about 35% of the matrix of the trachyphonolites, Finally, oxides and hydroxides were described

as composing about 15% of the rock, probably coming from the supergenic alteration of aegirine-augite and opaque trace.

Although the association of this group of rocks with the phonolites studied is suggested, it should not be admitted as conclusive. Firstly, no in situ outcrop of this lithology was found, making it impossible to verify the contact relations between these alkaline rocks. Secondly, this type of alteration is considered unusual for the geological situation of the Anta Gorda alkali dyke. Blocks of this lithotype were found at point 123 (a and b) of the study area as macroscopically pale yellowish or greenish rocks, with breccia appearance, a very fine textured matrix (aphanitic) and thin quartz veins, showing some red “breccia fragments”.

In the epidote fenites (Figure 5(e), Figure 5(f)) several small/micro quartz veins or venules were observed, with less frequent opaque minerals; Very fine inequigranular texture was seen in the substitution mineral mass, and fine in the quartz crystals in the veins. The rock is deformed with high undulating extinction and fracturing in the quartz.

Quartz is the main mineral in the composition of this rock and it appears both as very fractured crystals with strong undulating extinction—probably remnant of the nesting rock of the TrêsCórregos Granite Complex—and, in smaller proportion, as microfractures filling, developing small/micro veins or venules. All quartz corresponds to approximately 45% of the rock.

A substitution mass formed by epidotes (35%), chlorite (10%) and opaque minerals (10%) completes the composition of the rocks. The epidote has light brown color, euhedral (prismatic) and anhedral habit, and occurs in aggregates (ghost texture) or crystals, better developed in the quartz veins. Traces of biotite are also observed in the samples. The fragments of breccia are, in fact, the form in which this substitution mass occurs; in the same habit of the mafic minerals and feldspars that have been replaced, giving the rocks a ghost texture.

The diffraction analyzes (Table 1) were made for three lithological groups: phonolites s.s., amygdaloidal phonolites and trachyphonolites. This technique was used to aid petrography and the interpretation of the mineral phases present in the rocks. It was exclusively qualitative and essential in distinguishing minerals with difficult characterization in petrographic analyses.

The characterization of the zeolites from the nepheline alteration was possible in these analyzes. It was revealed that a first phase of hydration was responsible for the formation of natrolite in phonolites s.s. As this process continued, mesolite, which is present in all samples, was crystallized. The technique didn’t reveal the presence or absence of fluorite crystals in the amygdaloidal phonoliths. Nevertheless, this mineral was included in the interpretation, as all peaks in the diffractogram suggested its presence. Finally, a white muscovite was characterized for the alteration of the sanidine and minerals previously treated as oxides and hydroxides, mainly hematite and goethite, respectively.

The same rocks were also subjected to X-ray Fluorescence analyzes (Table 2). Sample 57 A corresponds to a phonolite s.sand was analyzed quantitatively, with reading only of the larger elements, which implied greater precision of results. The amygdaloidal phonolite (57C) and trachyphonolite (125) were analyzed semi-quantitatively, with reading of larger elements and some trace elements, implying less accurate results. Discrepant responses were observed mainly in silica, sodium, carbonate and iron oxide. The trace element contents are only cited for comparison purposes, as the accuracy of the reading is very low, making the use of the data unfeasible.

The classifications adopted for the studied rocks also originated from the results of chemical data projections, mainly TAS diagrams {(Na₂O + K₂O) − SiO₂}. The data was plotted in the diagram R1R2 {R1 = 4Si − 11(Na + K) − 2(Fe + Ti) and R2 = 6Ca − 2Mg + Al}, proposed by De La Roche et al. (1980) [8] . The responses were: sample 57 A (phonolith s.s.) in the field of phonolith, and samples 57C (amyloid phonolith) and 125 (trachyphonolith) in the field of trachyphonolith. Because the diagram is drawn with many of the elements considered for imprecise samples 57C and 125, it will not be illustrated. However, the phonolite s.s classification for sample 57 A is valid.

The TAS diagrams used in the geochemical classification are from Le Bas (1989 [9] ; Figure 7) and Le Maitre (1989) [10] . However, in this case, the maximum value of alkalis considered by the author is 16%. That is why sample 57A of phonolite s.s, with the sum Na₂O + K₂O equal to 17.3% is not projected in the

diagram. For the Le Maitre TAS diagram (Figure 8). Alkali values of up to 20% are considered in the rocks. According to schemes of bothauthors, the rock 57A should be classified as a phonolite and 57C and 125 as trachytes.

The agpaitic index {I.A = (Na₂O + K₂O)/Al₂O₃} is best represented by the mean value of 0.80, calculated from the quantitative analysis. When this value is greater than 1, the phonolites should be treated as peralkaline phonolites. The studied lithotypes of the Anta Gorda alkali dyke were individualized into four groups, the classification adopted is based on at least two striking characteristics for each group. Textural, structural, mineralogical or geochemical composition and possible associations with hydrothermal fluids were considered important.

Some considerations regarding the geological evolution of the Anta Gorda alkali dyke will be mentioned in this chapter, but it should be emphasized that this is not the main objective of the present research. The development of the magmatic and metasomatic events is interpreted as the main forming agents of the studied rocks, and the proposition of the evolutionary model for the alkali dyke is admitted as “embryonic”, lacking in information and refinement.

The main processes involved in the formation of alkaline rocks are magmatic, directly responsible for the crystallization of phonolites s.s., amygdaloidal phonolites and trachyphonolites. The magmatic activity generating these lithologies is preceded by the tectonism that allowed the lodging of this intrusion at crustal level, in which it is found. Probably, the rise mechanism in this case is decompressive, with mantle fusion from the pressure relief.

The first magmatic event is attributed to the generation of ultrabasic magma, with a carbonated lherzolite source [1] [11] in deep mantle (100 - 200 km). From this magma, the first differentiation event occurs; after the fractioning of mafic and alkalis, a nefelinitic immiscible liquid is created. In this situation, the fractional crystallization in a closed system, that is, without crustal contribution, is responsible for the rest of the magmatic differentiation in these rocks. Also Ruberti et al. 2012 concluded that the parental magmas of both series of Banhadão Alkaline Complex (Ca- and Ti-rich series and Ca- and Ti-poor series) were generated from a CO₂-rich potassic metasomatized lithospheric mantle enriched with rutile-bearing phlogopite clinopyroxenite veins and that the low-Ca series was likely crystallized from a phonolite magma that represents the evolved fraction of primary potassic nephelinite magma [12] .

The Vasconcellos (1995) model is accepted in this work, with differentiation from phonolithic nephelinites (Figure 9) for the phonolites studied. Petrographic and geochemical evidences suggest that for the rocks of the Anta Gorda alkali dyke, trachyphonolites are less evolved than phonolites s.s.

The process proposed for the generation of the epidotes fenites studied involves the participation of fluid with alkaline magmatic origin. The alteration paragenesis found does not correspond to products that would be expected from the action of potassium or sodium fluids in this geological situation. Therefore, the calcium origin must be attributed to the nesting rock or to a possible carbonated magmatic pulse.

The description of this type of rock is considered of great importance, as it is a clue for the characterization of the fluid phase that accompanies the magmatic activity. Other guides would be the analysis of fluid inclusions and description of late minerals that fill holes or microfractures in the rocks. Amygdaloidal phonolites are rocks that may indicate some of these fluid characteristics, since they have hydrothermal minerals, such as calcite, celadonite and possibly fluorite.

The similarities between the Anta Gorda alkali dyke and the Mato Preto Alkaline Complex go beyond a possible fluoride-enriched fluid. Jenkins II (1987) [5]

described some important characteristics of the fluorite ore body, which is directly associated with phonolite intrusions, with 40% - 70% nepheline, 30% - 40% aegirine-augite, 25 % alkali feldspar, and fluorite “enveloping” the phonolites. In a first hydrothermal alteration phase, the phonolites would have produced an aureole of epidote alteration (Figure 10).

The phonolites studied are so similar to the ones from Mato Preto, that [3] the study of much of the alkaline rocks of the region showed that the only phonoliths s.s. (I.A less than 1) and trachyphonoliths are upwelling in the Mato Preto Complex. The phonolites s.s. of Anta Gorda have I.A of 0.80 and are also described in this dyke trachyphonolite dyke.

During field studies, no levels were found with representative concentrations of fluorite, nor is the presence of this mineral in the samples collected. Between the purple color of the amygdaloidal phonolites and the X-ray diffraction analysis, there is still little information regarding this matter.

Considering the similarities between Anta Gorda and Mato Preto phonolites, the most promising economic potential lies in mineralizations of rare earth elements, which were not passable of chemical analysis. The concentrations of Er, Tm, Yb and Lu in the trachyphonolites are up to 945 ppm in Mato Preto. Vasconcellos (1995) [3] also cites significant concentrations of trace elements in the peralkaline phonolites and trachyphonolites. Apparently, the highest concentration found in the rocks of Anta Gorda is 190 ppm of Y₂O₃.

Weathering processes are identified on phonolites of the Anta Gorda alkali dyke, and may be responsible for supergenic concentrations, mainly of alumina and potassium. This type of enriched material, with low iron oxide content is widely used in the white and refractory ceramic industries. It is mainly found at point 125 and may contain up to 11.16% K₂O; 22.17% Al₂O₃ and 1.93% Fe₂O₃, corresponding to altered trachyphonolites.

Finally, the concentration of zeolites is considered of economic interest as well. Both natrolite and mesolite have applications as filters for oils, among other things. These minerals are also products of chemical weathering, mainly of nepheline hydration. A zeolite count was performed on a petrographical analyse for a phonolite s.s, and for every five nepheline crystals, there is one of zeolite (Zeo/Ne = 1/5 ratio). In altered trachyphonolites this ratio is reversed.

All hypotheses presented for mineralizations in the studied region should be considered conclusions of a first stage of research. The characteristics observed and described do not make the Anta Gorda alkaline dyke a mineral occurrence, only a research target.