Summary:
Bégin-Lamarche is an anorthosite massif-hosted phosphate (apatite) mineral deposit, similar to the nearby Lac à l’Orignal Deposit.
The Bégin-Lamarche Phosphate Deposit is hosted in oxide-apatite peridotite intrusions within the LSJA Suite. The Bégin-Lamarche Phosphate Deposit as defined in drilling extends for 2.5 km along strike and dips are steeply to the west-northwest or east-southeast. The Deposit is internally offset along cross-cutting faults into three main phosphate mineralized zones: 1) the Southern Zone; 2) the Northern Zone; and 3) the Mountain Zone.
The Southern Zone, the largest of the three mineralized zones, consists of four phosphate layers, one of which has a thickness of up to 200 m and extends for 1.7 km along strike. The Northern Zone consists of two phosphate layers (Northeast and Northwest Domains) that are up to 100 m thick and extend for 500 m along strike. The Mountain Zone is a single phosphate-bearing mass up to 200 m in diameter and 250 m in length with a northwest dip. Drilling at the Mountain Zone intersected massive apatite (phosphate-bearing mineral) layers up to 2 m thick.
Mineralization
The mineral apatite at the Bégin-Lamarche Deposit is hosted in peridotite. The major minerals forming the peridotite are olivine, orthopyroxene, clinopyroxene, plagioclase, amphibole, apatite, ilmenite, and magnetite. Biotite, Al-spinel and pyrrhotite occur as accessory minerals (<5%). Common alteration minerals are serpentine and iddingsite. Cross-cutting calcite and serpentine veins are also common in some samples with high whole-rock Cl contents.
Based on the modal abundances of olivine, orthopyroxene, clinopyroxene, and plagioclase, the peridotite host rock can be more specifically characterized as olivine gabbronorite with phaneritic texture.
Olivine (Mg2SiO4 - Fe2SiO4) is the dominant mineral. Olivine crystals of the peridotite are subhedral in shape. The crystals range in size from ~0.2 to 3 mm. Equant crystals of olivine are common. Olivine crystals are generally fractured, and the fractures are filled with serpentine. Many olivine crystals from rock samples with high whole-rock Cl contents are altered to iddingsite. Additional alteration products of olivine are saponite and celadonite (Laverne et al., 2006; Singh et al., 2023), which occur in samples with high and low whole-rock Cl contents. Olivine has end-member compositions of forsterite (Mg2SiO4: 49.1 to 54.6 mol%) and fayalite (Fe2SiO4: 45.4 to 50.9 mol%). There is little variability in contents of SiO2 (34.4 to 35.2%), MnO (0.56 to 0.59%), and CaO (0.01 to 0.03%).
Orthopyroxene (Mg2Si2O6 - Fe2Si2O6) crystals are mostly subhedral in shape and equant in size (~0.5 to 2.0 mm). They are associated with magnetite, ilmenite and apatite, and are less altered than olivine.
Clinopyroxene ((Ca,Mg,Fe)(Mg,Fe)Si2O6) crystals are less abundant than orthopyroxene and are also generally subhedral. Crystals range in size from ~0.5 to 2 mm.
Plagioclase (NaAlSi3O8 - CaAl2Si2O8) crystals are euhedral to subhedral and mostly equant in size ranging from ~0.5 to 2 mm. End-member compositions are (44.1 to 48.4 mol.% anorthite, 51.5 to 52.7 mol.% albite, and 0.1 to 0.22 mol.% orthoclase. There is little variability in contents of SiO2 (55.0 to 55.8%) and Al2O3 (27.9 to 28.2%).
Amphibole (Na, K)0-1(Na, Ca, Mg, Fe2+, Mn2+, Li)2(Mg, Fe2+, Fe3+, Al, Mn, Zn, Cr, Ti, Li)5(Si, Al)8O22(OH, F, Cl, O)2) crystals are identified under the optical microscope from the samples with low whole-rock Cl contents. The crystals are mostly subhedral in shape. Alteration of amphibole and plagioclase to clay minerals has been reported (Proust et al., 2006).
Apatite (Ca5(PO4)3F) mostly occurs as individual crystals associated with oxide minerals, orthopyroxene, olivine, and amphibole. Inclusions of apatite within olivine and orthopyroxene occur locally. Apatite crystals are euhedral to subhedral in shape and range in size from ~0.3 to 3 mm. Late-stage veinlets of serpentine cross-cutting apatite are common in the peridotite. Apatite crystals have narrow ranges of CaO (54.8 to 55.4%) and P2O5 (41.8 to 42.4%) contents. The contents of SiO2 (=0.04%), MnO (0.05 to 0.16%), MgO (0.04 to 0.15%), and FeO (0.14 to 0.31%) are low. The contents of F (2.63 to 3.00%) are higher than Cl (660 to 860 ppm) and H2O (0.34 to 0.53%). Mole fraction values of fluorapatite (XApFAp) range from 0.71 to 0.80, as calculated following the procedure of Piccoli and Candela (2002).
Ilmenite (FeTiO3) and magnetite (Fe2O3) are the two dominant oxide minerals. Both minerals occur as euhedral to subhedral grains that range in size from ~0.2 to 1.5 mm in diameter.